The Transport Layer Security (TLS) Protocol Version 1.3
draft-ietf-tls-tls13-13
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8446.
|
|
|---|---|---|---|
| Author | Eric Rescorla | ||
| Last updated | 2016-05-22 | ||
| Replaces | draft-ietf-tls-rfc5246-bis | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-tls-tls13-13
Network Working Group E. Rescorla
Internet-Draft RTFM, Inc.
Obsoletes: 5077, 5246, 5746 (if May 22, 2016
approved)
Updates: 4492, 6066, 6961 (if approved)
Intended status: Standards Track
Expires: November 23, 2016
The Transport Layer Security (TLS) Protocol Version 1.3
draft-ietf-tls-tls13-13
Abstract
This document specifies Version 1.3 of the Transport Layer Security
(TLS) protocol. The TLS protocol 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 November 23, 2016.
Copyright Notice
Copyright (c) 2016 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
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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 . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Conventions and Terminology . . . . . . . . . . . . . . . 5
1.2. Major Differences from TLS 1.2 . . . . . . . . . . . . . 6
2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3. Goals of This Document . . . . . . . . . . . . . . . . . . . 10
4. Presentation Language . . . . . . . . . . . . . . . . . . . . 11
4.1. Basic Block Size . . . . . . . . . . . . . . . . . . . . 11
4.2. Miscellaneous . . . . . . . . . . . . . . . . . . . . . . 11
4.3. Vectors . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.4. Numbers . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.5. Enumerateds . . . . . . . . . . . . . . . . . . . . . . . 13
4.6. Constructed Types . . . . . . . . . . . . . . . . . . . . 14
4.6.1. Variants . . . . . . . . . . . . . . . . . . . . . . 14
4.7. Constants . . . . . . . . . . . . . . . . . . . . . . . . 15
4.8. Cryptographic Attributes . . . . . . . . . . . . . . . . 15
4.8.1. Digital Signing . . . . . . . . . . . . . . . . . . . 16
4.8.2. Authenticated Encryption with Additional Data (AEAD) 17
5. The TLS Record Protocol . . . . . . . . . . . . . . . . . . . 17
5.1. Connection States . . . . . . . . . . . . . . . . . . . . 18
5.2. Record Layer . . . . . . . . . . . . . . . . . . . . . . 20
5.2.1. Fragmentation . . . . . . . . . . . . . . . . . . . . 20
5.2.2. Record Payload Protection . . . . . . . . . . . . . . 22
5.2.3. Record Padding . . . . . . . . . . . . . . . . . . . 24
6. The TLS Handshaking Protocols . . . . . . . . . . . . . . . . 25
6.1. Alert Protocol . . . . . . . . . . . . . . . . . . . . . 26
6.1.1. Closure Alerts . . . . . . . . . . . . . . . . . . . 27
6.1.2. Error Alerts . . . . . . . . . . . . . . . . . . . . 29
6.2. Handshake Protocol Overview . . . . . . . . . . . . . . . 32
6.2.1. Incorrect DHE Share . . . . . . . . . . . . . . . . . 35
6.2.2. Resumption and Pre-Shared Key (PSK) . . . . . . . . . 36
6.2.3. Zero-RTT Data . . . . . . . . . . . . . . . . . . . . 38
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6.3. Handshake Protocol . . . . . . . . . . . . . . . . . . . 39
6.3.1. Key Exchange Messages . . . . . . . . . . . . . . . . 40
6.3.2. Hello Extensions . . . . . . . . . . . . . . . . . . 46
6.3.3. Server Parameters . . . . . . . . . . . . . . . . . . 60
6.3.4. Authentication Messages . . . . . . . . . . . . . . . 63
6.3.5. Post-Handshake Messages . . . . . . . . . . . . . . . 71
7. Cryptographic Computations . . . . . . . . . . . . . . . . . 74
7.1. Key Schedule . . . . . . . . . . . . . . . . . . . . . . 74
7.2. Updating Traffic Keys and IVs . . . . . . . . . . . . . . 76
7.3. Traffic Key Calculation . . . . . . . . . . . . . . . . . 76
7.3.1. Diffie-Hellman . . . . . . . . . . . . . . . . . . . 77
7.3.2. Elliptic Curve Diffie-Hellman . . . . . . . . . . . . 78
7.3.3. Exporters . . . . . . . . . . . . . . . . . . . . . . 78
8. Mandatory Algorithms . . . . . . . . . . . . . . . . . . . . 79
8.1. MTI Cipher Suites . . . . . . . . . . . . . . . . . . . . 79
8.2. MTI Extensions . . . . . . . . . . . . . . . . . . . . . 79
9. Application Data Protocol . . . . . . . . . . . . . . . . . . 80
10. Security Considerations . . . . . . . . . . . . . . . . . . . 80
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 80
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 83
12.1. Normative References . . . . . . . . . . . . . . . . . . 83
12.2. Informative References . . . . . . . . . . . . . . . . . 86
Appendix A. Protocol Data Structures and Constant Values . . . . 92
A.1. Record Layer . . . . . . . . . . . . . . . . . . . . . . 92
A.2. Alert Messages . . . . . . . . . . . . . . . . . . . . . 92
A.3. Handshake Protocol . . . . . . . . . . . . . . . . . . . 94
A.3.1. Key Exchange Messages . . . . . . . . . . . . . . . . 94
A.3.2. Server Parameters Messages . . . . . . . . . . . . . 98
A.3.3. Authentication Messages . . . . . . . . . . . . . . . 99
A.3.4. Ticket Establishment . . . . . . . . . . . . . . . . 99
A.4. Cipher Suites . . . . . . . . . . . . . . . . . . . . . . 100
A.4.1. Unauthenticated Operation . . . . . . . . . . . . . . 105
A.5. The Security Parameters . . . . . . . . . . . . . . . . . 105
A.6. Changes to RFC 4492 . . . . . . . . . . . . . . . . . . . 106
Appendix B. Implementation Notes . . . . . . . . . . . . . . . . 107
B.1. Random Number Generation and Seeding . . . . . . . . . . 107
B.2. Certificates and Authentication . . . . . . . . . . . . . 107
B.3. Cipher Suite Support . . . . . . . . . . . . . . . . . . 107
B.4. Implementation Pitfalls . . . . . . . . . . . . . . . . . 107
B.5. Client Tracking Prevention . . . . . . . . . . . . . . . 109
Appendix C. Backward Compatibility . . . . . . . . . . . . . . . 109
C.1. Negotiating with an older server . . . . . . . . . . . . 110
C.2. Negotiating with an older client . . . . . . . . . . . . 111
C.3. Zero-RTT backwards compatibility . . . . . . . . . . . . 111
C.4. Backwards Compatibility Security Restrictions . . . . . . 111
Appendix D. Security Analysis . . . . . . . . . . . . . . . . . 112
D.1. Handshake Protocol . . . . . . . . . . . . . . . . . . . 113
D.1.1. Authentication and Key Exchange . . . . . . . . . . . 113
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D.1.2. Version Rollback Attacks . . . . . . . . . . . . . . 114
D.1.3. Detecting Attacks Against the Handshake Protocol . . 114
D.2. Protecting Application Data . . . . . . . . . . . . . . . 114
D.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 115
D.4. Final Notes . . . . . . . . . . . . . . . . . . . . . . . 115
Appendix E. Working Group Information . . . . . . . . . . . . . 115
Appendix F. Contributors . . . . . . . . . . . . . . . . . . . . 115
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 119
1. Introduction
DISCLAIMER: This is a WIP draft of TLS 1.3 and has not yet seen
significant security analysis.
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 the TLS protocol is to provide privacy and data
integrity between two communicating peers. The TLS protocol is
composed of two layers: the TLS Record Protocol and the TLS Handshake
Protocol. At the lowest level, layered on top of some reliable
transport protocol (e.g., TCP [RFC0793]), is the TLS Record Protocol.
The TLS Record Protocol provides connection security that has two
basic properties:
- The connection is private. Symmetric cryptography is used for
data encryption (e.g., AES [AES]). The keys for this symmetric
encryption are generated uniquely for each connection and are
based on a secret negotiated by another the TLS Handshake
Protocol.
- The connection is reliable. Messages include an authentication
tag which protects them against modification.
Note: The TLS Record Protocol can operate in an insecure mode but is
generally only used in this mode while another protocol is using the
TLS Record Protocol as a transport for negotiating security
parameters.
The TLS Record Protocol is used for encapsulation of various higher-
level protocols. One such encapsulated protocol, the TLS Handshake
Protocol, allows the server and client to authenticate each other and
to negotiate an encryption algorithm and cryptographic keys before
the application protocol transmits or receives its first byte of
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data. The TLS Handshake Protocol provides connection security that
has three basic properties:
- The peer's identity can be authenticated using asymmetric (public
key) cryptography (e.g., RSA [RSA], ECDSA [ECDSA]) or a pre-shared
symmetric key. The TLS server is always authenticated; client
authentication is optional.
- The negotiation of a shared secret is secure: the negotiated
secret is unavailable to eavesdroppers, and for any authenticated
connection the secret cannot be obtained, even by an attacker who
can place himself in the middle of the connection.
- The negotiation is reliable: no attacker can modify the
negotiation communication without being detected by the parties to
the communication.
One advantage of TLS is that it is application protocol independent.
Higher-level protocols can layer on top of the TLS protocol
transparently. The TLS standard, however, does not specify how
protocols add security with TLS; the decisions on 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.
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 RFC
2119 [RFC2119].
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 transactions.
peer: An endpoint. When discussing a particular endpoint, "peer"
refers to the endpoint that is remote to the primary subject of
discussion.
receiver: An endpoint that is receiving records.
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sender: An endpoint that is transmitting records.
session: An association between a client and a server resulting from
a handshake.
server: The endpoint which did not initiate the TLS connection.
1.2. Major Differences from TLS 1.2
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 EarlyDataIndiation
- Turn PSK-resumption response into an index to save room
- 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.
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- 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.
- 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.
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- 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.
- 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.
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- 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.
- 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.
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- Remove support for static RSA and DH key exchange.
- Remove support for non-AEAD ciphers.
2. Goals
The goals of the TLS protocol, in order of priority, are as follows:
1. Cryptographic security: TLS should be used to establish a secure
connection between two parties.
2. Interoperability: Independent programmers should be able to
develop applications utilizing TLS that can successfully exchange
cryptographic parameters without knowledge of one another's code.
3. Extensibility: TLS seeks to provide a framework into which new
public key and record protection methods can be incorporated as
necessary. This will also accomplish two sub-goals: preventing
the need to create a new protocol (and risking the introduction
of possible new weaknesses) and avoiding the need to implement an
entire new security library.
4. Relative efficiency: Cryptographic operations tend to be highly
CPU intensive, particularly public key operations. For this
reason, the TLS protocol has incorporated an optional session
caching scheme to reduce the number of connections that need to
be established from scratch. Additionally, care has been taken
to reduce network activity.
3. Goals of This Document
This document and the TLS protocol itself have evolved from the SSL
3.0 Protocol Specification as published by Netscape. The differences
between this version and previous versions are significant enough
that the various versions of TLS and SSL 3.0 do not interoperate
(although each protocol incorporates a mechanism by which an
implementation can back down to prior versions). This document is
intended primarily for readers who will be implementing the protocol
and for those doing cryptographic analysis of it. The specification
has been written with this in mind, and it is intended to reflect the
needs of those two groups. For that reason, many of the algorithm-
dependent data structures and rules are included in the body of the
text (as opposed to in an appendix), providing easier access to them.
This document is not intended to supply any details of service
definition or of interface definition, although it does cover select
areas of policy as they are required for the maintenance of solid
security.
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4. 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. The syntax draws from
several sources in its structure. Although it resembles the
programming language "C" in its syntax and XDR [RFC4506] in both its
syntax and intent, it would be risky to draw too many parallels. The
purpose of this presentation language is to document TLS only; it has
no general application beyond that particular goal.
4.1. 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.
4.2. 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.
4.3. 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];
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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 4.4). On the
other hand, 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 even multiple of the length of a single
element (for example, 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 */
4.4. 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 4.1 and are also unsigned. The
following numeric types are predefined.
uint8 uint16[2];
uint8 uint24[3];
uint8 uint32[4];
uint8 uint64[8];
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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.
Note that in some cases (e.g., DH parameters) it is necessary to
represent integers as opaque vectors. In such cases, they are
represented as unsigned integers (i.e., additional leading zero
octets are not used even if the most significant bit is set).
4.5. Enumerateds
An additional sparse data type is available called enum. A field of
type enum can only assume the values declared in the definition.
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;
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.
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
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 */
For enumerateds that are never converted to external representation,
the numerical information may be omitted.
enum { low, medium, high } Amount;
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4.6. 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]];
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.
Structure definitions may be embedded.
4.6.1. 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. There
must be a case arm for every element of the enumeration declared in
the select. Case arms have limited fall-through: if two case arms
follow in immediate succession with no fields in between, then they
both contain the same fields. Thus, in the example below, "orange"
and "banana" both contain V2. Note that this is a new piece of
syntax in TLS 1.2.
The body of the variant structure may be given a label for reference.
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;
case e2: Te2;
case e3: case e4: Te3;
....
case en: Ten;
} [[fv]];
} [[Tv]];
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For example:
enum { apple, orange, banana } VariantTag;
struct {
uint16 number;
opaque string<0..10>; /* variable length */
} V1;
struct {
uint32 number;
opaque string[10]; /* fixed length */
} V2;
struct {
select (VariantTag) { /* value of selector is implicit */
case apple:
V1; /* VariantBody, tag = apple */
case orange:
case banana:
V2; /* VariantBody, tag = orange or banana */
} variant_body; /* optional label on variant */
} VariantRecord;
4.7. Constants
Typed constants can be defined for purposes of specification by
declaring a symbol of the desired type and assigning values to it.
Under-specified types (opaque, variable-length vectors, and
structures that contain opaque) cannot be assigned values. No fields
of a multi-element structure or vector may be elided.
For example:
struct {
uint8 f1;
uint8 f2;
} Example1;
Example1 ex1 = {1, 4}; /* assigns f1 = 1, f2 = 4 */
4.8. Cryptographic Attributes
The two cryptographic operations -- digital signing, and
authenticated encryption with additional data (AEAD) -- are
designated digitally-signed, and aead-ciphered, respectively. A
field's cryptographic processing is specified by prepending an
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appropriate key word designation before the field's type
specification. Cryptographic keys are implied by the current session
state (see Section 5.1).
4.8.1. Digital Signing
A digitally-signed element is encoded as a struct DigitallySigned:
struct {
SignatureScheme algorithm;
opaque signature<0..2^16-1>;
} DigitallySigned;
The algorithm field specifies the algorithm used (see Section 6.3.2.2
for the definition of this field). The signature is a digital
signature using those algorithms over the contents of the element.
The contents themselves do not appear on the wire but are simply
calculated. The length of the signature is specified by the signing
algorithm and key.
In previous versions of TLS, the ServerKeyExchange format meant that
attackers can obtain a signature of a message with a chosen, 32-byte
prefix. Because TLS 1.3 servers are likely to also implement prior
versions, the contents of the element always start with 64 bytes of
octet 32 in order to clear that chosen-prefix.
Following that padding is a context string used to disambiguate
signatures for different purposes. The context string will be
specified whenever a digitally-signed element is used. A single 0
byte is appended to the context to act as a separator.
Finally, the specified contents of the digitally-signed structure
follow the 0 byte after the context string. (See the example at the
end of this section.)
The combined input is then fed into the corresponding signature
algorithm to produce the signature value on the wire. See
Section 6.3.2.2 for algorithms defined in this specification.
In the following example
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struct {
uint8 field1;
uint8 field2;
digitally-signed opaque {
uint8 field3<0..255>;
uint8 field4;
};
} UserType;
Assume that the context string for the signature was specified as
"Example". The input for the signature/hash algorithm would be:
2020202020202020202020202020202020202020202020202020202020202020
2020202020202020202020202020202020202020202020202020202020202020
4578616d706c6500
followed by the encoding of the inner struct (field3 and field4).
The length of the structure, in bytes, would be equal to two bytes
for field1 and field2, plus two bytes for the signature algorithm,
plus two bytes for the length of the signature, plus the length of
the output of the signing algorithm. The length of the signature is
known because the algorithm and key used for the signing are known
prior to encoding or decoding this structure.
4.8.2. Authenticated Encryption with Additional Data (AEAD)
In AEAD encryption, the plaintext is simultaneously encrypted and
integrity protected. The input may be of any length, and aead-
ciphered output is generally larger than the input in order to
accommodate the integrity check value.
5. The TLS 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 decrypted and verified, reassembled,
and then delivered to higher-level clients.
Three protocols that use the TLS Record Protocol are described in
this document: the TLS Handshake Protocol, the Alert Protocol, and
the application data protocol. In order to allow extension of the
TLS protocol, additional record content types can be supported by the
TLS Record Protocol. New record content type values are assigned by
IANA in the TLS Content Type Registry as described in Section 11.
Implementations MUST NOT send record types not defined in this
document unless negotiated by some extension. If a TLS
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implementation receives an unexpected record type, it MUST send an
"unexpected_message" alert.
Any protocol designed for use over TLS must be carefully designed to
deal with all possible attacks against it. As a practical matter,
this means that the protocol designer must be aware of what security
properties TLS does and does not provide and cannot safely rely on
the latter.
Note in particular that the length of a record or absence of traffic
itself is not protected by encryption unless the sender uses the
supplied padding mechanism - see Section 5.2.3 for more details.
5.1. Connection States
[[TODO: I plan to totally rewrite or remove this. IT seems like just
cruft.]]
A TLS connection state is the operating environment of the TLS Record
Protocol. It specifies a record protection algorithm and its
parameters as well as the record protection keys and IVs for the
connection in both the read and the write directions. The security
parameters are set by the TLS Handshake Protocol, which also
determines when new cryptographic keys are installed and used for
record protection. The initial current state always specifies that
records are not protected.
The security parameters for a TLS Connection read and write state are
set by providing the following values:
connection end
Whether this entity is considered the "client" or the "server" in
this connection.
Hash algorithm
An algorithm used to generate keys from the appropriate secret
(see Section 7.1 and Section 7.3).
record protection algorithm
The algorithm to be used for record protection. This algorithm
must be of the AEAD type and thus provides integrity and
confidentiality as a single primitive. This specification
includes the key size of this algorithm and of the nonce for the
AEAD algorithm.
master secret
A 48-byte secret shared between the two peers in the connection
and used to generate keys for protecting data.
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client random
A 32-byte value provided by the client.
server random
A 32-byte value provided by the server.
These parameters are defined in the presentation language as:
enum { server, client } ConnectionEnd;
enum { tls_kdf_sha256, tls_kdf_sha384 } KDFAlgorithm;
enum { aes_gcm } RecordProtAlgorithm;
/* The algorithms specified in KDFAlgorithm and
RecordProtAlgorithm may be added to. */
struct {
ConnectionEnd entity;
KDFAlgorithm kdf_algorithm;
RecordProtAlgorithm record_prot_algorithm;
uint8 enc_key_length;
uint8 iv_length;
opaque hs_master_secret[48];
opaque master_secret[48];
opaque client_random[32];
opaque server_random[32];
} SecurityParameters;
[TODO: update this to handle new key hierarchy.]
The connection state will use the security parameters to generate the
following four items:
client write key
server write key
client write iv
server write iv
The client write parameters are used by the server when receiving and
processing records and vice versa. The algorithm used for generating
these items from the security parameters is described in Section 7.3.
Once the security parameters have been set and the keys have been
generated, the connection states can be instantiated by making them
the current states. These current states MUST be updated for each
record processed. Each connection state includes the following
elements:
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cipher state
The current state of the encryption algorithm. This will consist
of the scheduled key for that connection.
sequence number
Each connection state contains a sequence number, which is
maintained separately for read and write states. The sequence
number is set to zero at the beginning of a connection, and
whenever the key is changed. The sequence number is incremented
after each record: specifically, the first record transmitted
under a particular connection state and record key MUST use
sequence number 0. Sequence numbers are of type uint64 and MUST
NOT exceed 2^64-1. Sequence numbers do not wrap. If a TLS
implementation would need to wrap a sequence number, it MUST
either rekey (Section 6.3.5.3) or terminate the connection.
5.2. Record Layer
The TLS record layer receives uninterpreted data from higher layers
in non-empty blocks of arbitrary size.
5.2.1. Fragmentation
The record layer fragments information blocks into TLSPlaintext
records carrying data in chunks of 2^14 bytes or less. Message
boundaries are not preserved in the record layer (i.e., multiple
messages of the same ContentType MAY be coalesced into a single
TLSPlaintext record, or a single message MAY be fragmented across
several records). Alert messages (Section 6.1) MUST NOT be
fragmented across records.
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struct {
uint8 major;
uint8 minor;
} ProtocolVersion;
enum {
alert(21),
handshake(22),
application_data(23)
(255)
} ContentType;
struct {
ContentType type;
ProtocolVersion record_version = { 3, 1 }; /* TLS v1.x */
uint16 length;
opaque fragment[TLSPlaintext.length];
} TLSPlaintext;
type
The higher-level protocol used to process the enclosed fragment.
record_version
The protocol version the current record is compatible with. This
value MUST be set to { 3, 1 } for all records. This field is
deprecated and MUST be ignored for all purposes.
length
The length (in bytes) of the following TLSPlaintext.fragment. The
length MUST NOT exceed 2^14.
fragment
The application data. This data is transparent and treated as an
independent block to be dealt with by the higher-level protocol
specified by the type field.
This document describes TLS Version 1.3, which uses the version { 3,
4 }. The version value 3.4 is historical, deriving from the use of {
3, 1 } for TLS 1.0 and { 3, 0 } for SSL 3.0. In order to maximize
backwards compatibility, the record layer version identifies as
simply TLS 1.0. Endpoints supporting other versions negotiate the
version to use by following the procedure and requirements in
Appendix C.
Implementations MUST NOT send zero-length fragments of Handshake or
Alert types, even if those fragments contain padding. Zero-length
fragments of Application data MAY be sent as they are potentially
useful as a traffic analysis countermeasure.
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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.
5.2.2. 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 a unified encryption
and authentication operation which turns plaintext into authenticated
ciphertext and back again.
AEAD ciphers 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 and in TLS 1.3 the
additional data input is empty (zero length).
struct {
ContentType opaque_type = application_data(23); /* see fragment.type */
ProtocolVersion record_version = { 3, 1 }; /* TLS v1.x */
uint16 length;
aead-ciphered struct {
opaque content[TLSPlaintext.length];
ContentType type;
uint8 zeros[length_of_padding];
} fragment;
} TLSCiphertext;
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 versions of TLS.
The actual content type of the record is found in fragment.type
after decryption.
record_version
The record_version field is identical to
TLSPlaintext.record_version and is always { 3, 1 }. Note that the
handshake protocol including the ClientHello and ServerHello
messages authenticates the protocol version, so this value is
redundant.
length
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The length (in bytes) of the following TLSCiphertext.fragment.
The length MUST NOT exceed 2^14 + 256. An endpoint that receives
a record that exceeds this length MUST generate a fatal
"record_overflow" alert.
fragment.content
The cleartext of TLSPlaintext.fragment.
fragment.type
The actual content type of the record.
fragment.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 5.2.3 for more
details.
fragment
The AEAD encrypted form of TLSPlaintext.fragment +
TLSPlaintext.type + zeros, where "+" denotes concatenation.
The length of the per-record nonce (iv_length) is set to max(8 bytes,
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 padded to the left with
zeroes 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.
Note: This is a different construction from that in TLS 1.2, which
specified a partially explicit nonce.
The plaintext is the concatenation of TLSPlaintext.fragment and
TLSPlaintext.type.
The AEAD output consists of the ciphertext output by the AEAD
encryption operation. The length of the plaintext is greater than
TLSPlaintext.length due to the inclusion of TLSPlaintext.type and
however much padding is supplied by the sender. The length of
aead_output will generally be larger than the plaintext, but by an
amount that varies with the AEAD cipher. Since the ciphers might
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incorporate padding, the amount of overhead could vary with different
lengths of plaintext. Symbolically,
AEADEncrypted =
AEAD-Encrypt(write_key, nonce, plaintext of fragment)
In order to decrypt and verify, the cipher takes as input the key,
nonce, and the AEADEncrypted value. The output is either the
plaintext or an error indicating that the decryption failed. There
is no separate integrity check. That is:
plaintext of fragment =
AEAD-Decrypt(write_key, nonce, AEADEncrypted)
If the decryption fails, a fatal "bad_record_mac" alert MUST be
generated.
An AEAD cipher MUST NOT produce an expansion of greater than 255
bytes. An endpoint that receives a record from its peer with
TLSCipherText.length larger than 2^14 + 256 octets MUST generate a
fatal "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.
5.2.3. 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 fragment.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 fragment.content.
The padding sent is automatically verified by the record protection
mechanism: Upon successful decryption of a TLSCiphertext.fragment,
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
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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 should treat the record as
having an unexpected ContentType, sending an "unexpected_message"
alert.
The presence of padding does not change the overall record size
limitations - the full fragment plaintext may not exceed 2^14 octets.
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 atop TLS permits 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.
6. The TLS Handshaking Protocols
TLS has two subprotocols that are used to allow peers to agree upon
security parameters for the record layer, to authenticate themselves,
to instantiate negotiated security parameters, and to report error
conditions to each other.
The TLS Handshake Protocol is responsible for negotiating a session,
which consists of the following items:
peer certificate
X509v3 [RFC5280] certificate of the peer. This element of the
state may be null.
cipher spec
Specifies the authentication and key establishment algorithms, the
hash for use with HKDF to generate keying material, and the record
protection algorithm (See Appendix A.5 for formal definition.)
resumption master secret
a secret shared between the client and server that can be used as
a pre-shared symmetric key (PSK) in future connections.
These items are then used to create security parameters for use by
the record layer when protecting application data. Many connections
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can be instantiated using the same session using a PSK established in
an initial handshake.
6.1. Alert Protocol
One of the content types supported by the TLS record layer is the
alert type. Alert messages convey the severity of the message
(warning or fatal) and a description of the alert. Alert messages
with a level of fatal result in the immediate termination of the
connection. In this case, other connections corresponding to the
session may continue, but the session identifier MUST be invalidated,
preventing the failed session from being used to establish new
connections. Like other messages, alert messages are encrypted as
specified by the current connection state.
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enum { warning(1), fatal(2), (255) } AlertLevel;
enum {
close_notify(0),
end_of_early_data(1),
unexpected_message(10), /* fatal */
bad_record_mac(20), /* fatal */
record_overflow(22), /* fatal */
handshake_failure(40), /* fatal */
bad_certificate(42),
unsupported_certificate(43),
certificate_revoked(44),
certificate_expired(45),
certificate_unknown(46),
illegal_parameter(47), /* fatal */
unknown_ca(48), /* fatal */
access_denied(49), /* fatal */
decode_error(50), /* fatal */
decrypt_error(51), /* fatal */
protocol_version(70), /* fatal */
insufficient_security(71), /* fatal */
internal_error(80), /* fatal */
inappropriate_fallback(86), /* fatal */
user_canceled(90),
missing_extension(109), /* fatal */
unsupported_extension(110), /* fatal */
certificate_unobtainable(111),
unrecognized_name(112),
bad_certificate_status_response(113), /* fatal */
bad_certificate_hash_value(114), /* fatal */
unknown_psk_identity(115),
(255)
} AlertDescription;
struct {
AlertLevel level;
AlertDescription description;
} Alert;
6.1.1. Closure Alerts
The client and the server must share knowledge that the connection is
ending in order to avoid a truncation attack. Failure to properly
close a connection does not prohibit a session from being resumed.
close_notify
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This alert notifies the recipient that the sender will not send
any more messages on this connection. Any data received after a
closure MUST be ignored.
end_of_early_data
This alert is sent by the client to indicate that all 0-RTT
application_data messages have been transmitted (or none will be
sent at all) and that this is the end of the flight. This alert
MUST be at the warning level. Servers MUST NOT send this alert
and clients receiving it MUST terminate the connection with an
"unexpected_message" alert.
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 by sending a "close_notify" alert.
Any data received after a closure alert is 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 the write
side of the connection, unless some other fatal alert has been
transmitted. The other party MUST respond with a "close_notify"
alert of its own and close down the connection immediately,
discarding any pending writes. The initiator of the close need not
wait for the responding "close_notify" alert before closing the 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 the responding
"close_notify" alert before indicating to the application layer that
the TLS connection has ended. If the application protocol will not
transfer any additional data, but will only close the underlying
transport connection, then the implementation MAY choose to close the
transport without waiting for the responding "close_notify". 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 a connection reliably delivers
pending data before destroying the transport.
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6.1.2. 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
immediately close the connection. Servers and clients MUST forget
any session-identifiers, keys, and secrets associated with a failed
connection. Thus, any connection terminated with a fatal alert MUST
NOT be resumed.
Whenever an implementation encounters a condition which is defined as
a fatal alert, it MUST send the appropriate alert prior to closing
the connection. For all errors where an alert level is not
explicitly specified, the sending party MAY determine at its
discretion whether to treat this as a fatal error or not. If the
implementation chooses to send an alert but intends to close the
connection immediately afterwards, it MUST send that alert at the
fatal alert level.
If an alert with a level of warning is sent and received, generally
the connection can continue normally. If the receiving party decides
not to proceed with the connection (e.g., after having received a
"user_canceled" alert that it is not willing to accept), it SHOULD
send a fatal alert to terminate the connection. Given this, the
sending peer cannot, in general, know how the receiving party will
behave. Therefore, warning alerts are not very useful when the
sending party wants to continue the connection, and thus are
sometimes omitted. For example, if a party decides to accept an
expired certificate (perhaps after confirming this with the user) and
wants to continue the connection, it would not generally send a
"certificate_expired" alert.
The following error alerts are defined:
unexpected_message
An inappropriate message was received. This alert is always fatal
and 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, this alert is used for all deprotection failures.
This alert is always fatal and should never be observed in
communication between proper implementations (except when messages
were corrupted in the network).
record_overflow
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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 is always fatal and should
never be observed in communication between proper implementations
(except when messages were corrupted in the network).
handshake_failure
Reception of a "handshake_failure" alert message indicates that
the sender was unable to negotiate an acceptable set of security
parameters given the options available. This alert is always
fatal.
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 out of range or inconsistent with
other fields. This alert is always fatal.
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 couldn't be matched with a known, trusted CA. This
alert is always fatal.
access_denied
A valid certificate or PSK was received, but when access control
was applied, the sender decided not to proceed with negotiation.
This alert is always fatal.
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 always fatal and should never be observed in
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communication between proper implementations (except when messages
were corrupted in the network).
decrypt_error
A handshake cryptographic operation failed, including being unable
to correctly verify a signature or validate a Finished message.
This alert is always fatal.
protocol_version
The protocol version the peer has attempted to negotiate is
recognized but not supported. (For example, old protocol versions
might be avoided for security reasons.) This alert is always
fatal.
insufficient_security
Returned instead of "handshake_failure" when a negotiation has
failed specifically because the server requires ciphers more
secure than those supported by the client. This alert is always
fatal.
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. This alert is always fatal.
inappropriate_fallback
Sent by a server in response to an invalid connection retry
attempt from a client. (see [RFC7507]) This alert is always fatal.
missing_extension
Sent by endpoints that receive a hello message not containing an
extension that is mandatory to send for the offered TLS version.
This message is always fatal. [[TODO: IANA Considerations.]]
unsupported_extension
Sent by endpoints receiving any hello message containing an
extension known to be prohibited for inclusion in the given hello
message, including any extensions in a ServerHello not first
offered in the corresponding ClientHello. This alert is always
fatal.
certificate_unobtainable
Sent by servers when unable to obtain a certificate from a URL
provided by the client via the "client_certificate_url" extension
[RFC6066].
unrecognized_name
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Sent by servers when no server exists identified by the name
provided by the client via the "server_name" extension [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
[RFC6066]. This alert is always fatal.
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 [RFC6066]. This alert is always fatal.
unknown_psk_identity
Sent by servers when a PSK cipher suite is selected 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.
New Alert values are assigned by IANA as described in Section 11.
6.2. Handshake Protocol Overview
The cryptographic parameters of the session state are produced by the
TLS Handshake Protocol, which operates on top of the TLS record
layer. When a TLS client and server first start communicating, they
agree on a protocol version, select cryptographic algorithms,
optionally authenticate each other, and establish shared secret
keying material.
TLS supports three basic key exchange modes:
- Diffie-Hellman (of both the finite field and elliptic curve
varieties).
- A pre-shared symmetric key (PSK)
- A combination of a symmetric key and Diffie-Hellman
Which mode is used depends on the negotiated cipher suite.
Conceptually, the handshake establishes three secrets which are used
to derive all the keys.
Figure 1 below shows the basic full TLS handshake.
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Client Server
Key ^ ClientHello
Exch | + key_share*
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 extensions sent in the
previously noted message.
* Indicates optional or situation-dependent
messages that are not always sent.
{} Indicates messages protected using keys
derived from handshake_traffic_secret.
[] Indicates messages protected using keys
derived from 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.
In the Key Exchange phase, the client sends the ClientHello
(Section 6.3.1.1) message, which contains a random nonce
(ClientHello.random), its offered protocol version, cipher suite, and
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extensions, and in general either one or more Diffie-Hellman key
shares (in the "key_share" extension Section 6.3.2.4), one or more
pre-shared key labels (in the "pre_shared_key" extension
Section 6.3.2.5), or both.
The server processes the ClientHello and determines the appropriate
cryptographic parameters for the connection. It then responds with
its own ServerHello which indicates the negotiated connection
parameters. [Section 6.3.1.2]. The combination of the ClientHello
and the ServerHello determines the values of ES and SS, as described
above. If either a pure (EC)DHE or (EC)DHE-PSK cipher suite is in
use, then the ServerHello will contain a "key_share" extension with
the server's ephemeral Diffie-Hellman share which MUST be in the same
group. If a pure PSK or an (EC)DHE-PSK cipher suite is negotiated,
then the ServerHello will contain a "pre_shared_key" extension
indicating which if the client's offered PSKs was selected.
The server then sends two messages to establish the Server
Parameters:
EncryptedExtensions responses to any extensions which are not
required in order to determine the cryptographic parameters.
[Section 6.3.3.1]
CertificateRequest if certificate-based client authentication is
desired, the desired parameters for that certificate. This
message will be omitted if client authentication is not desired.
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. This message is omitted if the
server is not authenticating with a certificate (i.e., with PSK or
(EC)DHE-PSK cipher suites). Note that if raw public keys
[RFC7250] or the cached information extension
[I-D.ietf-tls-cached-info] 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 6.3.4.1]
CertificateVerify
a signature over the entire handshake using the public key in the
Certificate message. This message is omitted if the server is not
authenticating via a certificate (i.e., with PSK or (EC)DHE-PSK
cipher suites). [Section 6.3.4.2]
Finished
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a MAC 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 6.3.4.3]
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
may exchange application layer data. Application data MUST NOT be
sent prior to sending the Finished message. 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.
[[TODO: Move this elsewhere? Note that higher layers should not be
overly reliant on whether TLS always negotiates the strongest
possible connection between two endpoints. There are a number of
ways in which a man-in-the-middle attacker can attempt to make two
entities drop down to the least secure method they support (i.e.,
perform a downgrade attack). The TLS protocol has been designed to
minimize this risk, but there are still attacks available: for
example, an attacker could block access to the port a secure service
runs on, or attempt to get the peers to negotiate an unauthenticated
connection. The fundamental rule is that higher levels must be
cognizant of what their security requirements are and never transmit
information over a channel less secure than what they require. The
TLS protocol is secure in that any cipher suite offers its promised
level of security: if you negotiate AES-GCM [GCM] with a 255-bit
ECDHE key exchange with a host whose certificate chain you have
verified, you can expect that to be reasonably "secure" against
algorithmic attacks, at least in the year 2015.]]
6.2.1. Incorrect DHE Share
If the client has not provided an appropriate "key_share" extension
(e.g. it includes only DHE or ECDHE groups unacceptable or
unsupported by the server), the server corrects the mismatch with a
HelloRetryRequest and the client will need 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 will
send a "handshake_failure" or "insufficient_security" fatal alert
(see Section 6.1).
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Client Server
ClientHello
+ key_share -------->
<-------- HelloRetryRequest
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 below.
6.2.2. Resumption and Pre-Shared Key (PSK)
Although TLS PSKs can be established out of band, PSKs can also be
established in a previous session and then reused ("session
resumption"). Once a handshake has completed, the server can send
the client a PSK identity which corresponds to a key derived from the
initial handshake (See Section 6.3.5.1). The client can then use
that PSK identity in future handshakes to negotiate use of the PSK;
if the server accepts it, then the security context of the original
connection is tied to the new connection. In TLS 1.2 and below, this
functionality was provided by "session resumption" and "session
tickets" [RFC5077]. Both mechanisms are obsoleted in TLS 1.3.
PSK cipher suites can either use PSK in combination with an (EC)DHE
exchange in order to provide forward secrecy in combination with
shared keys, or can use PSKs alone, at the cost of losing forward
secrecy.
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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. When a client offers resumption
via PSK it SHOULD also supply a "key_share" extension to the server
as well; this allows server to decline resumption and fall back to a
full handshake. A "key_share" extension MUST also be sent if the
client is attempting to negotiate an (EC)DHE-PSK cipher suite.
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6.2.3. Zero-RTT Data
When resuming via a PSK with an appropriate ticket (i.e., one with
the "allow_early_data" flag), clients can also send data on their
first flight ("early data"). This data is encrypted solely under
keys derived using the PSK as the static secret. As shown in
Figure 4, the Zero-RTT data is just added to the 1-RTT handshake in
the first flight, the rest of the handshake uses the same messages.
Client Server
ClientHello
+ early_data
+ key_share*
(EncryptedExtensions)
(Finished)
(Application Data*)
(end_of_early_data) -------->
ServerHello
+ early_data
+ key_share
{EncryptedExtensions}
{CertificateRequest*}
{Finished}
<-------- [Application Data*]
{Certificate*}
{CertificateVerify*}
{Finished} -------->
[Application Data] <-------> [Application Data]
* Indicates optional or situation-dependent
messages that are not always sent.
() Indicates messages protected using keys
derived from early_traffic_secret.
{} Indicates messages protected using keys
derived from handshake_traffic_secret.
[] Indicates messages protected using keys
derived from traffic_secret_N
Figure 4: Message flow for a zero round trip handshake
[[OPEN ISSUE: Should it be possible to combine 0-RTT with the server
authenticating via a signature https://github.com/tlswg/tls13-spec/
issues/443]]
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IMPORTANT NOTE: The security properties for 0-RTT data (regardless of
the cipher suite) are weaker than those for other kinds of TLS data.
Specifically:
1. This data is not forward secret, because it is encrypted solely
with the PSK.
2. There are no guarantees of non-replay between connections.
Unless the server takes special measures outside those provided
by TLS (See Section 6.3.2.7.2), the server has no guarantee that
the same 0-RTT data was not transmitted on multiple 0-RTT
connections. This is especially relevant if the data is
authenticated either with TLS client authentication or inside the
application layer protocol. However, 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.)
The contents and significance of each message will be presented in
detail in the following sections.
6.3. Handshake Protocol
The TLS Handshake Protocol is one of the defined higher-level clients
of the TLS Record Protocol. This protocol is used to negotiate the
secure attributes of a session. 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 session state.
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enum {
client_hello(1),
server_hello(2),
session_ticket(4),
hello_retry_request(6),
encrypted_extensions(8),
certificate(11),
certificate_request(13),
certificate_verify(15),
finished(20),
key_update(24),
(255)
} HandshakeType;
struct {
HandshakeType msg_type; /* handshake type */
uint24 length; /* bytes in message */
select (HandshakeType) {
case client_hello: ClientHello;
case server_hello: ServerHello;
case hello_retry_request: HelloRetryRequest;
case encrypted_extensions: EncryptedExtensions;
case certificate_request: CertificateRequest;
case certificate: Certificate;
case certificate_verify: CertificateVerify;
case finished: Finished;
case session_ticket: NewSessionTicket;
case key_update: KeyUpdate;
} body;
} Handshake;
The TLS Handshake Protocol messages are presented below in the order
they MUST be sent; sending handshake messages in an unexpected order
results in an "unexpected_message" fatal error. Unneeded handshake
messages can be omitted, however.
New handshake message types are assigned by IANA as described in
Section 11.
6.3.1. Key Exchange Messages
The key exchange messages are used to exchange security capabilities
between the client and server and to establish the traffic keys used
to protect the handshake and the data.
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6.3.1.1. Client Hello
When this message will be sent:
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 ServerHello that selects cryptographic parameters that don't
match the client's "key_share" extension. In that case, the
client MUST send the same ClientHello (without modification)
except including a new KeyShareEntry as the lowest priority share
(i.e., appended to the list of shares in the "key_share"
extension). If a server receives a ClientHello at any other time,
it MUST send a fatal "unexpected_message" alert and close the
connection.
Structure of this message:
The ClientHello message includes a random structure, which is used
later in the protocol.
The cipher suite list, passed from the client to the server in the
ClientHello message, contains the combinations of cryptographic
algorithms supported by the client in order of the client's
preference (favorite choice first). Each cipher suite defines a key
exchange algorithm, a record protection algorithm (including secret
key length) and a hash to be used with HKDF. The server will select
a cipher suite or, if no acceptable choices are presented, return a
"handshake_failure" alert and close the connection. If the list
contains cipher suites the server does not recognize, support, or
wish to use, the server MUST ignore those cipher suites, and process
the remaining ones as usual.
struct {
opaque random_bytes[32];
} Random;
uint8 CipherSuite[2]; /* Cryptographic suite selector */
struct {
ProtocolVersion client_version = { 3, 4 }; /* TLS v1.3 */
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<0..2^16-1>;
} ClientHello;
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TLS allows extensions to follow the compression_methods field in an
extensions block. The presence of extensions can be detected by
determining whether there are bytes following the compression_methods
at the end of the ClientHello. Note that this method of detecting
optional data differs from the normal TLS method of having a
variable-length field, but it is used for compatibility with TLS
before extensions were defined. As of TLS 1.3, all clients and
servers will send at least one extension (at least "key_share" or
"pre_shared_key").
client_version
The version of the TLS protocol by which the client wishes to
communicate during this session. This SHOULD be the latest
(highest valued) version supported by the client. For this
version of the specification, the version will be { 3, 4 }. (See
Appendix C for details about backward compatibility.)
random
32 bytes generated by a secure random number generator. See
Appendix B 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 6.2.2). This field MUST be ignored by a server
negotiating TLS 1.3 and SHOULD be set as a zero length vector
(i.e., a single zero byte length field) by clients which do not
have a cached session ID set by a pre-TLS 1.3 server.
cipher_suites
This is a list of the cryptographic options supported by the
client, with the client's first preference first. Values are
defined in Appendix A.4.
legacy_compression_methods
Versions of TLS before 1.3 supported compression and the list of
compression methods was supplied in this field. For any 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 generate a fatal
"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.
extensions
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Clients request extended functionality from servers by sending
data in the extensions field. The actual "Extension" format is
defined in Section 6.3.2.
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. Note: TLS 1.3 ClientHello messages
MUST always contain extensions, and a TLS 1.3 server MUST respond to
any TLS 1.3 ClientHello without extensions with a fatal
"decode_error" alert. TLS 1.3 servers may receive TLS 1.2
ClientHello messages without extensions. If negotiating TLS 1.2, a
server MUST check that the amount of data in the message precisely
matches one of these formats; if not, then it MUST send a fatal
"decode_error" alert.
After sending the ClientHello message, the client waits for a
ServerHello or HelloRetryRequest message.
6.3.1.2. Server Hello
When this message will be sent:
The server will send this message in response to a ClientHello
message when it was able to find an acceptable set of algorithms
and the client's "key_share" extension was acceptable. If the
client proposed groups are not acceptable by the server, it will
respond with a "handshake_failure" fatal alert.
Structure of this message:
struct {
ProtocolVersion server_version;
Random random;
CipherSuite cipher_suite;
Extension extensions<0..2^16-1>;
} ServerHello;
In prior versions of TLS, the extensions field could be omitted
entirely if not needed, similar to ClientHello. As of TLS 1.3, all
clients and servers will send at least one extension (at least
"key_share" or "pre_shared_key").
server_version
This field will contain the lower of that suggested by the client
in the ClientHello and the highest supported by the server. For
this version of the specification, the version is { 3, 4 }. (See
Appendix C for details about backward compatibility.)
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random
This structure is generated by the server and MUST be generated
independently of the ClientHello.random.
cipher_suite
The single cipher suite selected by the server from the list in
ClientHello.cipher_suites. For resumed sessions, this field is
the value from the state of the session being resumed. [[TODO:
interaction with PSK.]]
extensions
A list of extensions. Note that only extensions offered by the
client can appear in the server's list. In TLS 1.3 as opposed to
previous versions of TLS, the server's extensions are split
between the ServerHello and the EncryptedExtensions
Section 6.3.3.1 message. The ServerHello MUST only include
extensions which are required to establish the cryptographic
context. Currently the only such extensions are "key_share",
"pre_shared_key", and "early_data". Clients MUST check the
ServerHello for the presence of any forbidden extensions and if
any are found MUST terminate the handshake with a
"illegal_parameter" alert.
TLS 1.3 server implementations which respond to a ClientHello with a
client_version indicating TLS 1.2 or below MUST set the first eight
bytes of their Random value to the bytes:
44 4F 57 4E 47 52 44 01
TLS 1.2 server implementations which respond to a ClientHello with a
client_version indicating TLS 1.1 or below SHOULD set the first eight
bytes of their Random value to the bytes:
44 4F 57 4E 47 52 44 00
TLS 1.3 clients receiving a TLS 1.2 or below ServerHello MUST check
that the top eight octets are not equal to either of these values.
TLS 1.2 clients SHOULD also perform this check if the ServerHello
indicates TLS 1.1 or below. If a match is found, the client MUST
abort the handshake with a fatal "illegal_parameter" alert. This
mechanism provides limited protection against downgrade attacks over
and above that provided by the Finished exchange: because the
ServerKeyExchange includes a signature over both random values, it is
not possible for an active attacker to modify the randoms without
detection as long as ephemeral ciphers are used. It does not provide
downgrade protection when static RSA is used.
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Note: This is an update to TLS 1.2 so in practice many TLS 1.2
clients and servers will not behave as specified above.
Note: Versions of TLS prior to TLS 1.3 used the top 32 bits of the
Random value to encode the time since the UNIX epoch. The sentinel
value above was selected to avoid conflicting with any valid TLS 1.2
Random value and to have a low (2^{-64}) probability of colliding
with randomly selected Random values.
6.3.1.3. Hello Retry Request
When this message will be sent:
Servers send this message in response to a ClientHello message
when it was able to find an acceptable set of algorithms and
groups that are mutually supported, but the client's KeyShare did
not contain an acceptable offer. If it cannot find such a match,
it will respond with a fatal "handshake_failure" alert.
Structure of this message:
struct {
ProtocolVersion server_version;
CipherSuite cipher_suite;
NamedGroup selected_group;
Extension extensions<0..2^16-1>;
} HelloRetryRequest;
selected_group
The mutually supported group the server intends to negotiate and
is requesting a retried ClientHello/KeyShare for.
The server_version, cipher_suite, and extensions fields have the same
meanings as their corresponding values in the ServerHello. 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.
Upon receipt of a HelloRetryRequest, the client MUST first verify
that the selected_group field corresponds to a group which was
provided in the "supported_groups" extension in the original
ClientHello. It MUST then verify that 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 a fatal
"handshake_failure" alert. Clients SHOULD also abort with
"handshake_failure" in response to any second HelloRetryRequest which
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was sent in the same connection (i.e., where the ClientHello was
itself in response to a HelloRetryRequest).
Otherwise, the client MUST send a ClientHello with an updated
KeyShare extension to the server. The client MUST append a new
KeyShareEntry for the group indicated in the selected_group field to
the groups in its original KeyShare.
Upon re-sending the ClientHello and receiving the server's
ServerHello/KeyShare, the client MUST verify that the selected
CipherSuite and NamedGroup match that supplied in the
HelloRetryRequest. If either of these values differ, the client MUST
abort the connection with a fatal "handshake_failure" alert.
6.3.2. Hello Extensions
The extension format is:
struct {
ExtensionType extension_type;
opaque extension_data<0..2^16-1>;
} Extension;
enum {
supported_groups(10),
signature_algorithms(13),
key_share(40),
pre_shared_key(41),
early_data(42),
ticket_age(43),
cookie (44),
(65535)
} ExtensionType;
Here:
- "extension_type" identifies the particular extension type.
- "extension_data" contains information specific to the particular
extension type.
The initial set of extensions is defined in [RFC6066]. The list of
extension types is maintained by IANA as described in Section 11.
An extension type MUST NOT appear in the ServerHello or
HelloRetryRequest unless the same extension type appeared in the
corresponding ClientHello. If a client receives an extension type in
ServerHello or HelloRetryRequest that it did not request in the
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associated ClientHello, it MUST abort the handshake with an
"unsupported_extension" fatal alert.
Nonetheless, "server-oriented" extensions may be provided in the
future within this framework. Such an extension (say, of type x)
would require the client to first send an extension of type x in a
ClientHello with empty extension_data to indicate that it supports
the extension type. In this case, the client is offering the
capability to understand the extension type, and the server is taking
the client up on its offer.
When multiple extensions of different types are present in the
ClientHello or ServerHello messages, the extensions MAY appear in any
order. There MUST NOT be more than one extension of the same type.
Finally, note that extensions can be sent both when starting a new
session and when requesting session resumption or 0-RTT mode.
Indeed, a client that requests session resumption does not in general
know whether the server will accept this request, and therefore it
SHOULD send the same extensions as it would send if it were not
attempting resumption.
In general, the specification of each extension type needs to
describe the effect of the extension both during full handshake and
session resumption. Most current TLS extensions are relevant only
when a session is initiated: when an older session is resumed, the
server does not process these extensions in ClientHello, and does not
include them in ServerHello. However, some extensions may specify
different behavior during session resumption. [[TODO: update this
and the previous paragraph to cover PSK-based resumption.]]
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
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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.
- It would be technically possible to use extensions to change major
aspects of the design of TLS; for example, the design of cipher
suite negotiation. This is not recommended; it would be more
appropriate to define a new version of TLS -- particularly since
the TLS handshake algorithms have specific protection against
version rollback attacks based on the version number, and the
possibility of version rollback should be a significant
consideration in any major design change.
6.3.2.1. Cookie
struct {
opaque cookie<0..255>;
} 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 does this by pickling that post-ClientHello hash state into
the 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 echo
the value of the extension. Clients MUST NOT use cookies in
subsequent connections.
6.3.2.2. Signature Algorithms
The client uses the "signature_algorithms" extension to indicate to
the server which signature algorithms may be used in digital
signatures.
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Clients which offer one or more cipher suites which use certificate
authentication (i.e., any non-PSK cipher suite) MUST send the
"signature_algorithms" extension. If this extension is not provided
and no alternative cipher suite is available, the server MUST close
the connection with a fatal "missing_extension" alert. (see
Section 8.2)
The "extension_data" field of this extension contains a
"supported_signature_algorithms" value:
enum {
/* RSASSA-PKCS-v1_5 algorithms */
rsa_pkcs1_sha1 (0x0201),
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 */
rsa_pss_sha256 (0x0700),
rsa_pss_sha384 (0x0701),
rsa_pss_sha512 (0x0702),
/* EdDSA algorithms */
ed25519 (0x0703),
ed448 (0x0704),
/* Reserved Code Points */
private_use (0xFE00..0xFFFF),
(0xFFFF)
} SignatureScheme;
SignatureScheme supported_signature_algorithms<2..2^16-2>;
Note: This production is named "SignatureScheme" because there is
already a SignatureAlgorithm type in TLS 1.2. 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 function and then
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proceed as usual. The code point groups listed above have the
following meanings:
RSASSA-PKCS-v1_5 algorithms
Indicates a signature algorithm using RSASSA-PKCS1-v1_5 [RFC3447]
with the corresponding hash algorithm as defined in [SHS]. These
values refer solely to signatures which appear in certificates
(see Section 6.3.4.1.1) and are not defined for use in signed TLS
handshake messages (see Section 4.8.1).
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 algorithms
Indicates a signature algorithm using RSASSA-PSS [RFC3447] with
MGF1. The digest used in the mask generation function and the
digest being signed are both the corresponding hash algorithm as
defined in [SHS]. When used in signed TLS handshake messages (see
Section 4.8.1), the length of the salt MUST be equal to the length
of the digest output.
EdDSA algorithms
Indicates a signature algorithm using EdDSA as defined in
[I-D.irtf-cfrg-eddsa] or its successors. Note that these
correspond to the "PureEdDSA" algorithms and not the "prehash"
variants.
The semantics of this extension are somewhat complicated because the
cipher suite adds additional constraints on signature algorithms.
Section 6.3.4.1.1 describes the appropriate rules.
rsa_pkcs1_sha1 and dsa_sha1 SHOULD NOT be offered. Clients offering
these values for backwards compatibility MUST list them as the lowest
priority (listed after all other algorithms in the
supported_signature_algorithms vector). TLS 1.3 servers MUST NOT
offer a SHA-1 signed certificate unless no valid certificate chain
can be produced without it (see Section 6.3.4.1.1).
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.
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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] and SHA-224 MUST NOT
be used.
- ecdsa_secp256r1_sha256, etc., align with TLS 1.2's ECDSA hash/
signature pairs. However, the old semantics did not constrain the
signing curve.
6.3.2.3. Negotiated Groups
When sent by the client, the "supported_groups" extension indicates
the named groups which the client supports, ordered from most
preferred to least preferred.
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 [I-D.ietf-tls-negotiated-ff-dhe]. This extension was
also used to negotiate ECDSA curves. Signature algorithms are now
negotiated independently (see Section 6.3.2.2).
Clients which offer one or more (EC)DHE cipher suites MUST send at
least one supported NamedGroup value and servers MUST NOT negotiate
any of these cipher suites unless a supported value was provided. If
this extension is not provided and no alternative cipher suite is
available, the server MUST close the connection with a fatal
"missing_extension" alert. (see Section 8.2) If the extension is
provided, but no compatible group is offered, the server MUST NOT
negotiate a cipher suite of the relevant type. For instance, if a
client supplies only ECDHE groups, the server MUST NOT negotiate
finite field Diffie-Hellman. If no acceptable group can be selected
across all cipher suites, then the server MUST generate a fatal
"handshake_failure" alert.
The "extension_data" field of this extension contains a
"NamedGroupList" value:
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enum {
/* Elliptic Curve Groups (ECDHE) */
secp256r1 (23), secp384r1 (24), secp521r1 (25),
x25519 (29), x448 (30),
/* Finite Field Groups (DHE) */
ffdhe2048 (256), ffdhe3072 (257), ffdhe4096 (258),
ffdhe6144 (259), ffdhe8192 (260),
/* Reserved Code Points */
ffdhe_private_use (0x01FC..0x01FF),
ecdhe_private_use (0xFE00..0xFEFF),
(0xFFFF)
} NamedGroup;
struct {
NamedGroup named_group_list<1..2^16-1>;
} NamedGroupList;
Elliptic Curve Groups (ECDHE)
Indicates support of the corresponding named curve. Note that
some curves are also recommended in ANSI X9.62 [X962] and FIPS
186-4 [DSS]. Others are recommended 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 [I-D.ietf-tls-negotiated-ff-dhe]. 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. If the server has a group it prefers to the
ones in the "key_share" extension but is still willing to accept the
ClientHello, it SHOULD send "supported_groups" to update the client's
view of its preferences. 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 offer to a server in
subsequent connections.
[[TODO: IANA Considerations.]]
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6.3.2.4. Key Share
The "key_share" extension contains the endpoint's cryptographic
parameters for non-PSK key establishment methods (currently DHE or
ECDHE).
Clients which offer one or more (EC)DHE cipher suites MUST send this
extension and SHOULD send at least one supported KeyShareEntry value.
Servers MUST NOT negotiate any of these cipher suites unless a
supported value was provided. If this extension is not provided in a
ServerHello or ClientHello, and the peer is offering (EC)DHE cipher
suites, then the endpoint MUST close the connection with a fatal
"missing_extension" alert. (see Section 8.2) 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 6.3.1.3)
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 6.3.2.4.1;
Elliptic Curve Diffie-Hellman parameters are described in
Section 6.3.2.4.2.
key_exchange
Key exchange information. The contents of this field are
determined by the specified group and its corresponding
definition. Endpoints MUST NOT send empty or otherwise invalid
key_exchange values for any reason.
The "extension_data" field of this extension contains a "KeyShare"
value:
struct {
select (role) {
case client:
KeyShareEntry client_shares<0..2^16-1>;
case server:
KeyShareEntry server_share;
}
} KeyShare;
client_shares
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A list of offered KeyShareEntry values in descending order of
client preference. This vector MAY be empty if the client is
requesting a HelloRetryRequest. The ordering of values here
SHOULD match that of the ordering of offered support in the
"supported_groups" extension.
server_share
A single KeyShareEntry value for the negotiated cipher suite.
Servers offer exactly one KeyShareEntry value, which corresponds to
the key exchange used for the negotiated cipher suite.
Clients 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 by
generated independently. Clients MUST NOT offer multiple
KeyShareEntry values for the same group and servers receiving
multiple KeyShareEntry values for the same group MUST abort the
connection with a fatal "illegal_parameter" alert. Clients and
servers MUST NOT offer or accept any KeyShareEntry values for groups
not listed in the client's "supported_groups" extension. Servers
MUST NOT offer a KeyShareEntry value for a group not offered by the
client in its corresponding KeyShare.
If the server selects an (EC)DHE cipher suite and no mutually
supported group is available between the two endpoints' KeyShare
offers, yet there is a mutually supported group that can be found via
the "supported_groups" extension, then the server MUST reply with a
HelloRetryRequest. If there is no mutually supported group at all,
the server MUST NOT negotiate an (EC)DHE cipher suite.
[[TODO: Recommendation about what the client offers. Presumably
which integer DH groups and which curves.]]
6.3.2.4.1. 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), encoded as a big-endian integer, padded
with zeros to the size of p.
Note: For a given Diffie-Hellman group, the padding results in all
public keys having the same length.
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6.3.2.4.2. 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 byte
string representation of an elliptic curve public value following the
conversion routine in Section 4.3.6 of ANSI X9.62 [X962].
Although X9.62 supports multiple point formats, any given curve MUST
specify only a single point format. All curves currently specified
in this document MUST only be used with the uncompressed point format
(the format for all ECDH functions is considered uncompressed).
For x25519 and x448, the contents 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 negotiation; TLS
1.3 removes this feature in favor of a single point format for each
curve.
6.3.2.5. 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 a PSK or (EC)DHE-PSK cipher suite (see [RFC4279] for
background).
Clients which offer one or more PSK cipher suites MUST send at least
one supported psk_identity value and servers MUST NOT negotiate any
of these cipher suites unless a supported value was provided. If
this extension is not provided and no alternative cipher suite is
available, the server MUST close the connection with a fatal
"missing_extension" alert. (see Section 8.2)
The "extension_data" field of this extension contains a
"PreSharedKeyExtension" value:
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opaque psk_identity<0..2^16-1>;
struct {
select (Role) {
case client:
psk_identity identities<2..2^16-1>;
case server:
uint16 selected_identity;
}
} PreSharedKeyExtension;
identities
A list of the identities (labels for keys) that the client is
willing to negotiate with the server.
selected_identity
The server's chosen identity expressed as a (0-based) index into
the identies in the client's list.
If no suitable identity is provided, the server MUST NOT negotiate a
PSK cipher suite and MAY respond with an "unknown_psk_identity" alert
message. Sending this alert is OPTIONAL; servers MAY instead choose
to send a "decrypt_error" alert to merely indicate an invalid PSK
identity or instead negotiate use of a non-PSK cipher suite, if
available.
If the server selects a PSK cipher suite, it MUST send a
"pre_shared_key" extension with the identity that it selected. The
client MUST verify that the server's selected_identity is within the
range supplied by the client. If any other value is returned, the
client MUST generate a fatal "unknown_psk_identity" alert and close
the connection.
6.3.2.6. OCSP Status Extensions
[RFC6066] and [RFC6961] provide extensions to negotiate the server
sending OCSP responses to the client. In TLS 1.2 and below, the
server sends 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 EncryptedExtensions. Specifically: The body of the
"status_request" or "status_request_v2" extension from the server
MUST be a CertificateStatus structure as defined in [RFC6066] and
[RFC6961] respectively.
Note: this means that the certificate status appears prior to the
certificates it applies to. This is slightly anomalous but matches
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the existing behavior for SignedCertificateTimestamps [RFC6962], and
is more easily extensible in the handshake state machine.
6.3.2.7. Early Data Indication
When PSK resumption is used, the client can send application data in
its first flight of messages. If the client opts to do so, it MUST
supply an "early_data" extension as well as the "pre_shared_key"
extension.
The "extension_data" field of this extension contains an
"EarlyDataIndication" value:
struct {
select (Role) {
case client:
opaque context<0..255>;
case server:
struct {};
}
} EarlyDataIndication;
context
An optional context value that can be used for anti-replay (see
below).
All of the parameters for the 0-RTT data (symmetric cipher suite,
ALPN, etc.) MUST be 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.
0-RTT messages sent in the first flight have the same content types
as their corresponding messages sent in other flights (handshake,
application_data, and alert respectively) but are protected under
different keys. After all the 0-RTT application data messages (if
any) have been sent, a "end_of_early_data" alert of type "warning" is
sent to indicate the end of the flight. 0-RTT MUST always be
followed by an "end_of_early_data" alert.
A server which receives an "early_data" extension can behave in one
of two ways:
- Ignore the extension and return no response. This indicates that
the server has ignored any early data and an ordinary 1-RTT
handshake is required.
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- Return an empty extension, 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.
[[OPEN ISSUE: are the rules below correct? https://github.com/tlswg/
tls13-spec/issues/451]] Prior to accepting the "early_data"
extension, the server MUST validate that the session ticket
parameters are consistent with its current configuration. It MUST
also validate that the extensions negotiated in the previous
connection are identical to those being negotiated in the
ServerHello, with the exception of the following extensions:
- The use of "signed_certificate_timestamp" [RFC6962] MUST be
identical but the server's SCT extension value may differ.
- The "padding" extension [RFC7685] MUST be ignored for this
purpose.
- The values of "key_share", "pre_shared_key", and "early_data",
which MUST be as defined in this document.
In addition, it MUST validate that the ticket_age is within a small
tolerance of the time since the ticket was issued (see
Section 6.3.2.7.2).
If any of these checks fail, the server MUST NOT respond with the
extension and must discard all the remaining first flight data (thus
falling back to 1-RTT). If the client attempts a 0-RTT handshake but
the server rejects it, it will generally not have the 0-RTT record
protection keys and must instead trial decrypt each record with the
1-RTT handshake keys until it finds one that decrypts properly, and
then pick up the handshake from that point.
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,
decryption failure of any 0-RTT record following an accepted
"early_data" extension MUST produce a fatal "bad_record_mac" alert as
per Section 5.2.2. Implementations SHOULD determine the security
parameters for the 1-RTT phase of the connection entirely before
processing the EncryptedExtensions and Finished, using those values
solely to determine whether to accept or reject 0-RTT data.
[[TODO: How does the client behave if the indication is rejected.]]
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6.3.2.7.1. Processing Order
Clients are permitted to "stream" 0-RTT data until they receive the
server's Finished, only then sending the "end_of_early_data" alert.
In order to avoid deadlock, when accepting "early_data", servers MUST
process the client's Finished and then immediately send the
ServerHello, rather than waiting for the client's "end_of_early_data"
alert.
6.3.2.7.2. Replay Properties
As noted in Section 6.2.3, TLS provides only a limited inter-
connection mechanism for replay protection for data sent by the
client in the first flight.
The "ticket_age" extension sent by the client SHOULD be used by
servers to limit the time over which the first flight might be
replayed. A server can store the time at which it sends a server
configuration to a client, or encode the time in a ticket. Then,
each time it receives an early_data extension, it can check to see if
the value used by the client matches its expectations.
The "ticket_age" value provided by the client will be shorter than
the actual time elapsed on the server by a single round trip time.
This difference is comprised of the delay in sending the
NewSessionTicket message to the client, plus the time taken to send
the ClientHello to the server. For this reason, a server SHOULD
measure the round trip time prior to sending the NewSessionTicket
message and account for that in the value it saves.
There are several potential sources of error that make an exact
measurement of time difficult. Variations in client and server
clocks are likely to be minimal, outside of gross time corrections.
Network propagation delays are 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.
A small allowance for errors in clocks and variations in measurements
is advisable. However, any allowance also increases the opportunity
for replay. In this case, it is better to reject early data than to
risk greater exposure to replay attacks.
6.3.2.8. Ticket Age
struct {
uint32 ticket_age;
} TicketAge;
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When the client sends the "early_data" extension, it MUST also send a
"ticket_age" extension in its EncryptedExtensions block. This value
contains the time elapsed since the client learned about the server
configuration that it is using, in milliseconds. This value can be
used by the server to limit the time over which early data can be
replayed. Note: because ticket lifetimes are restricted to a week,
32 bits is enough to represent any plausible age, even in
milliseconds.
6.3.3. Server Parameters
6.3.3.1. Encrypted Extensions
When this message will be sent:
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
handshake_traffic_secret. If the client indicates "early_data" in
its ClientHello, it MUST also send EncryptedExtensions immediately
following the ClientHello and immediately prior to the Finished.
Meaning of this message:
The EncryptedExtensions message contains any extensions which
should be protected, i.e., any which are not needed to establish
the cryptographic context.
The same extension types MUST NOT appear in both the ServerHello and
EncryptedExtensions. If the same extension appears in both
locations, the client MUST rely only on the value in the
EncryptedExtensions block. All server-sent extensions other than
those explicitly listed in Section 6.3.1.2 or designated in the IANA
registry MUST only appear in EncryptedExtensions. Extensions which
are designated to appear in ServerHello MUST NOT appear in
EncryptedExtensions. Clients MUST check EncryptedExtensions for the
presence of any forbidden extensions and if any are found MUST
terminate the handshake with an "illegal_parameter" alert.
The client's EncryptedExtensions apply only to the early data with
which they appear. Servers MUST NOT use them to negotiate the rest
of the handshake. Only those extensions explicitly designated as
being included in 0-RTT Encrypted Extensions in the IANA registry can
be sent in the client's EncryptedExtensions.
Structure of this message:
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struct {
Extension extensions<0..2^16-1>;
} EncryptedExtensions;
extensions
A list of extensions.
6.3.3.2. Certificate Request
When this message will be sent:
A non-anonymous server can optionally request a certificate from
the client, if appropriate for the selected cipher suite. This
message, if sent, will follow EncryptedExtensions.
Structure of this message:
opaque DistinguishedName<1..2^16-1>;
struct {
opaque certificate_extension_oid<1..2^8-1>;
opaque certificate_extension_values<0..2^16-1>;
} CertificateExtension;
struct {
opaque certificate_request_context<0..2^8-1>;
SignatureScheme
supported_signature_algorithms<2..2^16-2>;
DistinguishedName certificate_authorities<0..2^16-1>;
CertificateExtension certificate_extensions<0..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).
supported_signature_algorithms
A list of the signature algorithms that the server is able to
verify, listed in descending order of preference. Any
certificates provided by the client MUST be signed using a
signature algorithm found in supported_signature_algorithms.
certificate_authorities
A list of the distinguished names [X501] of acceptable
certificate_authorities, represented in DER-encoded [X690] format.
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These distinguished names may specify a desired distinguished name
for a root CA or for a subordinate CA; thus, this message can be
used to describe known roots as well as a desired authorization
space. If the certificate_authorities list is empty, then the
client MAY send any certificate that meets the rest of the
selection criteria in the CertificateRequest, unless there is some
external arrangement to the contrary.
certificate_extensions
A list of certificate extension OIDs [RFC5280] with their allowed
values, 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
certificate_extensions list, the client certificate MUST contain
all of the specified extension OIDs that the client recognizes.
For each extension OID recognized by the client, all of the
specified 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 has 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
session without client authentication, or terminate the session
with a fatal 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]:
o 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.
o 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.
Note: It is a fatal "handshake_failure" alert for an anonymous server
to request client authentication.
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6.3.4. Authentication Messages
As discussed in Section 6.2, TLS uses a common set of messages for
authentication, key confirmation, and handshake integrity:
Certificate, CertificateVerify, and Finished. These 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.
The computations for the Authentication messages all uniformly take
the following inputs:
- The certificate and signing key to be used.
- A Handshake Context based on the hash of the handshake messages
- 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 the 0-RTT case.
CertificateVerify
A signature over the value Hash(Handshake Context + Certificate) +
Hash(resumption_context) See Section 6.3.5.1 for the definition of
resumption_context.
Finished
A MAC over the value Hash(Handshake Context + Certificate +
CertificateVerify) + Hash(resumption_context) using a MAC key
derived from the base key.
Because the CertificateVerify signs the Handshake Context +
Certificate and the Finished MACs the Handshake Context + Certificate
+ CertificateVerify, this is mostly equivalent to keeping a running
hash of the handshake messages (exactly so in the pure 1-RTT cases).
Note, however, that subsequent post-handshake authentications do not
include each other, just the messages through the end of the main
handshake.
The following table defines the Handshake Context and MAC Base Key
for each scenario:
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+------------+--------------------------------+---------------------+
| Mode | Handshake Context | Base Key |
+------------+--------------------------------+---------------------+
| 0-RTT | ClientHello | early_traffic_secre |
| | | t |
| | | |
| 1-RTT | ClientHello ... later of Encry | handshake_traffic_s |
| (Server) | ptedExtensions/CertificateRequ | ecret |
| | est | |
| | | |
| 1-RTT | ClientHello ... ServerFinished | handshake_traffic_s |
| (Client) | | ecret |
| | | |
| Post- | ClientHello ... ClientFinished | traffic_secret_0 |
| Handshake | + CertificateRequest | |
+------------+--------------------------------+---------------------+
Note: The Handshake Context for the last three rows does not include
any 0-RTT handshake messages, regardless of whether 0-RTT is used.
6.3.4.1. Certificate
When this message will be sent:
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 server
has requested client authentication via a CertificateRequest
message (Section 6.3.3.2). 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).
Meaning of this message:
This message conveys the endpoint's certificate chain to the peer.
The certificate MUST be appropriate for the negotiated cipher
suite's key exchange algorithm and any negotiated extensions.
Structure of this message:
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opaque ASN1Cert<1..2^24-1>;
struct {
opaque certificate_request_context<0..2^8-1>;
ASN1Cert 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 or client authentication in 0-RTT,
this field SHALL be zero length.
certificate_list
This is a sequence (chain) of certificates. The sender's
certificate MUST come first in the list. Each following
certificate SHOULD 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.
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.
6.3.4.1.1. 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., [RFC5081]).
- The server's end-entity certificate's public key (and associated
restrictions) MUST be compatible with the selected key exchange
algorithm.
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+----------------------+---------------------------+
| Key Exchange Alg. | Certificate Key Type |
+----------------------+---------------------------+
| DHE_RSA or ECDHE_RSA | RSA public key |
| | |
| ECDHE_ECDSA | ECDSA or EdDSA public key |
+----------------------+---------------------------+
- 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" extension.
- The "server_name" and "trusted_ca_keys" extensions [RFC6066] are
used to guide certificate selection. As servers MAY require the
presence of the "server_name" extension, clients SHOULD send this
extension.
All certificates provided by the server MUST be signed by a signature
algorithm that appears in the "signature_algorithms" extension
provided by the client, if they are able to provide such a chain (see
Section 6.3.2.2). 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 MAY use the deprecated SHA-1 hash
algorithm only if the "signature_algorithms" extension provided by
the client permits it. If the client cannot construct an acceptable
chain using the provided certificates and decides to abort the
handshake, then it MUST send an "unsupported_certificate" alert
message and close the connection.
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).
As cipher suites that specify new key exchange methods are specified
for the TLS protocol, they will imply the certificate format and the
required encoded keying information.
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6.3.4.1.2. Client Certificate Selection
The following rules apply to certificates sent by the client:
In particular:
- The certificate type MUST be X.509v3 [RFC5280], unless explicitly
negotiated otherwise (e.g., [RFC5081]).
- If the certificate_authorities list in the certificate request
message was non-empty, 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 hash/
signature algorithm pair, as described in Section 6.3.3.2. Note
that this relaxes the constraints on certificate-signing
algorithms found in prior versions of TLS.
- If the certificate_extensions list in the certificate request
message was non-empty, the end-entity certificate MUST match the
extension OIDs recognized by the client, as described in
Section 6.3.3.2.
Note that, as with the server certificate, there are certificates
that use algorithm combinations that cannot be currently used with
TLS.
6.3.4.1.3. 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
terminate the handshake with a fatal "decode_error" alert.
If the client does not send any certificates, the server MAY at its
discretion either continue the handshake without client
authentication, or respond with a fatal "handshake_failure" 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 send a fatal alert.
Any endpoint receiving any certificate signed using any signature
algorithm using an MD5 hash MUST send a "bad_certificate" alert
message and close the connection.
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SHA-1 is deprecated and therefore NOT RECOMMENDED. Endpoints that
reject certification paths due to use of a deprecated hash MUST send
a fatal "bad_certificate" alert message before closing the
connection. 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).
6.3.4.2. Certificate Verify
When this message will be sent:
This message is used to provide explicit proof that an endpoint
possesses the private key corresponding to its certificate and
also provides integrity for the handshake up to this point.
Servers MUST send this message when using a cipher suite which is
authenticated 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 {
digitally-signed struct {
opaque hashed_data[hash_length];
};
} CertificateVerify;
Where hashed_data is the hash output described in Section 6.3.4,
namely Hash(Handshake Context + Certificate) +
Hash(resumption_context). For concreteness, this means that the
value that is signed is:
padding + context_string + 00 + hashed_data
The context string for a server signature is "TLS 1.3, server
CertificateVerify" and for a client signature is "TLS 1.3, client
CertificateVerify". A hash of the handshake messages is signed
rather than the messages themselves because the digitally-signed
format requires padding and context bytes at the beginning of the
input. Thus, by signing a digest of the messages, an
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implementation only needs to maintain a single running hash per
hash type for CertificateVerify, Finished and other messages.
If sent by a server, the signature algorithm MUST be one offered
in the client's "signature_algorithms" extension unless no valid
certificate chain can be produced without unsupported algorithms
(see Section 6.3.2.2). Note that there is a possibility for
inconsistencies here. For instance, the client might offer
ECDHE_ECDSA key exchange but omit any ECDSA and EdDSA values from
its "signature_algorithms" extension. In order to negotiate
correctly, the server MUST check any candidate cipher suites
against the "signature_algorithms" extension before selecting
them. This is somewhat inelegant but is a compromise designed to
minimize changes to the original cipher suite design.
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 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-PKCS-
v1_5 algorithms appear in "signature_algorithms". SHA-1 MUST NOT
be used in any signatures in CertificateVerify. (Note that
rsa_pkcs1_sha1 and dsa_sha1, the only defined SHA-1 signature
algorithms, are undefined for CertificateVerify signatures.)
Note: When used with non-certificate-based handshakes (e.g., PSK),
the client's signature does not cover the server's certificate
directly, although it does cover the server's Finished message, which
transitively includes the server's certificate when the PSK derives
from a certificate-authenticated handshake. [PSK-FINISHED] describes
a concrete attack on this mode if the Finished is omitted from the
signature. It is unsafe to use certificate-based client
authentication when the client might potentially share the same PSK/
key-id pair with two different endpoints. In order to ensure this,
implementations MUST NOT mix certificate-based client authentication
with pure PSK modes (i.e., those where the PSK was not derived from a
previous non-PSK handshake).
6.3.4.3. Finished
When this message will be sent:
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.
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Meaning of this message:
Recipients of Finished messages MUST verify that the contents are
correct. 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.
The key used to compute the finished message is computed from the
Base key defined in Section 6.3.4 using HKDF (see Section 7.1).
Specifically:
client_finished_key =
HKDF-Expand-Label(BaseKey, "client finished", "", L)
server_finished_key =
HKDF-Expand-Label(BaseKey, "server finished", "", L)
Structure of this message:
struct {
opaque verify_data[verify_data_length];
} Finished;
The verify_data value is computed as follows:
verify_data =
HMAC(finished_key, Hash(
Handshake Context + Certificate* + CertificateVerify*
) + Hash(resumption_context)
)
* Only included if present.
Where 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 the current version of TLS, 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.
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6.3.5. 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 application traffic key.
6.3.5.1. 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 pre-shared key (PSK) binding between the ticket value and
the following two values derived from the resumption master secret:
resumption_psk = HKDF-Expand-Label(resumption_secret,
"resumption psk", "", L)
resumption_context = HKDF-Expand-Label(resumption_secret,
"resumption context", "", L)
The client MAY use this PSK for future handshakes by including the
ticket value in the "pre_shared_key" extension in its ClientHello
(Section 6.3.2.5) and supplying a suitable PSK cipher suite. Servers
may send multiple tickets on a single connection, for instance after
post-handshake authentication. For handshakes that do not use a
resumption_psk, the resumption_context is a string of L zeroes.
enum { (65535) } TicketExtensionType;
struct {
TicketExtensionType extension_type;
opaque extension_data<0..2^16-1>;
} TicketExtension;
enum {
allow_early_data(1)
allow_dhe_resumption(2),
allow_psk_resumption(4)
} TicketFlags;
struct {
uint32 ticket_lifetime;
uint32 flags;
TicketExtension extensions<2..2^16-2>;
opaque ticket<0..2^16-1>;
} NewSessionTicket;
flags
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A 32-bit value indicating the ways in which this ticket may be
used (as an OR of the flags values).
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 more than 604800 seconds (7 days). The value of
zero indicates that the ticket should be discarded immediately.
Clients MUST NOT cache session tickets for longer than 7 days,
regardless of the ticket_lifetime. It 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_extensions
A placeholder for extensions in the ticket. Clients MUST ignore
unrecognized extensions.
ticket
The value of the ticket to be used as the PSK identifier. 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.
The meanings of the flags are as follows:
allow_early_data
When resuming with this ticket, the client MAY send data in its
first flight (early data) encrypted under a key derived from this
PSK.
allow_dhe_resumption
This ticket MAY be used with (EC)DHE-PSK cipher suite
allow_psk_resumption
This ticket MAY be used with a pure PSK cipher suite.
In all cases, the PSK or (EC)DHE-PSK cipher suites that the client
offers/uses MUST have the same symmetric parameters (cipher/hash) as
the cipher suite negotiated for this connection. If no flags are set
that the client recognizes, it MUST ignore the ticket.
6.3.5.2. Post-Handshake Authentication
The server is permitted to request client authentication at any time
after the handshake has completed by sending a CertificateRequest
message. The client SHOULD respond with the appropriate
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Authentication messages. 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.
Note: Because client authentication may require 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).
6.3.5.3. Key and IV Update
struct {} KeyUpdate;
The KeyUpdate handshake message is used to indicate that the sender
is updating its sending cryptographic keys. This message can be sent
by the server after sending its first flight and the client after
sending its second flight. Implementations that receive a KeyUpdate
message prior to receiving a Finished message as part of the 1-RTT
handshake MUST generate a fatal "unexpected_message" alert. After
sending a KeyUpdate message, the sender SHALL send all its traffic
using the next generation of keys, computed as described in
Section 7.2. Upon receiving a KeyUpdate, the receiver MUST update
their receiving keys and if they have not already updated their
sending state up to or past the then current receiving generation
MUST send their own KeyUpdate prior to sending any other messages.
This mechanism allows either side to force an update to the entire
connection. Note that implementations may receive an arbitrary
number of messages between sending a KeyUpdate and receiving the
peer's KeyUpdate because those messages may already be in flight.
Note that if implementations independently send their own KeyUpdates
and they cross in flight, this only results in an update of one
generation; when each side receives the other side's update it just
updates its receive keys and notes that the generations match and
thus no send update is needed.
Note that the side which sends its KeyUpdate first needs to retain
the traffic keys (though not the traffic secret) for the previous
generation of keys until it receives the KeyUpdate from the other
side.
Both sender and receiver MUST encrypt their KeyUpdate messages with
the old keys. Additionally, both sides MUST enforce that a KeyUpdate
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with the old key is received before accepting any messages encrypted
with the new key. Failure to do so may allow message truncation
attacks.
7. Cryptographic Computations
In order to begin connection protection, the TLS Record Protocol
requires specification of a suite of algorithms, a master secret, and
the client and server random values. The authentication, key
exchange, and record protection algorithms are determined by the
cipher_suite selected by the server and revealed in the ServerHello
message. The random values are exchanged in the hello messages. All
that remains is to calculate the key schedule.
7.1. Key Schedule
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 makes use of the following
functions, based on HKDF [RFC5869]:
HKDF-Extract(Salt, IKM) as defined in {{RFC5869}}.
HKDF-Expand-Label(Secret, Label, Messages, Length) =
HKDF-Expand(Secret, HkdfLabel, Length)
Where HkdfLabel is specified as:
struct HkdfLabel {
uint16 length;
opaque label<9..255>;
opaque hash_value<0..255>;
};
- HkdfLabel.length is Length
- HkdfLabel.label is "TLS 1.3, " + Label
- HkdfLabel.hash_value is HashValue.
Derive-Secret(Secret, Label, Messages) =
HKDF-Expand-Label(Secret, Label,
Hash(Messages) + Hash(resumption_context), L))
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 0s as
long as the size of the Hash that is the basis for the HKDF.
Concretely, for the present version of TLS 1.3, secrets are added in
the following order:
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- PSK
- (EC)DHE shared secret
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.
- Derive-Secret's Secret argument is indicated by the arrow coming
in from the left. For instance, the Early Secret is the Secret
for generating the early_traffic-secret.
0
|
v
PSK -> HKDF-Extract
|
v
Early Secret --> Derive-Secret(., "early traffic secret",
| ClientHello)
| = early_traffic_secret
v
(EC)DHE -> HKDF-Extract
|
v
Handshake
Secret -----> Derive-Secret(., "handshake traffic secret",
| ClientHello + ServerHello)
| = handshake_traffic_secret
v
0 -> HKDF-Extract
|
v
Master Secret
|
+---------> Derive-Secret(., "application traffic secret",
| ClientHello...Server Finished)
| = traffic_secret_0
|
+---------> Derive-Secret(., "exporter master secret",
| ClientHello...Client Finished)
| = exporter_secret
|
+---------> Derive-Secret(., "resumption master secret",
ClientHello...Client Finished)
= resumption_secret
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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 L zeroes is used.
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
Section 6.3.5.3. The next generation of traffic keys is computed by
generating traffic_secret_N+1 from traffic_secret_N as described in
this section then re-deriving the traffic keys as described in
Section 7.3.
The next-generation traffic_secret is computed as:
traffic_secret_N+1 = HKDF-Expand-Label(traffic_secret_N, "application
traffic secret", "", L)
Once traffic_secret_N+1 and its associated traffic keys have been
computed, implementations SHOULD delete traffic_secret_N. Once the
directional keys are no longer needed, they SHOULD be deleted as
well.
7.3. Traffic Key Calculation
The traffic keying material is generated from the following input
values:
- A secret value
- A phase value indicating the phase of the protocol the keys are
being generated for.
- A purpose value indicating the specific value being generated
- The length of the key.
The keying material is computed using:
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key = HKDF-Expand-Label(Secret,
phase + ", " + purpose, "",
key_length)
The following table describes the inputs to the key calculation for
each class of traffic keys:
+-------------+--------------------------+--------------------------+
| Record Type | Secret | Phase |
+-------------+--------------------------+--------------------------+
| 0-RTT | early_traffic_secret | "early handshake key |
| Handshake | | expansion" |
| | | |
| 0-RTT | early_traffic_secret | "early application data |
| Application | | key expansion" |
| | | |
| Handshake | handshake_traffic_secret | "handshake key |
| | | expansion" |
| | | |
| Application | traffic_secret_N | "application data key |
| Data | | expansion" |
+-------------+--------------------------+--------------------------+
The following table indicates the purpose values for each type of
key:
+------------------+--------------------+
| Key Type | Purpose |
+------------------+--------------------+
| Client Write Key | "client write key" |
| | |
| Server Write Key | "server write key" |
| | |
| Client Write IV | "client write iv" |
| | |
| Server Write IV | "server write iv" |
+------------------+--------------------+
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).
7.3.1. Diffie-Hellman
A conventional Diffie-Hellman computation is performed. The
negotiated key (Z) is converted to byte string by encoding in big-
endian, padded with zeros up to the size of the prime. This byte
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string is used as the shared secret, and is used in the key schedule
as specified above.
Note that this construction differs from previous versions of TLS
which remove leading zeros.
7.3.2. 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 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 ECDH 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, see [RFC7748].
7.3.3. Exporters
[RFC5705] defines keying material exporters for TLS in terms of the
TLS PRF. This document replaces the PRF with HKDF, thus requiring a
new construction. The exporter interface remains the same, however
the value is computed as:
HKDF-Expand-Label(exporter_secret,
label, context_value, key_length)
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8. Mandatory Algorithms
8.1. MTI Cipher Suites
In the absence of an application profile standard specifying
otherwise, a TLS-compliant application MUST implement the following
cipher suites:
TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256
TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
These cipher suites MUST support both digital signatures and key
exchange with secp256r1 (NIST P-256) and SHOULD support key exchange
with X25519 [RFC7748].
A TLS-compliant application SHOULD implement the following cipher
suites:
TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384
TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256
TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256
8.2. MTI Extensions
In the absence of an application profile standard specifying
otherwise, a TLS-compliant application MUST implement the following
TLS extensions:
- Signature Algorithms ("signature_algorithms"; Section 6.3.2.2)
- Negotiated Groups ("supported_groups"; Section 6.3.2.3)
- Key Share ("key_share"; Section 6.3.2.4)
- Pre-Shared Key ("pre_shared_key"; Section 6.3.2.5)
- Server Name Indication ("server_name"; Section 3 of [RFC6066])
- Cookie ("cookie"; Section 6.3.2.1)
All implementations MUST send and use these extensions when offering
applicable cipher suites:
- "signature_algorithms" is REQUIRED for certificate authenticated
cipher suites
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- "supported_groups" and "key_share" are REQUIRED for DHE or ECDHE
cipher suites
- "pre_shared_key" is REQUIRED for PSK cipher suites
- "cookie" is REQUIRED for all cipher suites.
When negotiating use of applicable cipher suites, endpoints MUST
abort the connection with a "missing_extension" alert if the required
extension was not provided. Any endpoint that receives any invalid
combination of cipher suites and extensions MAY abort the connection
with a "missing_extension" alert, regardless of negotiated
parameters.
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 with a fatal "missing_extension"
alert.
Servers MUST NOT send the "signature_algorithms" extension; if a
client receives this extension it MUST respond with a fatal
"unsupported_extension" alert and close the connection.
9. Application Data Protocol
Application data messages are carried by the record layer and are
fragmented and encrypted based on the current connection state. The
messages are treated as transparent data to the record layer.
10. Security Considerations
Security issues are discussed throughout this memo, especially in
Appendices B, C, and D.
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 [RFC2434].
Values with the first byte 255 (decimal) are reserved for Private
Use [RFC2434]. IANA [SHALL add/has added] a "Recommended" column
to the cipher suite registry. All cipher suites listed in
Appendix A.4 are marked as "Yes". All other cipher suites are
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marked as "No". IANA [SHALL add/has added] add a note to this
column reading:
Cipher suites marked as "Yes" are those allocated via Standards
Track RFCs. Cipher suites marked as "No" are not; cipher
suites marked "No" range from "good" to "bad" from a
cryptographic standpoint.
- TLS ContentType Registry: Future values are allocated via
Standards Action [RFC2434].
- TLS Alert Registry: Future values are allocated via Standards
Action [RFC2434].
- TLS HandshakeType Registry: Future values are allocated via
Standards Action [RFC2434].
This document also uses a registry originally created in [RFC4366].
IANA has updated it to reference this document. The registry and its
allocation policy is listed below:
- TLS ExtensionType Registry: Values with the first byte in the
range 0-254 (decimal) are assigned via Specification Required
[RFC2434]. Values with the first byte 255 (decimal) are reserved
for Private Use [RFC2434]. IANA [SHALL update/has updated] this
registry to include the "key_share", "pre_shared_key", and
"early_data" extensions as defined in this document.
IANA [shall update/has updated] this registry to include a "TLS
1.3" column with the following four values: "Client", indicating
that the server shall not send them. "Clear", indicating that
they shall be in the ServerHello. "Encrypted", indicating that
they shall be in the EncryptedExtensions block, "Early",
indicating that they shall be only in the client's 0-RTT
EncryptedExtensions block, and "No" indicating that they are not
used in TLS 1.3. This column [shall be/has been] initially
populated with the values in this document. IANA [shall update/
has updated] this registry to add a "Recommended" column. IANA
[shall/has] initially populated this column with the values in the
table below. This table has been generated by marking Standards
Track RFCs as "Yes" and all others as "No".
+-------------------------------+-----------+-----------------------+
| Extension | Recommend | TLS 1.3 |
| | ed | |
+-------------------------------+-----------+-----------------------+
| server_name [RFC6066] | Yes | Encrypted |
| | | |
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| max_fragment_length [RFC6066] | Yes | Encrypted |
| | | |
| client_certificate_url | Yes | Encrypted |
| [RFC6066] | | |
| | | |
| trusted_ca_keys [RFC6066] | Yes | Encrypted |
| | | |
| truncated_hmac [RFC6066] | Yes | No |
| | | |
| status_request [RFC6066] | Yes | No |
| | | |
| user_mapping [RFC4681] | Yes | Encrypted |
| | | |
| client_authz [RFC5878] | No | Encrypted |
| | | |
| server_authz [RFC5878] | No | Encrypted |
| | | |
| cert_type [RFC6091] | Yes | Encrypted |
| | | |
| supported_groups [RFC-ietf- | Yes | Encrypted |
| tls-negotiated-ff-dhe] | | |
| | | |
| ec_point_formats [RFC4492] | Yes | No |
| | | |
| srp [RFC5054] | No | No |
| | | |
| signature_algorithms | Yes | Client |
| [RFC5246] | | |
| | | |
| use_srtp [RFC5764] | Yes | Encrypted |
| | | |
| heartbeat [RFC6520] | Yes | Encrypted |
| | | |
| application_layer_protocol_ne | Yes | Encrypted |
| gotiation [RFC7301] | | |
| | | |
| status_request_v2 [RFC6961] | Yes | Encrypted |
| | | |
| signed_certificate_timestamp | No | Encrypted |
| [RFC6962] | | |
| | | |
| client_certificate_type | Yes | Encrypted |
| [RFC7250] | | |
| | | |
| server_certificate_type | Yes | Encrypted |
| [RFC7250] | | |
| | | |
| padding [RFC7685] | Yes | Client |
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| | | |
| encrypt_then_mac [RFC7366] | Yes | No |
| | | |
| extended_master_secret | Yes | No |
| [RFC7627] | | |
| | | |
| SessionTicket TLS [RFC4507] | Yes | No |
| | | |
| renegotiation_info [RFC5746] | Yes | No |
| | | |
| key_share [[this document]] | Yes | Clear |
| | | |
| pre_shared_key [[this | Yes | Clear |
| document]] | | |
| | | |
| early_data [[this document]] | Yes | Clear |
| | | |
| ticket_age [[this document]] | Yes | Early |
| | | |
| cookie [[this document]] | Yes | Encrypted/HelloRetryR |
| | | equest |
+-------------------------------+-----------+-----------------------+
In addition, this document defines two new registries to be
maintained by IANA
- TLS SignatureScheme Registry: Values with the first byte in the
range 0-254 (decimal) are assigned via Specification Required
[RFC2434]. Values with the first byte 255 (decimal) are reserved
for Private Use [RFC2434]. This registry SHALL have a
"Recommended" column. The registry [shall be/ has been] initially
populated with the values described in Section 6.3.2.2. 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
[AES] National Institute of Standards and Technology,
"Specification for the Advanced Encryption Standard
(AES)", NIST FIPS 197, November 2001.
[DH] Diffie, W. and M. Hellman, "New Directions in
Cryptography", IEEE Transactions on Information Theory,
V.IT-22 n.6 , June 1977.
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[I-D.ietf-tls-chacha20-poly1305]
Langley, A., Chang, W., Mavrogiannopoulos, N.,
Strombergson, J., and S. Josefsson, "ChaCha20-Poly1305
Cipher Suites for Transport Layer Security (TLS)", draft-
ietf-tls-chacha20-poly1305-04 (work in progress), December
2015.
[I-D.irtf-cfrg-eddsa]
Josefsson, S. and I. Liusvaara, "Edwards-curve Digital
Signature Algorithm (EdDSA)", draft-irtf-cfrg-eddsa-05
(work in progress), March 2016.
[I-D.mattsson-tls-ecdhe-psk-aead]
Mattsson, J. and D. Migault, "ECDHE_PSK with AES-GCM and
AES-CCM Cipher Suites for Transport Layer Security (TLS)",
draft-mattsson-tls-ecdhe-psk-aead-05 (work in progress),
April 2016.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<http://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,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 2434,
DOI 10.17487/RFC2434, October 1998,
<http://www.rfc-editor.org/info/rfc2434>.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February
2003, <http://www.rfc-editor.org/info/rfc3447>.
[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,
<http://www.rfc-editor.org/info/rfc5280>.
[RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
DOI 10.17487/RFC5288, August 2008,
<http://www.rfc-editor.org/info/rfc5288>.
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[RFC5289] Rescorla, E., "TLS Elliptic Curve Cipher Suites with SHA-
256/384 and AES Galois Counter Mode (GCM)", RFC 5289,
DOI 10.17487/RFC5289, August 2008,
<http://www.rfc-editor.org/info/rfc5289>.
[RFC5487] Badra, M., "Pre-Shared Key Cipher Suites for TLS with SHA-
256/384 and AES Galois Counter Mode", RFC 5487,
DOI 10.17487/RFC5487, March 2009,
<http://www.rfc-editor.org/info/rfc5487>.
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
March 2010, <http://www.rfc-editor.org/info/rfc5705>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<http://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,
<http://www.rfc-editor.org/info/rfc6066>.
[RFC6209] Kim, W., Lee, J., Park, J., and D. Kwon, "Addition of the
ARIA Cipher Suites to Transport Layer Security (TLS)",
RFC 6209, DOI 10.17487/RFC6209, April 2011,
<http://www.rfc-editor.org/info/rfc6209>.
[RFC6367] Kanno, S. and M. Kanda, "Addition of the Camellia Cipher
Suites to Transport Layer Security (TLS)", RFC 6367,
DOI 10.17487/RFC6367, September 2011,
<http://www.rfc-editor.org/info/rfc6367>.
[RFC6655] McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for
Transport Layer Security (TLS)", RFC 6655,
DOI 10.17487/RFC6655, July 2012,
<http://www.rfc-editor.org/info/rfc6655>.
[RFC6961] Pettersen, Y., "The Transport Layer Security (TLS)
Multiple Certificate Status Request Extension", RFC 6961,
DOI 10.17487/RFC6961, June 2013,
<http://www.rfc-editor.org/info/rfc6961>.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
<http://www.rfc-editor.org/info/rfc6962>.
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[RFC7251] McGrew, D., Bailey, D., Campagna, M., and R. Dugal, "AES-
CCM Elliptic Curve Cryptography (ECC) Cipher Suites for
TLS", RFC 7251, DOI 10.17487/RFC7251, June 2014,
<http://www.rfc-editor.org/info/rfc7251>.
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <http://www.rfc-editor.org/info/rfc7748>.
[SHS] National Institute of Standards and Technology, U.S.
Department of Commerce, "Secure Hash Standard", NIST FIPS
PUB 180-4, March 2012.
[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.
[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
[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.
[FI06] Finney, H., "Bleichenbacher's RSA signature forgery based
on implementation error", August 2006,
<https://www.ietf.org/mail-archive/web/openpgp/current/
msg00999.html>.
[GCM] Dworkin, M., "Recommendation for Block Cipher Modes of
Operation: Galois/Counter Mode (GCM) and GMAC",
NIST Special Publication 800-38D, November 2007.
[I-D.ietf-tls-cached-info]
Santesson, S. and H. Tschofenig, "Transport Layer Security
(TLS) Cached Information Extension", draft-ietf-tls-
cached-info-23 (work in progress), May 2016.
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Internet-Draft TLS May 2016
[I-D.ietf-tls-negotiated-ff-dhe]
Gillmor, D., "Negotiated Finite Field Diffie-Hellman
Ephemeral Parameters for TLS", draft-ietf-tls-negotiated-
ff-dhe-10 (work in progress), June 2015.
[IEEE1363]
IEEE, "Standard Specifications for Public Key
Cryptography", IEEE 1363 , 2000.
[PKCS6] RSA Laboratories, "PKCS #6: RSA Extended Certificate
Syntax Standard, version 1.5", November 1993.
[PKCS7] RSA Laboratories, "PKCS #7: RSA Cryptographic Message
Syntax Standard, version 1.5", November 1993.
[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>.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<http://www.rfc-editor.org/info/rfc793>.
[RFC1948] Bellovin, S., "Defending Against Sequence Number Attacks",
RFC 1948, DOI 10.17487/RFC1948, May 1996,
<http://www.rfc-editor.org/info/rfc1948>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<http://www.rfc-editor.org/info/rfc4086>.
[RFC4279] Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key
Ciphersuites for Transport Layer Security (TLS)",
RFC 4279, DOI 10.17487/RFC4279, December 2005,
<http://www.rfc-editor.org/info/rfc4279>.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
DOI 10.17487/RFC4302, December 2005,
<http://www.rfc-editor.org/info/rfc4302>.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, DOI 10.17487/RFC4303, December 2005,
<http://www.rfc-editor.org/info/rfc4303>.
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[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346,
DOI 10.17487/RFC4346, April 2006,
<http://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,
<http://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,
<http://www.rfc-editor.org/info/rfc4492>.
[RFC4506] Eisler, M., Ed., "XDR: External Data Representation
Standard", STD 67, RFC 4506, DOI 10.17487/RFC4506, May
2006, <http://www.rfc-editor.org/info/rfc4506>.
[RFC4507] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 4507, DOI 10.17487/RFC4507, May
2006, <http://www.rfc-editor.org/info/rfc4507>.
[RFC4681] Santesson, S., Medvinsky, A., and J. Ball, "TLS User
Mapping Extension", RFC 4681, DOI 10.17487/RFC4681,
October 2006, <http://www.rfc-editor.org/info/rfc4681>.
[RFC5054] Taylor, D., Wu, T., Mavrogiannopoulos, N., and T. Perrin,
"Using the Secure Remote Password (SRP) Protocol for TLS
Authentication", RFC 5054, DOI 10.17487/RFC5054, November
2007, <http://www.rfc-editor.org/info/rfc5054>.
[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, <http://www.rfc-editor.org/info/rfc5077>.
[RFC5081] Mavrogiannopoulos, N., "Using OpenPGP Keys for Transport
Layer Security (TLS) Authentication", RFC 5081,
DOI 10.17487/RFC5081, November 2007,
<http://www.rfc-editor.org/info/rfc5081>.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<http://www.rfc-editor.org/info/rfc5116>.
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[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
<http://www.rfc-editor.org/info/rfc5746>.
[RFC5763] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
for Establishing a Secure Real-time Transport Protocol
(SRTP) Security Context Using Datagram Transport Layer
Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May
2010, <http://www.rfc-editor.org/info/rfc5763>.
[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,
<http://www.rfc-editor.org/info/rfc5764>.
[RFC5878] Brown, M. and R. Housley, "Transport Layer Security (TLS)
Authorization Extensions", RFC 5878, DOI 10.17487/RFC5878,
May 2010, <http://www.rfc-editor.org/info/rfc5878>.
[RFC5929] Altman, J., Williams, N., and L. Zhu, "Channel Bindings
for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010,
<http://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,
<http://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, <http://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, <http://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,
<http://www.rfc-editor.org/info/rfc6520>.
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[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,
<http://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, <http://www.rfc-editor.org/info/rfc7250>.
[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, <http://www.rfc-editor.org/info/rfc7301>.
[RFC7366] Gutmann, P., "Encrypt-then-MAC for Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", RFC 7366, DOI 10.17487/RFC7366, September 2014,
<http://www.rfc-editor.org/info/rfc7366>.
[RFC7465] Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465,
DOI 10.17487/RFC7465, February 2015,
<http://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,
<http://www.rfc-editor.org/info/rfc7568>.
[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,
<http://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, <http://www.rfc-editor.org/info/rfc7685>.
[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.
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[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.
12.3. URIs
[1] mailto:tls@ietf.org
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Appendix A. 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
here for completeness. TLS 1.3 implementations MUST NOT send them
but might receive them from older TLS implementations.
A.1. Record Layer
struct {
uint8 major;
uint8 minor;
} ProtocolVersion;
enum {
invalid_RESERVED(0),
change_cipher_spec_RESERVED(20),
alert(21),
handshake(22),
application_data(23)
(255)
} ContentType;
struct {
ContentType type;
ProtocolVersion record_version = { 3, 1 }; /* TLS v1.x */
uint16 length;
opaque fragment[TLSPlaintext.length];
} TLSPlaintext;
struct {
ContentType opaque_type = application_data(23); /* see fragment.type */
ProtocolVersion record_version = { 3, 1 }; /* TLS v1.x */
uint16 length;
aead-ciphered struct {
opaque content[TLSPlaintext.length];
ContentType type;
uint8 zeros[length_of_padding];
} fragment;
} TLSCiphertext;
A.2. Alert Messages
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enum { warning(1), fatal(2), (255) } AlertLevel;
enum {
close_notify(0),
end_of_early_data(1),
unexpected_message(10), /* fatal */
bad_record_mac(20), /* fatal */
decryption_failed_RESERVED(21), /* fatal */
record_overflow(22), /* fatal */
decompression_failure_RESERVED(30), /* fatal */
handshake_failure(40), /* fatal */
no_certificate_RESERVED(41), /* fatal */
bad_certificate(42),
unsupported_certificate(43),
certificate_revoked(44),
certificate_expired(45),
certificate_unknown(46),
illegal_parameter(47), /* fatal */
unknown_ca(48), /* fatal */
access_denied(49), /* fatal */
decode_error(50), /* fatal */
decrypt_error(51), /* fatal */
export_restriction_RESERVED(60), /* fatal */
protocol_version(70), /* fatal */
insufficient_security(71), /* fatal */
internal_error(80), /* fatal */
inappropriate_fallback(86), /* fatal */
user_canceled(90),
no_renegotiation_RESERVED(100), /* fatal */
missing_extension(109), /* fatal */
unsupported_extension(110), /* fatal */
certificate_unobtainable(111),
unrecognized_name(112),
bad_certificate_status_response(113), /* fatal */
bad_certificate_hash_value(114), /* fatal */
unknown_psk_identity(115),
(255)
} AlertDescription;
struct {
AlertLevel level;
AlertDescription description;
} Alert;
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A.3. Handshake Protocol
enum {
hello_request_RESERVED(0),
client_hello(1),
server_hello(2),
session_ticket(4),
hello_retry_request(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),
(255)
} HandshakeType;
struct {
HandshakeType msg_type; /* handshake type */
uint24 length; /* bytes in message */
select (HandshakeType) {
case client_hello: ClientHello;
case server_hello: ServerHello;
case hello_retry_request: HelloRetryRequest;
case encrypted_extensions: EncryptedExtensions;
case certificate_request: CertificateRequest;
case certificate: Certificate;
case certificate_verify: CertificateVerify;
case finished: Finished;
case session_ticket: NewSessionTicket;
case key_update: KeyUpdate;
} body;
} Handshake;
A.3.1. Key Exchange Messages
struct {
opaque random_bytes[32];
} Random;
uint8 CipherSuite[2]; /* Cryptographic suite selector */
struct {
ProtocolVersion client_version = { 3, 4 }; /* TLS v1.3 */
Random random;
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opaque legacy_session_id<0..32>;
CipherSuite cipher_suites<2..2^16-2>;
opaque legacy_compression_methods<1..2^8-1>;
Extension extensions<0..2^16-1>;
} ClientHello;
struct {
ProtocolVersion server_version;
Random random;
CipherSuite cipher_suite;
Extension extensions<0..2^16-1>;
} ServerHello;
struct {
ProtocolVersion server_version;
CipherSuite cipher_suite;
NamedGroup selected_group;
Extension extensions<0..2^16-1>;
} HelloRetryRequest;
struct {
ExtensionType extension_type;
opaque extension_data<0..2^16-1>;
} Extension;
enum {
supported_groups(10),
signature_algorithms(13),
key_share(40),
pre_shared_key(41),
early_data(42),
ticket_age(43),
cookie (44),
(65535)
} ExtensionType;
struct {
NamedGroup group;
opaque key_exchange<1..2^16-1>;
} KeyShareEntry;
struct {
select (role) {
case client:
KeyShareEntry client_shares<0..2^16-1>;
case server:
KeyShareEntry server_share;
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}
} KeyShare;
opaque psk_identity<0..2^16-1>;
struct {
select (Role) {
case client:
psk_identity identities<2..2^16-1>;
case server:
uint16 selected_identity;
}
} PreSharedKeyExtension;
struct {
select (Role) {
case client:
opaque context<0..255>;
case server:
struct {};
}
} EarlyDataIndication;
struct {
uint32 ticket_age;
} TicketAge;
A.3.1.1. Cookie Extension
struct {
opaque cookie<0..255>;
} Cookie;
A.3.1.2. Signature Algorithm Extension
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enum {
/* RSASSA-PKCS-v1_5 algorithms */
rsa_pkcs1_sha1 (0x0201),
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 */
rsa_pss_sha256 (0x0700),
rsa_pss_sha384 (0x0701),
rsa_pss_sha512 (0x0702),
/* EdDSA algorithms */
ed25519 (0x0703),
ed448 (0x0704),
/* Reserved Code Points */
dsa_sha1_RESERVED (0x0202),
dsa_sha256_RESERVED (0x0402),
dsa_sha384_RESERVED (0x0502),
dsa_sha512_RESERVED (0x0602),
obsolete_RESERVED (0x0000..0x0200),
obsolete_RESERVED (0x0203..0x0400),
obsolete_RESERVED (0x0404..0x0500),
obsolete_RESERVED (0x0504..0x0600),
obsolete_RESERVED (0x0604..0x06FF),
private_use (0xFE00..0xFFFF),
(0xFFFF)
} SignatureScheme;
SignatureScheme supported_signature_algorithms<2..2^16-2>;
A.3.1.3. Named Group Extension
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enum {
/* Elliptic Curve Groups (ECDHE) */
obsolete_RESERVED (1..22),
secp256r1 (23), secp384r1 (24), secp521r1 (25),
obsolete_RESERVED (26..28),
x25519 (29), x448 (30),
/* Finite Field Groups (DHE) */
ffdhe2048 (256), ffdhe3072 (257), ffdhe4096 (258),
ffdhe6144 (259), ffdhe8192 (260),
/* Reserved Code Points */
ffdhe_private_use (0x01FC..0x01FF),
ecdhe_private_use (0xFE00..0xFEFF),
obsolete_RESERVED (0xFF01..0xFF02),
(0xFFFF)
} NamedGroup;
struct {
NamedGroup named_group_list<1..2^16-1>;
} NamedGroupList;
Values within "obsolete_RESERVED" ranges were 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.
A.3.1.4. Deprecated Extensions
The following extensions are no longer applicable to TLS 1.3,
although TLS 1.3 clients MAY send them if they are willing to
negotiate them with prior versions of TLS. TLS 1.3 servers MUST
ignore these extensions if they are negotiating TLS 1.3:
truncated_hmac [RFC6066], srp [RFC5054], encrypt_then_mac [RFC7366],
extended_master_secret [RFC7627], SessionTicket [RFC5077], and
renegotiation_info [RFC5746].
A.3.2. Server Parameters Messages
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struct {
Extension extensions<0..2^16-1>;
} EncryptedExtensions;
opaque DistinguishedName<1..2^16-1>;
struct {
opaque certificate_extension_oid<1..2^8-1>;
opaque certificate_extension_values<0..2^16-1>;
} CertificateExtension;
struct {
opaque certificate_request_context<0..2^8-1>;
SignatureScheme
supported_signature_algorithms<2..2^16-2>;
DistinguishedName certificate_authorities<0..2^16-1>;
CertificateExtension certificate_extensions<0..2^16-1>;
} CertificateRequest;
A.3.3. Authentication Messages
opaque ASN1Cert<1..2^24-1>;
struct {
opaque certificate_request_context<0..2^8-1>;
ASN1Cert certificate_list<0..2^24-1>;
} Certificate;
struct {
digitally-signed struct {
opaque hashed_data[hash_length];
};
} CertificateVerify;
struct {
opaque verify_data[verify_data_length];
} Finished;
A.3.4. Ticket Establishment
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enum { (65535) } TicketExtensionType;
struct {
TicketExtensionType extension_type;
opaque extension_data<0..2^16-1>;
} TicketExtension;
enum {
allow_early_data(1)
allow_dhe_resumption(2),
allow_psk_resumption(4)
} TicketFlags;
struct {
uint32 ticket_lifetime;
uint32 flags;
TicketExtension extensions<2..2^16-2>;
opaque ticket<0..2^16-1>;
} NewSessionTicket;
A.4. Cipher Suites
A cipher suite defines a cipher specification supported in TLS and
negotiated via hello messages in the TLS handshake. Cipher suite
names follow a general naming convention composed of a series of
component algorithm names separated by underscores:
CipherSuite TLS_KEA_AUTH_WITH_CIPHER_HASH = VALUE;
+-----------+-------------------------------------------------------+
| Component | Contents |
+-----------+-------------------------------------------------------+
| TLS | The string "TLS" |
| | |
| KEA | The key exchange algorithm (e.g. ECDHE, DHE) |
| | |
| AUTH | The authentication algorithm (e.g. certificates, PSK) |
| | |
| WITH | The string "WITH" |
| | |
| CIPHER | The symmetric cipher used for record protection |
| | |
| HASH | The hash algorithm used with HKDF |
| | |
| VALUE | The two byte ID assigned for this cipher suite |
+-----------+-------------------------------------------------------+
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The "CIPHER" component commonly has sub-components used to designate
the cipher name, bits, and mode, if applicable. For example,
"AES_256_GCM" represents 256-bit AES in the GCM mode of operation.
Cipher suite names that lack a "HASH" value that are defined for use
with TLS 1.2 or later use the SHA-256 hash algorithm by default.
The primary key exchange algorithm used in TLS is Ephemeral Diffie-
Hellman [DH]. The finite field based version is denoted "DHE" and
the elliptic curve based version is denoted "ECDHE". Prior versions
of TLS supported non-ephemeral key exchanges, however these are not
supported by TLS 1.3.
See the definitions of each cipher suite in its specification
document for the full details of each combination of algorithms that
is specified.
The following is a list of standards track server-authenticated (and
optionally client-authenticated) cipher suites which are currently
available in TLS 1.3:
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+-------------------------------+----------+------------------------+
| Cipher Suite Name | Value | Specification |
+-------------------------------+----------+------------------------+
| TLS_DHE_RSA_WITH_AES_128_GCM_ | {0x00,0x | [RFC5288] |
| SHA256 | 9E} | |
| | | |
| TLS_DHE_RSA_WITH_AES_256_GCM_ | {0x00,0x | [RFC5288] |
| SHA384 | 9F} | |
| | | |
| TLS_ECDHE_ECDSA_WITH_AES_128_ | {0xC0,0x | [RFC5289] |
| GCM_SHA256 | 2B} | |
| | | |
| TLS_ECDHE_ECDSA_WITH_AES_256_ | {0xC0,0x | [RFC5289] |
| GCM_SHA384 | 2C} | |
| | | |
| TLS_ECDHE_RSA_WITH_AES_128_GC | {0xC0,0x | [RFC5289] |
| M_SHA256 | 2F} | |
| | | |
| TLS_ECDHE_RSA_WITH_AES_256_GC | {0xC0,0x | [RFC5289] |
| M_SHA384 | 30} | |
| | | |
| TLS_DHE_RSA_WITH_AES_128_CCM | {0xC0,0x | [RFC6655] |
| | 9E} | |
| | | |
| TLS_DHE_RSA_WITH_AES_256_CCM | {0xC0,0x | [RFC6655] |
| | 9F} | |
| | | |
| TLS_DHE_RSA_WITH_AES_128_CCM_ | {0xC0,0x | [RFC6655] |
| 8 | A2} | |
| | | |
| TLS_DHE_RSA_WITH_AES_256_CCM_ | {0xC0,0x | [RFC6655] |
| 8 | A3} | |
| | | |
| TLS_ECDHE_RSA_WITH_CHACHA20_P | {0xCC,0x | [I-D.ietf-tls-chacha20 |
| OLY1305_SHA256 | A8} | -poly1305] |
| | | |
| TLS_ECDHE_ECDSA_WITH_CHACHA20 | {0xCC,0x | [I-D.ietf-tls-chacha20 |
| _POLY1305_SHA256 | A9} | -poly1305] |
| | | |
| TLS_DHE_RSA_WITH_CHACHA20_POL | {0xCC,0x | [I-D.ietf-tls-chacha20 |
| Y1305_SHA256 | AA} | -poly1305] |
+-------------------------------+----------+------------------------+
Note: The values listed for ChaCha/Poly are preliminary but are being
or will be used for interop testing and therefore are likely to be
assigned.
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Note: ECDHE AES GCM was not yet standards track prior to the
publication of this specification. This document promotes the above-
listed ciphers to standards track.
The following is a list of standards track ephemeral pre-shared key
cipher suites which are currently available in TLS 1.3:
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+------------------------------+----------+-------------------------+
| Cipher Suite Name | Value | Specification |
+------------------------------+----------+-------------------------+
| TLS_DHE_PSK_WITH_AES_128_GCM | {0x00,0x | [RFC5487] |
| _SHA256 | AA} | |
| | | |
| TLS_DHE_PSK_WITH_AES_256_GCM | {0x00,0x | [RFC5487] |
| _SHA384 | AB} | |
| | | |
| TLS_DHE_PSK_WITH_AES_128_CCM | {0xC0,0x | [RFC6655] |
| | A6} | |
| | | |
| TLS_DHE_PSK_WITH_AES_256_CCM | {0xC0,0x | [RFC6655] |
| | A7} | |
| | | |
| TLS_PSK_DHE_WITH_AES_128_CCM | {0xC0,0x | [RFC6655] |
| _8 | AA} | |
| | | |
| TLS_PSK_DHE_WITH_AES_256_CCM | {0xC0,0x | [RFC6655] |
| _8 | AB} | |
| | | |
| TLS_ECDHE_PSK_WITH_AES_128_G | {0xD0,0x | [I-D.mattsson-tls-ecdhe |
| CM_SHA256 | 01} | -psk-aead] |
| | | |
| TLS_ECDHE_PSK_WITH_AES_256_G | {0xD0,0x | [I-D.mattsson-tls-ecdhe |
| CM_SHA384 | 02} | -psk-aead] |
| | | |
| TLS_ECDHE_PSK_WITH_AES_128_C | {0xD0,0x | [I-D.mattsson-tls-ecdhe |
| CM_8_SHA256 | 03} | -psk-aead] |
| | | |
| TLS_ECDHE_PSK_WITH_AES_128_C | {0xD0,0x | [I-D.mattsson-tls-ecdhe |
| CM_SHA256 | 04} | -psk-aead] |
| | | |
| TLS_ECDHE_PSK_WITH_AES_256_C | {0xD0,0x | [I-D.mattsson-tls-ecdhe |
| CM_SHA384 | 05} | -psk-aead] |
| | | |
| TLS_ECDHE_PSK_WITH_CHACHA20_ | {0xCC,0x | [I-D.ietf-tls-chacha20- |
| POLY1305_SHA256 | AC} | poly1305] |
| | | |
| TLS_DHE_PSK_WITH_CHACHA20_PO | {0xCC,0x | [I-D.ietf-tls-chacha20- |
| LY1305_SHA256 | AD} | poly1305] |
+------------------------------+----------+-------------------------+
Note: The values listed for ECDHE and ChaCha/Poly are preliminary but
are being or will be used for interop testing and therefore are
likely to be assigned.
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Note: [RFC6655] is inconsistent with respect to the ordering of
components within PSK AES CCM cipher suite names. The names above
are as defined.
All cipher suites in this section are specified for use with both TLS
1.2 and TLS 1.3, as well as the corresponding versions of DTLS. (see
Appendix C)
New cipher suite values are assigned by IANA as described in
Section 11.
A.4.1. Unauthenticated Operation
Previous versions of TLS offered explicitly unauthenticated cipher
suites based on anonymous Diffie-Hellman. These cipher suites have
been deprecated in TLS 1.3. However, it is still possible to
negotiate cipher suites 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 are 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 channel bindings [RFC5929]. [[NOTE: TLS 1.3
needs a new channel binding definition that has not yet been
defined.]] 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.
A.5. The Security Parameters
These security parameters are determined by the TLS Handshake
Protocol and provided as parameters to the TLS record layer in order
to initialize a connection state. SecurityParameters includes:
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enum { server, client } ConnectionEnd;
enum { tls_kdf_sha256, tls_kdf_sha384 } KDFAlgorithm;
enum { aes_gcm } RecordProtAlgorithm;
/* The algorithms specified in KDFAlgorithm and
RecordProtAlgorithm may be added to. */
struct {
ConnectionEnd entity;
KDFAlgorithm kdf_algorithm;
RecordProtAlgorithm record_prot_algorithm;
uint8 enc_key_length;
uint8 iv_length;
opaque hs_master_secret[48];
opaque master_secret[48];
opaque client_random[32];
opaque server_random[32];
} SecurityParameters;
A.6. Changes to RFC 4492
RFC 4492 [RFC4492] adds Elliptic Curve cipher suites to TLS. This
document changes some of the structures used in that document. This
section details the required changes for implementors of both RFC
4492 and TLS 1.2. Implementors of TLS 1.2 who are not implementing
RFC 4492 do not need to read this section.
This document adds an "algorithm" field to the digitally-signed
element in order to identify the signature and digest algorithms used
to create a signature. This change applies to digital signatures
formed using ECDSA as well, thus allowing ECDSA signatures to be used
with digest algorithms other than SHA-1, provided such use is
compatible with the certificate and any restrictions imposed by
future revisions of [RFC5280].
As described in Section 6.3.4.1.1, the restrictions on the signature
algorithms used to sign certificates are no longer tied to the cipher
suite. Thus, the restrictions on the algorithm used to sign
certificates specified in Sections 2 and 3 of RFC 4492 are also
relaxed. As in this document, the restrictions on the keys in the
end-entity certificate remain.
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Appendix B. Implementation Notes
The TLS protocol cannot prevent many common security mistakes. This
section provides several recommendations to assist implementors.
B.1. Random Number Generation and Seeding
TLS requires a cryptographically secure pseudorandom number generator
(PRNG). Care must be taken in designing and seeding PRNGs. PRNGs
based on secure hash operations, most notably SHA-256, are
acceptable, but cannot provide more security than the size of the
random number generator state.
To estimate the amount of seed material being produced, add the
number of bits of unpredictable information in each seed byte. For
example, keystroke timing values taken from a PC compatible 18.2 Hz
timer provide 1 or 2 secure bits each, even though the total size of
the counter value is 16 bits or more. Seeding a 128-bit PRNG would
thus require approximately 100 such timer values.
[RFC4086] provides guidance on the generation of random values.
B.2. Certificates and Authentication
Implementations are responsible for verifying the integrity of
certificates and should generally support certificate revocation
messages. Certificates should always be verified to ensure proper
signing by a trusted Certificate Authority (CA). The selection and
addition of trusted CAs should be done very carefully. Users should
be able to view information about the certificate and root CA.
B.3. Cipher Suite Support
TLS supports a range of key sizes and security levels, including some
that provide no or minimal security. A proper implementation will
probably not support many cipher suites. 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. See also
Appendix C.4.
B.4. 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
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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 5.2.1)? 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 certificate request
handshake messages can be large enough to require fragmentation.
- Do you ignore the TLS record layer version number in all TLS
records? (see Appendix C)
- Have you ensured that all support for SSL, RC4, EXPORT ciphers,
and MD5 (via the "signature_algorithm" 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 C)
- Do you handle TLS extensions in ClientHello correctly, including
unknown extensions or omitting the extensions field completely?
- 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 6.3.4.1.2)?
- 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?
- When processing a ClientHello containing a version of { 3, 5 } or
higher, do you respond with the highest common version of TLS
rather than requiring an exact match?
- Do you ignore unrecognized cipher suites (see Section 6.3.1.1),
named groups (see Section 6.3.2.3), and signature algorithms (see
Section 6.3.2.2)?
Cryptographic details:
- What countermeasures do you use to prevent timing attacks against
RSA signing operations [TIMING]?
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- When verifying RSA signatures, do you accept both NULL and missing
parameters (see Section 4.8)? Do you verify that the RSA padding
doesn't have additional data after the hash value? [FI06]
- When using Diffie-Hellman key exchange, do you correctly preserve
leading zero bytes in the negotiated key (see Section 7.3.1)?
- Does your TLS client check that the Diffie-Hellman parameters sent
by the server are acceptable (see Appendix D.1.1.1)?
- Do you use a strong and, most importantly, properly seeded random
number generator (see Appendix B.1) Diffie-Hellman private values,
the ECDSA "k" parameter, and other security-critical values?
- Do you zero-pad Diffie-Hellman public key values to the group size
(see Section 6.3.2.4.1)?
B.5. Client Tracking Prevention
Clients SHOULD NOT reuse a session ticket for multiple connections.
Reuse of a session ticket allows passive observers to correlate
different connections. Servers that issue session tickets SHOULD
offer at least as many session 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 session tickets with every connection. This ensures that
clients are always able to use a new session ticket when creating a
new connection.
Appendix C. 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 the client supports the
highest protocol version available in the server.
Prior versions of TLS used the record layer version number for
various purposes. (TLSPlaintext.record_version &
TLSCiphertext.record_version) As of TLS 1.3, this field is deprecated
and its value MUST be ignored by all implementations. Version
negotiation is performed using only the handshake versions.
(ClientHello.client_version & ServerHello.server_version) 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
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number to 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 6.3.4.1.1)
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.
C.1. Negotiating with an older server
A TLS 1.3 client who wishes to negotiate with such older servers will
send a normal TLS 1.3 ClientHello containing { 3, 4 } (TLS 1.3) in
ClientHello.client_version. If the server does not support this
version 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 resuming a session SHOULD initiate the connection using the
version that was previously negotiated.
Note that 0-RTT data is not compatible with older servers. See
Appendix C.3.
If the version chosen by the server is not supported by the client
(or not acceptable), the client MUST send a "protocol_version" alert
message and close the connection.
If a TLS server receives a ClientHello containing a version number
greater than the highest version supported by the server, it MUST
reply according to the highest version supported by the server.
Some legacy server implementations are known to not implement the TLS
specification properly and might abort connections upon encountering
TLS extensions or versions which it is 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.
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C.2. Negotiating with an older client
A TLS server can also receive a ClientHello containing a version
number smaller than the highest supported version. If the server
wishes to negotiate with old clients, it will proceed as appropriate
for the highest version supported by the server that is not greater
than ClientHello.client_version. For example, if the server supports
TLS 1.0, 1.1, and 1.2, and client_version is TLS 1.0, the server will
proceed with a TLS 1.0 ServerHello. If the server only supports
versions greater than client_version, it MUST send a
"protocol_version" alert message and close the connection.
Note that earlier versions of TLS did not clearly specify the record
layer version number value in all cases
(TLSPlaintext.record_version). Servers will receive various TLS 1.x
versions in this field, however its value MUST always be ignored.
C.3. Zero-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 fail to complete the
handshake. This can cause issues when a client offers 0-RTT,
particularly against multi-server deployments. For example, a
deployment may deploy TLS 1.3 gradually with some servers
implementing TLS 1.3 and some implementing TLS 1.2, or a TLS 1.3
deployment may be downgraded to TLS 1.2.
If a client accepts older versions of TLS and receives an older
ServerHello after sending a ClientHello with 0-RTT data, it MAY retry
the connection without 0-RTT. It is NOT RECOMMENDED to retry the
connection in response to a more generic error or advertise lower
versions of TLS.
Multi-server deployments are RECOMMENDED to ensure a stable
deployment of TLS 1.3 without 0-RTT prior to enabling 0-RTT.
C.4. Backwards Compatibility Security Restrictions
If an implementation negotiates use of TLS 1.2, then negotiation of
cipher suites also supported by TLS 1.3 SHOULD be preferred, if
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.
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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 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
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 or accept any records with a version
less than { 3, 0 }.
The security of SSL 3.0 [SSL3] is considered insufficient for the
reasons enumerated in [RFC7568], and MUST NOT be negotiated for any
reason.
Implementations MUST NOT send a ClientHello.client_version or
ServerHello.server_version set to { 3, 0 } or less. Any endpoint
receiving a Hello message with ClientHello.client_version or
ServerHello.server_version set to { 3, 0 } MUST respond with a
"protocol_version" alert message and close the connection.
Implementations MUST NOT use the Truncated HMAC extension, defined in
Section 7 of [RFC6066], as it is not applicable to AEAD ciphers and
has been shown to be insecure in some scenarios.
Appendix D. Security Analysis
[[TODO: The entire security analysis needs a rewrite.]]
The TLS protocol is designed to establish a secure connection between
a client and a server communicating over an insecure channel. This
document makes several traditional assumptions, including that
attackers have substantial computational resources and cannot obtain
secret information from sources outside the protocol. Attackers are
assumed to have the ability to capture, modify, delete, replay, and
otherwise tamper with messages sent over the communication channel.
This appendix outlines how TLS has been designed to resist a variety
of attacks.
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D.1. Handshake Protocol
The TLS Handshake Protocol is responsible for selecting a cipher spec
and generating a master secret, which together comprise the primary
cryptographic parameters associated with a secure session. The TLS
Handshake Protocol can also optionally authenticate parties who have
certificates signed by a trusted certificate authority.
D.1.1. Authentication and Key Exchange
TLS supports three authentication modes: authentication of both
parties, server authentication with an unauthenticated client, and
total anonymity. Whenever the server is authenticated, the channel
is secure against man-in-the-middle attacks, but completely anonymous
sessions are inherently vulnerable to such attacks. Anonymous
servers cannot authenticate clients. If the server is authenticated,
its certificate message must provide a valid certificate chain
leading to an acceptable certificate authority. Similarly,
authenticated clients must supply an acceptable certificate to the
server. Each party is responsible for verifying that the other's
certificate is valid and has not expired or been revoked.
[[TODO: Rewrite this because the master_secret is not used this way
any more after Hugo's changes.]] The general goal of the key exchange
process is to create a master_secret known to the communicating
parties and not to attackers (see Section 7.1). The master_secret is
required to generate the Finished messages and record protection keys
(see Section 6.3.4.3 and Section 7.3). By sending a correct Finished
message, parties thus prove that they know the correct master_secret.
D.1.1.1. Diffie-Hellman Key Exchange with Authentication
When Diffie-Hellman key exchange is used, the client and server use
the "key_share" extension to send temporary Diffie-Hellman
parameters. The signature in the certificate verify message (if
present) covers the entire handshake up to that point and thus
attests the certificate holder's desire to use the ephemeral DHE
keys.
Peers SHOULD validate each other's public key Y by ensuring that 1 <
Y < p-1. This simple check ensures that the remote peer is properly
behaved and isn't forcing the local system into a small subgroup.
Additionally, using a fresh key for each handshake provides Perfect
Forward Secrecy. Implementations SHOULD generate a new X for each
handshake when using DHE cipher suites.
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D.1.2. Version Rollback Attacks
Because TLS includes substantial improvements over SSL Version 2.0,
attackers may try to make TLS-capable clients and servers fall back
to Version 2.0. This attack can occur if (and only if) two TLS-
capable parties use an SSL 2.0 handshake. (See also Appendix C.4.)
Although the solution using non-random PKCS #1 block type 2 message
padding is inelegant, it provides a reasonably secure way for Version
3.0 servers to detect the attack. This solution is not secure
against attackers who can brute-force the key and substitute a new
ENCRYPTED-KEY-DATA message containing the same key (but with normal
padding) before the application-specified wait threshold has expired.
Altering the padding of the least-significant 8 bytes of the PKCS
padding does not impact security for the size of the signed hashes
and RSA key lengths used in the protocol, since this is essentially
equivalent to increasing the input block size by 8 bytes.
D.1.3. Detecting Attacks Against the Handshake Protocol
An attacker might try to influence the handshake exchange to make the
parties select different encryption algorithms than they would
normally choose.
For this attack, an attacker must actively change one or more
handshake messages. If this occurs, the client and server will
compute different values for the handshake message hashes. As a
result, the parties will not accept each others' Finished messages.
Without the static secret, the attacker cannot repair the Finished
messages, so the attack will be discovered.
D.2. Protecting Application Data
The shared secrets are hashed with the handshake transcript to
produce unique record protection secrets for each connection.
Outgoing data is protected using an AEAD algorithm before
transmission. The authentication data includes the sequence number,
message type, message length, and the message contents. The message
type field is necessary to ensure that messages intended for one TLS
record layer client are not redirected to another. The sequence
number ensures that attempts to delete or reorder messages will be
detected. Since sequence numbers are 64 bits long, they should never
overflow. Messages from one party cannot be inserted into the
other's output, since they use independent keys.
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D.3. Denial of Service
TLS is susceptible to a number of denial-of-service (DoS) attacks.
In particular, an attacker who initiates a large number of TCP
connections can cause a server to consume large amounts of CPU doing
asymmetric crypto operations. However, because TLS is generally used
over TCP, it is difficult for the attacker to hide their point of
origin if proper TCP SYN randomization is used [RFC1948] by the TCP
stack.
Because TLS runs over TCP, it is also susceptible to a number of DoS
attacks on individual connections. In particular, attackers can
forge RSTs, thereby terminating connections, or forge partial TLS
records, thereby causing the connection to stall. These attacks
cannot in general be defended against by a TCP-using protocol.
Implementors or users who are concerned with this class of attack
should use IPsec AH [RFC4302] or ESP [RFC4303].
D.4. Final Notes
For TLS to be able to provide a secure connection, both the client
and server systems, keys, and applications must be secure. In
addition, the implementation must be free of security errors.
The system is only as strong as the weakest key exchange and
authentication algorithm supported, and only trustworthy
cryptographic functions should be used. Short public keys and
anonymous servers should be used with great caution. Implementations
and users must be careful when deciding which certificates and
certificate authorities are acceptable; a dishonest certificate
authority can do tremendous damage.
Appendix E. 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://www1.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 F. Contributors
- Martin Abadi
University of California, Santa Cruz
abadi@cs.ucsc.edu
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- Christopher Allen (co-editor of TLS 1.0)
Alacrity Ventures
ChristopherA@AlacrityManagement.com
- Steven M. Bellovin
Columbia University
smb@cs.columbia.edu
- David Benjamin
Google
davidben@google.com
- Benjamin Beurdouche
- 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
- Pete Chown
Skygate Technology Ltd
pc@skygate.co.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
Securify
taher@securify.com
- Pasi Eronen
Nokia
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pasi.eronen@nokia.com
- Cedric Fournet
Microsoft
fournet@microsoft.com
- Anil Gangolli
anil@busybuddha.org
- David M. Garrett
- 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
- Kipp Hickman
- Alfred Hoenes
- David Hopwood
Independent Consultant
david.hopwood@blueyonder.co.uk
- Subodh Iyengar
Facebook
subodh@fb.com
- Daniel Kahn Gillmor
ACLU
dkg@fifthhorseman.net
- Phil Karlton (co-author of SSL 3.0)
- Paul Kocher (co-author of SSL 3.0)
Cryptography Research
paul@cryptography.com
- Hugo Krawczyk
IBM
hugo@ee.technion.ac.il
- Adam Langley (co-author of [RFC7627])
Google
agl@google.com
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- Ilari Liusvaara
Independent
ilariliusvaara@welho.com
- Jan Mikkelsen
Transactionware
janm@transactionware.com
- Bodo Moeller (co-author of [RFC4492])
Google
bodo@openssl.org
- Erik Nygren
Akamai Technologies
erik+ietf@nygren.org
- Magnus Nystrom
RSA Security
magnus@rsasecurity.com
- 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
- Jim Roskind
Netscape Communications
jar@netscape.com
- Michael Sabin
- Dan Simon
Microsoft, Inc.
dansimon@microsoft.com
- Nick Sullivan
CloudFlare Inc.
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nick@cloudflare.com
- Bjoern Tackmann
University of California, San Diego
btackmann@eng.ucsd.edu
- Martin Thomson
Mozilla
mt@mozilla.com
- Tom Weinstein
- Hoeteck Wee
Ecole Normale Superieure, Paris
hoeteck@alum.mit.edu
- Tim Wright
Vodafone
timothy.wright@vodafone.com
Author's Address
Eric Rescorla
RTFM, Inc.
EMail: ekr@rtfm.com
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