The Transport Layer Security (TLS) Protocol Version 1.3
draft-ietf-tls-tls13-24
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (tls WG) | |
|---|---|---|---|
| Author | Eric Rescorla | ||
| Last updated | 2018-03-02 (Latest revision 2018-02-15) | ||
| Replaces | draft-ietf-tls-rfc5246-bis | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text xml htmlized pdfized bibtex | ||
| Reviews |
GENART Last Call review
Ready with Nits
SECDIR Last Call review
Ready
|
||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Sean Turner | ||
| Shepherd write-up | Show Last changed 2018-03-01 | ||
| IESG | IESG state | Waiting for AD Go-Ahead | |
| Consensus boilerplate | Yes | ||
| Telechat date |
(None)
Needs a YES. Needs 8 more YES or NO OBJECTION positions to pass. |
||
| Responsible AD | Kathleen Moriarty | ||
| Send notices to | Sean Turner <sean@sn3rd.com> | ||
| IANA | IANA review state | IANA - Not OK |
draft-ietf-tls-tls13-24
Network Working Group E. Rescorla
Internet-Draft RTFM, Inc.
Obsoletes: 5077, 5246 (if approved) February 15, 2018
Updates: 4492, 5705, 6066, 6961 (if
approved)
Intended status: Standards Track
Expires: August 19, 2018
The Transport Layer Security (TLS) Protocol Version 1.3
draft-ietf-tls-tls13-24
Abstract
This document specifies version 1.3 of the Transport Layer Security
(TLS) protocol. TLS allows client/server applications to communicate
over the Internet in a way that is designed to prevent eavesdropping,
tampering, and message forgery.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 19, 2018.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
Rescorla Expires August 19, 2018 [Page 1]
Internet-Draft TLS February 2018
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Conventions and Terminology . . . . . . . . . . . . . . . 6
1.2. Change Log . . . . . . . . . . . . . . . . . . . . . . . 7
1.3. Major Differences from TLS 1.2 . . . . . . . . . . . . . 15
1.4. Updates Affecting TLS 1.2 . . . . . . . . . . . . . . . . 17
2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 17
2.1. Incorrect DHE Share . . . . . . . . . . . . . . . . . . . 20
2.2. Resumption and Pre-Shared Key (PSK) . . . . . . . . . . . 21
2.3. 0-RTT Data . . . . . . . . . . . . . . . . . . . . . . . 23
3. Presentation Language . . . . . . . . . . . . . . . . . . . . 25
3.1. Basic Block Size . . . . . . . . . . . . . . . . . . . . 25
3.2. Miscellaneous . . . . . . . . . . . . . . . . . . . . . . 25
3.3. Vectors . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.4. Numbers . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.5. Enumerateds . . . . . . . . . . . . . . . . . . . . . . . 27
3.6. Constructed Types . . . . . . . . . . . . . . . . . . . . 28
3.7. Constants . . . . . . . . . . . . . . . . . . . . . . . . 28
3.8. Variants . . . . . . . . . . . . . . . . . . . . . . . . 29
4. Handshake Protocol . . . . . . . . . . . . . . . . . . . . . 30
4.1. Key Exchange Messages . . . . . . . . . . . . . . . . . . 31
4.1.1. Cryptographic Negotiation . . . . . . . . . . . . . . 31
4.1.2. Client Hello . . . . . . . . . . . . . . . . . . . . 32
4.1.3. Server Hello . . . . . . . . . . . . . . . . . . . . 35
4.1.4. Hello Retry Request . . . . . . . . . . . . . . . . . 37
4.2. Extensions . . . . . . . . . . . . . . . . . . . . . . . 39
4.2.1. Supported Versions . . . . . . . . . . . . . . . . . 42
4.2.2. Cookie . . . . . . . . . . . . . . . . . . . . . . . 44
4.2.3. Signature Algorithms . . . . . . . . . . . . . . . . 44
4.2.4. Certificate Authorities . . . . . . . . . . . . . . . 48
4.2.5. OID Filters . . . . . . . . . . . . . . . . . . . . . 49
4.2.6. Post-Handshake Client Authentication . . . . . . . . 50
Rescorla Expires August 19, 2018 [Page 2]
Internet-Draft TLS February 2018
4.2.7. Negotiated Groups . . . . . . . . . . . . . . . . . . 50
4.2.8. Key Share . . . . . . . . . . . . . . . . . . . . . . 52
4.2.9. Pre-Shared Key Exchange Modes . . . . . . . . . . . . 55
4.2.10. Early Data Indication . . . . . . . . . . . . . . . . 56
4.2.11. Pre-Shared Key Extension . . . . . . . . . . . . . . 58
4.3. Server Parameters . . . . . . . . . . . . . . . . . . . . 62
4.3.1. Encrypted Extensions . . . . . . . . . . . . . . . . 62
4.3.2. Certificate Request . . . . . . . . . . . . . . . . . 63
4.4. Authentication Messages . . . . . . . . . . . . . . . . . 64
4.4.1. The Transcript Hash . . . . . . . . . . . . . . . . . 65
4.4.2. Certificate . . . . . . . . . . . . . . . . . . . . . 66
4.4.3. Certificate Verify . . . . . . . . . . . . . . . . . 71
4.4.4. Finished . . . . . . . . . . . . . . . . . . . . . . 73
4.5. End of Early Data . . . . . . . . . . . . . . . . . . . . 75
4.6. Post-Handshake Messages . . . . . . . . . . . . . . . . . 75
4.6.1. New Session Ticket Message . . . . . . . . . . . . . 75
4.6.2. Post-Handshake Authentication . . . . . . . . . . . . 78
4.6.3. Key and IV Update . . . . . . . . . . . . . . . . . . 78
5. Record Protocol . . . . . . . . . . . . . . . . . . . . . . . 79
5.1. Record Layer . . . . . . . . . . . . . . . . . . . . . . 80
5.2. Record Payload Protection . . . . . . . . . . . . . . . . 82
5.3. Per-Record Nonce . . . . . . . . . . . . . . . . . . . . 84
5.4. Record Padding . . . . . . . . . . . . . . . . . . . . . 85
5.5. Limits on Key Usage . . . . . . . . . . . . . . . . . . . 86
6. Alert Protocol . . . . . . . . . . . . . . . . . . . . . . . 86
6.1. Closure Alerts . . . . . . . . . . . . . . . . . . . . . 88
6.2. Error Alerts . . . . . . . . . . . . . . . . . . . . . . 89
7. Cryptographic Computations . . . . . . . . . . . . . . . . . 92
7.1. Key Schedule . . . . . . . . . . . . . . . . . . . . . . 92
7.2. Updating Traffic Keys and IVs . . . . . . . . . . . . . . 95
7.3. Traffic Key Calculation . . . . . . . . . . . . . . . . . 96
7.4. (EC)DHE Shared Secret Calculation . . . . . . . . . . . . 96
7.4.1. Finite Field Diffie-Hellman . . . . . . . . . . . . . 97
7.4.2. Elliptic Curve Diffie-Hellman . . . . . . . . . . . . 97
7.5. Exporters . . . . . . . . . . . . . . . . . . . . . . . . 98
8. 0-RTT and Anti-Replay . . . . . . . . . . . . . . . . . . . . 98
8.1. Single-Use Tickets . . . . . . . . . . . . . . . . . . . 100
8.2. Client Hello Recording . . . . . . . . . . . . . . . . . 100
8.3. Freshness Checks . . . . . . . . . . . . . . . . . . . . 101
9. Compliance Requirements . . . . . . . . . . . . . . . . . . . 103
9.1. Mandatory-to-Implement Cipher Suites . . . . . . . . . . 103
9.2. Mandatory-to-Implement Extensions . . . . . . . . . . . . 103
9.3. Protocol Invariants . . . . . . . . . . . . . . . . . . . 104
10. Security Considerations . . . . . . . . . . . . . . . . . . . 106
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 106
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 107
12.1. Normative References . . . . . . . . . . . . . . . . . . 107
12.2. Informative References . . . . . . . . . . . . . . . . . 110
Rescorla Expires August 19, 2018 [Page 3]
Internet-Draft TLS February 2018
Appendix A. State Machine . . . . . . . . . . . . . . . . . . . 118
A.1. Client . . . . . . . . . . . . . . . . . . . . . . . . . 118
A.2. Server . . . . . . . . . . . . . . . . . . . . . . . . . 119
Appendix B. Protocol Data Structures and Constant Values . . . . 119
B.1. Record Layer . . . . . . . . . . . . . . . . . . . . . . 120
B.2. Alert Messages . . . . . . . . . . . . . . . . . . . . . 120
B.3. Handshake Protocol . . . . . . . . . . . . . . . . . . . 122
B.3.1. Key Exchange Messages . . . . . . . . . . . . . . . . 122
B.3.2. Server Parameters Messages . . . . . . . . . . . . . 127
B.3.3. Authentication Messages . . . . . . . . . . . . . . . 128
B.3.4. Ticket Establishment . . . . . . . . . . . . . . . . 129
B.3.5. Updating Keys . . . . . . . . . . . . . . . . . . . . 129
B.4. Cipher Suites . . . . . . . . . . . . . . . . . . . . . . 130
Appendix C. Implementation Notes . . . . . . . . . . . . . . . . 131
C.1. Random Number Generation and Seeding . . . . . . . . . . 131
C.2. Certificates and Authentication . . . . . . . . . . . . . 132
C.3. Implementation Pitfalls . . . . . . . . . . . . . . . . . 132
C.4. Client Tracking Prevention . . . . . . . . . . . . . . . 133
C.5. Unauthenticated Operation . . . . . . . . . . . . . . . . 134
Appendix D. Backward Compatibility . . . . . . . . . . . . . . . 134
D.1. Negotiating with an older server . . . . . . . . . . . . 135
D.2. Negotiating with an older client . . . . . . . . . . . . 136
D.3. 0-RTT backwards compatibility . . . . . . . . . . . . . . 136
D.4. Middlebox Compatibility Mode . . . . . . . . . . . . . . 136
D.5. Backwards Compatibility Security Restrictions . . . . . . 137
Appendix E. Overview of Security Properties . . . . . . . . . . 138
E.1. Handshake . . . . . . . . . . . . . . . . . . . . . . . . 138
E.1.1. Key Derivation and HKDF . . . . . . . . . . . . . . . 141
E.1.2. Client Authentication . . . . . . . . . . . . . . . . 142
E.1.3. 0-RTT . . . . . . . . . . . . . . . . . . . . . . . . 142
E.1.4. Exporter Independence . . . . . . . . . . . . . . . . 142
E.1.5. Post-Compromise Security . . . . . . . . . . . . . . 143
E.1.6. External References . . . . . . . . . . . . . . . . . 143
E.2. Record Layer . . . . . . . . . . . . . . . . . . . . . . 143
E.2.1. External References . . . . . . . . . . . . . . . . . 144
E.3. Traffic Analysis . . . . . . . . . . . . . . . . . . . . 144
E.4. Side Channel Attacks . . . . . . . . . . . . . . . . . . 145
E.5. Replay Attacks on 0-RTT . . . . . . . . . . . . . . . . . 146
E.5.1. Replay and Exporters . . . . . . . . . . . . . . . . 147
E.6. Attacks on Static RSA . . . . . . . . . . . . . . . . . . 148
Appendix F. Working Group Information . . . . . . . . . . . . . 148
Appendix G. Contributors . . . . . . . . . . . . . . . . . . . . 148
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 155
Rescorla Expires August 19, 2018 [Page 4]
Internet-Draft TLS February 2018
1. Introduction
RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH The source for this
draft is maintained in GitHub. Suggested changes should be submitted
as pull requests at https://github.com/tlswg/tls13-spec.
Instructions are on that page as well. Editorial changes can be
managed in GitHub, but any substantive change should be discussed on
the TLS mailing list.
The primary goal of TLS is to provide a secure channel between two
communicating peers. Specifically, the channel should provide the
following properties:
- Authentication: The server side of the channel is always
authenticated; the client side is optionally authenticated.
Authentication can happen via asymmetric cryptography (e.g., RSA
[RSA], ECDSA [ECDSA], EdDSA [RFC8032]) or a pre-shared key (PSK).
- Confidentiality: Data sent over the channel after establishment is
only visible to the endpoints. TLS does not hide the length of
the data it transmits, though endpoints are able to pad TLS
records in order to obscure lengths and improve protection against
traffic analysis techniques.
- Integrity: Data sent over the channel after establishment cannot
be modified by attackers.
These properties should be true even in the face of an attacker who
has complete control of the network, as described in [RFC3552]. See
Appendix E for a more complete statement of the relevant security
properties.
TLS consists of two primary components:
- A handshake protocol (Section 4) that authenticates the
communicating parties, negotiates cryptographic modes and
parameters, and establishes shared keying material. The handshake
protocol is designed to resist tampering; an active attacker
should not be able to force the peers to negotiate different
parameters than they would if the connection were not under
attack.
- A record protocol (Section 5) that uses the parameters established
by the handshake protocol to protect traffic between the
communicating peers. The record protocol divides traffic up into
a series of records, each of which is independently protected
using the traffic keys.
Rescorla Expires August 19, 2018 [Page 5]
Internet-Draft TLS February 2018
TLS is application protocol independent; higher-level protocols can
layer on top of TLS transparently. The TLS standard, however, does
not specify how protocols add security with TLS; how to initiate TLS
handshaking and how to interpret the authentication certificates
exchanged are left to the judgment of the designers and implementors
of protocols that run on top of TLS.
This document defines TLS version 1.3. While TLS 1.3 is not directly
compatible with previous versions, all versions of TLS incorporate a
versioning mechanism which allows clients and servers to
interoperably negotiate a common version if one is supported by both
peers.
This document supersedes and obsoletes previous versions of TLS
including version 1.2 [RFC5246]. It also obsoletes the TLS ticket
mechanism defined in [RFC5077] and replaces it with the mechanism
defined in Section 2.2. Section 4.2.7 updates [RFC4492] by modifying
the protocol attributes used to negotiate Elliptic Curves. Because
TLS 1.3 changes the way keys are derived it updates [RFC5705] as
described in Section 7.5 it also changes how OCSP messages are
carried and therefore updates [RFC6066] and obsoletes [RFC6961] as
described in section Section 4.4.2.1.
1.1. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
The following terms are used:
client: The endpoint initiating the TLS connection.
connection: A transport-layer connection between two endpoints.
endpoint: Either the client or server of the connection.
handshake: An initial negotiation between client and server that
establishes the parameters of their subsequent interactions.
peer: An endpoint. When discussing a particular endpoint, "peer"
refers to the endpoint that is not the primary subject of discussion.
receiver: An endpoint that is receiving records.
sender: An endpoint that is transmitting records.
Rescorla Expires August 19, 2018 [Page 6]
Internet-Draft TLS February 2018
server: The endpoint which did not initiate the TLS connection.
1.2. Change Log
RFC EDITOR PLEASE DELETE THIS SECTION.
(*) indicates changes to the wire protocol which may require
implementations to update.
draft-24
- Require that CH2 have version 0303 (*)
- Some clarifications
draft-23 - Renumber key_share (*)
- Add a new extension and new code points to allow negotiating PSS
separately for certificates and CertificateVerify (*)
- Slightly restrict when CCS must be accepted to make implementation
easier.
- Document protocol invariants
- Add some text on the security of static RSA.
draft-22 - Implement changes for improved middlebox penetration (*)
- Move server_certificate_type to encrypted extensions (*)
- Allow resumption with a different SNI (*)
- Padding extension can change on HRR (*)
- Allow an empty ticket_nonce (*)
- Remove requirement to immediately respond to close_notify with
close_notify (allowing half-close)
draft-21
- Add a per-ticket nonce so that each ticket is associated with a
different PSK (*).
- Clarify that clients should send alerts with the handshake key if
possible.
Rescorla Expires August 19, 2018 [Page 7]
Internet-Draft TLS February 2018
- Update state machine to show rekeying events
- Add discussion of 0-RTT and replay. Recommend that
implementations implement some anti-replay mechanism.
draft-20
- Add "post_handshake_auth" extension to negotiate post-handshake
authentication (*).
- Shorten labels for HKDF-Expand-Label so that we can fit within one
compression block (*).
- Define how RFC 7250 works (*).
- Re-enable post-handshake client authentication even when you do
PSK. The previous prohibition was editorial error.
- Remove cert_type and user_mapping, which don't work on TLS 1.3
anyway.
- Added the no_application_protocol alert from [RFC7301] to the list
of extensions.
- Added discussion of traffic analysis and side channel attacks.
draft-19
- Hash context_value input to Exporters (*)
- Add an additional Derive-Secret stage to Exporters (*).
- Hash ClientHello1 in the transcript when HRR is used. This
reduces the state that needs to be carried in cookies. (*)
- Restructure CertificateRequest to have the selectors in
extensions. This also allowed defining a
"certificate_authorities" extension which can be used by the
client instead of trusted_ca_keys (*).
- Tighten record framing requirements and require checking of them
(*).
- Consolidate "ticket_early_data_info" and "early_data" into a
single extension (*).
- Change end_of_early_data to be a handshake message (*).
Rescorla Expires August 19, 2018 [Page 8]
Internet-Draft TLS February 2018
- Add pre-extract Derive-Secret stages to key schedule (*).
- Remove spurious requirement to implement "pre_shared_key".
- Clarify location of "early_data" from server (it goes in EE, as
indicated by the table in S 10).
- Require peer public key validation
- Add state machine diagram.
draft-18
- Remove unnecessary resumption_psk which is the only thing expanded
from the resumption master secret. (*).
- Fix signature_algorithms entry in extensions table.
- Restate rule from RFC 6066 that you can't resume unless SNI is the
same.
draft-17
- Remove 0-RTT Finished and resumption_context, and replace with a
psk_binder field in the PSK itself (*)
- Restructure PSK key exchange negotiation modes (*)
- Add max_early_data_size field to TicketEarlyDataInfo (*)
- Add a 0-RTT exporter and change the transcript for the regular
exporter (*)
- Merge TicketExtensions and Extensions registry. Changes
ticket_early_data_info code point (*)
- Replace Client.key_shares in response to HRR (*)
- Remove redundant labels for traffic key derivation (*)
- Harmonize requirements about cipher suite matching: for resumption
you need to match KDF but for 0-RTT you need whole cipher suite.
This allows PSKs to actually negotiate cipher suites. (*)
- Move SCT and OCSP into Certificate.extensions (*)
- Explicitly allow non-offered extensions in NewSessionTicket
Rescorla Expires August 19, 2018 [Page 9]
Internet-Draft TLS February 2018
- Explicitly allow predicting client Finished for NST
- Clarify conditions for allowing 0-RTT with PSK
draft-16
- Revise version negotiation (*)
- Change RSASSA-PSS and EdDSA SignatureScheme codepoints for better
backwards compatibility (*)
- Move HelloRetryRequest.selected_group to an extension (*)
- Clarify the behavior of no exporter context and make it the same
as an empty context.(*)
- New KeyUpdate format that allows for requesting/not-requesting an
answer. This also means changes to the key schedule to support
independent updates (*)
- New certificate_required alert (*)
- Forbid CertificateRequest with 0-RTT and PSK.
- Relax requirement to check SNI for 0-RTT.
draft-15
- New negotiation syntax as discussed in Berlin (*)
- Require CertificateRequest.context to be empty during handshake
(*)
- Forbid empty tickets (*)
- Forbid application data messages in between post-handshake
messages from the same flight (*)
- Clean up alert guidance (*)
- Clearer guidance on what is needed for TLS 1.2.
- Guidance on 0-RTT time windows.
- Rename a bunch of fields.
- Remove old PRNG text.
Rescorla Expires August 19, 2018 [Page 10]
Internet-Draft TLS February 2018
- Explicitly require checking that handshake records not span key
changes.
draft-14
- Allow cookies to be longer (*)
- Remove the "context" from EarlyDataIndication as it was undefined
and nobody used it (*)
- Remove 0-RTT EncryptedExtensions and replace the ticket_age
extension with an obfuscated version. Also necessitates a change
to NewSessionTicket (*).
- Move the downgrade sentinel to the end of ServerHello.Random to
accommodate tlsdate (*).
- Define ecdsa_sha1 (*).
- Allow resumption even after fatal alerts. This matches current
practice.
- Remove non-closure warning alerts. Require treating unknown
alerts as fatal.
- Make the rules for accepting 0-RTT less restrictive.
- Clarify 0-RTT backward-compatibility rules.
- Clarify how 0-RTT and PSK identities interact.
- Add a section describing the data limits for each cipher.
- Major editorial restructuring.
- Replace the Security Analysis section with a WIP draft.
draft-13
- Allow server to send SupportedGroups.
- Remove 0-RTT client authentication
- Remove (EC)DHE 0-RTT.
- Flesh out 0-RTT PSK mode and shrink EarlyDataIndication
- Turn PSK-resumption response into an index to save room
Rescorla Expires August 19, 2018 [Page 11]
Internet-Draft TLS February 2018
- Move CertificateStatus to an extension
- Extra fields in NewSessionTicket.
- Restructure key schedule and add a resumption_context value.
- Require DH public keys and secrets to be zero-padded to the size
of the group.
- Remove the redundant length fields in KeyShareEntry.
- Define a cookie field for HRR.
draft-12
- Provide a list of the PSK cipher suites.
- Remove the ability for the ServerHello to have no extensions (this
aligns the syntax with the text).
- Clarify that the server can send application data after its first
flight (0.5 RTT data)
- Revise signature algorithm negotiation to group hash, signature
algorithm, and curve together. This is backwards compatible.
- Make ticket lifetime mandatory and limit it to a week.
- Make the purpose strings lower-case. This matches how people are
implementing for interop.
- Define exporters.
- Editorial cleanup
draft-11
- Port the CFRG curves & signatures work from RFC4492bis.
- Remove sequence number and version from additional_data, which is
now empty.
- Reorder values in HkdfLabel.
- Add support for version anti-downgrade mechanism.
- Update IANA considerations section and relax some of the policies.
Rescorla Expires August 19, 2018 [Page 12]
Internet-Draft TLS February 2018
- Unify authentication modes. Add post-handshake client
authentication.
- Remove early_handshake content type. Terminate 0-RTT data with an
alert.
- Reset sequence number upon key change (as proposed by Fournet et
al.)
draft-10
- Remove ClientCertificateTypes field from CertificateRequest and
add extensions.
- Merge client and server key shares into a single extension.
draft-09
- Change to RSA-PSS signatures for handshake messages.
- Remove support for DSA.
- Update key schedule per suggestions by Hugo, Hoeteck, and Bjoern
Tackmann.
- Add support for per-record padding.
- Switch to encrypted record ContentType.
- Change HKDF labeling to include protocol version and value
lengths.
- Shift the final decision to abort a handshake due to incompatible
certificates to the client rather than having servers abort early.
- Deprecate SHA-1 with signatures.
- Add MTI algorithms.
draft-08
- Remove support for weak and lesser used named curves.
- Remove support for MD5 and SHA-224 hashes with signatures.
- Update lists of available AEAD cipher suites and error alerts.
Rescorla Expires August 19, 2018 [Page 13]
Internet-Draft TLS February 2018
- Reduce maximum permitted record expansion for AEAD from 2048 to
256 octets.
- Require digital signatures even when a previous configuration is
used.
- Merge EarlyDataIndication and KnownConfiguration.
- Change code point for server_configuration to avoid collision with
server_hello_done.
- Relax certificate_list ordering requirement to match current
practice.
draft-07
- Integration of semi-ephemeral DH proposal.
- Add initial 0-RTT support.
- Remove resumption and replace with PSK + tickets.
- Move ClientKeyShare into an extension.
- Move to HKDF.
draft-06
- Prohibit RC4 negotiation for backwards compatibility.
- Freeze & deprecate record layer version field.
- Update format of signatures with context.
- Remove explicit IV.
draft-05
- Prohibit SSL negotiation for backwards compatibility.
- Fix which MS is used for exporters.
draft-04
- Modify key computations to include session hash.
- Remove ChangeCipherSpec.
Rescorla Expires August 19, 2018 [Page 14]
Internet-Draft TLS February 2018
- Renumber the new handshake messages to be somewhat more consistent
with existing convention and to remove a duplicate registration.
- Remove renegotiation.
- Remove point format negotiation.
draft-03
- Remove GMT time.
- Merge in support for ECC from RFC 4492 but without explicit
curves.
- Remove the unnecessary length field from the AD input to AEAD
ciphers.
- Rename {Client,Server}KeyExchange to {Client,Server}KeyShare.
- Add an explicit HelloRetryRequest to reject the client's.
draft-02
- Increment version number.
- Rework handshake to provide 1-RTT mode.
- Remove custom DHE groups.
- Remove support for compression.
- Remove support for static RSA and DH key exchange.
- Remove support for non-AEAD ciphers.
1.3. Major Differences from TLS 1.2
The following is a list of the major functional differences between
TLS 1.2 and TLS 1.3. It is not intended to be exhaustive and there
are many minor differences.
- The list of supported symmetric algorithms has been pruned of all
algorithms that are considered legacy. Those that remain all use
Authenticated Encryption with Associated Data (AEAD) algorithms.
The ciphersuite concept has been changed to separate the
authentication and key exchange mechanisms from the record
protection algorithm (including secret key length) and a hash to
be used with the key derivation function and HMAC.
Rescorla Expires August 19, 2018 [Page 15]
Internet-Draft TLS February 2018
- A 0-RTT mode was added, saving a round-trip at connection setup
for some application data, at the cost of certain security
properties.
- Static RSA and Diffie-Hellman cipher suites have been removed; all
public-key based key exchange mechanisms now provide forward
secrecy.
- All handshake messages after the ServerHello are now encrypted.
The newly introduced EncryptedExtension message allows various
extensions previously sent in clear in the ServerHello to also
enjoy confidentiality protection from active attackers.
- The key derivation functions have been re-designed. The new
design allows easier analysis by cryptographers due to their
improved key separation properties. The HMAC-based Extract-and-
Expand Key Derivation Function (HKDF) is used as an underlying
primitive.
- The handshake state machine has been significantly restructured to
be more consistent and to remove superfluous messages such as
ChangeCipherSpec.
- Elliptic curve algorithms are now in the base spec and includes
new signature algorithms, such as ed25519 and ed448. TLS 1.3
removed point format negotiation in favor of a single point format
for each curve.
- Other cryptographic improvements including the removal of
compression and custom DHE groups, changing the RSA padding to use
PSS, and the removal of DSA.
- The TLS 1.2 version negotiation mechanism has been deprecated in
favor of a version list in an extension. This increases
compatibility with servers which incorrectly implemented version
negotiation.
- Session resumption with and without server-side state as well as
the PSK-based ciphersuites of earlier TLS versions have been
replaced by a single new PSK exchange.
- Updated references to point to the updated versions of RFCs, as
appropriate (e.g., RFC 5280 rather than RFC 3280).
Rescorla Expires August 19, 2018 [Page 16]
Internet-Draft TLS February 2018
1.4. Updates Affecting TLS 1.2
This document defines several changes that optionally affect
implementations of TLS 1.2:
- A version downgrade protection mechanism is described in
Section 4.1.3.
- RSASSA-PSS signature schemes are defined in Section 4.2.3.
- The "supported_versions" ClientHello extension can be used to
negotiate the version of TLS to use, in preference to the
legacy_version field of the ClientHello.
An implementation of TLS 1.3 that also supports TLS 1.2 might need to
include changes to support these changes even when TLS 1.3 is not in
use. See the referenced sections for more details.
Additionally, this document clarifies some compliance requirements
for earlier versions of TLS; see Section 9.3.
2. Protocol Overview
The cryptographic parameters used by the secure channel are produced
by the TLS handshake protocol. This sub-protocol of TLS is used by
the client and server when first communicating with each other. The
handshake protocol allows peers to negotiate a protocol version,
select cryptographic algorithms, optionally authenticate each other,
and establish shared secret keying material. Once the handshake is
complete, the peers use the established keys to protect the
application layer traffic.
A failure of the handshake or other protocol error triggers the
termination of the connection, optionally preceded by an alert
message (Section 6).
TLS supports three basic key exchange modes:
- (EC)DHE (Diffie-Hellman over either finite fields or elliptic
curves)
- PSK-only
- PSK with (EC)DHE
Figure 1 below shows the basic full TLS handshake:
Rescorla Expires August 19, 2018 [Page 17]
Internet-Draft TLS February 2018
Client Server
Key ^ ClientHello
Exch | + key_share*
| + signature_algorithms*
| + psk_key_exchange_modes*
v + pre_shared_key* -------->
ServerHello ^ Key
+ key_share* | Exch
+ pre_shared_key* v
{EncryptedExtensions} ^ Server
{CertificateRequest*} v Params
{Certificate*} ^
{CertificateVerify*} | Auth
{Finished} v
<-------- [Application Data*]
^ {Certificate*}
Auth | {CertificateVerify*}
v {Finished} -------->
[Application Data] <-------> [Application Data]
+ Indicates noteworthy extensions sent in the
previously noted message.
* Indicates optional or situation-dependent
messages/extensions that are not always sent.
{} Indicates messages protected using keys
derived from a [sender]_handshake_traffic_secret.
[] Indicates messages protected using keys
derived from [sender]_application_traffic_secret_N
Figure 1: Message flow for full TLS Handshake
The handshake can be thought of as having three phases (indicated in
the diagram above):
- Key Exchange: Establish shared keying material and select the
cryptographic parameters. Everything after this phase is
encrypted.
- Server Parameters: Establish other handshake parameters (whether
the client is authenticated, application layer protocol support,
etc.).
- Authentication: Authenticate the server (and optionally the
client) and provide key confirmation and handshake integrity.
Rescorla Expires August 19, 2018 [Page 18]
Internet-Draft TLS February 2018
In the Key Exchange phase, the client sends the ClientHello
(Section 4.1.2) message, which contains a random nonce
(ClientHello.random); its offered protocol versions; a list of
symmetric cipher/HKDF hash pairs; either a set of Diffie-Hellman key
shares (in the "key_share" extension Section 4.2.8), a set of pre-
shared key labels (in the "pre_shared_key" extension Section 4.2.11)
or both; and potentially additional extensions.
The server processes the ClientHello and determines the appropriate
cryptographic parameters for the connection. It then responds with
its own ServerHello (Section 4.1.3), which indicates the negotiated
connection parameters. The combination of the ClientHello and the
ServerHello determines the shared keys. If (EC)DHE key establishment
is in use, then the ServerHello contains a "key_share" extension with
the server's ephemeral Diffie-Hellman share which MUST be in the same
group as one of the client's shares. If PSK key establishment is in
use, then the ServerHello contains a "pre_shared_key" extension
indicating which of the client's offered PSKs was selected. Note
that implementations can use (EC)DHE and PSK together, in which case
both extensions will be supplied.
The server then sends two messages to establish the Server
Parameters:
EncryptedExtensions: responses to ClientHello extensions that are
not required to determine the cryptographic parameters, other than
those that are specific to individual certificates.
[Section 4.3.1]
CertificateRequest: if certificate-based client authentication is
desired, the desired parameters for that certificate. This
message is omitted if client authentication is not desired.
[Section 4.3.2]
Finally, the client and server exchange Authentication messages. TLS
uses the same set of messages every time that authentication is
needed. Specifically:
Certificate: the certificate of the endpoint and any per-certificate
extensions. This message is omitted by the server if not
authenticating with a certificate and by the client if the server
did not send CertificateRequest (thus indicating that the client
should not authenticate with a certificate). Note that if raw
public keys [RFC7250] or the cached information extension
[RFC7924] are in use, then this message will not contain a
certificate but rather some other value corresponding to the
server's long-term key. [Section 4.4.2]
Rescorla Expires August 19, 2018 [Page 19]
Internet-Draft TLS February 2018
CertificateVerify: a signature over the entire handshake using the
private key corresponding to the public key in the Certificate
message. This message is omitted if the endpoint is not
authenticating via a certificate. [Section 4.4.3]
Finished: a MAC (Message Authentication Code) over the entire
handshake. This message provides key confirmation, binds the
endpoint's identity to the exchanged keys, and in PSK mode also
authenticates the handshake. [Section 4.4.4]
Upon receiving the server's messages, the client responds with its
Authentication messages, namely Certificate and CertificateVerify (if
requested), and Finished.
At this point, the handshake is complete, and the client and server
derive the keying material required by the record layer to exchange
application-layer data protected through authenticated encryption.
Application data MUST NOT be sent prior to sending the Finished
message and until the record layer starts using encryption keys.
Note that while the server may send application data prior to
receiving the client's Authentication messages, any data sent at that
point is, of course, being sent to an unauthenticated peer.
2.1. Incorrect DHE Share
If the client has not provided a sufficient "key_share" extension
(e.g., it includes only DHE or ECDHE groups unacceptable to or
unsupported by the server), the server corrects the mismatch with a
HelloRetryRequest and the client needs to restart the handshake with
an appropriate "key_share" extension, as shown in Figure 2. If no
common cryptographic parameters can be negotiated, the server MUST
abort the handshake with an appropriate alert.
Rescorla Expires August 19, 2018 [Page 20]
Internet-Draft TLS February 2018
Client Server
ClientHello
+ key_share -------->
<-------- HelloRetryRequest
+ key_share
ClientHello
+ key_share -------->
ServerHello
+ key_share
{EncryptedExtensions}
{CertificateRequest*}
{Certificate*}
{CertificateVerify*}
{Finished}
<-------- [Application Data*]
{Certificate*}
{CertificateVerify*}
{Finished} -------->
[Application Data] <-------> [Application Data]
Figure 2: Message flow for a full handshake with mismatched
parameters
Note: The handshake transcript includes the initial ClientHello/
HelloRetryRequest exchange; it is not reset with the new ClientHello.
TLS also allows several optimized variants of the basic handshake, as
described in the following sections.
2.2. Resumption and Pre-Shared Key (PSK)
Although TLS PSKs can be established out of band, PSKs can also be
established in a previous connection and then reused ("session
resumption"). Once a handshake has completed, the server can send to
the client a PSK identity that corresponds to a unique key derived
from the initial handshake (see Section 4.6.1). The client can then
use that PSK identity in future handshakes to negotiate the use of
the associated PSK. If the server accepts it, then the security
context of the new connection is cryptographically tied to the
original connection and the key derived from the initial handshake is
used to bootstrap the cryptographic state instead of a full
handshake. In TLS 1.2 and below, this functionality was provided by
"session IDs" and "session tickets" [RFC5077]. Both mechanisms are
obsoleted in TLS 1.3.
Rescorla Expires August 19, 2018 [Page 21]
Internet-Draft TLS February 2018
PSKs can be used with (EC)DHE key exchange in order to provide
forward secrecy in combination with shared keys, or can be used
alone, at the cost of losing forward secrecy for the application
data.
Figure 3 shows a pair of handshakes in which the first establishes a
PSK and the second uses it:
Client Server
Initial Handshake:
ClientHello
+ key_share -------->
ServerHello
+ key_share
{EncryptedExtensions}
{CertificateRequest*}
{Certificate*}
{CertificateVerify*}
{Finished}
<-------- [Application Data*]
{Certificate*}
{CertificateVerify*}
{Finished} -------->
<-------- [NewSessionTicket]
[Application Data] <-------> [Application Data]
Subsequent Handshake:
ClientHello
+ key_share*
+ pre_shared_key -------->
ServerHello
+ pre_shared_key
+ key_share*
{EncryptedExtensions}
{Finished}
<-------- [Application Data*]
{Finished} -------->
[Application Data] <-------> [Application Data]
Figure 3: Message flow for resumption and PSK
As the server is authenticating via a PSK, it does not send a
Certificate or a CertificateVerify message. When a client offers
resumption via PSK, it SHOULD also supply a "key_share" extension to
the server to allow the server to decline resumption and fall back to
a full handshake, if needed. The server responds with a
Rescorla Expires August 19, 2018 [Page 22]
Internet-Draft TLS February 2018
"pre_shared_key" extension to negotiate use of PSK key establishment
and can (as shown here) respond with a "key_share" extension to do
(EC)DHE key establishment, thus providing forward secrecy.
When PSKs are provisioned out of band, the PSK identity and the KDF
hash algorithm to be used with the PSK MUST also be provisioned.
Note: When using an out-of-band provisioned pre-shared secret, a
critical consideration is using sufficient entropy during the key
generation, as discussed in [RFC4086]. Deriving a shared secret
from a password or other low-entropy sources is not secure. A
low-entropy secret, or password, is subject to dictionary attacks
based on the PSK binder. The specified PSK authentication is not
a strong password-based authenticated key exchange even when used
with Diffie-Hellman key establishment.
2.3. 0-RTT Data
When clients and servers share a PSK (either obtained externally or
via a previous handshake), TLS 1.3 allows clients to send data on the
first flight ("early data"). The client uses the PSK to authenticate
the server and to encrypt the early data.
As shown in Figure 4, the 0-RTT data is just added to the 1-RTT
handshake in the first flight. The rest of the handshake uses the
same messages as with a 1-RTT handshake with PSK resumption.
Rescorla Expires August 19, 2018 [Page 23]
Internet-Draft TLS February 2018
Client Server
ClientHello
+ early_data
+ key_share*
+ psk_key_exchange_modes
+ pre_shared_key
(Application Data*) -------->
ServerHello
+ pre_shared_key
+ key_share*
{EncryptedExtensions}
+ early_data*
{Finished}
<-------- [Application Data*]
(EndOfEarlyData)
{Finished} -------->
[Application Data] <-------> [Application Data]
+ Indicates noteworthy extensions sent in the
previously noted message.
* Indicates optional or situation-dependent
messages/extensions that are not always sent.
() Indicates messages protected using keys
derived from client_early_traffic_secret.
{} Indicates messages protected using keys
derived from a [sender]_handshake_traffic_secret.
[] Indicates messages protected using keys
derived from [sender]_application_traffic_secret_N
Figure 4: Message flow for a zero round trip handshake
IMPORTANT NOTE: The security properties for 0-RTT data are weaker
than those for other kinds of TLS data. Specifically:
1. This data is not forward secret, as it is encrypted solely under
keys derived using the offered PSK.
2. There are no guarantees of non-replay between connections.
Protection against replay for ordinary TLS 1.3 1-RTT data is
provided via the server's Random value, but 0-RTT data does not
depend on the ServerHello and therefore has weaker guarantees.
This is especially relevant if the data is authenticated either
Rescorla Expires August 19, 2018 [Page 24]
Internet-Draft TLS February 2018
with TLS client authentication or inside the application
protocol. The same warnings apply to any use of the
early_exporter_master_secret.
0-RTT data cannot be duplicated within a connection (i.e., the server
will not process the same data twice for the same connection) and an
attacker will not be able to make 0-RTT data appear to be 1-RTT data
(because it is protected with different keys.) Appendix E.5 contains
a description of potential attacks and Section 8 describes mechanisms
which the server can use to limit the impact of replay.
3. Presentation Language
This document deals with the formatting of data in an external
representation. The following very basic and somewhat casually
defined presentation syntax will be used.
3.1. Basic Block Size
The representation of all data items is explicitly specified. The
basic data block size is one byte (i.e., 8 bits). Multiple byte data
items are concatenations of bytes, from left to right, from top to
bottom. From the byte stream, a multi-byte item (a numeric in the
example) is formed (using C notation) by:
value = (byte[0] << 8*(n-1)) | (byte[1] << 8*(n-2)) |
... | byte[n-1];
This byte ordering for multi-byte values is the commonplace network
byte order or big-endian format.
3.2. Miscellaneous
Comments begin with "/*" and end with "*/".
Optional components are denoted by enclosing them in "[[ ]]" double
brackets.
Single-byte entities containing uninterpreted data are of type
opaque.
A type alias T' for an existing type T is defined by:
T T';
Rescorla Expires August 19, 2018 [Page 25]
Internet-Draft TLS February 2018
3.3. Vectors
A vector (single-dimensioned array) is a stream of homogeneous data
elements. The size of the vector may be specified at documentation
time or left unspecified until runtime. In either case, the length
declares the number of bytes, not the number of elements, in the
vector. The syntax for specifying a new type, T', that is a fixed-
length vector of type T is
T T'[n];
Here, T' occupies n bytes in the data stream, where n is a multiple
of the size of T. The length of the vector is not included in the
encoded stream.
In the following example, Datum is defined to be three consecutive
bytes that the protocol does not interpret, while Data is three
consecutive Datum, consuming a total of nine bytes.
opaque Datum[3]; /* three uninterpreted bytes */
Datum Data[9]; /* 3 consecutive 3-byte vectors */
Variable-length vectors are defined by specifying a subrange of legal
lengths, inclusively, using the notation <floor..ceiling>. When
these are encoded, the actual length precedes the vector's contents
in the byte stream. The length will be in the form of a number
consuming as many bytes as required to hold the vector's specified
maximum (ceiling) length. A variable-length vector with an actual
length field of zero is referred to as an empty vector.
T T'<floor..ceiling>;
In the following example, mandatory is a vector that must contain
between 300 and 400 bytes of type opaque. It can never be empty.
The actual length field consumes two bytes, a uint16, which is
sufficient to represent the value 400 (see Section 3.4). Similarly,
longer can represent up to 800 bytes of data, or 400 uint16 elements,
and it may be empty. Its encoding will include a two-byte actual
length field prepended to the vector. The length of an encoded
vector must be an exact multiple of the length of a single element
(e.g., a 17-byte vector of uint16 would be illegal).
opaque mandatory<300..400>;
/* length field is 2 bytes, cannot be empty */
uint16 longer<0..800>;
/* zero to 400 16-bit unsigned integers */
Rescorla Expires August 19, 2018 [Page 26]
Internet-Draft TLS February 2018
3.4. Numbers
The basic numeric data type is an unsigned byte (uint8). All larger
numeric data types are formed from fixed-length series of bytes
concatenated as described in Section 3.1 and are also unsigned. The
following numeric types are predefined.
uint8 uint16[2];
uint8 uint24[3];
uint8 uint32[4];
uint8 uint64[8];
All values, here and elsewhere in the specification, are stored in
network byte (big-endian) order; the uint32 represented by the hex
bytes 01 02 03 04 is equivalent to the decimal value 16909060.
3.5. Enumerateds
An additional sparse data type is available called enum. Each
definition is a different type. Only enumerateds of the same type
may be assigned or compared. Every element of an enumerated must be
assigned a value, as demonstrated in the following example. Since
the elements of the enumerated are not ordered, they can be assigned
any unique value, in any order.
enum { e1(v1), e2(v2), ... , en(vn) [[, (n)]] } Te;
Future extensions or additions to the protocol may define new values.
Implementations need to be able to parse and ignore unknown values
unless the definition of the field states otherwise.
An enumerated occupies as much space in the byte stream as would its
maximal defined ordinal value. The following definition would cause
one byte to be used to carry fields of type Color.
enum { red(3), blue(5), white(7) } Color;
One may optionally specify a value without its associated tag to
force the width definition without defining a superfluous element.
In the following example, Taste will consume two bytes in the data
stream but can only assume the values 1, 2, or 4 in the current
version of the protocol.
enum { sweet(1), sour(2), bitter(4), (32000) } Taste;
The names of the elements of an enumeration are scoped within the
defined type. In the first example, a fully qualified reference to
Rescorla Expires August 19, 2018 [Page 27]
Internet-Draft TLS February 2018
the second element of the enumeration would be Color.blue. Such
qualification is not required if the target of the assignment is well
specified.
Color color = Color.blue; /* overspecified, legal */
Color color = blue; /* correct, type implicit */
The names assigned to enumerateds do not need to be unique. The
numerical value can describe a range over which the same name
applies. The value includes the minimum and maximum inclusive values
in that range, separated by two period characters. This is
principally useful for reserving regions of the space.
enum { sad(0), meh(1..254), happy(255) } Mood;
3.6. Constructed Types
Structure types may be constructed from primitive types for
convenience. Each specification declares a new, unique type. The
syntax for definition is much like that of C.
struct {
T1 f1;
T2 f2;
...
Tn fn;
} T;
Fixed- and variable-length vector fields are allowed using the
standard vector syntax. Structures V1 and V2 in the variants example
below demonstrate this.
The fields within a structure may be qualified using the type's name,
with a syntax much like that available for enumerateds. For example,
T.f2 refers to the second field of the previous declaration.
3.7. Constants
Fields and variables may be assigned a fixed value using "=", as in:
struct {
T1 f1 = 8; /* T.f1 must always be 8 */
T2 f2;
} T;
Rescorla Expires August 19, 2018 [Page 28]
Internet-Draft TLS February 2018
3.8. Variants
Defined structures may have variants based on some knowledge that is
available within the environment. The selector must be an enumerated
type that defines the possible variants the structure defines. Each
arm of the select specifies the type of that variant's field and an
optional field label. The mechanism by which the variant is selected
at runtime is not prescribed by the presentation language.
struct {
T1 f1;
T2 f2;
....
Tn fn;
select (E) {
case e1: Te1 [[fe1]];
case e2: Te2 [[fe2]];
....
case en: Ten [[fen]];
};
} Tv;
For example:
enum { apple(0), orange(1) } VariantTag;
struct {
uint16 number;
opaque string<0..10>; /* variable length */
} V1;
struct {
uint32 number;
opaque string[10]; /* fixed length */
} V2;
struct {
VariantTag type;
select (VariantRecord.type) {
case apple: V1;
case orange: V2;
};
} VariantRecord;
Rescorla Expires August 19, 2018 [Page 29]
Internet-Draft TLS February 2018
4. Handshake Protocol
The handshake protocol is used to negotiate the security parameters
of a connection. Handshake messages are supplied to the TLS record
layer, where they are encapsulated within one or more TLSPlaintext or
TLSCiphertext structures, which are processed and transmitted as
specified by the current active connection state.
enum {
client_hello(1),
server_hello(2),
new_session_ticket(4),
end_of_early_data(5),
encrypted_extensions(8),
certificate(11),
certificate_request(13),
certificate_verify(15),
finished(20),
key_update(24),
message_hash(254),
(255)
} HandshakeType;
struct {
HandshakeType msg_type; /* handshake type */
uint24 length; /* bytes in message */
select (Handshake.msg_type) {
case client_hello: ClientHello;
case server_hello: ServerHello;
case end_of_early_data: EndOfEarlyData;
case encrypted_extensions: EncryptedExtensions;
case certificate_request: CertificateRequest;
case certificate: Certificate;
case certificate_verify: CertificateVerify;
case finished: Finished;
case new_session_ticket: NewSessionTicket;
case key_update: KeyUpdate;
};
} Handshake;
Protocol messages MUST be sent in the order defined in Section 4.4.1
and shown in the diagrams in Section 2. A peer which receives a
handshake message in an unexpected order MUST abort the handshake
with an "unexpected_message" alert.
New handshake message types are assigned by IANA as described in
Section 11.
Rescorla Expires August 19, 2018 [Page 30]
Internet-Draft TLS February 2018
4.1. Key Exchange Messages
The key exchange messages are used to determine the security
capabilities of the client and the server and to establish shared
secrets including the traffic keys used to protect the rest of the
handshake and the data.
4.1.1. Cryptographic Negotiation
In TLS, the cryptographic negotiation proceeds by the client offering
the following four sets of options in its ClientHello:
- A list of cipher suites which indicates the AEAD algorithm/HKDF
hash pairs which the client supports.
- A "supported_groups" (Section 4.2.7) extension which indicates the
(EC)DHE groups which the client supports and a "key_share"
(Section 4.2.8) extension which contains (EC)DHE shares for some
or all of these groups.
- A "signature_algorithms" (Section 4.2.3) extension which indicates
the signature algorithms which the client can accept.
- A "pre_shared_key" (Section 4.2.11) extension which contains a
list of symmetric key identities known to the client and a
"psk_key_exchange_modes" (Section 4.2.9) extension which indicates
the key exchange modes that may be used with PSKs.
If the server does not select a PSK, then the first three of these
options are entirely orthogonal: the server independently selects a
cipher suite, an (EC)DHE group and key share for key establishment,
and a signature algorithm/certificate pair to authenticate itself to
the client. If there is no overlap between the received
"supported_groups" and the groups supported by the server then the
server MUST abort the handshake with a "handshake_failure" or an
"insufficient_security" alert.
If the server selects a PSK, then it MUST also select a key
establishment mode from the set indicated by client's
"psk_key_exchange_modes" extension (at present, PSK alone or with
(EC)DHE). Note that if the PSK can be used without (EC)DHE then non-
overlap in the "supported_groups" parameters need not be fatal, as it
is in the non-PSK case discussed in the previous paragraph.
If the server selects an (EC)DHE group and the client did not offer a
compatible "key_share" extension in the initial ClientHello, the
server MUST respond with a HelloRetryRequest (Section 4.1.4) message.
Rescorla Expires August 19, 2018 [Page 31]
Internet-Draft TLS February 2018
If the server successfully selects parameters and does not require a
HelloRetryRequest, it indicates the selected parameters in the
ServerHello as follows:
- If PSK is being used, then the server will send a "pre_shared_key"
extension indicating the selected key.
- If PSK is not being used, then (EC)DHE and certificate-based
authentication are always used.
- When (EC)DHE is in use, the server will also provide a "key_share"
extension.
- When authenticating via a certificate, the server will send the
Certificate (Section 4.4.2) and CertificateVerify (Section 4.4.3)
messages. In TLS 1.3 as defined by this document, either a PSK or
a certificate is always used, but not both. Future documents may
define how to use them together.
If the server is unable to negotiate a supported set of parameters
(i.e., there is no overlap between the client and server parameters),
it MUST abort the handshake with either a "handshake_failure" or
"insufficient_security" fatal alert (see Section 6).
4.1.2. Client Hello
When a client first connects to a server, it is REQUIRED to send the
ClientHello as its first message. The client will also send a
ClientHello when the server has responded to its ClientHello with a
HelloRetryRequest. In that case, the client MUST send the same
ClientHello (without modification) except:
- If a "key_share" extension was supplied in the HelloRetryRequest,
replacing the list of shares with a list containing a single
KeyShareEntry from the indicated group.
- Removing the "early_data" extension (Section 4.2.10) if one was
present. Early data is not permitted after HelloRetryRequest.
- Including a "cookie" extension if one was provided in the
HelloRetryRequest.
- Updating the "pre_shared_key" extension if present by recomputing
the "obfuscated_ticket_age" and binder values and (optionally)
removing any PSKs which are incompatible with the server's
indicated cipher suite.
Rescorla Expires August 19, 2018 [Page 32]
Internet-Draft TLS February 2018
- Optionally adding, removing, or changing the length of the
"padding" extension [RFC7685].
Because TLS 1.3 forbids renegotiation, if a server has negotiated TLS
1.3 and receives a ClientHello at any other time, it MUST terminate
the connection with an "unexpected_message" alert.
If a server established a TLS connection with a previous version of
TLS and receives a TLS 1.3 ClientHello in a renegotiation, it MUST
retain the previous protocol version. In particular, it MUST NOT
negotiate TLS 1.3.
Structure of this message:
uint16 ProtocolVersion;
opaque Random[32];
uint8 CipherSuite[2]; /* Cryptographic suite selector */
struct {
ProtocolVersion legacy_version = 0x0303; /* TLS v1.2 */
Random random;
opaque legacy_session_id<0..32>;
CipherSuite cipher_suites<2..2^16-2>;
opaque legacy_compression_methods<1..2^8-1>;
Extension extensions<8..2^16-1>;
} ClientHello;
legacy_version In previous versions of TLS, this field was used for
version negotiation and represented the highest version number
supported by the client. Experience has shown that many servers
do not properly implement version negotiation, leading to "version
intolerance" in which the server rejects an otherwise acceptable
ClientHello with a version number higher than it supports. In TLS
1.3, the client indicates its version preferences in the
"supported_versions" extension (Section 4.2.1) and the
legacy_version field MUST be set to 0x0303, which is the version
number for TLS 1.2. (See Appendix D for details about backward
compatibility.)
random 32 bytes generated by a secure random number generator. See
Appendix C for additional information.
legacy_session_id Versions of TLS before TLS 1.3 supported a
"session resumption" feature which has been merged with Pre-Shared
Keys in this version (see Section 2.2). A client which has a
cached session ID set by a pre-TLS 1.3 server SHOULD set this
field to that value. In compatibility mode (see Appendix D.4)
Rescorla Expires August 19, 2018 [Page 33]
Internet-Draft TLS February 2018
this field MUST be non-empty, so a client not offering a pre-TLS
1.3 session MUST generate a new 32-byte value. This value need
not be random but SHOULD be unpredictable to avoid implementations
fixating on a specific value (also known as ossification).
Otherwise, it MUST be set as a zero length vector (i.e., a single
zero byte length field).
cipher_suites This is a list of the symmetric cipher options
supported by the client, specifically the record protection
algorithm (including secret key length) and a hash to be used with
HKDF, in descending order of client preference. If the list
contains cipher suites that the server does not recognize, support
or wish to use, the server MUST ignore those cipher suites and
process the remaining ones as usual. Values are defined in
Appendix B.4. If the client is attempting a PSK key
establishment, it SHOULD advertise at least one cipher suite
indicating a Hash associated with the PSK.
legacy_compression_methods Versions of TLS before 1.3 supported
compression with the list of supported compression methods being
sent in this field. For every TLS 1.3 ClientHello, this vector
MUST contain exactly one byte set to zero, which corresponds to
the "null" compression method in prior versions of TLS. If a TLS
1.3 ClientHello is received with any other value in this field,
the server MUST abort the handshake with an "illegal_parameter"
alert. Note that TLS 1.3 servers might receive TLS 1.2 or prior
ClientHellos which contain other compression methods and MUST
follow the procedures for the appropriate prior version of TLS.
TLS 1.3 ClientHellos are identified as having a legacy_version of
0x0303 and a supported_versions extension present with 0x0304 as
the highest version indicated therein.
extensions Clients request extended functionality from servers by
sending data in the extensions field. The actual "Extension"
format is defined in Section 4.2. In TLS 1.3, use of certain
extensions is mandatory, as functionality is moved into extensions
to preserve ClientHello compatibility with previous versions of
TLS. Servers MUST ignore unrecognized extensions.
All versions of TLS allow an extensions field to optionally follow
the compression_methods field. TLS 1.3 ClientHello messages always
contain extensions (minimally "supported_versions", otherwise they
will be interpreted as TLS 1.2 ClientHello messages). However, TLS
1.3 servers might receive ClientHello messages without an extensions
field from prior versions of TLS. The presence of extensions can be
detected by determining whether there are bytes following the
compression_methods field at the end of the ClientHello. Note that
this method of detecting optional data differs from the normal TLS
Rescorla Expires August 19, 2018 [Page 34]
Internet-Draft TLS February 2018
method of having a variable-length field, but it is used for
compatibility with TLS before extensions were defined. TLS 1.3
servers will need to perform this check first and only attempt to
negotiate TLS 1.3 if the "supported_versions" extension is present.
If negotiating a version of TLS prior to 1.3, a server MUST check
that the message either contains no data after
legacy_compression_methods or that it contains a valid extensions
block with no data following. If not, then it MUST abort the
handshake with a "decode_error" alert.
In the event that a client requests additional functionality using
extensions, and this functionality is not supplied by the server, the
client MAY abort the handshake.
After sending the ClientHello message, the client waits for a
ServerHello or HelloRetryRequest message. If early data is in use,
the client may transmit early application data (Section 2.3) while
waiting for the next handshake message.
4.1.3. Server Hello
The server will send this message in response to a ClientHello
message to proceed with the handshake if it is able to negotiate an
acceptable set of handshake parameters based on the ClientHello.
Structure of this message:
struct {
ProtocolVersion legacy_version = 0x0303; /* TLS v1.2 */
Random random;
opaque legacy_session_id_echo<0..32>;
CipherSuite cipher_suite;
uint8 legacy_compression_method = 0;
Extension extensions<6..2^16-1>;
} ServerHello;
legacy_version In previous versions of TLS, this field was used for
version negotiation and represented the selected version number
for the connection. Unfortunately, some middleboxes fail when
presented with new values. In TLS 1.3, the TLS server indicates
its version using the "supported_versions" extension
(Section 4.2.1), and the legacy_version field MUST be set to
0x0303, which is the version number for TLS 1.2. (See Appendix D
for details about backward compatibility.)
random 32 bytes generated by a secure random number generator. See
Appendix C for additional information. The last eight bytes MUST
be overwritten as described below if negotiating TLS 1.2 or TLS
Rescorla Expires August 19, 2018 [Page 35]
Internet-Draft TLS February 2018
1.1, but the remaining bytes MUST be random. This structure is
generated by the server and MUST be generated independently of the
ClientHello.random.
legacy_session_id_echo The contents of the client's
legacy_session_id field. Note that this field is echoed even if
the client's value corresponded to a cached pre-TLS 1.3 session
which the server has chosen not to resume. A client which
receives a legacy_session_id field that does not match what it
sent in the ClientHello MUST abort the handshake with an
"illegal_parameter" alert.
cipher_suite The single cipher suite selected by the server from the
list in ClientHello.cipher_suites. A client which receives a
cipher suite that was not offered MUST abort the handshake with an
"illegal_parameter" alert.
legacy_compression_method A single byte which MUST have the value 0.
extensions A list of extensions. The ServerHello MUST only include
extensions which are required to establish the cryptographic
context and negotiate the protocol version. All TLS 1.3
ServerHello messages MUST contain the "supported_versions"
extension. Current ServerHello messages contain either the
"pre_shared_key" or "key_share" extensions, or both when using a
PSK with (EC)DHE key establishment. The remaining extensions are
sent separately in the EncryptedExtensions message.
For backward compatibility reasons with middleboxes (see
Appendix D.4) the HelloRetryRequest message uses the same structure
as the ServerHello, but with Random set to the special value of the
SHA-256 of "HelloRetryRequest":
CF 21 AD 74 E5 9A 61 11 BE 1D 8C 02 1E 65 B8 91
C2 A2 11 16 7A BB 8C 5E 07 9E 09 E2 C8 A8 33 9C
Upon receiving a message with type server_hello, implementations MUST
first examine the Random value and if it matches this value, process
it as described in Section 4.1.4).
TLS 1.3 has a downgrade protection mechanism embedded in the server's
random value. TLS 1.3 servers which negotiate TLS 1.2 or below in
response to a ClientHello MUST set the last eight bytes of their
Random value specially.
If negotiating TLS 1.2, TLS 1.3 servers MUST set the last eight bytes
of their Random value to the bytes:
Rescorla Expires August 19, 2018 [Page 36]
Internet-Draft TLS February 2018
44 4F 57 4E 47 52 44 01
If negotiating TLS 1.1 or below, TLS 1.3 servers MUST and TLS 1.2
servers SHOULD set the last eight bytes of their Random value to the
bytes:
44 4F 57 4E 47 52 44 00
TLS 1.3 clients receiving a ServerHello indicating TLS 1.2 or below
MUST check that the last eight bytes are not equal to either of these
values. TLS 1.2 clients SHOULD also check that the last eight bytes
are not equal to the second value if the ServerHello indicates TLS
1.1 or below. If a match is found, the client MUST abort the
handshake with an "illegal_parameter" alert. This mechanism provides
limited protection against downgrade attacks over and above what is
provided by the Finished exchange: because the ServerKeyExchange, a
message present in TLS 1.2 and below, includes a signature over both
random values, it is not possible for an active attacker to modify
the random values without detection as long as ephemeral ciphers are
used. It does not provide downgrade protection when static RSA is
used.
Note: This is a change from [RFC5246], so in practice many TLS 1.2
clients and servers will not behave as specified above.
A legacy TLS client performing renegotiation with TLS 1.2 or prior
and which receives a TLS 1.3 ServerHello during renegotiation MUST
abort the handshake with a "protocol_version" alert. Note that
renegotiation is not possible when TLS 1.3 has been negotiated.
RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH Implementations of
draft versions (see Section 4.2.1.1) of this specification SHOULD NOT
implement this mechanism on either client and server. A pre-RFC
client connecting to RFC servers, or vice versa, will appear to
downgrade to TLS 1.2. With the mechanism enabled, this will cause an
interoperability failure.
4.1.4. Hello Retry Request
The server will send this message in response to a ClientHello
message if it is able to find an acceptable set of parameters but the
ClientHello does not contain sufficient information to proceed with
the handshake. As discussed in Section 4.1.3, the HelloRetryRequest
has the same format as a ServerHello message, and the legacy_version,
legacy_session_id_echo, cipher_suite, and legacy_compression methods
fields have the same meaning. However, for convenience we discuss
HelloRetryRequest throughout this document as if it were a distinct
message.
Rescorla Expires August 19, 2018 [Page 37]
Internet-Draft TLS February 2018
The server's extensions MUST contain "supported_versions" and
otherwise the server SHOULD send only the extensions necessary for
the client to generate a correct ClientHello pair. As with
ServerHello, a HelloRetryRequest MUST NOT contain any extensions that
were not first offered by the client in its ClientHello, with the
exception of optionally the "cookie" (see Section 4.2.2) extension.
Upon receipt of a HelloRetryRequest, the client MUST perform the
checks specified in Section 4.1.3 and then process the extensions,
starting with determining the version using "supported_versions".
Clients MUST abort the handshake with an "illegal_parameter" alert if
the HelloRetryRequest would not result in any change in the
ClientHello. If a client receives a second HelloRetryRequest in the
same connection (i.e., where the ClientHello was itself in response
to a HelloRetryRequest), it MUST abort the handshake with an
"unexpected_message" alert.
Otherwise, the client MUST process all extensions in the
HelloRetryRequest and send a second updated ClientHello. The
HelloRetryRequest extensions defined in this specification are:
- supported_versions (see Section 4.2.1)
- cookie (see Section 4.2.2)
- key_share (see Section 4.2.8)
In addition, in its updated ClientHello, the client SHOULD NOT offer
any pre-shared keys associated with a hash other than that of the
selected cipher suite. This allows the client to avoid having to
compute partial hash transcripts for multiple hashes in the second
ClientHello. A client which receives a cipher suite that was not
offered MUST abort the handshake. Servers MUST ensure that they
negotiate the same cipher suite when receiving a conformant updated
ClientHello (if the server selects the cipher suite as the first step
in the negotiation, then this will happen automatically). Upon
receiving the ServerHello, clients MUST check that the cipher suite
supplied in the ServerHello is the same as that in the
HelloRetryRequest and otherwise abort the handshake with an
"illegal_parameter" alert.
The value of selected_version in the HelloRetryRequest
"supported_versions" extension MUST be retained in the ServerHello,
and a client MUST abort the handshake with an "illegal_parameter"
alert if the value changes.
Rescorla Expires August 19, 2018 [Page 38]
Internet-Draft TLS February 2018
4.2. Extensions
A number of TLS messages contain tag-length-value encoded extensions
structures.
struct {
ExtensionType extension_type;
opaque extension_data<0..2^16-1>;
} Extension;
enum {
server_name(0), /* RFC 6066 */
max_fragment_length(1), /* RFC 6066 */
status_request(5), /* RFC 6066 */
supported_groups(10), /* RFC 4492, 7919 */
signature_algorithms(13), /* [[this document]] */
use_srtp(14), /* RFC 5764 */
heartbeat(15), /* RFC 6520 */
application_layer_protocol_negotiation(16), /* RFC 7301 */
signed_certificate_timestamp(18), /* RFC 6962 */
client_certificate_type(19), /* RFC 7250 */
server_certificate_type(20), /* RFC 7250 */
padding(21), /* RFC 7685 */
pre_shared_key(41), /* [[this document]] */
early_data(42), /* [[this document]] */
supported_versions(43), /* [[this document]] */
cookie(44), /* [[this document]] */
psk_key_exchange_modes(45), /* [[this document]] */
certificate_authorities(47), /* [[this document]] */
oid_filters(48), /* [[this document]] */
post_handshake_auth(49), /* [[this document]] */
signature_algorithms_cert(50), /* [[this document]] */
key_share(51), /* [[this document]] */
(65535)
} ExtensionType;
Here:
- "extension_type" identifies the particular extension type.
- "extension_data" contains information specific to the particular
extension type.
The list of extension types is maintained by IANA as described in
Section 11.
Extensions are generally structured in a request/response fashion,
though some extensions are just indications with no corresponding
Rescorla Expires August 19, 2018 [Page 39]
Internet-Draft TLS February 2018
response. The client sends its extension requests in the ClientHello
message and the server sends its extension responses in the
ServerHello, EncryptedExtensions, HelloRetryRequest and Certificate
messages. The server sends extension requests in the
CertificateRequest message which a client MAY respond to with a
Certificate message. The server MAY also send unsolicited extensions
in the NewSessionTicket, though the client does not respond directly
to these.
Implementations MUST NOT send extension responses if the remote
endpoint did not send the corresponding extension requests, with the
exception of the "cookie" extension in HelloRetryRequest. Upon
receiving such an extension, an endpoint MUST abort the handshake
with an "unsupported_extension" alert.
The table below indicates the messages where a given extension may
appear, using the following notation: CH (ClientHello), SH
(ServerHello), EE (EncryptedExtensions), CT (Certificate), CR
(CertificateRequest), NST (NewSessionTicket) and HRR
(HelloRetryRequest). If an implementation receives an extension
which it recognizes and which is not specified for the message in
which it appears it MUST abort the handshake with an
"illegal_parameter" alert.
Rescorla Expires August 19, 2018 [Page 40]
Internet-Draft TLS February 2018
+--------------------------------------------------+-------------+
| Extension | TLS 1.3 |
+--------------------------------------------------+-------------+
| server_name [RFC6066] | CH, EE |
| | |
| max_fragment_length [RFC6066] | CH, EE |
| | |
| status_request [RFC6066] | CH, CR, CT |
| | |
| supported_groups [RFC7919] | CH, EE |
| | |
| signature_algorithms [RFC5246] | CH, CR |
| | |
| use_srtp [RFC5764] | CH, EE |
| | |
| heartbeat [RFC6520] | CH, EE |
| | |
| application_layer_protocol_negotiation [RFC7301] | CH, EE |
| | |
| signed_certificate_timestamp [RFC6962] | CH, CR, CT |
| | |
| client_certificate_type [RFC7250] | CH, EE |
| | |
| server_certificate_type [RFC7250] | CH, EE |
| | |
| padding [RFC7685] | CH |
| | |
| key_share [[this document]] | CH, SH, HRR |
| | |
| pre_shared_key [[this document]] | CH, SH |
| | |
| psk_key_exchange_modes [[this document]] | CH |
| | |
| early_data [[this document]] | CH, EE, NST |
| | |
| cookie [[this document]] | CH, HRR |
| | |
| supported_versions [[this document]] | CH, SH, HRR |
| | |
| certificate_authorities [[this document]] | CH, CR |
| | |
| oid_filters [[this document]] | CR |
| | |
| post_handshake_auth [[this document]] | CH |
| | |
| signature_algorithms_cert [[this document]] | CH, CR |
+--------------------------------------------------+-------------+
Rescorla Expires August 19, 2018 [Page 41]
Internet-Draft TLS February 2018
When multiple extensions of different types are present, the
extensions MAY appear in any order, with the exception of
"pre_shared_key" Section 4.2.11 which MUST be the last extension in
the ClientHello. There MUST NOT be more than one extension of the
same type in a given extension block.
In TLS 1.3, unlike TLS 1.2, extensions are negotiated for each
handshake even when in resumption-PSK mode. However, 0-RTT
parameters are those negotiated in the previous handshake; mismatches
may require rejecting 0-RTT (see Section 4.2.10).
There are subtle (and not so subtle) interactions that may occur in
this protocol between new features and existing features which may
result in a significant reduction in overall security. The following
considerations should be taken into account when designing new
extensions:
- Some cases where a server does not agree to an extension are error
conditions, and some are simply refusals to support particular
features. In general, error alerts should be used for the former
and a field in the server extension response for the latter.
- Extensions should, as far as possible, be designed to prevent any
attack that forces use (or non-use) of a particular feature by
manipulation of handshake messages. This principle should be
followed regardless of whether the feature is believed to cause a
security problem. Often the fact that the extension fields are
included in the inputs to the Finished message hashes will be
sufficient, but extreme care is needed when the extension changes
the meaning of messages sent in the handshake phase. Designers
and implementors should be aware of the fact that until the
handshake has been authenticated, active attackers can modify
messages and insert, remove, or replace extensions.
4.2.1. Supported Versions
struct {
select (Handshake.msg_type) {
case client_hello:
ProtocolVersion versions<2..254>;
case server_hello: /* and HelloRetryRequest */
ProtocolVersion selected_version;
};
} SupportedVersions;
The "supported_versions" extension is used by the client to indicate
which versions of TLS it supports and by the server to indicate which
Rescorla Expires August 19, 2018 [Page 42]
Internet-Draft TLS February 2018
version it is using. The extension contains a list of supported
versions in preference order, with the most preferred version first.
Implementations of this specification MUST send this extension
containing all versions of TLS which they are prepared to negotiate
(for this specification, that means minimally 0x0304, but if previous
versions of TLS are allowed to be negotiated, they MUST be present as
well).
If this extension is not present, servers which are compliant with
this specification MUST negotiate TLS 1.2 or prior as specified in
[RFC5246], even if ClientHello.legacy_version is 0x0304 or later.
Servers MAY abort the handshake upon receiving a ClientHello with
legacy_version 0x0304 or later.
If this extension is present, servers MUST ignore the
ClientHello.legacy_version value and MUST use only the
"supported_versions" extension to determine client preferences.
Servers MUST only select a version of TLS present in that extension
and MUST ignore any unknown versions that are present in that
extension. Note that this mechanism makes it possible to negotiate a
version prior to TLS 1.2 if one side supports a sparse range.
Implementations of TLS 1.3 which choose to support prior versions of
TLS SHOULD support TLS 1.2. Servers should be prepared to receive
ClientHellos that include this extension but do not include 0x0304 in
the list of versions.
A server which negotiates TLS 1.3 MUST respond by sending a
"supported_versions" extension containing the selected version value
(0x0304). It MUST set the ServerHello.legacy_version field to 0x0303
(TLS 1.2). Clients MUST check for this extension prior to processing
the rest of the ServerHello (although they will have to parse the
ServerHello in order to read the extension). If this extension is
present, clients MUST ignore the ServerHello.legacy_version value and
MUST use only the "supported_versions" extension to determine client
preferences. If the "supported_versions" extension contains a
version not offered by the client, the client MUST abort the
handshake with an "illegal_parameter" alert.
4.2.1.1. Draft Version Indicator
RFC EDITOR: PLEASE REMOVE THIS SECTION
While the eventual version indicator for the RFC version of TLS 1.3
will be 0x0304, implementations of draft versions of this
specification SHOULD instead advertise 0x7f00 | draft_version in the
ServerHello and HelloRetryRequest "supported_versions" extension.
For instance, draft-17 would be encoded as the 0x7f11. This allows
Rescorla Expires August 19, 2018 [Page 43]
Internet-Draft TLS February 2018
pre-RFC implementations to safely negotiate with each other, even if
they would otherwise be incompatible.
4.2.2. Cookie
struct {
opaque cookie<1..2^16-1>;
} Cookie;
Cookies serve two primary purposes:
- Allowing the server to force the client to demonstrate
reachability at their apparent network address (thus providing a
measure of DoS protection). This is primarily useful for non-
connection-oriented transports (see [RFC6347] for an example of
this).
- Allowing the server to offload state to the client, thus allowing
it to send a HelloRetryRequest without storing any state. The
server can do this by storing the hash of the ClientHello in the
HelloRetryRequest cookie (protected with some suitable integrity
algorithm).
When sending a HelloRetryRequest, the server MAY provide a "cookie"
extension to the client (this is an exception to the usual rule that
the only extensions that may be sent are those that appear in the
ClientHello). When sending the new ClientHello, the client MUST copy
the contents of the extension received in the HelloRetryRequest into
a "cookie" extension in the new ClientHello. Clients MUST NOT use
cookies in their initial ClientHello in subsequent connections.
When a server is operating statelessly it may receive an unprotected
record of type change_cipher_spec between the first and second
ClientHello (see Section 5). Since the server is not storing any
state this will appear as if it were the first message to be
received. Servers operating statelessly MUST ignore these records.
4.2.3. Signature Algorithms
TLS 1.3 provides two extensions for indicating which signature
algorithms may be used in digital signatures. The
"signature_algorithms_cert" extension applies to signatures in
certificates and the "signature_algorithms" extension, which
originally appeared in TLS 1.2, applies to signatures in
CertificateVerify messages. The keys found in certificates MUST also
be of appropriate type for the signature algorithms they are used
with. This is a particular issue for RSA keys and PSS signatures, as
described below. If no "signature_algorithms_cert" extension is
Rescorla Expires August 19, 2018 [Page 44]
Internet-Draft TLS February 2018
present, then the "signature_algorithms" extension also applies to
signatures appearing in certificates. Clients which desire the
server to authenticate itself via a certificate MUST send
"signature_algorithms". If a server is authenticating via a
certificate and the client has not sent a "signature_algorithms"
extension, then the server MUST abort the handshake with a
"missing_extension" alert (see Section 9.2).
The "signature_algorithms_cert" extension was added to allow
implementations which supported different sets of algorithms for
certificates and in TLS itself to clearly signal their capabilities.
TLS 1.2 implementations SHOULD also process this extension.
Implementations which have the same policy in both cases MAY omit the
"signature_algorithms_cert" extension.
The "extension_data" field of these extension contains a
SignatureSchemeList value:
Rescorla Expires August 19, 2018 [Page 45]
Internet-Draft TLS February 2018
enum {
/* RSASSA-PKCS1-v1_5 algorithms */
rsa_pkcs1_sha256(0x0401),
rsa_pkcs1_sha384(0x0501),
rsa_pkcs1_sha512(0x0601),
/* ECDSA algorithms */
ecdsa_secp256r1_sha256(0x0403),
ecdsa_secp384r1_sha384(0x0503),
ecdsa_secp521r1_sha512(0x0603),
/* RSASSA-PSS algorithms with public key OID rsaEncryption */
rsa_pss_rsae_sha256(0x0804),
rsa_pss_rsae_sha384(0x0805),
rsa_pss_rsae_sha512(0x0806),
/* EdDSA algorithms */
ed25519(0x0807),
ed448(0x0808),
/* RSASSA-PSS algorithms with public key OID RSASSA-PSS */
rsa_pss_pss_sha256(0x0809),
rsa_pss_pss_sha384(0x080a),
rsa_pss_pss_sha512(0x080b),
/* Legacy algorithms */
rsa_pkcs1_sha1(0x0201),
ecdsa_sha1(0x0203),
/* Reserved Code Points */
private_use(0xFE00..0xFFFF),
(0xFFFF)
} SignatureScheme;
struct {
SignatureScheme supported_signature_algorithms<2..2^16-2>;
} SignatureSchemeList;
Note: This enum is named "SignatureScheme" because there is already a
"SignatureAlgorithm" type in TLS 1.2, which this replaces. We use
the term "signature algorithm" throughout the text.
Each SignatureScheme value lists a single signature algorithm that
the client is willing to verify. The values are indicated in
descending order of preference. Note that a signature algorithm
takes as input an arbitrary-length message, rather than a digest.
Algorithms which traditionally act on a digest should be defined in
TLS to first hash the input with a specified hash algorithm and then
Rescorla Expires August 19, 2018 [Page 46]
Internet-Draft TLS February 2018
proceed as usual. The code point groups listed above have the
following meanings:
RSASSA-PKCS1-v1_5 algorithms Indicates a signature algorithm using
RSASSA-PKCS1-v1_5 [RFC8017] with the corresponding hash algorithm
as defined in [SHS]. These values refer solely to signatures
which appear in certificates (see Section 4.4.2.2) and are not
defined for use in signed TLS handshake messages, although they
MAY appear in "signature_algorithms" and
"signature_algorithms_cert" for backward compatibility with TLS
1.2,
ECDSA algorithms Indicates a signature algorithm using ECDSA
[ECDSA], the corresponding curve as defined in ANSI X9.62 [X962]
and FIPS 186-4 [DSS], and the corresponding hash algorithm as
defined in [SHS]. The signature is represented as a DER-encoded
[X690] ECDSA-Sig-Value structure.
RSASSA-PSS RSAE algorithms Indicates a signature algorithm using
RSASSA-PSS [RFC8017] with mask generation function 1. The digest
used in the mask generation function and the digest being signed
are both the corresponding hash algorithm as defined in [SHS].
The length of the salt MUST be equal to the length of the digest
algorithm. If the public key is carried in an X.509 certificate,
it MUST use the rsaEncryption OID [RFC5280].
EdDSA algorithms Indicates a signature algorithm using EdDSA as
defined in [RFC8032] or its successors. Note that these
correspond to the "PureEdDSA" algorithms and not the "prehash"
variants.
RSASSA-PSS PSS algorithms Indicates a signature algorithm using
RSASSA-PSS [RFC8017] with mask generation function 1. The digest
used in the mask generation function and the digest being signed
are both the corresponding hash algorithm as defined in [SHS].
The length of the salt MUST be equal to the length of the digest
algorithm. If the public key is carried in an X.509 certificate,
it MUST use the RSASSA-PSS OID [RFC5756]. When used in
certificate signatures, the algorithm parameters MUST be DER
encoded. If the corresponding public key's parameters present,
then the parameters in the signature MUST be identical to those in
the public key.
Legacy algorithms Indicates algorithms which are being deprecated
because they use algorithms with known weaknesses, specifically
SHA-1 which is used in this context with either with RSA using
RSASSA-PKCS1-v1_5 or ECDSA. These values refer solely to
signatures which appear in certificates (see Section 4.4.2.2) and
Rescorla Expires August 19, 2018 [Page 47]
Internet-Draft TLS February 2018
are not defined for use in signed TLS handshake messages, even if
they appear in the "signature_algorithms" list (this is necessary
for backward compatibility with TLS 1.2). Endpoints SHOULD NOT
negotiate these algorithms but are permitted to do so solely for
backward compatibility. Clients offering these values MUST list
them as the lowest priority (listed after all other algorithms in
SignatureSchemeList). TLS 1.3 servers MUST NOT offer a SHA-1
signed certificate unless no valid certificate chain can be
produced without it (see Section 4.4.2.2).
The signatures on certificates that are self-signed or certificates
that are trust anchors are not validated since they begin a
certification path (see [RFC5280], Section 3.2). A certificate that
begins a certification path MAY use a signature algorithm that is not
advertised as being supported in the "signature_algorithms"
extension.
Note that TLS 1.2 defines this extension differently. TLS 1.3
implementations willing to negotiate TLS 1.2 MUST behave in
accordance with the requirements of [RFC5246] when negotiating that
version. In particular:
- TLS 1.2 ClientHellos MAY omit this extension.
- In TLS 1.2, the extension contained hash/signature pairs. The
pairs are encoded in two octets, so SignatureScheme values have
been allocated to align with TLS 1.2's encoding. Some legacy
pairs are left unallocated. These algorithms are deprecated as of
TLS 1.3. They MUST NOT be offered or negotiated by any
implementation. In particular, MD5 [SLOTH], SHA-224, and DSA MUST
NOT be used.
- ECDSA signature schemes align with TLS 1.2's ECDSA hash/signature
pairs. However, the old semantics did not constrain the signing
curve. If TLS 1.2 is negotiated, implementations MUST be prepared
to accept a signature that uses any curve that they advertised in
the "supported_groups" extension.
- Implementations that advertise support for RSASSA-PSS (which is
mandatory in TLS 1.3), MUST be prepared to accept a signature
using that scheme even when TLS 1.2 is negotiated. In TLS 1.2,
RSASSA-PSS is used with RSA cipher suites.
4.2.4. Certificate Authorities
The "certificate_authorities" extension is used to indicate the
certificate authorities which an endpoint supports and which SHOULD
be used by the receiving endpoint to guide certificate selection.
Rescorla Expires August 19, 2018 [Page 48]
Internet-Draft TLS February 2018
The body of the "certificate_authorities" extension consists of a
CertificateAuthoritiesExtension structure.
opaque DistinguishedName<1..2^16-1>;
struct {
DistinguishedName authorities<3..2^16-1>;
} CertificateAuthoritiesExtension;
authorities A list of the distinguished names [X501] of acceptable
certificate authorities, represented in DER-encoded [X690] format.
These distinguished names specify a desired distinguished name for
trust anchor or subordinate CA; thus, this message can be used to
describe known trust anchors as well as a desired authorization
space.
The client MAY send the "certificate_authorities" extension in the
ClientHello message. The server MAY send it in the
CertificateRequest message.
The "trusted_ca_keys" extension, which serves a similar purpose
[RFC6066], but is more complicated, is not used in TLS 1.3 (although
it may appear in ClientHello messages from clients which are offering
prior versions of TLS).
4.2.5. OID Filters
The "oid_filters" extension allows servers to provide a set of OID/
value pairs which it would like the client's certificate to match.
This extension, if provided by the server, MUST only be sent in the
CertificateRequest message.
struct {
opaque certificate_extension_oid<1..2^8-1>;
opaque certificate_extension_values<0..2^16-1>;
} OIDFilter;
struct {
OIDFilter filters<0..2^16-1>;
} OIDFilterExtension;
filters A list of certificate extension OIDs [RFC5280] with their
allowed values and represented in DER-encoded [X690] format. Some
certificate extension OIDs allow multiple values (e.g., Extended
Key Usage). If the server has included a non-empty filters list,
the client certificate included in the response MUST contain all
of the specified extension OIDs that the client recognizes. For
each extension OID recognized by the client, all of the specified
Rescorla Expires August 19, 2018 [Page 49]
Internet-Draft TLS February 2018
values MUST be present in the client certificate (but the
certificate MAY have other values as well). However, the client
MUST ignore and skip any unrecognized certificate extension OIDs.
If the client ignored some of the required certificate extension
OIDs and supplied a certificate that does not satisfy the request,
the server MAY at its discretion either continue the connection
without client authentication, or abort the handshake with an
"unsupported_certificate" alert.
PKIX RFCs define a variety of certificate extension OIDs and their
corresponding value types. Depending on the type, matching
certificate extension values are not necessarily bitwise-equal. It
is expected that TLS implementations will rely on their PKI libraries
to perform certificate selection using certificate extension OIDs.
This document defines matching rules for two standard certificate
extensions defined in [RFC5280]:
- The Key Usage extension in a certificate matches the request when
all key usage bits asserted in the request are also asserted in
the Key Usage certificate extension.
- The Extended Key Usage extension in a certificate matches the
request when all key purpose OIDs present in the request are also
found in the Extended Key Usage certificate extension. The
special anyExtendedKeyUsage OID MUST NOT be used in the request.
Separate specifications may define matching rules for other
certificate extensions.
4.2.6. Post-Handshake Client Authentication
The "post_handshake_auth" extension is used to indicate that a client
is willing to perform post-handshake authentication Section 4.6.2.
Servers MUST NOT send a post-handshake CertificateRequest to clients
which do not offer this extension. Servers MUST NOT send this
extension.
struct {} PostHandshakeAuth;
The "extension_data" field of the "post_handshake_auth" extension is
zero length.
4.2.7. Negotiated Groups
When sent by the client, the "supported_groups" extension indicates
the named groups which the client supports for key exchange, ordered
from most preferred to least preferred.
Rescorla Expires August 19, 2018 [Page 50]
Internet-Draft TLS February 2018
Note: In versions of TLS prior to TLS 1.3, this extension was named
"elliptic_curves" and only contained elliptic curve groups. See
[RFC4492] and [RFC7919]. This extension was also used to negotiate
ECDSA curves. Signature algorithms are now negotiated independently
(see Section 4.2.3).
The "extension_data" field of this extension contains a
"NamedGroupList" value:
enum {
/* Elliptic Curve Groups (ECDHE) */
secp256r1(0x0017), secp384r1(0x0018), secp521r1(0x0019),
x25519(0x001D), x448(0x001E),
/* Finite Field Groups (DHE) */
ffdhe2048(0x0100), ffdhe3072(0x0101), ffdhe4096(0x0102),
ffdhe6144(0x0103), ffdhe8192(0x0104),
/* Reserved Code Points */
ffdhe_private_use(0x01FC..0x01FF),
ecdhe_private_use(0xFE00..0xFEFF),
(0xFFFF)
} NamedGroup;
struct {
NamedGroup named_group_list<2..2^16-1>;
} NamedGroupList;
Elliptic Curve Groups (ECDHE) Indicates support for the
corresponding named curve, defined either in FIPS 186-4 [DSS] or
in [RFC7748]. Values 0xFE00 through 0xFEFF are reserved for
private use.
Finite Field Groups (DHE) Indicates support of the corresponding
finite field group, defined in [RFC7919]. Values 0x01FC through
0x01FF are reserved for private use.
Items in named_group_list are ordered according to the client's
preferences (most preferred choice first).
As of TLS 1.3, servers are permitted to send the "supported_groups"
extension to the client. Clients MUST NOT act upon any information
found in "supported_groups" prior to successful completion of the
handshake but MAY use the information learned from a successfully
completed handshake to change what groups they use in their
"key_share" extension in subsequent connections. If the server has a
group it prefers to the ones in the "key_share" extension but is
Rescorla Expires August 19, 2018 [Page 51]
Internet-Draft TLS February 2018
still willing to accept the ClientHello, it SHOULD send
"supported_groups" to update the client's view of its preferences;
this extension SHOULD contain all groups the server supports,
regardless of whether they are currently supported by the client.
4.2.8. Key Share
The "key_share" extension contains the endpoint's cryptographic
parameters.
Clients MAY send an empty client_shares vector in order to request
group selection from the server at the cost of an additional round
trip. (see Section 4.1.4)
struct {
NamedGroup group;
opaque key_exchange<1..2^16-1>;
} KeyShareEntry;
group The named group for the key being exchanged. Finite Field
Diffie-Hellman [DH] parameters are described in Section 4.2.8.1;
Elliptic Curve Diffie-Hellman parameters are described in
Section 4.2.8.2.
key_exchange Key exchange information. The contents of this field
are determined by the specified group and its corresponding
definition.
In the ClientHello message, the "extension_data" field of this
extension contains a "KeyShareClientHello" value:
struct {
KeyShareEntry client_shares<0..2^16-1>;
} KeyShareClientHello;
client_shares A list of offered KeyShareEntry values in descending
order of client preference.
This vector MAY be empty if the client is requesting a
HelloRetryRequest. Each KeyShareEntry value MUST correspond to a
group offered in the "supported_groups" extension and MUST appear in
the same order. However, the values MAY be a non-contiguous subset
of the "supported_groups" extension and MAY omit the most preferred
groups. Such a situation could arise if the most preferred groups
are new and unlikely to be supported in enough places to make
pregenerating key shares for them efficient.
Rescorla Expires August 19, 2018 [Page 52]
Internet-Draft TLS February 2018
Clients can offer an arbitrary number of KeyShareEntry values, each
representing a single set of key exchange parameters. For instance,
a client might offer shares for several elliptic curves or multiple
FFDHE groups. The key_exchange values for each KeyShareEntry MUST be
generated independently. Clients MUST NOT offer multiple
KeyShareEntry values for the same group. Clients MUST NOT offer any
KeyShareEntry values for groups not listed in the client's
"supported_groups" extension. Servers MAY check for violations of
these rules and abort the handshake with an "illegal_parameter" alert
if one is violated.
In a HelloRetryRequest message, the "extension_data" field of this
extension contains a KeyShareHelloRetryRequest value:
struct {
NamedGroup selected_group;
} KeyShareHelloRetryRequest;
selected_group The mutually supported group the server intends to
negotiate and is requesting a retried ClientHello/KeyShare for.
Upon receipt of this extension in a HelloRetryRequest, the client
MUST verify that (1) the selected_group field corresponds to a group
which was provided in the "supported_groups" extension in the
original ClientHello; and (2) the selected_group field does not
correspond to a group which was provided in the "key_share" extension
in the original ClientHello. If either of these checks fails, then
the client MUST abort the handshake with an "illegal_parameter"
alert. Otherwise, when sending the new ClientHello, the client MUST
replace the original "key_share" extension with one containing only a
new KeyShareEntry for the group indicated in the selected_group field
of the triggering HelloRetryRequest.
In a ServerHello message, the "extension_data" field of this
extension contains a KeyShareServerHello value:
struct {
KeyShareEntry server_share;
} KeyShareServerHello;
server_share A single KeyShareEntry value that is in the same group
as one of the client's shares.
If using (EC)DHE key establishment, servers offer exactly one
KeyShareEntry in the ServerHello. This value MUST be in the same
group as the KeyShareEntry value offered by the client that the
server has selected for the negotiated key exchange. Servers MUST
NOT send a KeyShareEntry for any group not indicated in the
Rescorla Expires August 19, 2018 [Page 53]
Internet-Draft TLS February 2018
"supported_groups" extension and MUST NOT send a KeyShareEntry when
using the "psk_ke" PskKeyExchangeMode. If a HelloRetryRequest was
received by the client, the client MUST verify that the selected
NamedGroup in the ServerHello is the same as that in the
HelloRetryRequest. If this check fails, the client MUST abort the
handshake with an "illegal_parameter" alert.
4.2.8.1. Diffie-Hellman Parameters
Diffie-Hellman [DH] parameters for both clients and servers are
encoded in the opaque key_exchange field of a KeyShareEntry in a
KeyShare structure. The opaque value contains the Diffie-Hellman
public value (Y = g^X mod p) for the specified group (see [RFC7919]
for group definitions) encoded as a big-endian integer and padded to
the left with zeros to the size of p in bytes.
Note: For a given Diffie-Hellman group, the padding results in all
public keys having the same length.
Peers MUST validate each other's public key Y by ensuring that 1 < Y
< p-1. This check ensures that the remote peer is properly behaved
and isn't forcing the local system into a small subgroup.
4.2.8.2. ECDHE Parameters
ECDHE parameters for both clients and servers are encoded in the the
opaque key_exchange field of a KeyShareEntry in a KeyShare structure.
For secp256r1, secp384r1 and secp521r1, the contents are the
serialized value of the following struct:
struct {
uint8 legacy_form = 4;
opaque X[coordinate_length];
opaque Y[coordinate_length];
} UncompressedPointRepresentation;
X and Y respectively are the binary representations of the X and Y
values in network byte order. There are no internal length markers,
so each number representation occupies as many octets as implied by
the curve parameters. For P-256 this means that each of X and Y use
32 octets, padded on the left by zeros if necessary. For P-384 they
take 48 octets each, and for P-521 they take 66 octets each.
For the curves secp256r1, secp384r1 and secp521r1, peers MUST
validate each other's public value Y by ensuring that the point is a
valid point on the elliptic curve. The appropriate validation
procedures are defined in Section 4.3.7 of [X962] and alternatively
Rescorla Expires August 19, 2018 [Page 54]
Internet-Draft TLS February 2018
in Section 5.6.2.3 of [KEYAGREEMENT]. This process consists of three
steps: (1) verify that Y is not the point at infinity (O), (2) verify
that for Y = (x, y) both integers are in the correct interval, (3)
ensure that (x, y) is a correct solution to the elliptic curve
equation. For these curves, implementers do not need to verify
membership in the correct subgroup.
For X25519 and X448, the contents of the public value are the byte
string inputs and outputs of the corresponding functions defined in
[RFC7748], 32 bytes for X25519 and 56 bytes for X448.
Note: Versions of TLS prior to 1.3 permitted point format
negotiation; TLS 1.3 removes this feature in favor of a single point
format for each curve.
4.2.9. Pre-Shared Key Exchange Modes
In order to use PSKs, clients MUST also send a
"psk_key_exchange_modes" extension. The semantics of this extension
are that the client only supports the use of PSKs with these modes,
which restricts both the use of PSKs offered in this ClientHello and
those which the server might supply via NewSessionTicket.
A client MUST provide a "psk_key_exchange_modes" extension if it
offers a "pre_shared_key" extension. If clients offer
"pre_shared_key" without a "psk_key_exchange_modes" extension,
servers MUST abort the handshake. Servers MUST NOT select a key
exchange mode that is not listed by the client. This extension also
restricts the modes for use with PSK resumption; servers SHOULD NOT
send NewSessionTicket with tickets that are not compatible with the
advertised modes; however, if a server does so, the impact will just
be that the client's attempts at resumption fail.
The server MUST NOT send a "psk_key_exchange_modes" extension.
enum { psk_ke(0), psk_dhe_ke(1), (255) } PskKeyExchangeMode;
struct {
PskKeyExchangeMode ke_modes<1..255>;
} PskKeyExchangeModes;
psk_ke PSK-only key establishment. In this mode, the server MUST
NOT supply a "key_share" value.
psk_dhe_ke PSK with (EC)DHE key establishment. In this mode, the
client and servers MUST supply "key_share" values as described in
Section 4.2.8.
Rescorla Expires August 19, 2018 [Page 55]
Internet-Draft TLS February 2018
4.2.10. Early Data Indication
When a PSK is used, the client can send application data in its first
flight of messages. If the client opts to do so, it MUST supply both
the "early_data" extension as well as the "pre_shared_key" extension.
The "extension_data" field of this extension contains an
"EarlyDataIndication" value.
struct {} Empty;
struct {
select (Handshake.msg_type) {
case new_session_ticket: uint32 max_early_data_size;
case client_hello: Empty;
case encrypted_extensions: Empty;
};
} EarlyDataIndication;
See Section 4.6.1 for the use of the max_early_data_size field.
The parameters for the 0-RTT data (version, symmetric cipher suite,
ALPN protocol, etc.) are those associated with the PSK in use. For
externally established PSKs, the associated values are those
provisioned along with the key. For PSKs established via a
NewSessionTicket message, the associated values are those which were
negotiated in the connection which established the PSK. The PSK used
to encrypt the early data MUST be the first PSK listed in the
client's "pre_shared_key" extension.
For PSKs provisioned via NewSessionTicket, a server MUST validate
that the ticket age for the selected PSK identity (computed by
subtracting ticket_age_add from PskIdentity.obfuscated_ticket_age
modulo 2^32) is within a small tolerance of the time since the ticket
was issued (see Section 8). If it is not, the server SHOULD proceed
with the handshake but reject 0-RTT, and SHOULD NOT take any other
action that assumes that this ClientHello is fresh.
0-RTT messages sent in the first flight have the same (encrypted)
content types as their corresponding messages sent in other flights
(handshake and application_data) but are protected under different
keys. After receiving the server's Finished message, if the server
has accepted early data, an EndOfEarlyData message will be sent to
indicate the key change. This message will be encrypted with the
0-RTT traffic keys.
A server which receives an "early_data" extension MUST behave in one
of three ways:
Rescorla Expires August 19, 2018 [Page 56]
Internet-Draft TLS February 2018
- Ignore the extension and return a regular 1-RTT response. The
server then ignores early data by attempting to decrypt received
records in the handshake traffic keys until it is able to receive
the client's second flight and complete an ordinary 1-RTT
handshake, skipping records that fail to decrypt, up to the
configured max_early_data_size.
- Request that the client send another ClientHello by responding
with a HelloRetryRequest. A client MUST NOT include the
"early_data" extension in its followup ClientHello. The server
then ignores early data by skipping all records with external
content type of "application_data" (indicating that they are
encrypted).
- Return its own extension in EncryptedExtensions, indicating that
it intends to process the early data. It is not possible for the
server to accept only a subset of the early data messages. Even
though the server sends a message accepting early data, the actual
early data itself may already be in flight by the time the server
generates this message.
In order to accept early data, the server MUST have accepted a PSK
cipher suite and selected the first key offered in the client's
"pre_shared_key" extension. In addition, it MUST verify that the
following values are consistent with those associated with the
selected PSK:
- The TLS version number
- The selected cipher suite
- The selected ALPN [RFC7301] protocol, if any
These requirements are a superset of those needed to perform a 1-RTT
handshake using the PSK in question. For externally established
PSKs, the associated values are those provisioned along with the key.
For PSKs established via a NewSessionTicket message, the associated
values are those negotiated in the connection during which the ticket
was established.
Future extensions MUST define their interaction with 0-RTT.
If any of these checks fail, the server MUST NOT respond with the
extension and must discard all the first flight data using one of the
first two mechanisms listed above (thus falling back to 1-RTT or
2-RTT). If the client attempts a 0-RTT handshake but the server
rejects it, the server will generally not have the 0-RTT record
protection keys and must instead use trial decryption (either with
Rescorla Expires August 19, 2018 [Page 57]
Internet-Draft TLS February 2018
the 1-RTT handshake keys or by looking for a cleartext ClientHello in
the case of HelloRetryRequest) to find the first non-0-RTT message.
If the server chooses to accept the "early_data" extension, then it
MUST comply with the same error handling requirements specified for
all records when processing early data records. Specifically, if the
server fails to decrypt any 0-RTT record following an accepted
"early_data" extension it MUST terminate the connection with a
"bad_record_mac" alert as per Section 5.2.
If the server rejects the "early_data" extension, the client
application MAY opt to retransmit early data once the handshake has
been completed. Note that automatic re-transmission of early data
could result in assumptions about the status of the connection being
incorrect. For instance, when the negotiated connection selects a
different ALPN protocol from what was used for the early data, an
application might need to construct different messages. Similarly,
if early data assumes anything about the connection state, it might
be sent in error after the handshake completes.
A TLS implementation SHOULD NOT automatically re-send early data;
applications are in a better position to decide when re-transmission
is appropriate. A TLS implementation MUST NOT automatically re-send
early data unless the negotiated connection selects the same ALPN
protocol.
4.2.11. Pre-Shared Key Extension
The "pre_shared_key" extension is used to indicate the identity of
the pre-shared key to be used with a given handshake in association
with PSK key establishment.
The "extension_data" field of this extension contains a
"PreSharedKeyExtension" value:
Rescorla Expires August 19, 2018 [Page 58]
Internet-Draft TLS February 2018
struct {
opaque identity<1..2^16-1>;
uint32 obfuscated_ticket_age;
} PskIdentity;
opaque PskBinderEntry<32..255>;
struct {
PskIdentity identities<7..2^16-1>;
PskBinderEntry binders<33..2^16-1>;
} OfferedPsks;
struct {
select (Handshake.msg_type) {
case client_hello: OfferedPsks;
case server_hello: uint16 selected_identity;
};
} PreSharedKeyExtension;
identity A label for a key. For instance, a ticket defined in
Appendix B.3.4 or a label for a pre-shared key established
externally.
obfuscated_ticket_age An obfuscated version of the age of the key.
Section 4.2.11.1 describes how to form this value for identities
established via the NewSessionTicket message. For identities
established externally an obfuscated_ticket_age of 0 SHOULD be
used, and servers MUST ignore the value.
identities A list of the identities that the client is willing to
negotiate with the server. If sent alongside the "early_data"
extension (see Section 4.2.10), the first identity is the one used
for 0-RTT data.
binders A series of HMAC values, one for each PSK offered in the
"pre_shared_keys" extension and in the same order, computed as
described below.
selected_identity The server's chosen identity expressed as a
(0-based) index into the identities in the client's list.
Each PSK is associated with a single Hash algorithm. For PSKs
established via the ticket mechanism (Section 4.6.1), this is the KDF
Hash algorithm on the connection where the ticket was established.
For externally established PSKs, the Hash algorithm MUST be set when
the PSK is established, or default to SHA-256 if no such algorithm is
defined. The server MUST ensure that it selects a compatible PSK (if
any) and cipher suite.
Rescorla Expires August 19, 2018 [Page 59]
Internet-Draft TLS February 2018
In TLS versions prior to TLS 1.3, the Server Name Identification
(SNI) value was intended to be associated with the session (Section 3
of [RFC6066]), with the server being required to enforce that the SNI
value associated with the session matches the one specified in the
resumption handshake. However, in reality the implementations were
not consistent on which of two supplied SNI values they would use,
leading to the consistency requirement being de-facto enforced by the
clients. In TLS 1.3, the SNI value is always explicitly specified in
the resumption handshake, and there is no need for the server to
associate an SNI value with the ticket. Clients, however, SHOULD
store the SNI with the PSK to fulfill the requirements of
Section 4.6.1.
Implementor's note: the most straightforward way to implement the
PSK/cipher suite matching requirements is to negotiate the cipher
suite first and then exclude any incompatible PSKs. Any unknown PSKs
(e.g., they are not in the PSK database or are encrypted with an
unknown key) SHOULD simply be ignored. If no acceptable PSKs are
found, the server SHOULD perform a non-PSK handshake if possible.
Prior to accepting PSK key establishment, the server MUST validate
the corresponding binder value (see Section 4.2.11.2 below). If this
value is not present or does not validate, the server MUST abort the
handshake. Servers SHOULD NOT attempt to validate multiple binders;
rather they SHOULD select a single PSK and validate solely the binder
that corresponds to that PSK. In order to accept PSK key
establishment, the server sends a "pre_shared_key" extension
indicating the selected identity.
Clients MUST verify that the server's selected_identity is within the
range supplied by the client, that the server selected a cipher suite
indicating a Hash associated with the PSK and that a server
"key_share" extension is present if required by the ClientHello
"psk_key_exchange_modes". If these values are not consistent the
client MUST abort the handshake with an "illegal_parameter" alert.
If the server supplies an "early_data" extension, the client MUST
verify that the server's selected_identity is 0. If any other value
is returned, the client MUST abort the handshake with an
"illegal_parameter" alert.
This extension MUST be the last extension in the ClientHello (this
facilitates implementation as described below). Servers MUST check
that it is the last extension and otherwise fail the handshake with
an "illegal_parameter" alert.
Rescorla Expires August 19, 2018 [Page 60]
Internet-Draft TLS February 2018
4.2.11.1. Ticket Age
The client's view of the age of a ticket is the time since the
receipt of the NewSessionTicket message. Clients MUST NOT attempt to
use tickets which have ages greater than the "ticket_lifetime" value
which was provided with the ticket. The "obfuscated_ticket_age"
field of each PskIdentity contains an obfuscated version of the
ticket age formed by taking the age in milliseconds and adding the
"ticket_age_add" value that was included with the ticket, see
Section 4.6.1 modulo 2^32. This addition prevents passive observers
from correlating connections unless tickets are reused. Note that
the "ticket_lifetime" field in the NewSessionTicket message is in
seconds but the "obfuscated_ticket_age" is in milliseconds. Because
ticket lifetimes are restricted to a week, 32 bits is enough to
represent any plausible age, even in milliseconds.
4.2.11.2. PSK Binder
The PSK binder value forms a binding between a PSK and the current
handshake, as well as between the handshake in which the PSK was
generated (if via a NewSessionTicket message) and the handshake where
it was used. Each entry in the binders list is computed as an HMAC
over a transcript hash (see Section 4.4.1) containing a partial
ClientHello up to and including the PreSharedKeyExtension.identities
field. That is, it includes all of the ClientHello but not the
binders list itself. The length fields for the message (including
the overall length, the length of the extensions block, and the
length of the "pre_shared_key" extension) are all set as if binders
of the correct lengths were present.
The PskBinderEntry is computed in the same way as the Finished
message (Section 4.4.4) but with the BaseKey being the binder_key
derived via the key schedule from the corresponding PSK which is
being offered (see Section 7.1).
If the handshake includes a HelloRetryRequest, the initial
ClientHello and HelloRetryRequest are included in the transcript
along with the new ClientHello. For instance, if the client sends
ClientHello1, its binder will be computed over:
Transcript-Hash(Truncate(ClientHello1))
Where Truncate() removes the binders list from the ClientHello.
If the server responds with HelloRetryRequest, and the client then
sends ClientHello2, its binder will be computed over:
Rescorla Expires August 19, 2018 [Page 61]
Internet-Draft TLS February 2018
Transcript-Hash(ClientHello1,
HelloRetryRequest,
Truncate(ClientHello2))
The full ClientHello1/ClientHello2 is included in all other handshake
hash computations. Note that in the first flight,
Truncate(ClientHello1) is hashed directly, but in the second flight,
ClientHello1 is hashed and then reinjected as a "message_hash"
message, as described in Section 4.4.1.
4.2.11.3. Processing Order
Clients are permitted to "stream" 0-RTT data until they receive the
server's Finished, only then sending the EndOfEarlyData message,
followed by the rest of the handshake. In order to avoid deadlocks,
when accepting "early_data", servers MUST process the client's
ClientHello and then immediately send the ServerHello, rather than
waiting for the client's EndOfEarlyData message.
4.3. Server Parameters
The next two messages from the server, EncryptedExtensions and
CertificateRequest, contain information from the server that
determines the rest of the handshake. These messages are encrypted
with keys derived from the server_handshake_traffic_secret.
4.3.1. Encrypted Extensions
In all handshakes, the server MUST send the EncryptedExtensions
message immediately after the ServerHello message. This is the first
message that is encrypted under keys derived from the
server_handshake_traffic_secret.
The EncryptedExtensions message contains extensions that can be
protected, i.e., any which are not needed to establish the
cryptographic context, but which are not associated with individual
certificates. The client MUST check EncryptedExtensions for the
presence of any forbidden extensions and if any are found MUST abort
the handshake with an "illegal_parameter" alert.
Structure of this message:
struct {
Extension extensions<0..2^16-1>;
} EncryptedExtensions;
extensions A list of extensions. For more information, see the
table in Section 4.2.
Rescorla Expires August 19, 2018 [Page 62]
Internet-Draft TLS February 2018
4.3.2. Certificate Request
A server which is authenticating with a certificate MAY optionally
request a certificate from the client. This message, if sent, MUST
follow EncryptedExtensions.
Structure of this message:
struct {
opaque certificate_request_context<0..2^8-1>;
Extension extensions<2..2^16-1>;
} CertificateRequest;
certificate_request_context An opaque string which identifies the
certificate request and which will be echoed in the client's
Certificate message. The certificate_request_context MUST be
unique within the scope of this connection (thus preventing replay
of client CertificateVerify messages). This field SHALL be zero
length unless used for the post-handshake authentication exchanges
described in Section 4.6.2. When requesting post-handshake
authentication, the server SHOULD make the context unpredictable
to the client (e.g., by randomly generating it) in order to
prevent an attacker who has temporary access to the client's
private key from pre-computing valid CertificateVerify messages.
extensions A set of extensions describing the parameters of the
certificate being requested. The "signature_algorithms" extension
MUST be specified, and other extensions may optionally be included
if defined for this message. Clients MUST ignore unrecognized
extensions.
In prior versions of TLS, the CertificateRequest message carried a
list of signature algorithms and certificate authorities which the
server would accept. In TLS 1.3 the former is expressed by sending
the "signature_algorithms" and "signature_algorithms_cert"
extensions. The latter is expressed by sending the
"certificate_authorities" extension (see Section 4.2.4).
Servers which are authenticating with a PSK MUST NOT send the
CertificateRequest message in the main handshake, though they MAY
send it in post-handshake authentication (see Section 4.6.2) provided
that the client has sent the "post_handshake_auth" extension (see
Section 4.2.6).
Rescorla Expires August 19, 2018 [Page 63]
Internet-Draft TLS February 2018
4.4. Authentication Messages
As discussed in Section 2, TLS generally uses a common set of
messages for authentication, key confirmation, and handshake
integrity: Certificate, CertificateVerify, and Finished. (The
PreSharedKey binders also perform key confirmation, in a similar
fashion.) These three messages are always sent as the last messages
in their handshake flight. The Certificate and CertificateVerify
messages are only sent under certain circumstances, as defined below.
The Finished message is always sent as part of the Authentication
block. These messages are encrypted under keys derived from
[sender]_handshake_traffic_secret.
The computations for the Authentication messages all uniformly take
the following inputs:
- The certificate and signing key to be used.
- A Handshake Context consisting of the set of messages to be
included in the transcript hash.
- A base key to be used to compute a MAC key.
Based on these inputs, the messages then contain:
Certificate The certificate to be used for authentication, and any
supporting certificates in the chain. Note that certificate-based
client authentication is not available in 0-RTT mode.
CertificateVerify A signature over the value Transcript-
Hash(Handshake Context, Certificate)
Finished A MAC over the value Transcript-Hash(Handshake Context,
Certificate, CertificateVerify) using a MAC key derived from the
base key.
The following table defines the Handshake Context and MAC Base Key
for each scenario:
Rescorla Expires August 19, 2018 [Page 64]
Internet-Draft TLS February 2018
+-----------+----------------------------+--------------------------+
| Mode | Handshake Context | Base Key |
+-----------+----------------------------+--------------------------+
| Server | ClientHello ... later of E | server_handshake_traffic |
| | ncryptedExtensions/Certifi | _secret |
| | cateRequest | |
| | | |
| Client | ClientHello ... later of | client_handshake_traffic |
| | server | _secret |
| | Finished/EndOfEarlyData | |
| | | |
| Post- | ClientHello ... client | client_application_traff |
| Handshake | Finished + | ic_secret_N |
| | CertificateRequest | |
+-----------+----------------------------+--------------------------+
4.4.1. The Transcript Hash
Many of the cryptographic computations in TLS make use of a
transcript hash. This value is computed by hashing the concatenation
of each included handshake message, including the handshake message
header carrying the handshake message type and length fields, but not
including record layer headers. I.e.,
Transcript-Hash(M1, M2, ... MN) = Hash(M1 || M2 ... MN)
As an exception to this general rule, when the server responds to a
ClientHello with a HelloRetryRequest, the value of ClientHello1 is
replaced with a special synthetic handshake message of handshake type
"message_hash" containing Hash(ClientHello1). I.e.,
Transcript-Hash(ClientHello1, HelloRetryRequest, ... MN) =
Hash(message_hash || /* Handshake type */
00 00 Hash.length || /* Handshake message length (bytes) */
Hash(ClientHello1) || /* Hash of ClientHello1 */
HelloRetryRequest ... MN)
The reason for this construction is to allow the server to do a
stateless HelloRetryRequest by storing just the hash of ClientHello1
in the cookie, rather than requiring it to export the entire
intermediate hash state (see Section 4.2.2).
For concreteness, the transcript hash is always taken from the
following sequence of handshake messages, starting at the first
ClientHello and including only those messages that were sent:
ClientHello, HelloRetryRequest, ClientHello, ServerHello,
EncryptedExtensions, server CertificateRequest, server Certificate,
Rescorla Expires August 19, 2018 [Page 65]
Internet-Draft TLS February 2018
server CertificateVerify, server Finished, EndOfEarlyData, client
Certificate, client CertificateVerify, client Finished.
In general, implementations can implement the transcript by keeping a
running transcript hash value based on the negotiated hash. Note,
however, that subsequent post-handshake authentications do not
include each other, just the messages through the end of the main
handshake.
4.4.2. Certificate
This message conveys the endpoint's certificate chain to the peer.
The server MUST send a Certificate message whenever the agreed-upon
key exchange method uses certificates for authentication (this
includes all key exchange methods defined in this document except
PSK).
The client MUST send a Certificate message if and only if the server
has requested client authentication via a CertificateRequest message
(Section 4.3.2). If the server requests client authentication but no
suitable certificate is available, the client MUST send a Certificate
message containing no certificates (i.e., with the "certificate_list"
field having length 0).
Structure of this message:
Rescorla Expires August 19, 2018 [Page 66]
Internet-Draft TLS February 2018
enum {
X509(0),
RawPublicKey(2),
(255)
} CertificateType;
struct {
select (certificate_type) {
case RawPublicKey:
/* From RFC 7250 ASN.1_subjectPublicKeyInfo */
opaque ASN1_subjectPublicKeyInfo<1..2^24-1>;
case X509:
opaque cert_data<1..2^24-1>;
};
Extension extensions<0..2^16-1>;
} CertificateEntry;
struct {
opaque certificate_request_context<0..2^8-1>;
CertificateEntry certificate_list<0..2^24-1>;
} Certificate;
certificate_request_context If this message is in response to a
CertificateRequest, the value of certificate_request_context in
that message. Otherwise (in the case of server authentication),
this field SHALL be zero length.
certificate_list This is a sequence (chain) of CertificateEntry
structures, each containing a single certificate and set of
extensions.
extensions: A set of extension values for the CertificateEntry. The
"Extension" format is defined in Section 4.2. Valid extensions
for server certificates include OCSP Status extension ([RFC6066])
and SignedCertificateTimestamps ([RFC6962]). Extensions in the
Certificate message from the server MUST correspond to one from
the ClientHello message. Extensions in the Certificate from the
client MUST correspond with an extension in the CertificateRequest
message from the server. If an extension applies to the entire
chain, it SHOULD be included in the first CertificateEntry.
If the corresponding certificate type extension
("server_certificate_type" or "client_certificate_type") was not
negotiated in Encrypted Extensions, or the X.509 certificate type was
negotiated, then each CertificateEntry contains a DER-encoded X.509
certificate. The sender's certificate MUST come in the first
CertificateEntry in the list. Each following certificate SHOULD
Rescorla Expires August 19, 2018 [Page 67]
Internet-Draft TLS February 2018
directly certify one preceding it. Because certificate validation
requires that trust anchors be distributed independently, a
certificate that specifies a trust anchor MAY be omitted from the
chain, provided that supported peers are known to possess any omitted
certificates.
Note: Prior to TLS 1.3, "certificate_list" ordering required each
certificate to certify the one immediately preceding it; however,
some implementations allowed some flexibility. Servers sometimes
send both a current and deprecated intermediate for transitional
purposes, and others are simply configured incorrectly, but these
cases can nonetheless be validated properly. For maximum
compatibility, all implementations SHOULD be prepared to handle
potentially extraneous certificates and arbitrary orderings from any
TLS version, with the exception of the end-entity certificate which
MUST be first.
If the RawPublicKey certificate type was negotiated, then the
certificate_list MUST contain no more than one CertificateEntry,
which contains an ASN1_subjectPublicKeyInfo value as defined in
[RFC7250], Section 3.
The OpenPGP certificate type [RFC6091] MUST NOT be used with TLS 1.3.
The server's certificate_list MUST always be non-empty. A client
will send an empty certificate_list if it does not have an
appropriate certificate to send in response to the server's
authentication request.
4.4.2.1. OCSP Status and SCT Extensions
[RFC6066] and [RFC6961] provide extensions to negotiate the server
sending OCSP responses to the client. In TLS 1.2 and below, the
server replies with an empty extension to indicate negotiation of
this extension and the OCSP information is carried in a
CertificateStatus message. In TLS 1.3, the server's OCSP information
is carried in an extension in the CertificateEntry containing the
associated certificate. Specifically: The body of the
"status_request" extension from the server MUST be a
CertificateStatus structure as defined in [RFC6066], which is
interpreted as defined in [RFC6960].
Note: status_request_v2 extension ([RFC6961]) is deprecated. TLS 1.3
servers MUST NOT act upon its presence or information in it when
processing Client Hello, in particular they MUST NOT send the
status_request_v2 extension in the Encrypted Extensions, Certificate
Request or the Certificate messages. TLS 1.3 servers MUST be able to
Rescorla Expires August 19, 2018 [Page 68]
Internet-Draft TLS February 2018
process Client Hello messages that include it, as it MAY be sent by
clients that wish to use it in earlier protocol versions.
A server MAY request that a client present an OCSP response with its
certificate by sending an empty "status_request" extension in its
CertificateRequest message. If the client opts to send an OCSP
response, the body of its "status_request" extension MUST be a
CertificateStatus structure as defined in [RFC6066].
Similarly, [RFC6962] provides a mechanism for a server to send a
Signed Certificate Timestamp (SCT) as an extension in the ServerHello
in TLS 1.2 and below. In TLS 1.3, the server's SCT information is
carried in an extension in CertificateEntry.
4.4.2.2. Server Certificate Selection
The following rules apply to the certificates sent by the server:
- The certificate type MUST be X.509v3 [RFC5280], unless explicitly
negotiated otherwise (e.g., [RFC7250]).
- The server's end-entity certificate's public key (and associated
restrictions) MUST be compatible with the selected authentication
algorithm (currently RSA, ECDSA, or EdDSA).
- The certificate MUST allow the key to be used for signing (i.e.,
the digitalSignature bit MUST be set if the Key Usage extension is
present) with a signature scheme indicated in the client's
"signature_algorithms"/"signature_algorithms_cert" extensions (see
Section 4.2.3).
- The "server_name" [RFC6066] and "certificate_authorities"
extensions are used to guide certificate selection. As servers
MAY require the presence of the "server_name" extension, clients
SHOULD send this extension, when applicable.
All certificates provided by the server MUST be signed by a signature
algorithm advertised by the client, if they are able to provide such
a chain (see Section 4.2.3). Certificates that are self-signed or
certificates that are expected to be trust anchors are not validated
as part of the chain and therefore MAY be signed with any algorithm.
If the server cannot produce a certificate chain that is signed only
via the indicated supported algorithms, then it SHOULD continue the
handshake by sending the client a certificate chain of its choice
that may include algorithms that are not known to be supported by the
client. This fallback chain SHOULD NOT use the deprecated SHA-1 hash
Rescorla Expires August 19, 2018 [Page 69]
Internet-Draft TLS February 2018
algorithm in general, but MAY do so if the client's advertisement
permits it, and MUST NOT do so otherwise.
If the client cannot construct an acceptable chain using the provided
certificates and decides to abort the handshake, then it MUST abort
the handshake with an appropriate certificate-related alert (by
default, "unsupported_certificate"; see Section 6.2 for more).
If the server has multiple certificates, it chooses one of them based
on the above-mentioned criteria (in addition to other criteria, such
as transport layer endpoint, local configuration and preferences).
4.4.2.3. Client Certificate Selection
The following rules apply to certificates sent by the client:
- The certificate type MUST be X.509v3 [RFC5280], unless explicitly
negotiated otherwise (e.g., [RFC7250]).
- If the "certificate_authorities" extension in the
CertificateRequest message was present, at least one of the
certificates in the certificate chain SHOULD be issued by one of
the listed CAs.
- The certificates MUST be signed using an acceptable signature
algorithm, as described in Section 4.3.2. Note that this relaxes
the constraints on certificate-signing algorithms found in prior
versions of TLS.
- If the CertificateRequest message contained a non-empty
"oid_filters" extension, the end-entity certificate MUST match the
extension OIDs recognized by the client, as described in
Section 4.2.5.
Note that, as with the server certificate, there are certificates
that use algorithm combinations that cannot be currently used with
TLS.
4.4.2.4. Receiving a Certificate Message
In general, detailed certificate validation procedures are out of
scope for TLS (see [RFC5280]). This section provides TLS-specific
requirements.
If the server supplies an empty Certificate message, the client MUST
abort the handshake with a "decode_error" alert.
Rescorla Expires August 19, 2018 [Page 70]
Internet-Draft TLS February 2018
If the client does not send any certificates, the server MAY at its
discretion either continue the handshake without client
authentication, or abort the handshake with a "certificate_required"
alert. Also, if some aspect of the certificate chain was
unacceptable (e.g., it was not signed by a known, trusted CA), the
server MAY at its discretion either continue the handshake
(considering the client unauthenticated) or abort the handshake.
Any endpoint receiving any certificate which it would need to
validate using any signature algorithm using an MD5 hash MUST abort
the handshake with a "bad_certificate" alert. SHA-1 is deprecated
and it is RECOMMENDED that any endpoint receiving any certificate
which it would need to validate using any signature algorithm using a
SHA-1 hash abort the handshake with a "bad_certificate" alert. For
clarity, this means that endpoints MAY accept these algorithms for
certificates that are self-signed or are trust anchors.
All endpoints are RECOMMENDED to transition to SHA-256 or better as
soon as possible to maintain interoperability with implementations
currently in the process of phasing out SHA-1 support.
Note that a certificate containing a key for one signature algorithm
MAY be signed using a different signature algorithm (for instance, an
RSA key signed with an ECDSA key).
4.4.3. Certificate Verify
This message is used to provide explicit proof that an endpoint
possesses the private key corresponding to its certificate. The
CertificateVerify message also provides integrity for the handshake
up to this point. Servers MUST send this message when authenticating
via a certificate. Clients MUST send this message whenever
authenticating via a certificate (i.e., when the Certificate message
is non-empty). When sent, this message MUST appear immediately after
the Certificate message and immediately prior to the Finished
message.
Structure of this message:
struct {
SignatureScheme algorithm;
opaque signature<0..2^16-1>;
} CertificateVerify;
The algorithm field specifies the signature algorithm used (see
Section 4.2.3 for the definition of this field). The signature is a
digital signature using that algorithm. The content that is covered
Rescorla Expires August 19, 2018 [Page 71]
Internet-Draft TLS February 2018
under the signature is the hash output as described in Section 4.4,
namely:
Transcript-Hash(Handshake Context, Certificate)
The digital signature is then computed over the concatenation of:
- A string that consists of octet 32 (0x20) repeated 64 times
- The context string
- A single 0 byte which serves as the separator
- The content to be signed
This structure is intended to prevent an attack on previous versions
of TLS in which the ServerKeyExchange format meant that attackers
could obtain a signature of a message with a chosen 32-byte prefix
(ClientHello.random). The initial 64-byte pad clears that prefix
along with the server-controlled ServerHello.random.
The context string for a server signature is "TLS 1.3, server
CertificateVerify" and for a client signature is "TLS 1.3, client
CertificateVerify". It is used to provide separation between
signatures made in different contexts, helping against potential
cross-protocol attacks.
For example, if the transcript hash was 32 bytes of 01 (this length
would make sense for SHA-256), the content covered by the digital
signature for a server CertificateVerify would be:
2020202020202020202020202020202020202020202020202020202020202020
2020202020202020202020202020202020202020202020202020202020202020
544c5320312e332c207365727665722043657274696669636174655665726966
79
00
0101010101010101010101010101010101010101010101010101010101010101
On the sender side the process for computing the signature field of
the CertificateVerify message takes as input:
- The content covered by the digital signature
- The private signing key corresponding to the certificate sent in
the previous message
If the CertificateVerify message is sent by a server, the signature
algorithm MUST be one offered in the client's "signature_algorithms"
Rescorla Expires August 19, 2018 [Page 72]
Internet-Draft TLS February 2018
extension unless no valid certificate chain can be produced without
unsupported algorithms (see Section 4.2.3).
If sent by a client, the signature algorithm used in the signature
MUST be one of those present in the supported_signature_algorithms
field of the "signature_algorithms" extension in the
CertificateRequest message.
In addition, the signature algorithm MUST be compatible with the key
in the sender's end-entity certificate. RSA signatures MUST use an
RSASSA-PSS algorithm, regardless of whether RSASSA-PKCS1-v1_5
algorithms appear in "signature_algorithms". The SHA-1 algorithm
MUST NOT be used in any signatures of CertificateVerify messages.
All SHA-1 signature algorithms in this specification are defined
solely for use in legacy certificates and are not valid for
CertificateVerify signatures.
The receiver of a CertificateVerify message MUST verify the signature
field. The verification process takes as input:
- The content covered by the digital signature
- The public key contained in the end-entity certificate found in
the associated Certificate message.
- The digital signature received in the signature field of the
CertificateVerify message
If the verification fails, the receiver MUST terminate the handshake
with a "decrypt_error" alert.
4.4.4. Finished
The Finished message is the final message in the authentication
block. It is essential for providing authentication of the handshake
and of the computed keys.
Recipients of Finished messages MUST verify that the contents are
correct and if incorrect MUST terminate the connection with a
"decrypt_error" alert.
Once a side has sent its Finished message and received and validated
the Finished message from its peer, it may begin to send and receive
application data over the connection. There are two settings in
which it is permitted to send data prior to receiving the peer's
Finished:
1. Clients sending 0-RTT data as described in Section 4.2.10.
Rescorla Expires August 19, 2018 [Page 73]
Internet-Draft TLS February 2018
2. Servers MAY send data after sending their first flight, but
because the handshake is not yet complete, they have no assurance
of either the peer's identity or of its liveness (i.e., the
ClientHello might have been replayed).
The key used to compute the finished message is computed from the
Base key defined in Section 4.4 using HKDF (see Section 7.1).
Specifically:
finished_key =
HKDF-Expand-Label(BaseKey, "finished", "", Hash.length)
Structure of this message:
struct {
opaque verify_data[Hash.length];
} Finished;
The verify_data value is computed as follows:
verify_data =
HMAC(finished_key,
Transcript-Hash(Handshake Context,
Certificate*, CertificateVerify*))
* Only included if present.
HMAC [RFC2104] uses the Hash algorithm for the handshake. As noted
above, the HMAC input can generally be implemented by a running hash,
i.e., just the handshake hash at this point.
In previous versions of TLS, the verify_data was always 12 octets
long. In TLS 1.3, it is the size of the HMAC output for the Hash
used for the handshake.
Note: Alerts and any other record types are not handshake messages
and are not included in the hash computations.
Any records following a 1-RTT Finished message MUST be encrypted
under the appropriate application traffic key as described in
Section 7.2. In particular, this includes any alerts sent by the
server in response to client Certificate and CertificateVerify
messages.
Rescorla Expires August 19, 2018 [Page 74]
Internet-Draft TLS February 2018
4.5. End of Early Data
struct {} EndOfEarlyData;
If the server sent an "early_data" extension, the client MUST send an
EndOfEarlyData message after receiving the server Finished. If the
server does not send an "early_data" extension, then the client MUST
NOT send an EndOfEarlyData message. This message indicates that all
0-RTT application_data messages, if any, have been transmitted and
that the following records are protected under handshake traffic
keys. Servers MUST NOT send this message and clients receiving it
MUST terminate the connection with an "unexpected_message" alert.
This message is encrypted under keys derived from the
client_early_traffic_secret.
4.6. Post-Handshake Messages
TLS also allows other messages to be sent after the main handshake.
These messages use a handshake content type and are encrypted under
the appropriate application traffic key.
4.6.1. New Session Ticket Message
At any time after the server has received the client Finished
message, it MAY send a NewSessionTicket message. This message
creates a unique association between the ticket value and a secret
PSK derived from the resumption master secret.
The client MAY use this PSK for future handshakes by including the
ticket value in the "pre_shared_key" extension in its ClientHello
(Section 4.2.11). Servers MAY send multiple tickets on a single
connection, either immediately after each other or after specific
events (see Appendix C.4). For instance, the server might send a new
ticket after post-handshake authentication in order to encapsulate
the additional client authentication state. Multiple tickets are
useful for clients for a variety of purposes, including:
- Opening multiple parallel HTTP connections.
- Performing connection racing across interfaces and address
families via, e.g., Happy Eyeballs [RFC8305] or related
techniques.
Any ticket MUST only be resumed with a cipher suite that has the same
KDF hash algorithm as that used to establish the original connection.
Clients MUST only resume if the new SNI value is valid for the server
certificate presented in the original session, and SHOULD only resume
Rescorla Expires August 19, 2018 [Page 75]
Internet-Draft TLS February 2018
if the SNI value matches the one used in the original session. The
latter is a performance optimization: normally, there is no reason to
expect that different servers covered by a single certificate would
be able to accept each other's tickets, hence attempting resumption
in that case would waste a single-use ticket. If such an indication
is provided (externally or by any other means), clients MAY resume
with a different SNI value.
On resumption, if reporting an SNI value to the calling application,
implementations MUST use the value sent in the resumption ClientHello
rather than the value sent in the previous session. Note that if a
server implementation declines all PSK identities with different SNI
values, these two values are always the same.
Note: Although the resumption master secret depends on the client's
second flight, servers which do not request client authentication MAY
compute the remainder of the transcript independently and then send a
NewSessionTicket immediately upon sending its Finished rather than
waiting for the client Finished. This might be appropriate in cases
where the client is expected to open multiple TLS connections in
parallel and would benefit from the reduced overhead of a resumption
handshake, for example.
struct {
uint32 ticket_lifetime;
uint32 ticket_age_add;
opaque ticket_nonce<0..255>;
opaque ticket<1..2^16-1>;
Extension extensions<0..2^16-2>;
} NewSessionTicket;
ticket_lifetime Indicates the lifetime in seconds as a 32-bit
unsigned integer in network byte order from the time of ticket
issuance. Servers MUST NOT use any value greater than 604800
seconds (7 days). The value of zero indicates that the ticket
should be discarded immediately. Clients MUST NOT cache tickets
for longer than 7 days, regardless of the ticket_lifetime, and MAY
delete the ticket earlier based on local policy. A server MAY
treat a ticket as valid for a shorter period of time than what is
stated in the ticket_lifetime.
ticket_age_add A securely generated, random 32-bit value that is
used to obscure the age of the ticket that the client includes in
the "pre_shared_key" extension. The client-side ticket age is
added to this value modulo 2^32 to obtain the value that is
transmitted by the client. The server MUST generate a fresh value
for each ticket it sends.
Rescorla Expires August 19, 2018 [Page 76]
Internet-Draft TLS February 2018
ticket_nonce A per-ticket value that is unique across all tickets
issued on this connection.
ticket The value of the ticket to be used as the PSK identity. The
ticket itself is an opaque label. It MAY either be a database
lookup key or a self-encrypted and self-authenticated value.
Section 4 of [RFC5077] describes a recommended ticket construction
mechanism.
extensions A set of extension values for the ticket. The
"Extension" format is defined in Section 4.2. Clients MUST ignore
unrecognized extensions.
The sole extension currently defined for NewSessionTicket is
"early_data", indicating that the ticket may be used to send 0-RTT
data (Section 4.2.10)). It contains the following value:
max_early_data_size The maximum amount of 0-RTT data that the client
is allowed to send when using this ticket, in bytes. Only
Application Data payload (i.e., plaintext but not padding or the
inner content type byte) is counted. A server receiving more than
max_early_data_size bytes of 0-RTT data SHOULD terminate the
connection with an "unexpected_message" alert. Note that servers
that reject early data due to lack of cryptographic material will
be unable to differentiate padding from content, so clients SHOULD
NOT depend on being able to send large quantities of padding in
early data records.
The PSK associated with the ticket is computed as:
HKDF-Expand-Label(resumption_master_secret,
"resumption", ticket_nonce, Hash.length)
Because the ticket_nonce value is distinct for each NewSessionTicket
message, a different PSK will be derived for each ticket.
Note that in principle it is possible to continue issuing new tickets
which indefinitely extend the lifetime of the keying material
originally derived from an initial non-PSK handshake (which was most
likely tied to the peer's certificate). It is RECOMMENDED that
implementations place limits on the total lifetime of such keying
material; these limits should take into account the lifetime of the
peer's certificate, the likelihood of intervening revocation, and the
time since the peer's online CertificateVerify signature.
Rescorla Expires August 19, 2018 [Page 77]
Internet-Draft TLS February 2018
4.6.2. Post-Handshake Authentication
When the client has sent the "post_handshake_auth" extension (see
Section 4.2.6), a server MAY request client authentication at any
time after the handshake has completed by sending a
CertificateRequest message. The client MUST respond with the
appropriate Authentication messages (see Section 4.4). If the client
chooses to authenticate, it MUST send Certificate, CertificateVerify,
and Finished. If it declines, it MUST send a Certificate message
containing no certificates followed by Finished. All of the client's
messages for a given response MUST appear consecutively on the wire
with no intervening messages of other types.
A client that receives a CertificateRequest message without having
sent the "post_handshake_auth" extension MUST send an
"unexpected_message" fatal alert.
Note: Because client authentication could involve prompting the user,
servers MUST be prepared for some delay, including receiving an
arbitrary number of other messages between sending the
CertificateRequest and receiving a response. In addition, clients
which receive multiple CertificateRequests in close succession MAY
respond to them in a different order than they were received (the
certificate_request_context value allows the server to disambiguate
the responses).
4.6.3. Key and IV Update
enum {
update_not_requested(0), update_requested(1), (255)
} KeyUpdateRequest;
struct {
KeyUpdateRequest request_update;
} KeyUpdate;
request_update Indicates whether the recipient of the KeyUpdate
should respond with its own KeyUpdate. If an implementation
receives any other value, it MUST terminate the connection with an
"illegal_parameter" alert.
The KeyUpdate handshake message is used to indicate that the sender
is updating its sending cryptographic keys. This message can be sent
by either peer after it has sent a Finished message. Implementations
that receive a KeyUpdate message prior to receiving a Finished
message MUST terminate the connection with an "unexpected_message"
alert. After sending a KeyUpdate message, the sender SHALL send all
its traffic using the next generation of keys, computed as described
Rescorla Expires August 19, 2018 [Page 78]
Internet-Draft TLS February 2018
in Section 7.2. Upon receiving a KeyUpdate, the receiver MUST update
its receiving keys.
If the request_update field is set to "update_requested" then the
receiver MUST send a KeyUpdate of its own with request_update set to
"update_not_requested" prior to sending its next application data
record. This mechanism allows either side to force an update to the
entire connection, but causes an implementation which receives
multiple KeyUpdates while it is silent to respond with a single
update. Note that implementations may receive an arbitrary number of
messages between sending a KeyUpdate with request_update set to
update_requested and receiving the peer's KeyUpdate, because those
messages may already be in flight. However, because send and receive
keys are derived from independent traffic secrets, retaining the
receive traffic secret does not threaten the forward secrecy of data
sent before the sender changed keys.
If implementations independently send their own KeyUpdates with
request_update set to "update_requested", and they cross in flight,
then each side will also send a response, with the result that each
side increments by two generations.
Both sender and receiver MUST encrypt their KeyUpdate messages with
the old keys. Additionally, both sides MUST enforce that a KeyUpdate
with the old key is received before accepting any messages encrypted
with the new key. Failure to do so may allow message truncation
attacks.
5. Record Protocol
The TLS record protocol takes messages to be transmitted, fragments
the data into manageable blocks, protects the records, and transmits
the result. Received data is verified, decrypted, reassembled, and
then delivered to higher-level clients.
TLS records are typed, which allows multiple higher-level protocols
to be multiplexed over the same record layer. This document
specifies four content types: handshake, application data, alert, and
change_cipher_spec. The change_cipher_spec record is used only for
compatibility purposes (see Appendix D.4).
An implementation may receive an unencrypted record of type
change_cipher_spec consisting of the single byte value 0x01 at any
time after the first ClientHello message has been sent or received
and before the peer's Finished message has been received and MUST
simply drop it without further processing. Note that this record may
appear at a point at the handshake where the implementation is
expecting protected records and so it is necessary to detect this
Rescorla Expires August 19, 2018 [Page 79]
Internet-Draft TLS February 2018
condition prior to attempting to deprotect the record. An
implementation which receives any other change_cipher_spec value or
which receives a protected change_cipher_spec record MUST abort the
handshake with an "unexpected_message" alert. A change_cipher_spec
record received before the first ClientHello message or after the
peer's Finished message MUST be treated as an unexpected record type.
Implementations MUST NOT send record types not defined in this
document unless negotiated by some extension. If a TLS
implementation receives an unexpected record type, it MUST terminate
the connection with an "unexpected_message" alert. New record
content type values are assigned by IANA in the TLS Content Type
Registry as described in Section 11.
5.1. Record Layer
The record layer fragments information blocks into TLSPlaintext
records carrying data in chunks of 2^14 bytes or less. Message
boundaries are handled differently depending on the underlying
ContentType. Any future content types MUST specify appropriate
rules. Note that these rules are stricter than what was enforced in
TLS 1.2.
Handshake messages MAY be coalesced into a single TLSPlaintext record
or fragmented across several records, provided that:
- Handshake messages MUST NOT be interleaved with other record
types. That is, if a handshake message is split over two or more
records, there MUST NOT be any other records between them.
- Handshake messages MUST NOT span key changes. Implementations
MUST verify that all messages immediately preceding a key change
align with a record boundary; if not, then they MUST terminate the
connection with an "unexpected_message" alert. Because the
ClientHello, EndOfEarlyData, ServerHello, Finished, and KeyUpdate
messages can immediately precede a key change, implementations
MUST send these messages in alignment with a record boundary.
Implementations MUST NOT send zero-length fragments of Handshake
types, even if those fragments contain padding.
Alert messages (Section 6) MUST NOT be fragmented across records and
multiple Alert messages MUST NOT be coalesced into a single
TLSPlaintext record. In other words, a record with an Alert type
MUST contain exactly one message.
Application Data messages contain data that is opaque to TLS.
Application Data messages are always protected. Zero-length
Rescorla Expires August 19, 2018 [Page 80]
Internet-Draft TLS February 2018
fragments of Application Data MAY be sent as they are potentially
useful as a traffic analysis countermeasure. Application Data
fragments MAY be split across multiple records or coalesced into a
single record.
enum {
invalid(0),
change_cipher_spec(20),
alert(21),
handshake(22),
application_data(23),
(255)
} ContentType;
struct {
ContentType type;
ProtocolVersion legacy_record_version;
uint16 length;
opaque fragment[TLSPlaintext.length];
} TLSPlaintext;
type The higher-level protocol used to process the enclosed
fragment.
legacy_record_version This value MUST be set to 0x0303 for all
records generated by a TLS 1.3 implementation other than an
initial ClientHello (i.e., one not generated after a
HelloRetryRequest), where it MAY also be 0x0301 for compatibility
purposes. This field is deprecated and MUST be ignored for all
purposes. Previous versions of TLS would use other values in this
field under some circumstances.
length The length (in bytes) of the following TLSPlaintext.fragment.
The length MUST NOT exceed 2^14 bytes. An endpoint that receives
a record that exceeds this length MUST terminate the connection
with a "record_overflow" alert.
fragment The data being transmitted. This value is transparent and
is treated as an independent block to be dealt with by the higher-
level protocol specified by the type field.
This document describes TLS 1.3, which uses the version 0x0304. This
version value is historical, deriving from the use of 0x0301 for TLS
1.0 and 0x0300 for SSL 3.0. In order to maximize backwards
compatibility, records containing an initial ClientHello MUST have
version 0x0301 and a record containing a second ClientHello or a
ServerHello MUST have version 0x0303, reflecting TLS 1.0 and TLS 1.2
Rescorla Expires August 19, 2018 [Page 81]
Internet-Draft TLS February 2018
respectively. When negotiating prior versions of TLS, endpoints
follow the procedure and requirements in Appendix D.
When record protection has not yet been engaged, TLSPlaintext
structures are written directly onto the wire. Once record
protection has started, TLSPlaintext records are protected and sent
as described in the following section.
5.2. Record Payload Protection
The record protection functions translate a TLSPlaintext structure
into a TLSCiphertext. The deprotection functions reverse the
process. In TLS 1.3, as opposed to previous versions of TLS, all
ciphers are modeled as "Authenticated Encryption with Additional
Data" (AEAD) [RFC5116]. AEAD functions provide an unified encryption
and authentication operation which turns plaintext into authenticated
ciphertext and back again. Each encrypted record consists of a
plaintext header followed by an encrypted body, which itself contains
a type and optional padding.
struct {
opaque content[TLSPlaintext.length];
ContentType type;
uint8 zeros[length_of_padding];
} TLSInnerPlaintext;
struct {
ContentType opaque_type = application_data; /* 23 */
ProtocolVersion legacy_record_version = 0x0303; /* TLS v1.2 */
uint16 length;
opaque encrypted_record[TLSCiphertext.length];
} TLSCiphertext;
content The byte encoding of a handshake or an alert message, or the
raw bytes of the application's data to send.
type The content type of the record.
zeros An arbitrary-length run of zero-valued bytes may appear in the
cleartext after the type field. This provides an opportunity for
senders to pad any TLS record by a chosen amount as long as the
total stays within record size limits. See Section 5.4 for more
details.
opaque_type The outer opaque_type field of a TLSCiphertext record is
always set to the value 23 (application_data) for outward
compatibility with middleboxes accustomed to parsing previous
Rescorla Expires August 19, 2018 [Page 82]
Internet-Draft TLS February 2018
versions of TLS. The actual content type of the record is found
in TLSInnerPlaintext.type after decryption.
legacy_record_version The legacy_record_version field is always
0x0303. TLS 1.3 TLSCiphertexts are not generated until after TLS
1.3 has been negotiated, so there are no historical compatibility
concerns where other values might be received. Implementations
MAY verify that the legacy_record_version field is 0x0303 and
abort the connection if it is not. Note that the handshake
protocol including the ClientHello and ServerHello messages
authenticates the protocol version, so this value is redundant.
length The length (in bytes) of the following
TLSCiphertext.encrypted_record, which is the sum of the lengths of
the content and the padding, plus one for the inner content type,
plus any expansion added by the AEAD algorithm. The length MUST
NOT exceed 2^14 + 256 bytes. An endpoint that receives a record
that exceeds this length MUST terminate the connection with a
"record_overflow" alert.
encrypted_record The AEAD-encrypted form of the serialized
TLSInnerPlaintext structure.
AEAD algorithms take as input a single key, a nonce, a plaintext, and
"additional data" to be included in the authentication check, as
described in Section 2.1 of [RFC5116]. The key is either the
client_write_key or the server_write_key, the nonce is derived from
the sequence number (see Section 5.3) and the client_write_iv or
server_write_iv, and the additional data input is empty (zero
length). Derivation of traffic keys is defined in Section 7.3.
The plaintext input to the AEAD algorithm is the encoded
TLSInnerPlaintext structure.
The AEAD output consists of the ciphertext output from the AEAD
encryption operation. The length of the plaintext is greater than
the corresponding TLSPlaintext.length due to the inclusion of
TLSInnerPlaintext.type and any padding supplied by the sender. The
length of the AEAD output will generally be larger than the
plaintext, but by an amount that varies with the AEAD algorithm.
Since the ciphers might incorporate padding, the amount of overhead
could vary with different lengths of plaintext. Symbolically,
AEADEncrypted =
AEAD-Encrypt(write_key, nonce, plaintext)
In order to decrypt and verify, the cipher takes as input the key,
nonce, and the AEADEncrypted value. The output is either the
Rescorla Expires August 19, 2018 [Page 83]
Internet-Draft TLS February 2018
plaintext or an error indicating that the decryption failed. There
is no separate integrity check. That is:
plaintext of encrypted_record =
AEAD-Decrypt(peer_write_key, nonce, AEADEncrypted)
If the decryption fails, the receiver MUST terminate the connection
with a "bad_record_mac" alert.
An AEAD algorithm used in TLS 1.3 MUST NOT produce an expansion
greater than 255 octets. An endpoint that receives a record from its
peer with TLSCiphertext.length larger than 2^14 + 256 octets MUST
terminate the connection with a "record_overflow" alert. This limit
is derived from the maximum TLSPlaintext length of 2^14 octets + 1
octet for ContentType + the maximum AEAD expansion of 255 octets.
5.3. Per-Record Nonce
A 64-bit sequence number is maintained separately for reading and
writing records. Each sequence number is set to zero at the
beginning of a connection and whenever the key is changed.
The appropriate sequence number is incremented by one after reading
or writing each record. The first record transmitted under a
particular traffic key MUST use sequence number 0.
Because the size of sequence numbers is 64-bit, they should not wrap.
If a TLS implementation would need to wrap a sequence number, it MUST
either re-key (Section 4.6.3) or terminate the connection.
Each AEAD algorithm will specify a range of possible lengths for the
per-record nonce, from N_MIN bytes to N_MAX bytes of input
([RFC5116]). The length of the TLS per-record nonce (iv_length) is
set to the larger of 8 bytes and N_MIN for the AEAD algorithm (see
[RFC5116] Section 4). An AEAD algorithm where N_MAX is less than 8
bytes MUST NOT be used with TLS. The per-record nonce for the AEAD
construction is formed as follows:
1. The 64-bit record sequence number is encoded in network byte
order and padded to the left with zeros to iv_length.
2. The padded sequence number is XORed with the static
client_write_iv or server_write_iv, depending on the role.
The resulting quantity (of length iv_length) is used as the per-
record nonce.
Rescorla Expires August 19, 2018 [Page 84]
Internet-Draft TLS February 2018
Note: This is a different construction from that in TLS 1.2, which
specified a partially explicit nonce.
5.4. Record Padding
All encrypted TLS records can be padded to inflate the size of the
TLSCiphertext. This allows the sender to hide the size of the
traffic from an observer.
When generating a TLSCiphertext record, implementations MAY choose to
pad. An unpadded record is just a record with a padding length of
zero. Padding is a string of zero-valued bytes appended to the
ContentType field before encryption. Implementations MUST set the
padding octets to all zeros before encrypting.
Application Data records may contain a zero-length
TLSInnerPlaintext.content if the sender desires. This permits
generation of plausibly-sized cover traffic in contexts where the
presence or absence of activity may be sensitive. Implementations
MUST NOT send Handshake or Alert records that have a zero-length
TLSInnerPlaintext.content; if such a message is received, the
receiving implementation MUST terminate the connection with an
"unexpected_message" alert.
The padding sent is automatically verified by the record protection
mechanism; upon successful decryption of a
TLSCiphertext.encrypted_record, the receiving implementation scans
the field from the end toward the beginning until it finds a non-zero
octet. This non-zero octet is the content type of the message. This
padding scheme was selected because it allows padding of any
encrypted TLS record by an arbitrary size (from zero up to TLS record
size limits) without introducing new content types. The design also
enforces all-zero padding octets, which allows for quick detection of
padding errors.
Implementations MUST limit their scanning to the cleartext returned
from the AEAD decryption. If a receiving implementation does not
find a non-zero octet in the cleartext, it MUST terminate the
connection with an "unexpected_message" alert.
The presence of padding does not change the overall record size
limitations - the full encoded TLSInnerPlaintext MUST NOT exceed 2^14
+ 1 octets. If the maximum fragment length is reduced, as for
example by the max_fragment_length extension from [RFC6066], then the
reduced limit applies to the full plaintext, including the content
type and padding.
Rescorla Expires August 19, 2018 [Page 85]
Internet-Draft TLS February 2018
Selecting a padding policy that suggests when and how much to pad is
a complex topic and is beyond the scope of this specification. If
the application layer protocol on top of TLS has its own padding, it
may be preferable to pad application_data TLS records within the
application layer. Padding for encrypted handshake and alert TLS
records must still be handled at the TLS layer, though. Later
documents may define padding selection algorithms or define a padding
policy request mechanism through TLS extensions or some other means.
5.5. Limits on Key Usage
There are cryptographic limits on the amount of plaintext which can
be safely encrypted under a given set of keys. [AEAD-LIMITS]
provides an analysis of these limits under the assumption that the
underlying primitive (AES or ChaCha20) has no weaknesses.
Implementations SHOULD do a key update as described in Section 4.6.3
prior to reaching these limits.
For AES-GCM, up to 2^24.5 full-size records (about 24 million) may be
encrypted on a given connection while keeping a safety margin of
approximately 2^-57 for Authenticated Encryption (AE) security. For
ChaCha20/Poly1305, the record sequence number would wrap before the
safety limit is reached.
6. Alert Protocol
One of the content types supported by the TLS record layer is the
alert type. Like other messages, alert messages are encrypted as
specified by the current connection state.
Alert messages convey a description of the alert and a legacy field
that conveyed the severity of the message in previous versions of
TLS. In TLS 1.3, the severity is implicit in the type of alert being
sent, and the 'level' field can safely be ignored. The
"close_notify" alert is used to indicate orderly closure of one
direction of the connection. Upon receiving such an alert, the TLS
implementation SHOULD indicate end-of-data to the application.
Error alerts indicate abortive closure of the connection (see
Section 6.2). Upon receiving an error alert, the TLS implementation
SHOULD indicate an error to the application and MUST NOT allow any
further data to be sent or received on the connection. Servers and
clients MUST forget keys and secrets associated with a failed
connection. Stateful implementations of tickets (as in many clients)
SHOULD discard tickets associated with failed connections.
Rescorla Expires August 19, 2018 [Page 86]
Internet-Draft TLS February 2018
All the alerts listed in Section 6.2 MUST be sent as fatal and MUST
be treated as fatal regardless of the AlertLevel in the message.
Unknown alert types MUST be treated as fatal.
Note: TLS defines two generic alerts (see Section 6) to use upon
failure to parse a message. Peers which receive a message which
cannot be parsed according to the syntax (e.g., have a length
extending beyond the message boundary or contain an out-of-range
length) MUST terminate the connection with a "decode_error" alert.
Peers which receive a message which is syntactically correct but
semantically invalid (e.g., a DHE share of p - 1, or an invalid enum)
MUST terminate the connection with an "illegal_parameter" alert.
Rescorla Expires August 19, 2018 [Page 87]
Internet-Draft TLS February 2018
enum { warning(1), fatal(2), (255) } AlertLevel;
enum {
close_notify(0),
unexpected_message(10),
bad_record_mac(20),
record_overflow(22),
handshake_failure(40),
bad_certificate(42),
unsupported_certificate(43),
certificate_revoked(44),
certificate_expired(45),
certificate_unknown(46),
illegal_parameter(47),
unknown_ca(48),
access_denied(49),
decode_error(50),
decrypt_error(51),
protocol_version(70),
insufficient_security(71),
internal_error(80),
inappropriate_fallback(86),
user_canceled(90),
missing_extension(109),
unsupported_extension(110),
certificate_unobtainable(111),
unrecognized_name(112),
bad_certificate_status_response(113),
bad_certificate_hash_value(114),
unknown_psk_identity(115),
certificate_required(116),
no_application_protocol(120),
(255)
} AlertDescription;
struct {
AlertLevel level;
AlertDescription description;
} Alert;
6.1. Closure Alerts
The client and the server must share knowledge that the connection is
ending in order to avoid a truncation attack.
close_notify This alert notifies the recipient that the sender will
not send any more messages on this connection. Any data received
after a closure alert has been received MUST be ignored.
Rescorla Expires August 19, 2018 [Page 88]
Internet-Draft TLS February 2018
user_canceled This alert notifies the recipient that the sender is
canceling the handshake for some reason unrelated to a protocol
failure. If a user cancels an operation after the handshake is
complete, just closing the connection by sending a "close_notify"
is more appropriate. This alert SHOULD be followed by a
"close_notify". This alert is generally a warning.
Either party MAY initiate a close of its write side of the connection
by sending a "close_notify" alert. Any data received after a closure
alert has been received MUST be ignored. If a transport-level close
is received prior to a "close_notify", the receiver cannot know that
all the data that was sent has been received.
Each party MUST send a "close_notify" alert before closing its write
side of the connection, unless it has already sent some other fatal
alert. This does not have any effect on its read side of the
connection. Note that this is a change from versions of TLS prior to
TLS 1.3 in which implementations were required to react to a
"close_notify" by discarding pending writes and sending an immediate
"close_notify" alert of their own. That previous requirement could
cause truncation in the read side. Both parties need not wait to
receive a "close_notify" alert before closing their read side of the
connection.
If the application protocol using TLS provides that any data may be
carried over the underlying transport after the TLS connection is
closed, the TLS implementation MUST receive a "close_notify" alert
before indicating end-of-data to the application-layer. No part of
this standard should be taken to dictate the manner in which a usage
profile for TLS manages its data transport, including when
connections are opened or closed.
Note: It is assumed that closing the write side of a connection
reliably delivers pending data before destroying the transport.
6.2. Error Alerts
Error handling in the TLS Handshake Protocol is very simple. When an
error is detected, the detecting party sends a message to its peer.
Upon transmission or receipt of a fatal alert message, both parties
MUST immediately close the connection.
Whenever an implementation encounters a fatal error condition, it
SHOULD send an appropriate fatal alert and MUST close the connection
without sending or receiving any additional data. In the rest of
this specification, when the phrases "terminate the connection" and
"abort the handshake" are used without a specific alert it means that
the implementation SHOULD send the alert indicated by the
Rescorla Expires August 19, 2018 [Page 89]
Internet-Draft TLS February 2018
descriptions below. The phrases "terminate the connection with a X
alert" and "abort the handshake with a X alert" mean that the
implementation MUST send alert X if it sends any alert. All alerts
defined in this section below, as well as all unknown alerts, are
universally considered fatal as of TLS 1.3 (see Section 6). The
implementation SHOULD provide a way to facilitate logging the sending
and receiving of alerts.
The following error alerts are defined:
unexpected_message An inappropriate message (e.g., the wrong
handshake message, premature application data, etc.) was received.
This alert should never be observed in communication between
proper implementations.
bad_record_mac This alert is returned if a record is received which
cannot be deprotected. Because AEAD algorithms combine decryption
and verification, and also to avoid side channel attacks, this
alert is used for all deprotection failures. This alert should
never be observed in communication between proper implementations,
except when messages were corrupted in the network.
record_overflow A TLSCiphertext record was received that had a
length more than 2^14 + 256 bytes, or a record decrypted to a
TLSPlaintext record with more than 2^14 bytes. This alert should
never be observed in communication between proper implementations,
except when messages were corrupted in the network.
handshake_failure Receipt of a "handshake_failure" alert message
indicates that the sender was unable to negotiate an acceptable
set of security parameters given the options available.
bad_certificate A certificate was corrupt, contained signatures that
did not verify correctly, etc.
unsupported_certificate A certificate was of an unsupported type.
certificate_revoked A certificate was revoked by its signer.
certificate_expired A certificate has expired or is not currently
valid.
certificate_unknown Some other (unspecified) issue arose in
processing the certificate, rendering it unacceptable.
illegal_parameter A field in the handshake was incorrect or
inconsistent with other fields. This alert is used for errors
Rescorla Expires August 19, 2018 [Page 90]
Internet-Draft TLS February 2018
which conform to the formal protocol syntax but are otherwise
incorrect.
unknown_ca A valid certificate chain or partial chain was received,
but the certificate was not accepted because the CA certificate
could not be located or could not be matched with a known trust
anchor.
access_denied A valid certificate or PSK was received, but when
access control was applied, the sender decided not to proceed with
negotiation.
decode_error A message could not be decoded because some field was
out of the specified range or the length of the message was
incorrect. This alert is used for errors where the message does
not conform to the formal protocol syntax. This alert should
never be observed in communication between proper implementations,
except when messages were corrupted in the network.
decrypt_error A handshake (not record-layer) cryptographic operation
failed, including being unable to correctly verify a signature or
validate a Finished message or a PSK binder.
protocol_version The protocol version the peer has attempted to
negotiate is recognized but not supported. (see Appendix D)
insufficient_security Returned instead of "handshake_failure" when a
negotiation has failed specifically because the server requires
parameters more secure than those supported by the client.
internal_error An internal error unrelated to the peer or the
correctness of the protocol (such as a memory allocation failure)
makes it impossible to continue.
inappropriate_fallback Sent by a server in response to an invalid
connection retry attempt from a client (see [RFC7507]).
missing_extension Sent by endpoints that receive a hello message not
containing an extension that is mandatory to send for the offered
TLS version or other negotiated parameters.
unsupported_extension Sent by endpoints receiving any hello message
containing an extension known to be prohibited for inclusion in
the given hello message, or including any extensions in a
ServerHello or Certificate not first offered in the corresponding
ClientHello.
Rescorla Expires August 19, 2018 [Page 91]
Internet-Draft TLS February 2018
certificate_unobtainable Sent by servers when unable to obtain a
certificate from a URL provided by the client via the
"client_certificate_url" extension (see [RFC6066]).
unrecognized_name Sent by servers when no server exists identified
by the name provided by the client via the "server_name" extension
(see [RFC6066]).
bad_certificate_status_response Sent by clients when an invalid or
unacceptable OCSP response is provided by the server via the
"status_request" extension (see [RFC6066]).
bad_certificate_hash_value Sent by servers when a retrieved object
does not have the correct hash provided by the client via the
"client_certificate_url" extension (see [RFC6066]).
unknown_psk_identity Sent by servers when PSK key establishment is
desired but no acceptable PSK identity is provided by the client.
Sending this alert is OPTIONAL; servers MAY instead choose to send
a "decrypt_error" alert to merely indicate an invalid PSK
identity.
certificate_required Sent by servers when a client certificate is
desired but none was provided by the client.
no_application_protocol Sent by servers when a client
"application_layer_protocol_negotiation" extension advertises only
protocols that the server does not support (see [RFC7301]).
New Alert values are assigned by IANA as described in Section 11.
7. Cryptographic Computations
The TLS handshake establishes one or more input secrets which are
combined to create the actual working keying material, as detailed
below. The key derivation process incorporates both the input
secrets and the handshake transcript. Note that because the
handshake transcript includes the random values from the Hello
messages, any given handshake will have different traffic secrets,
even if the same input secrets are used, as is the case when the same
PSK is used for multiple connections.
7.1. Key Schedule
The key derivation process makes use of the HKDF-Extract and HKDF-
Expand functions as defined for HKDF [RFC5869], as well as the
functions defined below:
Rescorla Expires August 19, 2018 [Page 92]
Internet-Draft TLS February 2018
HKDF-Expand-Label(Secret, Label, Context, Length) =
HKDF-Expand(Secret, HkdfLabel, Length)
Where HkdfLabel is specified as:
struct {
uint16 length = Length;
opaque label<7..255> = "tls13 " + Label;
opaque context<0..255> = Context;
} HkdfLabel;
Derive-Secret(Secret, Label, Messages) =
HKDF-Expand-Label(Secret, Label,
Transcript-Hash(Messages), Hash.length)
The Hash function used by Transcript-Hash and HKDF is the cipher
suite hash algorithm. Hash.length is its output length in bytes.
Messages are the concatenation of the indicated handshake messages,
including the handshake message type and length fields, but not
including record layer headers. Note that in some cases a zero-
length Context (indicated by "") is passed to HKDF-Expand-Label. The
Labels specified in this document are all ASCII strings, and do not
include a trailing NUL byte.
Note: with common hash functions, any label longer than 12 characters
requires an additional iteration of the hash function to compute.
The labels in this specification have all been chosen to fit within
this limit.
Given a set of n InputSecrets, the final "master secret" is computed
by iteratively invoking HKDF-Extract with InputSecret_1,
InputSecret_2, etc. The initial secret is simply a string of
Hash.length bytes set to zeros. Concretely, for the present version
of TLS 1.3, secrets are added in the following order:
- PSK (a pre-shared key established externally or derived from the
resumption_master_secret value from a previous connection)
- (EC)DHE shared secret (Section 7.4)
This produces a full key derivation schedule shown in the diagram
below. In this diagram, the following formatting conventions apply:
- HKDF-Extract is drawn as taking the Salt argument from the top and
the IKM argument from the left.
Rescorla Expires August 19, 2018 [Page 93]
Internet-Draft TLS February 2018
- Derive-Secret's Secret argument is indicated by the incoming
arrow. For instance, the Early Secret is the Secret for
generating the client_early_traffic_secret.
0
|
v
PSK -> HKDF-Extract = Early Secret
|
+-----> Derive-Secret(.,
| "ext binder" |
| "res binder",
| "")
| = binder_key
|
+-----> Derive-Secret(., "c e traffic",
| ClientHello)
| = client_early_traffic_secret
|
+-----> Derive-Secret(., "e exp master",
| ClientHello)
| = early_exporter_master_secret
v
Derive-Secret(., "derived", "")
|
v
(EC)DHE -> HKDF-Extract = Handshake Secret
|
+-----> Derive-Secret(., "c hs traffic",
| ClientHello...ServerHello)
| = client_handshake_traffic_secret
|
+-----> Derive-Secret(., "s hs traffic",
| ClientHello...ServerHello)
| = server_handshake_traffic_secret
v
Derive-Secret(., "derived", "")
|
v
0 -> HKDF-Extract = Master Secret
|
+-----> Derive-Secret(., "c ap traffic",
| ClientHello...server Finished)
| = client_application_traffic_secret_0
|
+-----> Derive-Secret(., "s ap traffic",
| ClientHello...server Finished)
| = server_application_traffic_secret_0
Rescorla Expires August 19, 2018 [Page 94]
Internet-Draft TLS February 2018
|
+-----> Derive-Secret(., "exp master",
| ClientHello...server Finished)
| = exporter_master_secret
|
+-----> Derive-Secret(., "res master",
ClientHello...client Finished)
= resumption_master_secret
The general pattern here is that the secrets shown down the left side
of the diagram are just raw entropy without context, whereas the
secrets down the right side include handshake context and therefore
can be used to derive working keys without additional context. Note
that the different calls to Derive-Secret may take different Messages
arguments, even with the same secret. In a 0-RTT exchange, Derive-
Secret is called with four distinct transcripts; in a 1-RTT-only
exchange with three distinct transcripts.
If a given secret is not available, then the 0-value consisting of a
string of Hash.length bytes set to zeros is used. Note that this
does not mean skipping rounds, so if PSK is not in use Early Secret
will still be HKDF-Extract(0, 0). For the computation of the
binder_secret, the label is "ext binder" for external PSKs (those
provisioned outside of TLS) and "res binder" for resumption PSKs
(those provisioned as the resumption master secret of a previous
handshake). The different labels prevent the substitution of one
type of PSK for the other.
There are multiple potential Early Secret values depending on which
PSK the server ultimately selects. The client will need to compute
one for each potential PSK; if no PSK is selected, it will then need
to compute the early secret corresponding to the zero PSK.
Once all the values which are to be derived from a given secret have
been computed, that secret SHOULD be erased.
7.2. Updating Traffic Keys and IVs
Once the handshake is complete, it is possible for either side to
update its sending traffic keys using the KeyUpdate handshake message
defined in Section 4.6.3. The next generation of traffic keys is
computed by generating client_/server_application_traffic_secret_N+1
from client_/server_application_traffic_secret_N as described in this
section then re-deriving the traffic keys as described in
Section 7.3.
The next-generation application_traffic_secret is computed as:
Rescorla Expires August 19, 2018 [Page 95]
Internet-Draft TLS February 2018
application_traffic_secret_N+1 =
HKDF-Expand-Label(application_traffic_secret_N,
"traffic upd", "", Hash.length)
Once client/server_application_traffic_secret_N+1 and its associated
traffic keys have been computed, implementations SHOULD delete
client_/server_application_traffic_secret_N and its associated
traffic keys.
7.3. Traffic Key Calculation
The traffic keying material is generated from the following input
values:
- A secret value
- A purpose value indicating the specific value being generated
- The length of the key
The traffic keying material is generated from an input traffic secret
value using:
[sender]_write_key = HKDF-Expand-Label(Secret, "key", "", key_length)
[sender]_write_iv = HKDF-Expand-Label(Secret, "iv" , "", iv_length)
[sender] denotes the sending side. The Secret value for each record
type is shown in the table below.
+-------------------+---------------------------------------+
| Record Type | Secret |
+-------------------+---------------------------------------+
| 0-RTT Application | client_early_traffic_secret |
| | |
| Handshake | [sender]_handshake_traffic_secret |
| | |
| Application Data | [sender]_application_traffic_secret_N |
+-------------------+---------------------------------------+
All the traffic keying material is recomputed whenever the underlying
Secret changes (e.g., when changing from the handshake to application
data keys or upon a key update).
7.4. (EC)DHE Shared Secret Calculation
Rescorla Expires August 19, 2018 [Page 96]
Internet-Draft TLS February 2018
7.4.1. Finite Field Diffie-Hellman
For finite field groups, a conventional Diffie-Hellman computation is
performed. The negotiated key (Z) is converted to a byte string by
encoding in big-endian and padded with zeros up to the size of the
prime. This byte string is used as the shared secret in the key
schedule as specified above.
Note that this construction differs from previous versions of TLS
which remove leading zeros.
7.4.2. Elliptic Curve Diffie-Hellman
For secp256r1, secp384r1 and secp521r1, ECDH calculations (including
parameter and key generation as well as the shared secret
calculation) are performed according to [IEEE1363] using the ECKAS-
DH1 scheme with the identity map as key derivation function (KDF), so
that the shared secret is the x-coordinate of the ECDH shared secret
elliptic curve point represented as an octet string. Note that this
octet string (Z in IEEE 1363 terminology) as output by FE2OSP, the
Field Element to Octet String Conversion Primitive, has constant
length for any given field; leading zeros found in this octet string
MUST NOT be truncated.
(Note that this use of the identity KDF is a technicality. The
complete picture is that ECDH is employed with a non-trivial KDF
because TLS does not directly use this secret for anything other than
for computing other secrets.)
ECDH functions are used as follows:
- The public key to put into the KeyShareEntry.key_exchange
structure is the result of applying the ECDH scalar multiplication
function to the secret key of appropriate length (into scalar
input) and the standard public basepoint (into u-coordinate point
input).
- The ECDH shared secret is the result of applying the ECDH scalar
multiplication function to the secret key (into scalar input) and
the peer's public key (into u-coordinate point input). The output
is used raw, with no processing.
For X25519 and X448, implementations SHOULD use the approach
specified in [RFC7748] to calculate the Diffie-Hellman shared secret.
Implementations MUST check whether the computed Diffie-Hellman shared
secret is the all-zero value and abort if so, as described in
Section 6 of [RFC7748]. If implementers use an alternative
Rescorla Expires August 19, 2018 [Page 97]
Internet-Draft TLS February 2018
implementation of these elliptic curves, they SHOULD perform the
additional checks specified in Section 7 of [RFC7748].
7.5. Exporters
[RFC5705] defines keying material exporters for TLS in terms of the
TLS pseudorandom function (PRF). This document replaces the PRF with
HKDF, thus requiring a new construction. The exporter interface
remains the same.
The exporter value is computed as:
TLS-Exporter(label, context_value, key_length) =
HKDF-Expand-Label(Derive-Secret(Secret, label, ""),
"exporter", Hash(context_value), key_length)
Where Secret is either the early_exporter_master_secret or the
exporter_master_secret. Implementations MUST use the
exporter_master_secret unless explicitly specified by the
application. The early_exporter_master_secret is defined for use in
settings where an exporter is needed for 0-RTT data. A separate
interface for the early exporter is RECOMMENDED, especially on a
server where a single interface can make the early exporter
inaccessible.
If no context is provided, the context_value is zero-length.
Consequently, providing no context computes the same value as
providing an empty context. This is a change from previous versions
of TLS where an empty context produced a different output to an
absent context. As of this document's publication, no allocated
exporter label is used both with and without a context. Future
specifications MUST NOT define a use of exporters that permit both an
empty context and no context with the same label. New uses of
exporters SHOULD provide a context in all exporter computations,
though the value could be empty.
Requirements for the format of exporter labels are defined in section
4 of [RFC5705].
8. 0-RTT and Anti-Replay
As noted in Section 2.3 and Appendix E.5, TLS does not provide
inherent replay protections for 0-RTT data. There are two potential
threats to be concerned with:
- Network attackers who mount a replay attack by simply duplicating
a flight of 0-RTT data.
Rescorla Expires August 19, 2018 [Page 98]
Internet-Draft TLS February 2018
- Network attackers who take advantage of client retry behavior to
arrange for the server to receive multiple copies of an
application message. This threat already exists to some extent
because clients that value robustness respond to network errors by
attempting to retry requests. However, 0-RTT adds an additional
dimension for any server system which does not maintain globally
consistent server state. Specifically, if a server system has
multiple zones where tickets from zone A will not be accepted in
zone B, then an attacker can duplicate a ClientHello and early
data intended for A to both A and B. At A, the data will be
accepted in 0-RTT, but at B the server will reject 0-RTT data and
instead force a full handshake. If the attacker blocks the
ServerHello from A, then the client will complete the handshake
with B and probably retry the request, leading to duplication on
the server system as a whole.
The first class of attack can be prevented by sharing state to
guarantee that the 0-RTT data is accepted at most once. Servers
SHOULD provide that level of replay safety, by implementing one of
the methods described in this section or by equivalent means. It is
understood, however, that due to operational concerns not all
deployments will maintain state at that level. Therefore, in normal
operation, clients will not know which, if any, of these mechanisms
servers actually implement and hence MUST only send early data which
they deem safe to be replayed.
In addition to the direct effects of replays, there is a class of
attacks where even operations normally considered idempotent could be
exploited by a large number of replays (timing attacks, resource
limit exhaustion and others described in Appendix E.5). Those can be
mitigated by ensuring that every 0-RTT payload can be replayed only a
limited number of times. The server MUST ensure that any instance of
it (be it a machine, a thread or any other entity within the relevant
serving infrastructure) would accept 0-RTT for the same 0-RTT
handshake at most once; this limits the number of replays to the
number of server instances in the deployment. Such a guarantee can
be accomplished by locally recording data from recently-received
ClientHellos and rejecting repeats, or by any other method that
provides the same or a stronger guarantee. The "at most once per
server instance" guarantee is a minimum requirement; servers SHOULD
limit 0-RTT replays further when feasible.
The second class of attack cannot be prevented at the TLS layer and
MUST be dealt with by any application. Note that any application
whose clients implement any kind of retry behavior already needs to
implement some sort of anti-replay defense.
Rescorla Expires August 19, 2018 [Page 99]
Internet-Draft TLS February 2018
8.1. Single-Use Tickets
The simplest form of anti-replay defense is for the server to only
allow each session ticket to be used once. For instance, the server
can maintain a database of all outstanding valid tickets; deleting
each ticket from the database as it is used. If an unknown ticket is
provided, the server would then fall back to a full handshake.
If the tickets are not self-contained but rather are database keys,
and the corresponding PSKs are deleted upon use, then connections
established using one PSK enjoy forward secrecy. This improves
security for all 0-RTT data and PSK usage when PSK is used without
(EC)DHE.
Because this mechanism requires sharing the session database between
server nodes in environments with multiple distributed servers, it
may be hard to achieve high rates of successful PSK 0-RTT connections
when compared to self-encrypted tickets. Unlike session databases,
session tickets can successfully do PSK-based session establishment
even without consistent storage, though when 0-RTT is allowed they
still require consistent storage for anti-replay of 0-RTT data, as
detailed in the following section.
8.2. Client Hello Recording
An alternative form of anti-replay is to record a unique value
derived from the ClientHello (generally either the random value or
the PSK binder) and reject duplicates. Recording all ClientHellos
causes state to grow without bound, but a server can instead record
ClientHellos within a given time window and use the
"obfuscated_ticket_age" to ensure that tickets aren't reused outside
that window.
In order to implement this, when a ClientHello is received, the
server first verifies the PSK binder as described Section 4.2.11. It
then computes the expected_arrival_time as described in the next
section and rejects 0-RTT if it is outside the recording window,
falling back to the 1-RTT handshake.
If the expected arrival time is in the window, then the server checks
to see if it has recorded a matching ClientHello. If one is found,
it either aborts the handshake with an "illegal_parameter" alert or
accepts the PSK but reject 0-RTT. If no matching ClientHello is
found, then it accepts 0-RTT and then stores the ClientHello for as
long as the expected_arrival_time is inside the window. Servers MAY
also implement data stores with false positives, such as Bloom
filters, in which case they MUST respond to apparent replay by
rejecting 0-RTT but MUST NOT abort the handshake.
Rescorla Expires August 19, 2018 [Page 100]
Internet-Draft TLS February 2018
The server MUST derive the storage key only from validated sections
of the ClientHello. If the ClientHello contains multiple PSK
identities, then an attacker can create multiple ClientHellos with
different binder values for the less-preferred identity on the
assumption that the server will not verify it, as recommended by
Section 4.2.11. I.e., if the client sends PSKs A and B but the
server prefers A, then the attacker can change the binder for B
without affecting the binder for A. This will cause the ClientHello
to be accepted, and may cause side effects such as replay cache
pollution, although any 0-RTT data will not be decryptable because it
will use different keys. If the validated binder or the
ClientHello.random are used as the storage key, then this attack is
not possible.
Because this mechanism does not require storing all outstanding
tickets, it may be easier to implement in distributed systems with
high rates of resumption and 0-RTT, at the cost of potentially weaker
anti-replay defense because of the difficulty of reliably storing and
retrieving the received ClientHello messages. In many such systems,
it is impractical to have globally consistent storage of all the
received ClientHellos. In this case, the best anti-replay protection
is provided by having a single storage zone be authoritative for a
given ticket and refusing 0-RTT for that ticket in any other zone.
This approach prevents simple replay by the attacker because only one
zone will accept 0-RTT data. A weaker design is to implement
separate storage for each zone but allow 0-RTT in any zone. This
approach limits the number of replays to once per zone. Application
message duplication of course remains possible with either design.
When implementations are freshly started, they SHOULD reject 0-RTT as
long as any portion of their recording window overlaps the startup
time. Otherwise, they run the risk of accepting replays which were
originally sent during that period.
Note: If the client's clock is running much faster than the server's
then a ClientHello may be received that is outside the window in the
future, in which case it might be accepted for 1-RTT, causing a
client retry, and then acceptable later for 0-RTT. This is another
variant of the second form of attack described above.
8.3. Freshness Checks
Because the ClientHello indicates the time at which the client sent
it, it is possible to efficiently determine whether a ClientHello was
likely sent reasonably recently and only accept 0-RTT for such a
ClientHello, otherwise falling back to a 1-RTT handshake. This is
necessary for the ClientHello storage mechanism described in
Section 8.2 because otherwise the server needs to store an unlimited
Rescorla Expires August 19, 2018 [Page 101]
Internet-Draft TLS February 2018
number of ClientHellos and is a useful optimization for single-use
tickets because it allows efficient rejection of ClientHellos which
cannot be used for 0-RTT.
In order to implement this mechanism, a server needs to store the
time that the server generated the session ticket, offset by an
estimate of the round trip time between client and server. I.e.,
adjusted_creation_time = creation_time + estimated_RTT
This value can be encoded in the ticket, thus avoiding the need to
keep state for each outstanding ticket. The server can determine the
client's view of the age of the ticket by subtracting the ticket's
"ticket_age_add value" from the "obfuscated_ticket_age" parameter in
the client's "pre_shared_key" extension. The server can determine
the "expected arrival time" of the ClientHello as:
expected_arrival_time = adjusted_creation_time + clients_ticket_age
When a new ClientHello is received, the expected_arrival_time is then
compared against the current server wall clock time and if they
differ by more than a certain amount, 0-RTT is rejected, though the
1-RTT handshake can be allowed to complete.
There are several potential sources of error that might cause
mismatches between the expected arrival time and the measured time.
Variations in client and server clock rates are likely to be minimal,
though potentially with gross time corrections. Network propagation
delays are the most likely causes of a mismatch in legitimate values
for elapsed time. Both the NewSessionTicket and ClientHello messages
might be retransmitted and therefore delayed, which might be hidden
by TCP. For clients on the Internet, this implies windows on the
order of ten seconds to account for errors in clocks and variations
in measurements; other deployment scenarios may have different needs.
Clock skew distributions are not symmetric, so the optimal tradeoff
may involve an asymmetric range of permissible mismatch values.
Note that freshness checking alone is not sufficient to prevent
replays because it does not detect them during the error window,
which, depending on bandwidth and system capacity could include
billions of replays in real-world settings. In addition, this
freshness checking is only done at the time the ClientHello is
received, and not when later early application data records are
received. After early data is accepted, records may continue to be
streamed to the server over a longer time period.
Rescorla Expires August 19, 2018 [Page 102]
Internet-Draft TLS February 2018
9. Compliance Requirements
9.1. Mandatory-to-Implement Cipher Suites
In the absence of an application profile standard specifying
otherwise, a TLS-compliant application MUST implement the
TLS_AES_128_GCM_SHA256 [GCM] cipher suite and SHOULD implement the
TLS_AES_256_GCM_SHA384 [GCM] and TLS_CHACHA20_POLY1305_SHA256
[RFC7539] cipher suites. (see Appendix B.4)
A TLS-compliant application MUST support digital signatures with
rsa_pkcs1_sha256 (for certificates), rsa_pss_rsae_sha256 (for
CertificateVerify and certificates), and ecdsa_secp256r1_sha256. A
TLS-compliant application MUST support key exchange with secp256r1
(NIST P-256) and SHOULD support key exchange with X25519 [RFC7748].
9.2. Mandatory-to-Implement Extensions
In the absence of an application profile standard specifying
otherwise, a TLS-compliant application MUST implement the following
TLS extensions:
- Supported Versions ("supported_versions"; Section 4.2.1)
- Cookie ("cookie"; Section 4.2.2)
- Signature Algorithms ("signature_algorithms"; Section 4.2.3)
- Signature Algorithms Certificate ("signature_algorithms_cert";
Section 4.2.3)
- Negotiated Groups ("supported_groups"; Section 4.2.7)
- Key Share ("key_share"; Section 4.2.8)
- Server Name Indication ("server_name"; Section 3 of [RFC6066])
All implementations MUST send and use these extensions when offering
applicable features:
- "supported_versions" is REQUIRED for all ClientHello, ServerHello
and HelloRetryRequest messages.
- "signature_algorithms" is REQUIRED for certificate authentication.
- "supported_groups" is REQUIRED for ClientHello messages using DHE
or ECDHE key exchange.
Rescorla Expires August 19, 2018 [Page 103]
Internet-Draft TLS February 2018
- "key_share" is REQUIRED for DHE or ECDHE key exchange.
- "pre_shared_key" is REQUIRED for PSK key agreement.
- "psk_key_exchange_modes" is REQUIRED for PSK key agreement.
A client is considered to be attempting to negotiate using this
specification if the ClientHello contains a "supported_versions"
extension with 0x0304 as the highest version number contained in its
body. Such a ClientHello message MUST meet the following
requirements:
- If not containing a "pre_shared_key" extension, it MUST contain
both a "signature_algorithms" extension and a "supported_groups"
extension.
- If containing a "supported_groups" extension, it MUST also contain
a "key_share" extension, and vice versa. An empty
KeyShare.client_shares vector is permitted.
Servers receiving a ClientHello which does not conform to these
requirements MUST abort the handshake with a "missing_extension"
alert.
Additionally, all implementations MUST support use of the
"server_name" extension with applications capable of using it.
Servers MAY require clients to send a valid "server_name" extension.
Servers requiring this extension SHOULD respond to a ClientHello
lacking a "server_name" extension by terminating the connection with
a "missing_extension" alert.
9.3. Protocol Invariants
This section describes invariants that TLS endpoints and middleboxes
MUST follow. It also applies to earlier versions, which assumed
these rules but did not document them.
TLS is designed to be securely and compatibly extensible. Newer
clients or servers, when communicating with newer peers, SHOULD
negotiate the most preferred common parameters. The TLS handshake
provides downgrade protection: Middleboxes passing traffic between a
newer client and newer server without terminating TLS should be
unable to influence the handshake (see Appendix E.1). At the same
time, deployments update at different rates, so a newer client or
server MAY continue to support older parameters, which would allow it
to interoperate with older endpoints.
Rescorla Expires August 19, 2018 [Page 104]
Internet-Draft TLS February 2018
For this to work, implementations MUST correctly handle extensible
fields:
- A client sending a ClientHello MUST support all parameters
advertised in it. Otherwise, the server may fail to interoperate
by selecting one of those parameters.
- A server receiving a ClientHello MUST correctly ignore all
unrecognized cipher suites, extensions, and other parameters.
Otherwise, it may fail to interoperate with newer clients. In TLS
1.3, a client receiving a CertificateRequest or NewSessionTicket
MUST also ignore all unrecognized extensions.
- A middlebox which terminates a TLS connection MUST behave as a
compliant TLS server (to the original client), including having a
certificate which the client is willing to accept, and as a
compliant TLS client (to the original server), including verifying
the original server's certificate. In particular, it MUST
generate its own ClientHello containing only parameters it
understands, and it MUST generate a fresh ServerHello random
value, rather than forwarding the endpoint's value.
Note that TLS's protocol requirements and security analysis only
apply to the two connections separately. Safely deploying a TLS
terminator requires additional security considerations which are
beyond the scope of this document.
- An middlebox which forwards ClientHello parameters it does not
understand MUST NOT process any messages beyond that ClientHello.
It MUST forward all subsequent traffic unmodified. Otherwise, it
may fail to interoperate with newer clients and servers.
Forwarded ClientHellos may contain advertisements for features not
supported by the middlebox, so the response may include future TLS
additions the middlebox does not recognize. These additions MAY
change any message beyond the ClientHello arbitrarily. In
particular, the values sent in the ServerHello might change, the
ServerHello format might change, and the TLSCiphertext format
might change.
The design of TLS 1.3 was constrained by widely-deployed non-
compliant TLS middleboxes (see Appendix D.4), however it does not
relax the invariants. Those middleboxes continue to be non-
compliant.
Rescorla Expires August 19, 2018 [Page 105]
Internet-Draft TLS February 2018
10. Security Considerations
Security issues are discussed throughout this memo, especially in
Appendix C, Appendix D, and Appendix E.
11. IANA Considerations
This document uses several registries that were originally created in
[RFC4346]. IANA has updated these to reference this document. The
registries and their allocation policies are below:
- TLS Cipher Suite Registry: values with the first byte in the range
0-254 (decimal) are assigned via Specification Required [RFC8126].
Values with the first byte 255 (decimal) are reserved for Private
Use [RFC8126].
IANA [SHALL add/has added] the cipher suites listed in
Appendix B.4 to the registry. The "Value" and "Description"
columns are taken from the table. The "DTLS-OK" and "Recommended"
columns are both marked as "Yes" for each new cipher suite.
[[This assumes [I-D.ietf-tls-iana-registry-updates] has been
applied.]]
- TLS ContentType Registry: Future values are allocated via
Standards Action [RFC8126].
- TLS Alert Registry: Future values are allocated via Standards
Action [RFC8126]. IANA [SHALL update/has updated] this registry
to include values for "missing_extension" and
"certificate_required".
- TLS HandshakeType Registry: Future values are allocated via
Standards Action [RFC8126]. IANA [SHALL update/has updated] this
registry to rename item 4 from "NewSessionTicket" to
"new_session_ticket" and to add the
"hello_retry_request_RESERVED", "encrypted_extensions",
"end_of_early_data", "key_update", and "message_hash" values.
This document also uses the TLS ExtensionType Registry originally
created in [RFC4366]. IANA has updated it to reference this
document. The registry and its allocation policy is listed below:
- IANA [SHALL update/has updated] this registry to include the
"key_share", "pre_shared_key", "psk_key_exchange_modes",
"early_data", "cookie", "supported_versions",
"certificate_authorities", "oid_filters", "post_handshake_auth",
and "signature_algorithms_cert", extensions with the values
defined in this document and the Recommended value of "Yes".
Rescorla Expires August 19, 2018 [Page 106]
Internet-Draft TLS February 2018
- IANA [SHALL update/has updated] this registry to include a "TLS
1.3" column which lists the messages in which the extension may
appear. This column [SHALL be/has been] initially populated from
the table in Section 4.2 with any extension not listed there
marked as "-" to indicate that it is not used by TLS 1.3.
In addition, this document defines a new registry to be maintained by
IANA:
- TLS SignatureScheme Registry: Values with the first byte in the
range 0-253 (decimal) are assigned via Specification Required
[RFC8126]. Values with the first byte 254 or 255 (decimal) are
reserved for Private Use [RFC8126]. Values with the first byte in
the range 0-6 or with the second byte in the range 0-3 that are
not currently allocated are reserved for backwards compatibility.
This registry SHALL have a "Recommended" column. The registry
[shall be/ has been] initially populated with the values described
in Section 4.2.3. The following values SHALL be marked as
"Recommended": ecdsa_secp256r1_sha256, ecdsa_secp384r1_sha384,
rsa_pss_sha256, rsa_pss_sha384, rsa_pss_sha512, ed25519.
12. References
12.1. Normative References
[DH] Diffie, W. and M. Hellman, "New Directions in
Cryptography", IEEE Transactions on Information Theory,
V.IT-22 n.6 , June 1977.
[GCM] Dworkin, M., "Recommendation for Block Cipher Modes of
Operation: Galois/Counter Mode (GCM) and GMAC",
NIST Special Publication 800-38D, November 2007.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997, <https://www.rfc-
editor.org/info/rfc2104>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
editor.org/info/rfc2119>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
Rescorla Expires August 19, 2018 [Page 107]
Internet-Draft TLS February 2018
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
March 2010, <https://www.rfc-editor.org/info/rfc5705>.
[RFC5756] Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
"Updates for RSAES-OAEP and RSASSA-PSS Algorithm
Parameters", RFC 5756, DOI 10.17487/RFC5756, January 2010,
<https://www.rfc-editor.org/info/rfc5756>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010, <https://www.rfc-
editor.org/info/rfc5869>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011, <https://www.rfc-
editor.org/info/rfc6066>.
[RFC6655] McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for
Transport Layer Security (TLS)", RFC 6655,
DOI 10.17487/RFC6655, July 2012, <https://www.rfc-
editor.org/info/rfc6655>.
[RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
Galperin, S., and C. Adams, "X.509 Internet Public Key
Infrastructure Online Certificate Status Protocol - OCSP",
RFC 6960, DOI 10.17487/RFC6960, June 2013,
<https://www.rfc-editor.org/info/rfc6960>.
[RFC6961] Pettersen, Y., "The Transport Layer Security (TLS)
Multiple Certificate Status Request Extension", RFC 6961,
DOI 10.17487/RFC6961, June 2013, <https://www.rfc-
editor.org/info/rfc6961>.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
<https://www.rfc-editor.org/info/rfc6962>.
[RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature
Algorithm (DSA) and Elliptic Curve Digital Signature
Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
2013, <https://www.rfc-editor.org/info/rfc6979>.
[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/info/rfc7301>.
Rescorla Expires August 19, 2018 [Page 108]
Internet-Draft TLS February 2018
[RFC7507] Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
Suite Value (SCSV) for Preventing Protocol Downgrade
Attacks", RFC 7507, DOI 10.17487/RFC7507, April 2015,
<https://www.rfc-editor.org/info/rfc7507>.
[RFC7539] Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
Protocols", RFC 7539, DOI 10.17487/RFC7539, May 2015,
<https://www.rfc-editor.org/info/rfc7539>.
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <https://www.rfc-editor.org/info/rfc7748>.
[RFC7919] Gillmor, D., "Negotiated Finite Field Diffie-Hellman
Ephemeral Parameters for Transport Layer Security (TLS)",
RFC 7919, DOI 10.17487/RFC7919, August 2016,
<https://www.rfc-editor.org/info/rfc7919>.
[RFC8017] Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
"PKCS #1: RSA Cryptography Specifications Version 2.2",
RFC 8017, DOI 10.17487/RFC8017, November 2016,
<https://www.rfc-editor.org/info/rfc8017>.
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017, <https://www.rfc-
editor.org/info/rfc8032>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[SHS] Dang, Q., "Secure Hash Standard", National Institute of
Standards and Technology report,
DOI 10.6028/nist.fips.180-4, July 2015.
[X690] ITU-T, "Information technology - ASN.1 encoding Rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER)", ISO/IEC 8825-1:2002, 2002.
Rescorla Expires August 19, 2018 [Page 109]
Internet-Draft TLS February 2018
[X962] ANSI, "Public Key Cryptography For The Financial Services
Industry: The Elliptic Curve Digital Signature Algorithm
(ECDSA)", ANSI X9.62, 1998.
12.2. Informative References
[AEAD-LIMITS]
Luykx, A. and K. Paterson, "Limits on Authenticated
Encryption Use in TLS", 2016,
<http://www.isg.rhul.ac.uk/~kp/TLS-AEbounds.pdf>.
[BBFKZG16]
Bhargavan, K., Brzuska, C., Fournet, C., Kohlweiss, M.,
Zanella-Beguelin, S., and M. Green, "Downgrade Resilience
in Key-Exchange Protocols", Proceedings of IEEE Symposium
on Security and Privacy (Oakland) 2016 , 2016.
[BBK17] Bhargavan, K., Blanchet, B., and N. Kobeissi, "Verified
Models and Reference Implementations for the TLS 1.3
Standard Candidate", Proceedings of IEEE Symposium on
Security and Privacy (Oakland) 2017 , 2017.
[BDFKPPRSZZ16]
Bhargavan, K., Delignat-Lavaud, A., Fournet, C.,
Kohlweiss, M., Pan, J., Protzenko, J., Rastogi, A., Swamy,
N., Zanella-Beguelin, S., and J. Zinzindohoue,
"Implementing and Proving the TLS 1.3 Record Layer",
Proceedings of IEEE Symposium on Security and Privacy
(Oakland) 2017 , December 2016,
<https://eprint.iacr.org/2016/1178>.
[Ben17a] Benjamin, D., "Presentation before the TLS WG at IETF
100", 2017,
<https://datatracker.ietf.org/meeting/100/materials/
slides-100-tls-sessa-tls13/>.
[Ben17b] Benjamin, D., "Additional TLS 1.3 results from Chrome",
2017, <https://www.ietf.org/mail-archive/web/tls/current/
msg25168.html>.
[BMMT15] Badertscher, C., Matt, C., Maurer, U., and B. Tackmann,
"Augmented Secure Channels and the Goal of the TLS 1.3
Record Layer", ProvSec 2015 , September 2015,
<https://eprint.iacr.org/2015/394>.
[BT16] Bellare, M. and B. Tackmann, "The Multi-User Security of
Authenticated Encryption: AES-GCM in TLS 1.3", Proceedings
of CRYPTO 2016 , 2016, <https://eprint.iacr.org/2016/564>.
Rescorla Expires August 19, 2018 [Page 110]
Internet-Draft TLS February 2018
[CCG16] Cohn-Gordon, K., Cremers, C., and L. Garratt, "On Post-
Compromise Security", IEEE Computer Security Foundations
Symposium , 2015.
[CHECKOWAY]
Checkoway, S., Shacham, H., Maskiewicz, J., Garman, C.,
Fried, J., Cohney, S., Green, M., Heninger, N., Weinmann,
R., and E. Rescorla, "A Systematic Analysis of the Juniper
Dual EC Incident", Proceedings of the 2016 ACM SIGSAC
Conference on Computer and Communications Security
- CCS'16, DOI 10.1145/2976749.2978395, 2016.
[CHHSV17] Cremers, C., Horvat, M., Hoyland, J., van der Merwe, T.,
and S. Scott, "Awkward Handshake: Possible mismatch of
client/server view on client authentication in post-
handshake mode in Revision 18", 2017,
<https://www.ietf.org/mail-archive/web/tls/current/
msg22382.html>.
[CHSV16] Cremers, C., Horvat, M., Scott, S., and T. van der Merwe,
"Automated Analysis and Verification of TLS 1.3: 0-RTT,
Resumption and Delayed Authentication", Proceedings of
IEEE Symposium on Security and Privacy (Oakland) 2016 ,
2016, <http://ieeexplore.ieee.org/document/7546518/>.
[CK01] Canetti, R. and H. Krawczyk, "Analysis of Key-Exchange
Protocols and Their Use for Building Secure Channels",
Proceedings of Eurocrypt 2001 , 2001.
[CLINIC] Miller, B., Huang, L., Joseph, A., and J. Tygar, "I Know
Why You Went to the Clinic: Risks and Realization of HTTPS
Traffic Analysis", Privacy Enhancing Technologies pp.
143-163, DOI 10.1007/978-3-319-08506-7_8, 2014.
[DFGS15] Dowling, B., Fischlin, M., Guenther, F., and D. Stebila,
"A Cryptographic Analysis of the TLS 1.3 draft-10 Full and
Pre-shared Key Handshake Protocol", Proceedings of ACM CCS
2015 , 2015, <https://eprint.iacr.org/2015/914>.
[DFGS16] Dowling, B., Fischlin, M., Guenther, F., and D. Stebila,
"A Cryptographic Analysis of the TLS 1.3 draft-10 Full and
Pre-shared Key Handshake Protocol", TRON 2016 , 2016,
<https://eprint.iacr.org/2016/081>.
[DOW92] Diffie, W., van Oorschot, P., and M. Wiener,
""Authentication and authenticated key exchanges"",
Designs, Codes and Cryptography , 1992.
Rescorla Expires August 19, 2018 [Page 111]
Internet-Draft TLS February 2018
[DSS] National Institute of Standards and Technology, U.S.
Department of Commerce, "Digital Signature Standard,
version 4", NIST FIPS PUB 186-4, 2013.
[ECDSA] American National Standards Institute, "Public Key
Cryptography for the Financial Services Industry: The
Elliptic Curve Digital Signature Algorithm (ECDSA)",
ANSI ANS X9.62-2005, November 2005.
[FG17] Fischlin, M. and F. Guenther, "Replay Attacks on Zero
Round-Trip Time: The Case of the TLS 1.3 Handshake
Candidates", Proceedings of Euro S"P 2017 , 2017,
<https://eprint.iacr.org/2017/082>.
[FGSW16] Fischlin, M., Guenther, F., Schmidt, B., and B. Warinschi,
"Key Confirmation in Key Exchange: A Formal Treatment and
Implications for TLS 1.3", Proceedings of IEEE Symposium
on Security and Privacy (Oakland) 2016 , 2016,
<http://ieeexplore.ieee.org/document/7546517/>.
[FW15] Florian Weimer, ., "Factoring RSA Keys With TLS Perfect
Forward Secrecy", September 2015.
[HCJ16] Husak, M., Čermak, M., Jirsik, T., and P.
Čeleda, "HTTPS traffic analysis and client
identification using passive SSL/TLS fingerprinting",
EURASIP Journal on Information Security Vol. 2016,
DOI 10.1186/s13635-016-0030-7, February 2016.
[HGFS15] Hlauschek, C., Gruber, M., Fankhauser, F., and C. Schanes,
"Prying Open Pandora's Box: KCI Attacks against TLS",
Proceedings of USENIX Workshop on Offensive Technologies ,
2015.
[I-D.ietf-tls-iana-registry-updates]
Salowey, J. and S. Turner, "IANA Registry Updates for TLS
and DTLS", draft-ietf-tls-iana-registry-updates-03 (work
in progress), January 2018.
[I-D.ietf-tls-tls13-vectors]
Thomson, M., "Example Handshake Traces for TLS 1.3",
draft-ietf-tls-tls13-vectors-03 (work in progress),
December 2017.
[IEEE1363]
IEEE, "Standard Specifications for Public Key
Cryptography", IEEE 1363 , 2000.
Rescorla Expires August 19, 2018 [Page 112]
Internet-Draft TLS February 2018
[JSS15] Jager, T., Schwenk, J., and J. Somorovsky, "On the
Security of TLS 1.3 and QUIC Against Weaknesses in PKCS#1
v1.5 Encryption", Proceedings of ACM CCS 2015 , 2015,
<https://www.nds.rub.de/media/nds/
veroeffentlichungen/2015/08/21/Tls13QuicAttacks.pdf>.
[KEYAGREEMENT]
Barker, E., Chen, L., Roginsky, A., and M. Smid,
"Recommendation for Pair-Wise Key Establishment Schemes
Using Discrete Logarithm Cryptography", National Institute
of Standards and Technology report,
DOI 10.6028/nist.sp.800-56ar2, May 2013.
[Kraw10] Krawczyk, H., "Cryptographic Extraction and Key
Derivation: The HKDF Scheme", Proceedings of CRYPTO 2010 ,
2010, <https://eprint.iacr.org/2010/264>.
[Kraw16] Krawczyk, H., "A Unilateral-to-Mutual Authentication
Compiler for Key Exchange (with Applications to Client
Authentication in TLS 1.3", Proceedings of ACM CCS 2016 ,
2016, <https://eprint.iacr.org/2016/711>.
[KW16] Krawczyk, H. and H. Wee, "The OPTLS Protocol and TLS 1.3",
Proceedings of Euro S"P 2016 , 2016,
<https://eprint.iacr.org/2015/978>.
[LXZFH16] Li, X., Xu, J., Feng, D., Zhang, Z., and H. Hu, "Multiple
Handshakes Security of TLS 1.3 Candidates", Proceedings of
IEEE Symposium on Security and Privacy (Oakland) 2016 ,
2016, <http://ieeexplore.ieee.org/document/7546519/>.
[Mac17] MacCarthaigh, C., "Security Review of TLS1.3 0-RTT", 2017,
<https://github.com/tlswg/tls13-spec/issues/1001>.
[PSK-FINISHED]
Cremers, C., Horvat, M., van der Merwe, T., and S. Scott,
"Revision 10: possible attack if client authentication is
allowed during PSK", 2015, <https://www.ietf.org/mail-
archive/web/tls/current/msg18215.html>.
[REKEY] Abdalla, M. and M. Bellare, "Increasing the Lifetime of a
Key: A Comparative Analysis of the Security of Re-keying
Techniques", ASIACRYPT2000 , October 2000.
[Res17a] Rescorla, E., "Preliminary data on Firefox TLS 1.3
Middlebox experiment", 2017, <https://www.ietf.org/mail-
archive/web/tls/current/msg25091.html>.
Rescorla Expires August 19, 2018 [Page 113]
Internet-Draft TLS February 2018
[Res17b] Rescorla, E., "More compatibility measurement results",
2017, <https://www.ietf.org/mail-archive/web/tls/current/
msg25179.html>.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003, <https://www.rfc-
editor.org/info/rfc3552>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005, <https://www.rfc-
editor.org/info/rfc4086>.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346,
DOI 10.17487/RFC4346, April 2006, <https://www.rfc-
editor.org/info/rfc4346>.
[RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 4366, DOI 10.17487/RFC4366, April 2006,
<https://www.rfc-editor.org/info/rfc4366>.
[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
for Transport Layer Security (TLS)", RFC 4492,
DOI 10.17487/RFC4492, May 2006, <https://www.rfc-
editor.org/info/rfc4492>.
[RFC4681] Santesson, S., Medvinsky, A., and J. Ball, "TLS User
Mapping Extension", RFC 4681, DOI 10.17487/RFC4681,
October 2006, <https://www.rfc-editor.org/info/rfc4681>.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
January 2008, <https://www.rfc-editor.org/info/rfc5077>.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<https://www.rfc-editor.org/info/rfc5116>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008, <https://www.rfc-
editor.org/info/rfc5246>.
Rescorla Expires August 19, 2018 [Page 114]
Internet-Draft TLS February 2018
[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764,
DOI 10.17487/RFC5764, May 2010, <https://www.rfc-
editor.org/info/rfc5764>.
[RFC5929] Altman, J., Williams, N., and L. Zhu, "Channel Bindings
for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010,
<https://www.rfc-editor.org/info/rfc5929>.
[RFC6091] Mavrogiannopoulos, N. and D. Gillmor, "Using OpenPGP Keys
for Transport Layer Security (TLS) Authentication",
RFC 6091, DOI 10.17487/RFC6091, February 2011,
<https://www.rfc-editor.org/info/rfc6091>.
[RFC6176] Turner, S. and T. Polk, "Prohibiting Secure Sockets Layer
(SSL) Version 2.0", RFC 6176, DOI 10.17487/RFC6176, March
2011, <https://www.rfc-editor.org/info/rfc6176>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[RFC6520] Seggelmann, R., Tuexen, M., and M. Williams, "Transport
Layer Security (TLS) and Datagram Transport Layer Security
(DTLS) Heartbeat Extension", RFC 6520,
DOI 10.17487/RFC6520, February 2012, <https://www.rfc-
editor.org/info/rfc6520>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
Weiler, S., and T. Kivinen, "Using Raw Public Keys in
Transport Layer Security (TLS) and Datagram Transport
Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
June 2014, <https://www.rfc-editor.org/info/rfc7250>.
[RFC7465] Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465,
DOI 10.17487/RFC7465, February 2015, <https://www.rfc-
editor.org/info/rfc7465>.
[RFC7568] Barnes, R., Thomson, M., Pironti, A., and A. Langley,
"Deprecating Secure Sockets Layer Version 3.0", RFC 7568,
DOI 10.17487/RFC7568, June 2015, <https://www.rfc-
editor.org/info/rfc7568>.
Rescorla Expires August 19, 2018 [Page 115]
Internet-Draft TLS February 2018
[RFC7627] Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
Langley, A., and M. Ray, "Transport Layer Security (TLS)
Session Hash and Extended Master Secret Extension",
RFC 7627, DOI 10.17487/RFC7627, September 2015,
<https://www.rfc-editor.org/info/rfc7627>.
[RFC7685] Langley, A., "A Transport Layer Security (TLS) ClientHello
Padding Extension", RFC 7685, DOI 10.17487/RFC7685,
October 2015, <https://www.rfc-editor.org/info/rfc7685>.
[RFC7924] Santesson, S. and H. Tschofenig, "Transport Layer Security
(TLS) Cached Information Extension", RFC 7924,
DOI 10.17487/RFC7924, July 2016, <https://www.rfc-
editor.org/info/rfc7924>.
[RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
Better Connectivity Using Concurrency", RFC 8305,
DOI 10.17487/RFC8305, December 2017, <https://www.rfc-
editor.org/info/rfc8305>.
[RSA] Rivest, R., Shamir, A., and L. Adleman, "A Method for
Obtaining Digital Signatures and Public-Key
Cryptosystems", Communications of the ACM v. 21, n. 2, pp.
120-126., February 1978.
[SIGMA] Krawczyk, H., "SIGMA: the 'SIGn-and-MAc' approach to
authenticated Diffie-Hellman and its use in the IKE
protocols", Proceedings of CRYPTO 2003 , 2003.
[SLOTH] Bhargavan, K. and G. Leurent, "Transcript Collision
Attacks: Breaking Authentication in TLS, IKE, and SSH",
Network and Distributed System Security Symposium (NDSS
2016) , 2016.
[SSL2] Hickman, K., "The SSL Protocol", February 1995.
[SSL3] Freier, A., Karlton, P., and P. Kocher, "The SSL 3.0
Protocol", November 1996.
[TIMING] Boneh, D. and D. Brumley, "Remote timing attacks are
practical", USENIX Security Symposium, 2003.
[X501] "Information Technology - Open Systems Interconnection -
The Directory: Models", ITU-T X.501, 1993.
Rescorla Expires August 19, 2018 [Page 116]
Internet-Draft TLS February 2018
12.3. URIs
[1] mailto:tls@ietf.org
Rescorla Expires August 19, 2018 [Page 117]
Internet-Draft TLS February 2018
Appendix A. State Machine
This section provides a summary of the legal state transitions for
the client and server handshakes. State names (in all capitals,
e.g., START) have no formal meaning but are provided for ease of
comprehension. Actions which are taken only in certain circumstances
are indicated in []. The notation "K_{send,recv} = foo" means "set
the send/recv key to the given key".
A.1. Client
START <----+
Send ClientHello | | Recv HelloRetryRequest
[K_send = early data] | |
v |
/ WAIT_SH ----+
| | Recv ServerHello
| | K_recv = handshake
Can | V
send | WAIT_EE
early | | Recv EncryptedExtensions
data | +--------+--------+
| Using | | Using certificate
| PSK | v
| | WAIT_CERT_CR
| | Recv | | Recv CertificateRequest
| | Certificate | v
| | | WAIT_CERT
| | | | Recv Certificate
| | v v
| | WAIT_CV
| | | Recv CertificateVerify
| +> WAIT_FINISHED <+
| | Recv Finished
\ | [Send EndOfEarlyData]
| K_send = handshake
| [Send Certificate [+ CertificateVerify]]
Can send | Send Finished
app data --> | K_send = K_recv = application
after here v
CONNECTED
Note that with the transitions as shown above, clients may send
alerts that derive from post-ServerHello messages in the clear or
with the early data keys. If clients need to send such alerts, they
SHOULD first rekey to the handshake keys if possible.
Rescorla Expires August 19, 2018 [Page 118]
Internet-Draft TLS February 2018
A.2. Server
START <-----+
Recv ClientHello | | Send HelloRetryRequest
v |
RECVD_CH ----+
| Select parameters
v
NEGOTIATED
| Send ServerHello
| K_send = handshake
| Send EncryptedExtensions
| [Send CertificateRequest]
Can send | [Send Certificate + CertificateVerify]
app data | Send Finished
after --> | K_send = application
here +--------+--------+
No 0-RTT | | 0-RTT
| |
K_recv = handshake | | K_recv = early data
[Skip decrypt errors] | +------> WAIT_EOED -+
| | Recv | | Recv EndOfEarlyData
| | early data | | K_recv = handshake
| +------------+ |
| |
+> WAIT_FLIGHT2 <--------+
|
+--------+--------+
No auth | | Client auth
| |
| v
| WAIT_CERT
| Recv | | Recv Certificate
| empty | v
| Certificate | WAIT_CV
| | | Recv
| v | CertificateVerify
+-> WAIT_FINISHED <---+
| Recv Finished
| K_recv = application
v
CONNECTED
Appendix B. Protocol Data Structures and Constant Values
This section describes protocol types and constants. Values listed
as _RESERVED were used in previous versions of TLS and are listed
Rescorla Expires August 19, 2018 [Page 119]
Internet-Draft TLS February 2018
here for completeness. TLS 1.3 implementations MUST NOT send them
but might receive them from older TLS implementations.
B.1. Record Layer
enum {
invalid(0),
change_cipher_spec(20),
alert(21),
handshake(22),
application_data(23),
(255)
} ContentType;
struct {
ContentType type;
ProtocolVersion legacy_record_version;
uint16 length;
opaque fragment[TLSPlaintext.length];
} TLSPlaintext;
struct {
opaque content[TLSPlaintext.length];
ContentType type;
uint8 zeros[length_of_padding];
} TLSInnerPlaintext;
struct {
ContentType opaque_type = application_data; /* 23 */
ProtocolVersion legacy_record_version = 0x0303; /* TLS v1.2 */
uint16 length;
opaque encrypted_record[TLSCiphertext.length];
} TLSCiphertext;
B.2. Alert Messages
Rescorla Expires August 19, 2018 [Page 120]
Internet-Draft TLS February 2018
enum { warning(1), fatal(2), (255) } AlertLevel;
enum {
close_notify(0),
unexpected_message(10),
bad_record_mac(20),
decryption_failed_RESERVED(21),
record_overflow(22),
decompression_failure_RESERVED(30),
handshake_failure(40),
no_certificate_RESERVED(41),
bad_certificate(42),
unsupported_certificate(43),
certificate_revoked(44),
certificate_expired(45),
certificate_unknown(46),
illegal_parameter(47),
unknown_ca(48),
access_denied(49),
decode_error(50),
decrypt_error(51),
export_restriction_RESERVED(60),
protocol_version(70),
insufficient_security(71),
internal_error(80),
inappropriate_fallback(86),
user_canceled(90),
no_renegotiation_RESERVED(100),
missing_extension(109),
unsupported_extension(110),
certificate_unobtainable(111),
unrecognized_name(112),
bad_certificate_status_response(113),
bad_certificate_hash_value(114),
unknown_psk_identity(115),
certificate_required(116),
no_application_protocol(120),
(255)
} AlertDescription;
struct {
AlertLevel level;
AlertDescription description;
} Alert;
Rescorla Expires August 19, 2018 [Page 121]
Internet-Draft TLS February 2018
B.3. Handshake Protocol
enum {
hello_request_RESERVED(0),
client_hello(1),
server_hello(2),
hello_verify_request_RESERVED(3),
new_session_ticket(4),
end_of_early_data(5),
hello_retry_request_RESERVED(6),
encrypted_extensions(8),
certificate(11),
server_key_exchange_RESERVED(12),
certificate_request(13),
server_hello_done_RESERVED(14),
certificate_verify(15),
client_key_exchange_RESERVED(16),
finished(20),
key_update(24),
message_hash(254),
(255)
} HandshakeType;
struct {
HandshakeType msg_type; /* handshake type */
uint24 length; /* bytes in message */
select (Handshake.msg_type) {
case client_hello: ClientHello;
case server_hello: ServerHello;
case end_of_early_data: EndOfEarlyData;
case encrypted_extensions: EncryptedExtensions;
case certificate_request: CertificateRequest;
case certificate: Certificate;
case certificate_verify: CertificateVerify;
case finished: Finished;
case new_session_ticket: NewSessionTicket;
case key_update: KeyUpdate;
};
} Handshake;
B.3.1. Key Exchange Messages
uint16 ProtocolVersion;
opaque Random[32];
uint8 CipherSuite[2]; /* Cryptographic suite selector */
struct {
Rescorla Expires August 19, 2018 [Page 122]
Internet-Draft TLS February 2018
ProtocolVersion legacy_version = 0x0303; /* TLS v1.2 */
Random random;
opaque legacy_session_id<0..32>;
CipherSuite cipher_suites<2..2^16-2>;
opaque legacy_compression_methods<1..2^8-1>;
Extension extensions<8..2^16-1>;
} ClientHello;
struct {
ProtocolVersion legacy_version = 0x0303; /* TLS v1.2 */
Random random;
opaque legacy_session_id_echo<0..32>;
CipherSuite cipher_suite;
uint8 legacy_compression_method = 0;
Extension extensions<6..2^16-1>;
} ServerHello;
struct {
ExtensionType extension_type;
opaque extension_data<0..2^16-1>;
} Extension;
enum {
server_name(0), /* RFC 6066 */
max_fragment_length(1), /* RFC 6066 */
status_request(5), /* RFC 6066 */
supported_groups(10), /* RFC 4492, 7919 */
signature_algorithms(13), /* [[this document]] */
use_srtp(14), /* RFC 5764 */
heartbeat(15), /* RFC 6520 */
application_layer_protocol_negotiation(16), /* RFC 7301 */
signed_certificate_timestamp(18), /* RFC 6962 */
client_certificate_type(19), /* RFC 7250 */
server_certificate_type(20), /* RFC 7250 */
padding(21), /* RFC 7685 */
RESERVED(40), /* Used but never assigned */
pre_shared_key(41), /* [[this document]] */
early_data(42), /* [[this document]] */
supported_versions(43), /* [[this document]] */
cookie(44), /* [[this document]] */
psk_key_exchange_modes(45), /* [[this document]] */
RESERVED(46), /* Used but never assigned */
certificate_authorities(47), /* [[this document]] */
oid_filters(48), /* [[this document]] */
post_handshake_auth(49), /* [[this document]] */
signature_algorithms_cert(50), /* [[this document]] */
key_share(51), /* [[this document]] */
(65535)
Rescorla Expires August 19, 2018 [Page 123]
Internet-Draft TLS February 2018
} ExtensionType;
struct {
NamedGroup group;
opaque key_exchange<1..2^16-1>;
} KeyShareEntry;
struct {
KeyShareEntry client_shares<0..2^16-1>;
} KeyShareClientHello;
struct {
NamedGroup selected_group;
} KeyShareHelloRetryRequest;
struct {
KeyShareEntry server_share;
} KeyShareServerHello;
struct {
uint8 legacy_form = 4;
opaque X[coordinate_length];
opaque Y[coordinate_length];
} UncompressedPointRepresentation;
enum { psk_ke(0), psk_dhe_ke(1), (255) } PskKeyExchangeMode;
struct {
PskKeyExchangeMode ke_modes<1..255>;
} PskKeyExchangeModes;
struct {} Empty;
struct {
select (Handshake.msg_type) {
case new_session_ticket: uint32 max_early_data_size;
case client_hello: Empty;
case encrypted_extensions: Empty;
};
} EarlyDataIndication;
struct {
opaque identity<1..2^16-1>;
uint32 obfuscated_ticket_age;
} PskIdentity;
opaque PskBinderEntry<32..255>;
Rescorla Expires August 19, 2018 [Page 124]
Internet-Draft TLS February 2018
struct {
PskIdentity identities<7..2^16-1>;
PskBinderEntry binders<33..2^16-1>;
} OfferedPsks;
struct {
select (Handshake.msg_type) {
case client_hello: OfferedPsks;
case server_hello: uint16 selected_identity;
};
} PreSharedKeyExtension;
B.3.1.1. Version Extension
struct {
select (Handshake.msg_type) {
case client_hello:
ProtocolVersion versions<2..254>;
case server_hello: /* and HelloRetryRequest */
ProtocolVersion selected_version;
};
} SupportedVersions;
B.3.1.2. Cookie Extension
struct {
opaque cookie<1..2^16-1>;
} Cookie;
B.3.1.3. Signature Algorithm Extension
Rescorla Expires August 19, 2018 [Page 125]
Internet-Draft TLS February 2018
enum {
/* RSASSA-PKCS1-v1_5 algorithms */
rsa_pkcs1_sha256(0x0401),
rsa_pkcs1_sha384(0x0501),
rsa_pkcs1_sha512(0x0601),
/* ECDSA algorithms */
ecdsa_secp256r1_sha256(0x0403),
ecdsa_secp384r1_sha384(0x0503),
ecdsa_secp521r1_sha512(0x0603),
/* RSASSA-PSS algorithms with public key OID rsaEncryption */
rsa_pss_rsae_sha256(0x0804),
rsa_pss_rsae_sha384(0x0805),
rsa_pss_rsae_sha512(0x0806),
/* EdDSA algorithms */
ed25519(0x0807),
ed448(0x0808),
/* RSASSA-PSS algorithms with public key OID RSASSA-PSS */
rsa_pss_pss_sha256(0x0809),
rsa_pss_pss_sha384(0x080a),
rsa_pss_pss_sha512(0x080b),
/* Legacy algorithms */
rsa_pkcs1_sha1(0x0201),
ecdsa_sha1(0x0203),
/* Reserved Code Points */
obsolete_RESERVED(0x0000..0x0200),
dsa_sha1_RESERVED(0x0202),
obsolete_RESERVED(0x0204..0x0400),
dsa_sha256_RESERVED(0x0402),
obsolete_RESERVED(0x0404..0x0500),
dsa_sha384_RESERVED(0x0502),
obsolete_RESERVED(0x0504..0x0600),
dsa_sha512_RESERVED(0x0602),
obsolete_RESERVED(0x0604..0x06FF),
private_use(0xFE00..0xFFFF),
(0xFFFF)
} SignatureScheme;
struct {
SignatureScheme supported_signature_algorithms<2..2^16-2>;
} SignatureSchemeList;
Rescorla Expires August 19, 2018 [Page 126]
Internet-Draft TLS February 2018
B.3.1.4. Supported Groups Extension
enum {
unallocated_RESERVED(0x0000),
/* Elliptic Curve Groups (ECDHE) */
obsolete_RESERVED(0x0001..0x0016),
secp256r1(0x0017), secp384r1(0x0018), secp521r1(0x0019),
obsolete_RESERVED(0x001A..0x001C),
x25519(0x001D), x448(0x001E),
/* Finite Field Groups (DHE) */
ffdhe2048(0x0100), ffdhe3072(0x0101), ffdhe4096(0x0102),
ffdhe6144(0x0103), ffdhe8192(0x0104),
/* Reserved Code Points */
ffdhe_private_use(0x01FC..0x01FF),
ecdhe_private_use(0xFE00..0xFEFF),
obsolete_RESERVED(0xFF01..0xFF02),
(0xFFFF)
} NamedGroup;
struct {
NamedGroup named_group_list<2..2^16-1>;
} NamedGroupList;
Values within "obsolete_RESERVED" ranges are used in previous
versions of TLS and MUST NOT be offered or negotiated by TLS 1.3
implementations. The obsolete curves have various known/theoretical
weaknesses or have had very little usage, in some cases only due to
unintentional server configuration issues. They are no longer
considered appropriate for general use and should be assumed to be
potentially unsafe. The set of curves specified here is sufficient
for interoperability with all currently deployed and properly
configured TLS implementations.
B.3.2. Server Parameters Messages
Rescorla Expires August 19, 2018 [Page 127]
Internet-Draft TLS February 2018
opaque DistinguishedName<1..2^16-1>;
struct {
DistinguishedName authorities<3..2^16-1>;
} CertificateAuthoritiesExtension;
struct {
opaque certificate_extension_oid<1..2^8-1>;
opaque certificate_extension_values<0..2^16-1>;
} OIDFilter;
struct {
OIDFilter filters<0..2^16-1>;
} OIDFilterExtension;
struct {} PostHandshakeAuth;
struct {
Extension extensions<0..2^16-1>;
} EncryptedExtensions;
struct {
opaque certificate_request_context<0..2^8-1>;
Extension extensions<2..2^16-1>;
} CertificateRequest;
B.3.3. Authentication Messages
Rescorla Expires August 19, 2018 [Page 128]
Internet-Draft TLS February 2018
enum {
X509(0),
OpenPGP_RESERVED(1),
RawPublicKey(2),
(255)
} CertificateType;
struct {
select (certificate_type) {
case RawPublicKey:
/* From RFC 7250 ASN.1_subjectPublicKeyInfo */
opaque ASN1_subjectPublicKeyInfo<1..2^24-1>;
case X509:
opaque cert_data<1..2^24-1>;
};
Extension extensions<0..2^16-1>;
} CertificateEntry;
struct {
opaque certificate_request_context<0..2^8-1>;
CertificateEntry certificate_list<0..2^24-1>;
} Certificate;
struct {
SignatureScheme algorithm;
opaque signature<0..2^16-1>;
} CertificateVerify;
struct {
opaque verify_data[Hash.length];
} Finished;
B.3.4. Ticket Establishment
struct {
uint32 ticket_lifetime;
uint32 ticket_age_add;
opaque ticket_nonce<0..255>;
opaque ticket<1..2^16-1>;
Extension extensions<0..2^16-2>;
} NewSessionTicket;
B.3.5. Updating Keys
Rescorla Expires August 19, 2018 [Page 129]
Internet-Draft TLS February 2018
struct {} EndOfEarlyData;
enum {
update_not_requested(0), update_requested(1), (255)
} KeyUpdateRequest;
struct {
KeyUpdateRequest request_update;
} KeyUpdate;
B.4. Cipher Suites
A symmetric cipher suite defines the pair of the AEAD algorithm and
hash algorithm to be used with HKDF. Cipher suite names follow the
naming convention:
CipherSuite TLS_AEAD_HASH = VALUE;
+-----------+------------------------------------------------+
| Component | Contents |
+-----------+------------------------------------------------+
| TLS | The string "TLS" |
| | |
| AEAD | The AEAD algorithm used for record protection |
| | |
| HASH | The hash algorithm used with HKDF |
| | |
| VALUE | The two byte ID assigned for this cipher suite |
+-----------+------------------------------------------------+
This specification defines the following cipher suites for use with
TLS 1.3.
+------------------------------+-------------+
| Description | Value |
+------------------------------+-------------+
| TLS_AES_128_GCM_SHA256 | {0x13,0x01} |
| | |
| TLS_AES_256_GCM_SHA384 | {0x13,0x02} |
| | |
| TLS_CHACHA20_POLY1305_SHA256 | {0x13,0x03} |
| | |
| TLS_AES_128_CCM_SHA256 | {0x13,0x04} |
| | |
| TLS_AES_128_CCM_8_SHA256 | {0x13,0x05} |
+------------------------------+-------------+
Rescorla Expires August 19, 2018 [Page 130]
Internet-Draft TLS February 2018
The corresponding AEAD algorithms AEAD_AES_128_GCM, AEAD_AES_256_GCM,
and AEAD_AES_128_CCM are defined in [RFC5116].
AEAD_CHACHA20_POLY1305 is defined in [RFC7539]. AEAD_AES_128_CCM_8
is defined in [RFC6655]. The corresponding hash algorithms are
defined in [SHS].
Although TLS 1.3 uses the same cipher suite space as previous
versions of TLS, TLS 1.3 cipher suites are defined differently, only
specifying the symmetric ciphers, and cannot be used for TLS 1.2.
Similarly, TLS 1.2 and lower cipher suites cannot be used with TLS
1.3.
New cipher suite values are assigned by IANA as described in
Section 11.
Appendix C. Implementation Notes
The TLS protocol cannot prevent many common security mistakes. This
section provides several recommendations to assist implementors.
[I-D.ietf-tls-tls13-vectors] provides test vectors for TLS 1.3
handshakes.
C.1. Random Number Generation and Seeding
TLS requires a cryptographically secure pseudorandom number generator
(CSPRNG). In most cases, the operating system provides an
appropriate facility such as /dev/urandom, which should be used
absent other (performance) concerns. It is RECOMMENDED to use an
existing CSPRNG implementation in preference to crafting a new one.
Many adequate cryptographic libraries are already available under
favorable license terms. Should those prove unsatisfactory,
[RFC4086] provides guidance on the generation of random values.
TLS uses random values both in public protocol fields such as the
public Random values in the ClientHello and ServerHello and to
generate keying material. With a properly functioning CSPRNG, this
does not present a security problem as it is not feasible to
determine the CSPRNG state from its output. However, with a broken
CSPRNG, it may be possible for an attacker to use the public output
to determine the CSPRNG internal state and thereby predict the keying
material, as documented in [CHECKOWAY]. Implementations can provide
extra security against this form of attack by using separate CSPRNGs
to generate public and private values.
Rescorla Expires August 19, 2018 [Page 131]
Internet-Draft TLS February 2018
C.2. Certificates and Authentication
Implementations are responsible for verifying the integrity of
certificates and should generally support certificate revocation
messages. Absent a specific indication from an application profile,
Certificates should always be verified to ensure proper signing by a
trusted Certificate Authority (CA). The selection and addition of
trust anchors should be done very carefully. Users should be able to
view information about the certificate and trust anchor.
Applications SHOULD also enforce minimum and maximum key sizes. For
example, certification paths containing keys or signatures weaker
than 2048-bit RSA or 224-bit ECDSA are not appropriate for secure
applications.
C.3. Implementation Pitfalls
Implementation experience has shown that certain parts of earlier TLS
specifications are not easy to understand and have been a source of
interoperability and security problems. Many of these areas have
been clarified in this document but this appendix contains a short
list of the most important things that require special attention from
implementors.
TLS protocol issues:
- Do you correctly handle handshake messages that are fragmented to
multiple TLS records (see Section 5.1)? Including corner cases
like a ClientHello that is split to several small fragments? Do
you fragment handshake messages that exceed the maximum fragment
size? In particular, the Certificate and CertificateRequest
handshake messages can be large enough to require fragmentation.
- Do you ignore the TLS record layer version number in all
unencrypted TLS records? (see Appendix D)
- Have you ensured that all support for SSL, RC4, EXPORT ciphers,
and MD5 (via the "signature_algorithms" extension) is completely
removed from all possible configurations that support TLS 1.3 or
later, and that attempts to use these obsolete capabilities fail
correctly? (see Appendix D)
- Do you handle TLS extensions in ClientHello correctly, including
unknown extensions?
- When the server has requested a client certificate, but no
suitable certificate is available, do you correctly send an empty
Certificate message, instead of omitting the whole message (see
Section 4.4.2.3)?
Rescorla Expires August 19, 2018 [Page 132]
Internet-Draft TLS February 2018
- When processing the plaintext fragment produced by AEAD-Decrypt
and scanning from the end for the ContentType, do you avoid
scanning past the start of the cleartext in the event that the
peer has sent a malformed plaintext of all-zeros?
- Do you properly ignore unrecognized cipher suites (Section 4.1.2),
hello extensions (Section 4.2), named groups (Section 4.2.7), key
shares (Section 4.2.8), supported versions (Section 4.2.1), and
signature algorithms (Section 4.2.3) in the ClientHello?
- As a server, do you send a HelloRetryRequest to clients which
support a compatible (EC)DHE group but do not predict it in the
"key_share" extension? As a client, do you correctly handle a
HelloRetryRequest from the server?
Cryptographic details:
- What countermeasures do you use to prevent timing attacks
[TIMING]?
- When using Diffie-Hellman key exchange, do you correctly preserve
leading zero bytes in the negotiated key (see Section 7.4.1)?
- Does your TLS client check that the Diffie-Hellman parameters sent
by the server are acceptable, (see Section 4.2.8.1)?
- Do you use a strong and, most importantly, properly seeded random
number generator (see Appendix C.1) when generating Diffie-Hellman
private values, the ECDSA "k" parameter, and other security-
critical values? It is RECOMMENDED that implementations implement
"deterministic ECDSA" as specified in [RFC6979].
- Do you zero-pad Diffie-Hellman public key values to the group size
(see Section 4.2.8.1)?
- Do you verify signatures after making them to protect against RSA-
CRT key leaks? [FW15]
C.4. Client Tracking Prevention
Clients SHOULD NOT reuse a ticket for multiple connections. Reuse of
a ticket allows passive observers to correlate different connections.
Servers that issue tickets SHOULD offer at least as many tickets as
the number of connections that a client might use; for example, a web
browser using HTTP/1.1 [RFC7230] might open six connections to a
server. Servers SHOULD issue new tickets with every connection.
This ensures that clients are always able to use a new ticket when
creating a new connection.
Rescorla Expires August 19, 2018 [Page 133]
Internet-Draft TLS February 2018
C.5. Unauthenticated Operation
Previous versions of TLS offered explicitly unauthenticated cipher
suites based on anonymous Diffie-Hellman. These modes have been
deprecated in TLS 1.3. However, it is still possible to negotiate
parameters that do not provide verifiable server authentication by
several methods, including:
- Raw public keys [RFC7250].
- Using a public key contained in a certificate but without
validation of the certificate chain or any of its contents.
Either technique used alone is vulnerable to man-in-the-middle
attacks and therefore unsafe for general use. However, it is also
possible to bind such connections to an external authentication
mechanism via out-of-band validation of the server's public key,
trust on first use, or a mechanism such as channel bindings (though
the channel bindings described in [RFC5929] are not defined for TLS
1.3). If no such mechanism is used, then the connection has no
protection against active man-in-the-middle attack; applications MUST
NOT use TLS in such a way absent explicit configuration or a specific
application profile.
Appendix D. Backward Compatibility
The TLS protocol provides a built-in mechanism for version
negotiation between endpoints potentially supporting different
versions of TLS.
TLS 1.x and SSL 3.0 use compatible ClientHello messages. Servers can
also handle clients trying to use future versions of TLS as long as
the ClientHello format remains compatible and and there is at least
one protocol version supported by both the client and the server.
Prior versions of TLS used the record layer version number for
various purposes. (TLSPlaintext.legacy_record_version and
TLSCiphertext.legacy_record_version) As of TLS 1.3, this field is
deprecated. The value of TLSPlaintext.legacy_record_version MUST be
ignored by all implementations. The value of
TLSCiphertext.legacy_record_version MAY be ignored, or MAY be
validated to match the fixed constant value. Version negotiation is
performed using only the handshake versions
(ClientHello.legacy_version, ServerHello.legacy_version, as well as
the ClientHello, HelloRetryRequest and ServerHello
"supported_versions" extensions). In order to maximize
interoperability with older endpoints, implementations that negotiate
the use of TLS 1.0-1.2 SHOULD set the record layer version number to
Rescorla Expires August 19, 2018 [Page 134]
Internet-Draft TLS February 2018
the negotiated version for the ServerHello and all records
thereafter.
For maximum compatibility with previously non-standard behavior and
misconfigured deployments, all implementations SHOULD support
validation of certification paths based on the expectations in this
document, even when handling prior TLS versions' handshakes. (see
Section 4.4.2.2)
TLS 1.2 and prior supported an "Extended Master Secret" [RFC7627]
extension which digested large parts of the handshake transcript into
the master secret. Because TLS 1.3 always hashes in the transcript
up to the server CertificateVerify, implementations which support
both TLS 1.3 and earlier versions SHOULD indicate the use of the
Extended Master Secret extension in their APIs whenever TLS 1.3 is
used.
D.1. Negotiating with an older server
A TLS 1.3 client who wishes to negotiate with servers that do not
support TLS 1.3 will send a normal TLS 1.3 ClientHello containing
0x0303 (TLS 1.2) in ClientHello.legacy_version but with the correct
version(s) in the "supported_versions" extension. If the server does
not support TLS 1.3 it will respond with a ServerHello containing an
older version number. If the client agrees to use this version, the
negotiation will proceed as appropriate for the negotiated protocol.
A client using a ticket for resumption SHOULD initiate the connection
using the version that was previously negotiated.
Note that 0-RTT data is not compatible with older servers and SHOULD
NOT be sent absent knowledge that the server supports TLS 1.3. See
Appendix D.3.
If the version chosen by the server is not supported by the client
(or not acceptable), the client MUST abort the handshake with a
"protocol_version" alert.
Some legacy server implementations are known to not implement the TLS
specification properly and might abort connections upon encountering
TLS extensions or versions which they are not aware of.
Interoperability with buggy servers is a complex topic beyond the
scope of this document. Multiple connection attempts may be required
in order to negotiate a backwards compatible connection; however,
this practice is vulnerable to downgrade attacks and is NOT
RECOMMENDED.
Rescorla Expires August 19, 2018 [Page 135]
Internet-Draft TLS February 2018
D.2. Negotiating with an older client
A TLS server can also receive a ClientHello indicating a version
number smaller than its highest supported version. If the
"supported_versions" extension is present, the server MUST negotiate
using that extension as described in Section 4.2.1. If the
"supported_versions" extension is not present, the server MUST
negotiate the minimum of ClientHello.legacy_version and TLS 1.2. For
example, if the server supports TLS 1.0, 1.1, and 1.2, and
legacy_version is TLS 1.0, the server will proceed with a TLS 1.0
ServerHello. If the "supported_versions" extension is absent and the
server only supports versions greater than
ClientHello.legacy_version, the server MUST abort the handshake with
a "protocol_version" alert.
Note that earlier versions of TLS did not clearly specify the record
layer version number value in all cases
(TLSPlaintext.legacy_record_version). Servers will receive various
TLS 1.x versions in this field, but its value MUST always be ignored.
D.3. 0-RTT backwards compatibility
0-RTT data is not compatible with older servers. An older server
will respond to the ClientHello with an older ServerHello, but it
will not correctly skip the 0-RTT data and will fail to complete the
handshake. This can cause issues when a client attempts to use
0-RTT, particularly against multi-server deployments. For example, a
deployment could deploy TLS 1.3 gradually with some servers
implementing TLS 1.3 and some implementing TLS 1.2, or a TLS 1.3
deployment could be downgraded to TLS 1.2.
A client that attempts to send 0-RTT data MUST fail a connection if
it receives a ServerHello with TLS 1.2 or older. A client that
attempts to repair this error SHOULD NOT send a TLS 1.2 ClientHello,
but instead send a TLS 1.3 ClientHello without 0-RTT data.
To avoid this error condition, multi-server deployments SHOULD ensure
a uniform and stable deployment of TLS 1.3 without 0-RTT prior to
enabling 0-RTT.
D.4. Middlebox Compatibility Mode
Field measurements [Ben17a], [Ben17b], [Res17a], [Res17b] have found
that a significant number of middleboxes misbehave when a TLS client/
server pair negotiates TLS 1.3. Implementations can increase the
chance of making connections through those middleboxes by making the
TLS 1.3 handshake look more like a TLS 1.2 handshake:
Rescorla Expires August 19, 2018 [Page 136]
Internet-Draft TLS February 2018
- The client always provides a non-empty session ID in the
ClientHello, as described in the legacy_session_id section of
Section 4.1.2.
- If not offering early data, the client sends a dummy
change_cipher_spec record (see the third paragraph of Section 5.1)
immediately before its second flight. This may either be before
its second ClientHello or before its encrypted handshake flight.
If offering early data, the record is placed immediately after the
first ClientHello.
- The server sends a dummy change_cipher_spec record immediately
after its first handshake message. This may either be after a
ServerHello or a HelloRetryRequest.
When put together, these changes make the TLS 1.3 handshake resemble
TLS 1.2 session resumption, which improves the chance of successfully
connecting through middleboxes. This "compatibility mode" is
partially negotiated: The client can opt to provide a session ID or
not and the server has to echo it. Either side can send
change_cipher_spec at any time during the handshake, as they must be
ignored by the peer, but if the client sends a non-empty session ID,
the server MUST send the change_cipher_spec as described in this
section.
D.5. Backwards Compatibility Security Restrictions
Implementations negotiating use of older versions of TLS SHOULD
prefer forward secret and AEAD cipher suites, when available.
The security of RC4 cipher suites is considered insufficient for the
reasons cited in [RFC7465]. Implementations MUST NOT offer or
negotiate RC4 cipher suites for any version of TLS for any reason.
Old versions of TLS permitted the use of very low strength ciphers.
Ciphers with a strength less than 112 bits MUST NOT be offered or
negotiated for any version of TLS for any reason.
The security of SSL 3.0 [SSL3] is considered insufficient for the
reasons enumerated in [RFC7568], and MUST NOT be negotiated for any
reason.
The security of SSL 2.0 [SSL2] is considered insufficient for the
reasons enumerated in [RFC6176], and MUST NOT be negotiated for any
reason.
Implementations MUST NOT send an SSL version 2.0 compatible CLIENT-
HELLO. Implementations MUST NOT negotiate TLS 1.3 or later using an
Rescorla Expires August 19, 2018 [Page 137]
Internet-Draft TLS February 2018
SSL version 2.0 compatible CLIENT-HELLO. Implementations are NOT
RECOMMENDED to accept an SSL version 2.0 compatible CLIENT-HELLO in
order to negotiate older versions of TLS.
Implementations MUST NOT send a ClientHello.legacy_version or
ServerHello.legacy_version set to 0x0300 or less. Any endpoint
receiving a Hello message with ClientHello.legacy_version or
ServerHello.legacy_version set to 0x0300 MUST abort the handshake
with a "protocol_version" alert.
Implementations MUST NOT send any records with a version less than
0x0300. Implementations SHOULD NOT accept any records with a version
less than 0x0300 (but may inadvertently do so if the record version
number is ignored completely).
Implementations MUST NOT use the Truncated HMAC extension, defined in
Section 7 of [RFC6066], as it is not applicable to AEAD algorithms
and has been shown to be insecure in some scenarios.
Appendix E. Overview of Security Properties
A complete security analysis of TLS is outside the scope of this
document. In this section, we provide an informal description the
desired properties as well as references to more detailed work in the
research literature which provides more formal definitions.
We cover properties of the handshake separately from those of the
record layer.
E.1. Handshake
The TLS handshake is an Authenticated Key Exchange (AKE) protocol
which is intended to provide both one-way authenticated (server-only)
and mutually authenticated (client and server) functionality. At the
completion of the handshake, each side outputs its view of the
following values:
- A set of "session keys" (the various secrets derived from the
master secret) from which can be derived a set of working keys.
- A set of cryptographic parameters (algorithms, etc.)
- The identities of the communicating parties.
We assume the attacker to be an active network attacker, which means
it has complete control over the network used to communicate between
the parties [RFC3552]. Even under these conditions, the handshake
should provide the properties listed below. Note that these
Rescorla Expires August 19, 2018 [Page 138]
Internet-Draft TLS February 2018
properties are not necessarily independent, but reflect the protocol
consumers' needs.
Establishing the same session keys. The handshake needs to output
the same set of session keys on both sides of the handshake,
provided that it completes successfully on each endpoint (See
[CK01]; defn 1, part 1).
Secrecy of the session keys. The shared session keys should be known
only to the communicating parties and not to the attacker (See
[CK01]; defn 1, part 2). Note that in a unilaterally
authenticated connection, the attacker can establish its own
session keys with the server, but those session keys are distinct
from those established by the client.
Peer Authentication. The client's view of the peer identity should
reflect the server's identity. If the client is authenticated,
the server's view of the peer identity should match the client's
identity.
Uniqueness of the session keys: Any two distinct handshakes should
produce distinct, unrelated session keys. Individual session keys
produced by a handshake should also be distinct and unrelated.
Downgrade protection. The cryptographic parameters should be the
same on both sides and should be the same as if the peers had been
communicating in the absence of an attack (See [BBFKZG16]; defns 8
and 9}).
Forward secret with respect to long-term keys If the long-term
keying material (in this case the signature keys in certificate-
based authentication modes or the external/resumption PSK in PSK
with (EC)DHE modes) is compromised after the handshake is
complete, this does not compromise the security of the session key
(See [DOW92]), as long as the session key itself has been erased.
The forward secrecy property is not satisfied when PSK is used in
the "psk_ke" PskKeyExchangeMode.
Key Compromise Impersonation (KCI) resistance In a mutually-
authenticated connection with certificates, peer authentication
should hold even if the local long-term secret was compromised
before the connection was established (see [HGFS15]). For
example, if a client's signature key is compromised, it should not
be possible to impersonate arbitrary servers to that client in
subsequent handshakes.
Protection of endpoint identities. The server's identity
(certificate) should be protected against passive attackers. The
Rescorla Expires August 19, 2018 [Page 139]
Internet-Draft TLS February 2018
client's identity should be protected against both passive and
active attackers.
Informally, the signature-based modes of TLS 1.3 provide for the
establishment of a unique, secret, shared key established by an
(EC)DHE key exchange and authenticated by the server's signature over
the handshake transcript, as well as tied to the server's identity by
a MAC. If the client is authenticated by a certificate, it also
signs over the handshake transcript and provides a MAC tied to both
identities. [SIGMA] describes the design and analysis of this type
of key exchange protocol. If fresh (EC)DHE keys are used for each
connection, then the output keys are forward secret.
The external PSK and resumption PSK bootstrap from a long-term shared
secret into a unique per-connection set of short-term session keys.
This secret may have been established in a previous handshake. If
PSK with (EC)DHE key establishment is used, these session keys will
also be forward secret. The resumption PSK has been designed so that
the resumption master secret computed by connection N and needed to
form connection N+1 is separate from the traffic keys used by
connection N, thus providing forward secrecy between the connections.
In addition, if multiple tickets are established on the same
connection, they are associated with different keys, so compromise of
the PSK associated with one ticket does not lead to the compromise of
connections established with PSKs associated with other tickets.
This property is most interesting if tickets are stored in a database
(and so can be deleted) rather than if they are self-encrypted.
The PSK binder value forms a binding between a PSK and the current
handshake, as well as between the session where the PSK was
established and the session where it was used. This binding
transitively includes the original handshake transcript, because that
transcript is digested into the values which produce the Resumption
Master Secret. This requires that both the KDF used to produce the
resumption master secret and the MAC used to compute the binder be
collision resistant. See Appendix E.1.1 for more on this. Note: The
binder does not cover the binder values from other PSKs, though they
are included in the Finished MAC.
Note: TLS does not currently permit the server to send a
certificate_request message in non-certificate-based handshakes
(e.g., PSK). If this restriction were to be relaxed in future, the
client's signature would not cover the server's certificate directly.
However, if the PSK was established through a NewSessionTicket, the
client's signature would transitively cover the server's certificate
through the PSK binder. [PSK-FINISHED] describes a concrete attack
on constructions that do not bind to the server's certificate (see
also [Kraw16]). It is unsafe to use certificate-based client
Rescorla Expires August 19, 2018 [Page 140]
Internet-Draft TLS February 2018
authentication when the client might potentially share the same PSK/
key-id pair with two different endpoints. Implementations MUST NOT
combine external PSKs with certificate-based authentication of either
the client or the server unless negotiated by some extension.
If an exporter is used, then it produces values which are unique and
secret (because they are generated from a unique session key).
Exporters computed with different labels and contexts are
computationally independent, so it is not feasible to compute one
from another or the session secret from the exported value. Note:
exporters can produce arbitrary-length values. If exporters are to
be used as channel bindings, the exported value MUST be large enough
to provide collision resistance. The exporters provided in TLS 1.3
are derived from the same handshake contexts as the early traffic
keys and the application traffic keys respectively, and thus have
similar security properties. Note that they do not include the
client's certificate; future applications which wish to bind to the
client's certificate may need to define a new exporter that includes
the full handshake transcript.
For all handshake modes, the Finished MAC (and where present, the
signature), prevents downgrade attacks. In addition, the use of
certain bytes in the random nonces as described in Section 4.1.3
allows the detection of downgrade to previous TLS versions. See
[BBFKZG16] for more detail on TLS 1.3 and downgrade.
As soon as the client and the server have exchanged enough
information to establish shared keys, the remainder of the handshake
is encrypted, thus providing protection against passive attackers,
even if the computed shared key is not authenticated. Because the
server authenticates before the client, the client can ensure that if
it authenticates to the server, it only reveals its identity to an
authenticated server. Note that implementations must use the
provided record padding mechanism during the handshake to avoid
leaking information about the identities due to length. The client's
proposed PSK identities are not encrypted, nor is the one that the
server selects.
E.1.1. Key Derivation and HKDF
Key derivation in TLS 1.3 uses the HKDF function defined in [RFC5869]
and its two components, HKDF-Extract and HKDF-Expand. The full
rationale for the HKDF construction can be found in [Kraw10] and the
rationale for the way it is used in TLS 1.3 in [KW16]. Throughout
this document, each application of HKDF-Extract is followed by one or
more invocations of HKDF-Expand. This ordering should always be
followed (including in future revisions of this document), in
particular, one SHOULD NOT use an output of HKDF-Extract as an input
Rescorla Expires August 19, 2018 [Page 141]
Internet-Draft TLS February 2018
to another application of HKDF-Extract without an HKDF-Expand in
between. Consecutive applications of HKDF-Expand are allowed as long
as these are differentiated via the key and/or the labels.
Note that HKDF-Expand implements a pseudorandom function (PRF) with
both inputs and outputs of variable length. In some of the uses of
HKDF in this document (e.g., for generating exporters and the
resumption_master_secret), it is necessary that the application of
HKDF-Expand be collision-resistant, namely, it should be infeasible
to find two different inputs to HKDF-Expand that output the same
value. This requires the underlying hash function to be collision
resistant and the output length from HKDF-Expand to be of size at
least 256 bits (or as much as needed for the hash function to prevent
finding collisions).
E.1.2. Client Authentication
A client that has sent authentication data to a server, either during
the handshake or in post-handshake authentication, cannot be sure if
the server afterwards considers the client to be authenticated or
not. If the client needs to determine if the server considers the
connection to be unilaterally or mutually authenticated, this has to
be provisioned by the application layer. See [CHHSV17] for details.
In addition, the analysis of post-handshake authentication from
[Kraw16] shows that the client identified by the certificate sent in
the post-handshake phase possesses the traffic key. This party is
therefore the client that participated in the original handshake or
one to whom the original client delegated the traffic key (assuming
that the traffic key has not been compromised).
E.1.3. 0-RTT
The 0-RTT mode of operation generally provides similar security
properties as 1-RTT data, with the two exceptions that the 0-RTT
encryption keys do not provide full forward secrecy and that the
server is not able to guarantee uniqueness of the handshake (non-
replayability) without keeping potentially undue amounts of state.
See Section 4.2.10 for one mechanism to limit the exposure to replay.
E.1.4. Exporter Independence
The exporter_master_secret and early_exporter_master_secret are
derived to be independent of the traffic keys and therefore do not
represent a threat to the security of traffic encrypted with those
keys. However, because these secrets can be used to compute any
exporter value, they SHOULD be erased as soon as possible. If the
total set of exporter labels is known, then implementations SHOULD
pre-compute the inner Derive-Secret stage of the exporter computation
Rescorla Expires August 19, 2018 [Page 142]
Internet-Draft TLS February 2018
for all those labels, then erase the [early_]exporter_master_secret,
followed by each inner values as soon as it is known that it will not
be needed again.
E.1.5. Post-Compromise Security
TLS does not provide security for handshakes which take place after
the peer's long-term secret (signature key or external PSK) is
compromised. It therefore does not provide post-compromise security
[CCG16], sometimes also referred to as backwards or future secrecy.
This is in contrast to KCI resistance, which describes the security
guarantees that a party has after its own long-term secret has been
compromised.
E.1.6. External References
The reader should refer to the following references for analysis of
the TLS handshake: [DFGS15] [CHSV16] [DFGS16] [KW16] [Kraw16]
[FGSW16] [LXZFH16] [FG17] [BBK17].
E.2. Record Layer
The record layer depends on the handshake producing strong traffic
secrets which can be used to derive bidirectional encryption keys and
nonces. Assuming that is true, and the keys are used for no more
data than indicated in Section 5.5 then the record layer should
provide the following guarantees:
Confidentiality. An attacker should not be able to determine the
plaintext contents of a given record.
Integrity. An attacker should not be able to craft a new record
which is different from an existing record which will be accepted
by the receiver.
Order protection/non-replayability An attacker should not be able to
cause the receiver to accept a record which it has already
accepted or cause the receiver to accept record N+1 without having
first processed record N.
Length concealment. Given a record with a given external length, the
attacker should not be able to determine the amount of the record
that is content versus padding.
Forward secrecy after key change. If the traffic key update
mechanism described in Section 4.6.3 has been used and the
previous generation key is deleted, an attacker who compromises
Rescorla Expires August 19, 2018 [Page 143]
Internet-Draft TLS February 2018
the endpoint should not be able to decrypt traffic encrypted with
the old key.
Informally, TLS 1.3 provides these properties by AEAD-protecting the
plaintext with a strong key. AEAD encryption [RFC5116] provides
confidentiality and integrity for the data. Non-replayability is
provided by using a separate nonce for each record, with the nonce
being derived from the record sequence number (Section 5.3), with the
sequence number being maintained independently at both sides thus
records which are delivered out of order result in AEAD deprotection
failures. In order to prevent mass cryptanalysis when the same
plaintext is repeatedly encrypted by different users under the same
key (as is commonly the case for HTTP), the nonce is formed by mixing
the sequence number with a secret per-connection initialization
vector derived along with the traffic keys. See [BT16] for analysis
of this construction.
The re-keying technique in TLS 1.3 (see Section 7.2) follows the
construction of the serial generator in [REKEY], which shows that re-
keying can allow keys to be used for a larger number of encryptions
than without re-keying. This relies on the security of the HKDF-
Expand-Label function as a pseudorandom function (PRF). In addition,
as long as this function is truly one way, it is not possible to
compute traffic keys from prior to a key change (forward secrecy).
TLS does not provide security for data which is communicated on a
connection after a traffic secret of that connection is compromised.
That is, TLS does not provide post-compromise security/future
secrecy/backward secrecy with respect to the traffic secret. Indeed,
an attacker who learns a traffic secret can compute all future
traffic secrets on that connection. Systems which want such
guarantees need to do a fresh handshake and establish a new
connection with an (EC)DHE exchange.
E.2.1. External References
The reader should refer to the following references for analysis of
the TLS record layer: [BMMT15] [BT16] [BDFKPPRSZZ16] [BBK17].
E.3. Traffic Analysis
TLS is susceptible to a variety of traffic analysis attacks based on
observing the length and timing of encrypted packets [CLINIC]
[HCJ16]. This is particularly easy when there is a small set of
possible messages to be distinguished, such as for a video server
hosting a fixed corpus of content, but still provides usable
information even in more complicated scenarios.
Rescorla Expires August 19, 2018 [Page 144]
Internet-Draft TLS February 2018
TLS does not provide any specific defenses against this form of
attack but does include a padding mechanism for use by applications:
The plaintext protected by the AEAD function consists of content plus
variable-length padding, which allows the application to produce
arbitrary length encrypted records as well as padding-only cover
traffic to conceal the difference between periods of transmission and
periods of silence. Because the padding is encrypted alongside the
actual content, an attacker cannot directly determine the length of
the padding, but may be able to measure it indirectly by the use of
timing channels exposed during record processing (i.e., seeing how
long it takes to process a record or trickling in records to see
which ones elicit a response from the server). In general, it is not
known how to remove all of these channels because even a constant
time padding removal function will likely feed the content into data-
dependent functions. At minimum, a fully constant time server or
client would require close cooperation with the application layer
protocol implementation, including making that higher level protocol
constant time.
Note: Robust traffic analysis defences will likely lead to inferior
performance due to delay in transmitting packets and increased
traffic volume.
E.4. Side Channel Attacks
In general, TLS does not have specific defenses against side-channel
attacks (i.e., those which attack the communications via secondary
channels such as timing) leaving those to the implementation of the
relevant cryptographic primitives. However, certain features of TLS
are designed to make it easier to write side-channel resistant code:
- Unlike previous versions of TLS which used a composite MAC-then-
encrypt structure, TLS 1.3 only uses AEAD algorithms, allowing
implementations to use self-contained constant-time
implementations of those primitives.
- TLS uses a uniform "bad_record_mac" alert for all decryption
errors, which is intended to prevent an attacker from gaining
piecewise insight into portions of the message. Additional
resistance is provided by terminating the connection on such
errors; a new connection will have different cryptographic
material, preventing attacks against the cryptographic primitives
that require multiple trials.
Information leakage through side channels can occur at layers above
TLS, in application protocols and the applications that use them.
Resistance to side-channel attacks depends on applications and
Rescorla Expires August 19, 2018 [Page 145]
Internet-Draft TLS February 2018
application protocols separately ensuring that confidential
information is not inadvertently leaked.
E.5. Replay Attacks on 0-RTT
Replayable 0-RTT data presents a number of security threats to TLS-
using applications, unless those applications are specifically
engineered to be safe under replay (minimally, this means idempotent,
but in many cases may also require other stronger conditions, such as
constant-time response). Potential attacks include:
- Duplication of actions which cause side effects (e.g., purchasing
an item or transferring money) to be duplicated, thus harming the
site or the user.
- Attackers can store and replay 0-RTT messages in order to re-order
them with respect to other messages (e.g., moving a delete to
after a create).
- Exploiting cache timing behavior to discover the content of 0-RTT
messages by replaying a 0-RTT message to a different cache node
and then using a separate connection to measure request latency,
to see if the two requests address the same resource.
If data can be replayed a large number of times, additional attacks
become possible, such as making repeated measurements of the the
speed of cryptographic operations. In addition, they may be able to
overload rate-limiting systems. For further description of these
attacks, see [Mac17].
Ultimately, servers have the responsibility to protect themselves
against attacks employing 0-RTT data replication. The mechanisms
described in Section 8 are intended to prevent replay at the TLS
layer but do not provide complete protection against receiving
multiple copies of client data. TLS 1.3 falls back to the 1-RTT
handshake when the server does not have any information about the
client, e.g., because it is in a different cluster which does not
share state or because the ticket has been deleted as described in
Section 8.1. If the application layer protocol retransmits data in
this setting, then it is possible for an attacker to induce message
duplication by sending the ClientHello to both the original cluster
(which processes the data immediately) and another cluster which will
fall back to 1-RTT and process the data upon application layer
replay. The scale of this attack is limited by the client's
willingness to retry transactions and therefore only allows a limited
amount of duplication, with each copy appearing as a new connection
at the server.
Rescorla Expires August 19, 2018 [Page 146]
Internet-Draft TLS February 2018
If implemented correctly, the mechanisms described in Section 8.1 and
Section 8.2 prevent a replayed ClientHello and its associated 0-RTT
data from being accepted multiple times by any cluster with
consistent state; for servers which limit the use of 0-RTT to one
cluster for a single ticket, then a given ClientHello and its
associated 0-RTT data will only be accepted once. However, if state
is not completely consistent, then an attacker might be able to have
multiple copies of the data be accepted during the replication
window. Because clients do not know the exact details of server
behavior, they MUST NOT send messages in early data which are not
safe to have replayed and which they would not be willing to retry
across multiple 1-RTT connections.
Application protocols MUST NOT use 0-RTT data without a profile that
defines its use. That profile needs to identify which messages or
interactions are safe to use with 0-RTT and how to handle the
situation when the server rejects 0-RTT and falls back to 1-RTT.
In addition, to avoid accidental misuse, TLS implementations MUST NOT
enable 0-RTT (either sending or accepting) unless specifically
requested by the application and MUST NOT automatically resend 0-RTT
data if it is rejected by the server unless instructed by the
application. Server-side applications may wish to implement special
processing for 0-RTT data for some kinds of application traffic
(e.g., abort the connection, request that data be resent at the
application layer, or delay processing until the handshake
completes). In order to allow applications to implement this kind of
processing, TLS implementations MUST provide a way for the
application to determine if the handshake has completed.
E.5.1. Replay and Exporters
Replays of the ClientHello produce the same early exporter, thus
requiring additional care by applications which use these exporters.
In particular, if these exporters are used as an authentication
channel binding (e.g., by signing the output of the exporter) an
attacker who compromises the PSK can transplant authenticators
between connections without compromising the authentication key.
In addition, the early exporter SHOULD NOT be used to generate
server-to-client encryption keys because that would entail the reuse
of those keys. This parallels the use of the early application
traffic keys only in the client-to-server direction.
Rescorla Expires August 19, 2018 [Page 147]
Internet-Draft TLS February 2018
E.6. Attacks on Static RSA
Although TLS 1.3 does not use RSA key transport and so is not
directly susceptible to Bleichenbacher-type attacks, if TLS 1.3
servers also support static RSA in the context of previous versions
of TLS, then it may be possible to impersonate the server for TLS 1.3
connections [JSS15]. TLS 1.3 implementations can prevent this attack
by disabling support for static RSA across all versions of TLS. In
principle, implementations might also be able to separate
certificates with different keyUsage bits for static RSA decryption
and RSA signature, but this technique relies on clients refusing to
accept signatures using keys in certificates that do not have the
digitalSignature bit set, and many clients do not enforce this
restriction.
Appendix F. Working Group Information
The discussion list for the IETF TLS working group is located at the
e-mail address tls@ietf.org [1]. Information on the group and
information on how to subscribe to the list is at
https://www.ietf.org/mailman/listinfo/tls
Archives of the list can be found at: https://www.ietf.org/mail-
archive/web/tls/current/index.html
Appendix G. Contributors
- Martin Abadi
University of California, Santa Cruz
abadi@cs.ucsc.edu
- Christopher Allen (co-editor of TLS 1.0)
Alacrity Ventures
ChristopherA@AlacrityManagement.com
- Richard Barnes
Cisco
rlb@ipv.sx
- Steven M. Bellovin
Columbia University
smb@cs.columbia.edu
- David Benjamin
Google
davidben@google.com
- Benjamin Beurdouche
Rescorla Expires August 19, 2018 [Page 148]
Internet-Draft TLS February 2018
INRIA & Microsoft Research
benjamin.beurdouche@ens.fr
- Karthikeyan Bhargavan (co-author of [RFC7627])
INRIA
karthikeyan.bhargavan@inria.fr
- Simon Blake-Wilson (co-author of [RFC4492])
BCI
sblakewilson@bcisse.com
- Nelson Bolyard (co-author of [RFC4492])
Sun Microsystems, Inc.
nelson@bolyard.com
- Ran Canetti
IBM
canetti@watson.ibm.com
- Matt Caswell
OpenSSL
matt@openssl.org
- Stephen Checkoway
University of Illinois at Chicago
sfc@uic.edu
- Pete Chown
Skygate Technology Ltd
pc@skygate.co.uk
- Katriel Cohn-Gordon
University of Oxford
me@katriel.co.uk
- Cas Cremers
University of Oxford
cas.cremers@cs.ox.ac.uk
- Antoine Delignat-Lavaud (co-author of [RFC7627])
INRIA
antoine.delignat-lavaud@inria.fr
- Tim Dierks (co-editor of TLS 1.0, 1.1, and 1.2)
Independent
tim@dierks.org
- Taher Elgamal
Rescorla Expires August 19, 2018 [Page 149]
Internet-Draft TLS February 2018
Securify
taher@securify.com
- Pasi Eronen
Nokia
pasi.eronen@nokia.com
- Cedric Fournet
Microsoft
fournet@microsoft.com
- Anil Gangolli
anil@busybuddha.org
- David M. Garrett
dave@nulldereference.com
- Illya Gerasymchuk
Independent
illya@iluxonchik.me
- Alessandro Ghedini
Cloudflare Inc.
alessandro@cloudflare.com
- Daniel Kahn Gillmor
ACLU
dkg@fifthhorseman.net
- Matthew Green
Johns Hopkins University
mgreen@cs.jhu.edu
- Jens Guballa
ETAS
jens.guballa@etas.com
- Felix Guenther
TU Darmstadt
mail@felixguenther.info
- Vipul Gupta (co-author of [RFC4492])
Sun Microsystems Laboratories
vipul.gupta@sun.com
- Chris Hawk (co-author of [RFC4492])
Corriente Networks LLC
chris@corriente.net
Rescorla Expires August 19, 2018 [Page 150]
Internet-Draft TLS February 2018
- Kipp Hickman
- Alfred Hoenes
- David Hopwood
Independent Consultant
david.hopwood@blueyonder.co.uk
- Marko Horvat
MPI-SWS
mhorvat@mpi-sws.org
- Jonathan Hoyland
Royal Holloway, University of London
- Subodh Iyengar
Facebook
subodh@fb.com
- Benjamin Kaduk
Akamai
kaduk@mit.edu
- Hubert Kario
Red Hat Inc.
hkario@redhat.com
- Phil Karlton (co-author of SSL 3.0)
- Leon Klingele
Independent
mail@leonklingele.de
- Paul Kocher (co-author of SSL 3.0)
Cryptography Research
paul@cryptography.com
- Hugo Krawczyk
IBM
hugokraw@us.ibm.com
- Adam Langley (co-author of [RFC7627])
Google
agl@google.com
- Olivier Levillain
ANSSI
olivier.levillain@ssi.gouv.fr
Rescorla Expires August 19, 2018 [Page 151]
Internet-Draft TLS February 2018
- Xiaoyin Liu
University of North Carolina at Chapel Hill
xiaoyin.l@outlook.com
- Ilari Liusvaara
Independent
ilariliusvaara@welho.com
- Atul Luykx
K.U. Leuven
atul.luykx@kuleuven.be
- Colm MacCarthaigh
Amazon Web Services
colm@allcosts.net
- Carl Mehner
USAA
carl.mehner@usaa.com
- Jan Mikkelsen
Transactionware
janm@transactionware.com
- Bodo Moeller (co-author of [RFC4492])
Google
bodo@openssl.org
- Kyle Nekritz
Facebook
knekritz@fb.com
- Erik Nygren
Akamai Technologies
erik+ietf@nygren.org
- Magnus Nystrom
Microsoft
mnystrom@microsoft.com
- Kazuho Oku
DeNA Co., Ltd.
kazuhooku@gmail.com
- Kenny Paterson
Royal Holloway, University of London
kenny.paterson@rhul.ac.uk
Rescorla Expires August 19, 2018 [Page 152]
Internet-Draft TLS February 2018
- Alfredo Pironti (co-author of [RFC7627])
INRIA
alfredo.pironti@inria.fr
- Andrei Popov
Microsoft
andrei.popov@microsoft.com
- Marsh Ray (co-author of [RFC7627])
Microsoft
maray@microsoft.com
- Robert Relyea
Netscape Communications
relyea@netscape.com
- Kyle Rose
Akamai Technologies
krose@krose.org
- Jim Roskind
Amazon
jroskind@amazon.com
- Michael Sabin
- Joe Salowey
Tableau Software
joe@salowey.net
- Rich Salz
Akamai
rsalz@akamai.com
- David Schinazi
Apple Inc.
dschinazi@apple.com
- Sam Scott
Royal Holloway, University of London
me@samjs.co.uk
- Dan Simon
Microsoft, Inc.
dansimon@microsoft.com
- Brian Smith
Independent
Rescorla Expires August 19, 2018 [Page 153]
Internet-Draft TLS February 2018
brian@briansmith.org
- Brian Sniffen
Akamai Technologies
ietf@bts.evenmere.org
- Nick Sullivan
Cloudflare Inc.
nick@cloudflare.com
- Bjoern Tackmann
University of California, San Diego
btackmann@eng.ucsd.edu
- Tim Taubert
Mozilla
ttaubert@mozilla.com
- Martin Thomson
Mozilla
mt@mozilla.com
- Sean Turner
sn3rd
sean@sn3rd.com
- Steven Valdez
Google
svaldez@google.com
- Filippo Valsorda
Cloudflare Inc.
filippo@cloudflare.com
- Thyla van der Merwe
Royal Holloway, University of London
tjvdmerwe@gmail.com
- Victor Vasiliev
Google
vasilvv@google.com
- Tom Weinstein
- Hoeteck Wee
Ecole Normale Superieure, Paris
hoeteck@alum.mit.edu
Rescorla Expires August 19, 2018 [Page 154]
Internet-Draft TLS February 2018
- David Wong
NCC Group
david.wong@nccgroup.trust
- Christopher A. Wood
Apple Inc.
cawood@apple.com
- Tim Wright
Vodafone
timothy.wright@vodafone.com
- Peter Wu
Independent
peter@lekensteyn.nl
- Kazu Yamamoto
Internet Initiative Japan Inc.
kazu@iij.ad.jp
Author's Address
Eric Rescorla
RTFM, Inc.
EMail: ekr@rtfm.com
Rescorla Expires August 19, 2018 [Page 155]