Network Working Group E. Rescorla
Internet-Draft RTFM, Inc.
Obsoletes: 5077, 5246, 5746 (if October 19, 2015
approved)
Updates: 4492 (if approved)
Intended status: Standards Track
Expires: April 21, 2016
The Transport Layer Security (TLS) Protocol Version 1.3
draft-ietf-tls-tls13-10
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
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This Internet-Draft will expire on April 21, 2016.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Conventions and Terminology . . . . . . . . . . . . . . . 5
1.2. Major Differences from TLS 1.2 . . . . . . . . . . . . . 6
2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3. Goals of This Document . . . . . . . . . . . . . . . . . . . 9
4. Presentation Language . . . . . . . . . . . . . . . . . . . . 9
4.1. Basic Block Size . . . . . . . . . . . . . . . . . . . . 10
4.2. Miscellaneous . . . . . . . . . . . . . . . . . . . . . . 10
4.3. Vectors . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.4. Numbers . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.5. Enumerateds . . . . . . . . . . . . . . . . . . . . . . . 12
4.6. Constructed Types . . . . . . . . . . . . . . . . . . . . 12
4.6.1. Variants . . . . . . . . . . . . . . . . . . . . . . 13
4.7. Constants . . . . . . . . . . . . . . . . . . . . . . . . 14
4.8. Primitive Types . . . . . . . . . . . . . . . . . . . . . 14
4.9. Cryptographic Attributes . . . . . . . . . . . . . . . . 15
4.9.1. Digital Signing . . . . . . . . . . . . . . . . . . . 15
4.9.2. Authenticated Encryption with Additional Data (AEAD) 16
5. The TLS Record Protocol . . . . . . . . . . . . . . . . . . . 16
5.1. Connection States . . . . . . . . . . . . . . . . . . . . 17
5.2. Record Layer . . . . . . . . . . . . . . . . . . . . . . 19
5.2.1. Fragmentation . . . . . . . . . . . . . . . . . . . . 19
5.2.2. Record Payload Protection . . . . . . . . . . . . . . 21
5.2.3. Record Padding . . . . . . . . . . . . . . . . . . . 23
6. The TLS Handshaking Protocols . . . . . . . . . . . . . . . . 24
6.1. Alert Protocol . . . . . . . . . . . . . . . . . . . . . 25
6.1.1. Closure Alerts . . . . . . . . . . . . . . . . . . . 26
6.1.2. Error Alerts . . . . . . . . . . . . . . . . . . . . 27
6.2. Handshake Protocol Overview . . . . . . . . . . . . . . . 31
6.2.1. Incorrect DHE Share . . . . . . . . . . . . . . . . . 34
6.2.2. Zero-RTT Exchange . . . . . . . . . . . . . . . . . . 35
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6.2.3. Resumption and PSK . . . . . . . . . . . . . . . . . 37
6.3. Handshake Protocol . . . . . . . . . . . . . . . . . . . 38
6.3.1. Hello Messages . . . . . . . . . . . . . . . . . . . 39
6.3.2. Hello Extensions . . . . . . . . . . . . . . . . . . 44
6.3.3. Encrypted Extensions . . . . . . . . . . . . . . . . 57
6.3.4. Server Certificate . . . . . . . . . . . . . . . . . 57
6.3.5. Certificate Request . . . . . . . . . . . . . . . . . 60
6.3.6. Server Configuration . . . . . . . . . . . . . . . . 62
6.3.7. Server Certificate Verify . . . . . . . . . . . . . . 63
6.3.8. Server Finished . . . . . . . . . . . . . . . . . . . 64
6.3.9. Client Certificate . . . . . . . . . . . . . . . . . 65
6.3.10. Client Certificate Verify . . . . . . . . . . . . . . 66
6.3.11. New Session Ticket Message . . . . . . . . . . . . . 67
7. Cryptographic Computations . . . . . . . . . . . . . . . . . 68
7.1. Key Schedule . . . . . . . . . . . . . . . . . . . . . . 68
7.2. Traffic Key Calculation . . . . . . . . . . . . . . . . . 70
7.2.1. The Handshake Hash . . . . . . . . . . . . . . . . . 71
7.2.2. Diffie-Hellman . . . . . . . . . . . . . . . . . . . 71
7.2.3. Elliptic Curve Diffie-Hellman . . . . . . . . . . . . 72
8. Mandatory Algorithms . . . . . . . . . . . . . . . . . . . . 72
8.1. MTI Cipher Suites . . . . . . . . . . . . . . . . . . . . 72
8.2. MTI Extensions . . . . . . . . . . . . . . . . . . . . . 72
9. Application Data Protocol . . . . . . . . . . . . . . . . . . 73
10. Security Considerations . . . . . . . . . . . . . . . . . . . 74
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 74
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 75
12.1. Normative References . . . . . . . . . . . . . . . . . . 75
12.2. Informative References . . . . . . . . . . . . . . . . . 77
Appendix A. Protocol Data Structures and Constant Values . . . . 81
A.1. Record Layer . . . . . . . . . . . . . . . . . . . . . . 81
A.2. Alert Messages . . . . . . . . . . . . . . . . . . . . . 81
A.3. Handshake Protocol . . . . . . . . . . . . . . . . . . . 82
A.3.1. Hello Messages . . . . . . . . . . . . . . . . . . . 83
A.3.2. Key Exchange Messages . . . . . . . . . . . . . . . . 87
A.3.3. Authentication Messages . . . . . . . . . . . . . . . 87
A.3.4. Handshake Finalization Messages . . . . . . . . . . . 88
A.3.5. Ticket Establishment . . . . . . . . . . . . . . . . 88
A.4. Cipher Suites . . . . . . . . . . . . . . . . . . . . . . 88
A.4.1. Unauthenticated Operation . . . . . . . . . . . . . . 90
A.5. The Security Parameters . . . . . . . . . . . . . . . . . 91
A.6. Changes to RFC 4492 . . . . . . . . . . . . . . . . . . . 91
Appendix B. Implementation Notes . . . . . . . . . . . . . . . . 92
B.1. Random Number Generation and Seeding . . . . . . . . . . 92
B.2. Certificates and Authentication . . . . . . . . . . . . . 92
B.3. Cipher Suite Support . . . . . . . . . . . . . . . . . . 93
B.4. Implementation Pitfalls . . . . . . . . . . . . . . . . . 93
Appendix C. Backward Compatibility . . . . . . . . . . . . . . . 94
C.1. Negotiating with an older server . . . . . . . . . . . . 95
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C.2. Negotiating with an older client . . . . . . . . . . . . 95
C.3. Backwards Compatibility Security Restrictions . . . . . . 96
Appendix D. Security Analysis . . . . . . . . . . . . . . . . . 96
D.1. Handshake Protocol . . . . . . . . . . . . . . . . . . . 97
D.1.1. Authentication and Key Exchange . . . . . . . . . . . 97
D.1.2. Version Rollback Attacks . . . . . . . . . . . . . . 98
D.1.3. Detecting Attacks Against the Handshake Protocol . . 98
D.2. Protecting Application Data . . . . . . . . . . . . . . . 98
D.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 99
D.4. Final Notes . . . . . . . . . . . . . . . . . . . . . . . 99
Appendix E. Working Group Information . . . . . . . . . . . . . 99
Appendix F. Contributors . . . . . . . . . . . . . . . . . . . . 100
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 protocol (such as 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.
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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
data. The TLS Handshake Protocol provides connection security that
has three basic properties:
- The peer's identity can be authenticated using asymmetric, or
public key, cryptography (e.g., RSA [RSA], ECDSA [ECDSA]). This
authentication can be made optional, but is generally required for
at least one of the peers.
- 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.
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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.
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-10
- Remove ClientCertificateTypes field from CertificateRequest and
add extensions.
- Merge client and server key shares into a single extension.
draft-09
- Change to RSA-PSS signatures for handshake messages.
- Remove support for DSA.
- Update key schedule per suggestions by Hugo, Hoeteck, and Bjoern
Tackmann.
- Add support for per-record padding.
- Switch to encrypted record ContentType.
- Change HKDF labeling to include protocol version and value
lengths.
- Shift the final decision to abort a handshake due to incompatible
certificates to the client rather than having servers abort early.
- Deprecate SHA-1 with signatures.
- Add MTI algorithms.
draft-08
- Remove support for weak and lesser used named curves.
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- 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.
- 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
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- 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.
- 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.
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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.
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.
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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];
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
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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];
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., leading zero octets are not
required even if the most significant bit is set).
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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;
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.
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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]];
For example:
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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. Primitive Types
The following common primitive types are defined and used
subsequently:
enum { false(0), true(1) } Boolean;
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4.9. 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
appropriate key word designation before the field's type
specification. Cryptographic keys are implied by the current session
state (see Section 5.1).
4.9.1. Digital Signing
A digitally-signed element is encoded as a struct DigitallySigned:
struct {
SignatureAndHashAlgorithm algorithm;
opaque signature<0..2^16-1>;
} DigitallySigned;
The algorithm field specifies the algorithm used (see Section 6.3.2.1
for the definition of this field). Note that the algorithm field was
introduced in TLS 1.2, and is not in earlier versions. 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 NUL-terminated context string in order to
disambiguate signatures for different purposes. The context string
will be specified whenever a digitally-signed element is used.
Finally, the specified contents of the digitally-signed structure
follow the NUL at the end of the context string. (See the example at
the end of this section.)
In RSA signing, the opaque vector contains the signature generated
using the RSASSA-PSS signature scheme defined in [RFC3447] with MGF1.
The digest used in the mask generation function MUST be the same as
the digest which is being signed (i.e., what appears in
algorithm.signature). The length of the salt MUST be equal to the
octet length of the digest output. Note that previous versions of
TLS used RSASSA-PKCS1-v1_5, not RSASSA-PSS.
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All ECDSA computations MUST be performed according to ANSI X9.62
[X962] or its successors. Data to be signed/verified is hashed, and
the result run directly through the ECDSA algorithm with no
additional hashing. The SignatureAndHashAlgorithm parameter in the
DigitallySigned object indicates the digest algorithm which was used
in the signature.
In the following example
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 and hash
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.
4.9.2. 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 is a layered protocol. At each layer,
messages may include fields for length, description, and content.
The TLS Record Protocol takes messages to be transmitted, fragments
the data into manageable blocks, protects the records, and transmits
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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
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
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An algorithm used to generate keys from the appropriate secret
(see Section 7.1 and Section 7.2).
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.
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:
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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 7.2.
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:
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
incremented by one thereafter. 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 terminate the connection. A sequence number is incremented
after each record: specifically, the first record transmitted
under a particular connection state MUST use sequence number 0.
NOTE: This is a change from previous versions of TLS, where
sequence numbers were reset whenever keys were changed.
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. Client
message boundaries are not preserved in the record layer (i.e.,
multiple client 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),
early_handshake(25),
(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
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fragments of Application data MAY be sent as they are potentially
useful as a traffic analysis countermeasure.
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.
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 used 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.
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length
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 additional authenticated data, which we denote as
additional_data, is defined as follows:
additional_data = seq_num + TLSPlaintext.record_version
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where "+" denotes concatenation.
Note: In versions of TLS prior to 1.3, the additional_data included a
length field. This presents a problem for cipher constructions with
data-dependent padding (such as CBC). TLS 1.3 removes the length
field and relies on the AEAD cipher to provide integrity for the
length of the data.
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
incorporate padding, the amount of overhead could vary with different
lengths of plaintext. Symbolically,
AEADEncrypted = AEAD-Encrypt(write_key, nonce, plaintext of fragment,
additional_data)
In order to decrypt and verify, the cipher takes as input the key,
nonce, the "additional_data", 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,
additional_data)
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
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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.
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.
Versions of TLS prior to 1.3 had limited support for padding. This
padding scheme was selected because it allows padding of any
encrypted TLS record by an arbitrary size (from zero up to TLS record
size limits) without introducing new content types. The design also
enforces all-zero padding octets, which allows for quick detection of
padding errors.
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 three 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.
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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 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
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),
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
This message notifies the recipient that the sender will not send
any more messages on this connection. Any data received after a
closure MUST be ignored.
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user_canceled
This message 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.
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
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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
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.
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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
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
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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
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.
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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.
[[TODO: This doesn't really make sense with the current PSK
negotiation scheme where the client provides multiple PSKs in
flight 1. https://github.com/tlswg/tls13-spec/issues/230]]
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 two secrets which are used to
derive all the keys.
Ephemeral Secret (ES): A secret which is derived from fresh (EC)DHE
shares for this connection. Keying material derived from ES is
intended to be forward secure (with the exception of pre-shared key
only modes).
Static Secret (SS): A secret which may be derived from static or
semi-static keying material, such as a pre-shared key or the server's
semi-static (EC)DH share.
In some cases, as with the DH handshake shown in Figure 1, these
secrets are the same, but having both allows for a uniform key
derivation scheme for all cipher modes.
The basic TLS Handshake for DH is shown in Figure 1:
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Client Server
ClientHello
+ KeyShare -------->
ServerHello
+ KeyShare
{EncryptedExtensions}
{ServerConfiguration*}
{Certificate*}
{CertificateRequest*}
{CertificateVerify*}
<-------- {Finished}
{Certificate*}
{CertificateVerify*}
{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 the ephemeral secret.
[] Indicates messages protected using keys
derived from the master secret.
Figure 1: Message flow for full TLS Handshake
The first message sent by the client is the ClientHello
Section 6.3.1.1 which contains a random nonce (ClientHello.random),
its offered protocol version, cipher suite, and extensions, and one
or more Diffie-Hellman key shares in the KeyShare extension
Section 6.3.2.3.
The server processes the ClientHello and determines the appropriate
cryptographic parameters for the connection. It then responds with
the following messages:
ServerHello
indicates the negotiated connection parameters. [Section 6.3.1.2]
If DH is in use, this will contain a KeyShare extension with the
server's ephemeral Diffie-Hellman share which MUST be in the same
group as one of the shares offered by the client. The server's
KeyShare and the client's KeyShare corresponding to the negotiated
key exchange are used together to derive the Static Secret and
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Ephemeral Secret (in this mode they are the same).
[Section 6.3.2.3]
ServerConfiguration
supplies a configuration for 0-RTT handshakes (see Section 6.2.2).
[Section 6.3.6]
EncryptedExtensions
responses to any extensions which are not required in order to
determine the cryptographic parameters. [Section 6.3.3]
Certificate
the server certificate. This message will be omitted if the
server is not authenticating via a certificates. [Section 6.3.4]
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. [[OPEN ISSUE: See
https://github.com/tlswg/tls13-spec/issues/184]]. [Section 6.3.5]
CertificateVerify
a signature over the entire handshake using the public key in the
Certificate message. This message will be omitted if the server
is not authenticating via a certificate. [Section 6.3.7]
Finished
a MAC over the entire handshake computed using the Static Secret.
This message provides key confirmation and In some modes (see
Section 6.2.2) it also authenticates the handshake using the the
Static Secret. [Section 6.3.8]
Upon receiving the server's messages, the client responds with his
final flight of messages:
Certificate
the client's certificate. This message will be omitted if the
client is not authenticating via a certificates. [Section 6.3.9]
CertificateVerify
a signature over the entire handshake using the private key
corresponding to the public key in the Certificate message. This
message will be omitted if the client is not authenticating via a
certificate. [Section 6.3.10]
Finished
a MAC over the entire handshake computed using the Static Secret
and providing key confirmation. [Section 6.3.8]
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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. If client authentication
is requested, the server MUST NOT send application data before it
receives the client's Finished.
[[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 KeyShare 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 KeyShare extension, as shown in Figure 2:
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Client Server
ClientHello
+ KeyShare -------->
<-------- HelloRetryRequest
ClientHello
+ KeyShare -------->
ServerHello
+ KeyShare
{EncryptedExtensions}
{ServerConfiguration*}
{Certificate*}
{CertificateRequest*}
{CertificateVerify*}
<-------- {Finished}
{Certificate*}
{CertificateVerify*}
{Finished} -------->
[Application Data] <-------> [Application Data]
Figure 2: Message flow for a full handshake with mismatched
parameters
[[OPEN ISSUE: Should we restart the handshake hash?
https://github.com/tlswg/tls13-spec/issues/104.]] [[OPEN ISSUE: We
need to make sure that this flow doesn't introduce downgrade issues.
Potential options include continuing the handshake hashes (as long as
clients don't change their opinion of the server's capabilities with
aborted handshakes) and requiring the client to send the same
ClientHello (as is currently done) and then checking you get the same
negotiated parameters.]]
If no common cryptographic parameters can be negotiated, the server
will send a "handshake_failure" or "insufficient_security" fatal
alert (see Section 6.1).
TLS also allows several optimized variants of the basic handshake, as
described below.
6.2.2. Zero-RTT Exchange
TLS 1.3 supports a "0-RTT" mode in which the client can send
application data as well as its Certificate and CertificateVerify (if
client authentication is requested) on its first flight, thus
reducing handshake latency. In order to enable this functionality,
the server provides a ServerConfiguration message containing a long-
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term (EC)DH share. On future connections to the same server, the
client can use that share to encrypt the first-flight data.
Client Server
ClientHello
+ KeyShare
+ EarlyDataIndication
(EncryptedExtensions)
(Certificate*)
(CertificateVerify*)
(Application Data) -------->
ServerHello
+ KeyShare
+ EarlyDataIndication
{EncryptedExtensions}
{ServerConfiguration*}
{Certificate*}
{CertificateRequest*}
{CertificateVerify*}
<-------- {Finished}
{Finished} -------->
[Application Data] <-------> [Application Data]
() Indicates messages protected using keys
derived from the static secret.
Figure 3: Message flow for a zero round trip handshake
Note: because sequence numbers continue to increment between the
initial (early) application data and the application data sent after
the handshake has completed, an attacker cannot remove early
application data messages.
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 secure, because it is encrypted solely
with the server's semi-static (EC)DH share.
2. There are no guarantees of non-replay between connections.
Unless the server takes special measures outside those provided
by TLS (See Section 6.3.2.5.2), 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
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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 also cannot be
sent as if it were ordinary TLS data.
3. If the server key is compromised, and client authentication is
used, then the attacker can impersonate the client to the server
(as it knows the traffic key).
6.2.3. Resumption and PSK
Finally, TLS provides a pre-shared key (PSK) mode which allows a
client and server who share an existing secret (e.g., a key
established out of band) to establish a connection authenticated by
that key. 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 6.3.11). 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.
Figure 4 shows a pair of handshakes in which the first establishes a
PSK and the second uses it:
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Client Server
Initial Handshake:
ClientHello
+ KeyShare -------->
ServerHello
+ KeyShare
{EncryptedExtensions}
{ServerConfiguration*}
{Certificate*}
{CertificateRequest*}
{CertificateVerify*}
<-------- {Finished}
{Certificate*}
{CertificateVerify*}
{Finished} -------->
<-------- [NewSessionTicket]
[Application Data] <-------> [Application Data]
Subsequent Handshake:
ClientHello
+ KeyShare
+ PreSharedKeyExtension -------->
ServerHello
+ PreSharedKeyExtension
{EncryptedExtensions}
<-------- {Finished}
{Finished} -------->
[Application Data] <-------> [Application Data]
Figure 4: Message flow for resumption and PSK
As the server is authenticating via a PSK, it does not send a
Certificate or a CertificateVerify. PSK-based resumption cannot be
used to provide a new ServerConfiguration. Note that the client
supplies a KeyShare to the server as well, which allows the server to
decline resumption and fall back to a full handshake.
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
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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.
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),
server_configuration(17),
finished(20),
(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 server_configuration:ServerConfiguration;
case certificate: Certificate;
case certificate_request: CertificateRequest;
case certificate_verify: CertificateVerify;
case finished: Finished;
case session_ticket: NewSessionTicket;
} 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. Hello Messages
The hello phase messages are used to exchange security enhancement
capabilities between the client and server. When a new session
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begins, the record layer's connection state AEAD algorithm is
initialized to NULL_NULL.
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 KeyShare 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 KeyShare message). [[OPEN
ISSUE: New random values? See: https://github.com/tlswg/tls13-
spec/issues/185]] 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.
struct {
opaque random_bytes[32];
} Random;
random_bytes
32 bytes generated by a secure random number generator. See
Appendix B for additional information.
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 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.
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uint8 CipherSuite[2]; /* Cryptographic suite selector */
enum { null(0), (255) } CompressionMethod;
struct {
ProtocolVersion client_version = { 3, 4 }; /* TLS v1.3 */
Random random;
SessionID session_id;
CipherSuite cipher_suites<2..2^16-2>;
CompressionMethod compression_methods<1..2^8-1>;
Extension extensions<0..2^16-1>;
} ClientHello;
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.
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
A client-generated random structure.
session_id
Versions of TLS prior to TLS 1.3 supported a session resumption
feature which has been merged with Pre-Shared Keys in this version
(see Section 6.2.3). 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.
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 field MUST contain only the "null" compression
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method with the code point of 0. 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 may 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
Clients MAY 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. A server MUST accept ClientHello
messages both with and without the extensions field, and (as for all
other messages) it 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 KeyShare 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;
select (extensions_present) {
case false:
struct {};
case true:
Extension extensions<0..2^16-1>;
};
} ServerHello;
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The presence of extensions can be detected by determining whether
there are bytes following the cipher_suite field at the end of the
ServerHello.
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.)
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
message. The ServerHello MUST only include extensions which are
required to establish the cryptographic context.
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 "handshake_failure" alert.
Structure of this message:
struct {
ProtocolVersion server_version;
CipherSuite cipher_suite;
NamedGroup selected_group;
Extension extensions<0..2^16-1>;
} HelloRetryRequest;
[[OPEN ISSUE: Merge in DTLS Cookies?]]
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selected_group
The group which the client MUST use for its new ClientHello.
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.
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
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 a new KeyShare
extension to the server. The client MUST append a new KeyShareEntry
list which is consistent with 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.
[[OPEN ISSUE: https://github.com/tlswg/tls13-spec/issues/104]]
6.3.2. Hello Extensions
The extension format is:
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struct {
ExtensionType extension_type;
opaque extension_data<0..2^16-1>;
} Extension;
enum {
supported_groups(10),
signature_algorithms(13),
early_data(TBD),
pre_shared_key(TBD),
key_share(TBD),
(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
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
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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
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.
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6.3.2.1. Signature Algorithms
The client uses the "signature_algorithms" extension to indicate to
the server which signature/hash algorithm pairs may be used in
digital signatures.
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 {
none(0),
sha1(2),
sha256(4), sha384(5), sha512(6),
(255)
} HashAlgorithm;
enum {
rsa(1),
dsa(2),
ecdsa(3),
rsapss(4),
(255)
} SignatureAlgorithm;
struct {
HashAlgorithm hash;
SignatureAlgorithm signature;
} SignatureAndHashAlgorithm;
SignatureAndHashAlgorithm
supported_signature_algorithms<2..2^16-2>;
[[TODO: IANA considerations for new SignatureAlgorithm value]]
Each SignatureAndHashAlgorithm value lists a single hash/signature
pair that the client is willing to verify. The values are indicated
in descending order of preference.
Note: Because not all signature algorithms and hash algorithms may be
accepted by an implementation (e.g., ECDSA with SHA-256, but not SHA-
384), algorithms here are listed in pairs.
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hash
This field indicates the hash algorithms which may be used. The
values indicate support for unhashed data, SHA-1, SHA-256, SHA-
384, and SHA-512 [SHS], respectively. The "none" value is
provided for future extensibility, in case of a signature
algorithm which does not require hashing before signing. Previous
versions of TLS supported MD5 and SHA-1. These algorithms are now
deprecated and MUST NOT be offered by TLS 1.3 implementations.
SHA-1 SHOULD NOT be offered, however clients willing to negotiate
use of TLS 1.2 MAY offer support for SHA-1 for backwards
compatibility with old servers.
signature
This field indicates the signature algorithm that may be used.
The values indicate RSASSA-PKCS1-v1_5 [RFC3447], DSA [DSS], ECDSA
[ECDSA], and RSASSA-PSS [RFC3447] respectively. Because all RSA
signatures used in signed TLS handshake messages (see
Section 4.9.1), as opposed to those in certificates, are RSASSA-
PSS, the "rsa" value refers solely to signatures which appear in
certificates. The use of DSA and anonymous is deprecated.
Previous versions of TLS supported DSA. DSA is deprecated as of
TLS 1.3 and SHOULD NOT be offered or negotiated by any
implementation.
The semantics of this extension are somewhat complicated because the
cipher suite indicates permissible signature algorithms but not hash
algorithms. Section 6.3.4 and Section 6.3.2.3 describe the
appropriate rules.
Clients offering support for SHA-1 for TLS 1.2 servers MUST do so by
listing those hash/signature pairs as the lowest priority (listed
after all other pairs 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).
Note: TLS 1.3 servers MAY receive TLS 1.2 ClientHellos which do not
contain this extension. If those servers are willing to negotiate
TLS 1.2, they MUST behave in accordance with the requirements of
[RFC5246] when negotiating that version.
6.3.2.2. 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.
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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].
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:
enum {
// Elliptic Curve Groups.
secp256r1 (23), secp384r1 (24), secp521r1 (25),
// Finite Field Groups.
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;
secp256r1, etc.
Indicates support of the corresponding named curve. Note that
some curves are also recommended in ANSI X9.62 [X962] and FIPS
186-4 [DSS]. Values 0xFE00 through 0xFEFF are reserved for
private use.
ffdhe2048, etc.
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.
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Items in named_curve_list are ordered according to the client's
preferences (most preferred choice first).
As an example, a client that only supports secp256r1 (aka NIST P-256;
value 23 = 0x0017) and secp384r1 (aka NIST P-384; value 24 = 0x0018)
and prefers to use secp256r1 would include a TLS extension consisting
of the following octets. Note that the first two octets indicate the
extension type (Supported Group Extension):
00 0A 00 06 00 04 00 17 00 18
NOTE: A server participating in an ECDHE-ECDSA key exchange may use
different curves for (i) the ECDSA key in its certificate, and (ii)
the ephemeral ECDH key in its KeyShare extension. The server must
consider the supported groups in both cases.
[[TODO: IANA Considerations.]]
6.3.2.3. 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 at
least one supported KeyShare value and 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 retried
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)
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 6.3.2.3.1;
Elliptic Curve Diffie-Hellman parameters are described in
Section 6.3.2.3.2.
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.
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The "extension_data" field of this extension contains a "KeyShare"
value:
struct {
select (role) {
case client:
KeyShareEntry client_shares<4..2^16-1>;
case server:
KeyShareEntry server_share;
}
} KeyShare;
client_shares
A list of offered KeyShareEntry values in descending order of
client preference. This vector MUST NOT be empty. Clients not
providing a KeyShare MUST instead omit this extension from the
ClientHello.
server_shares
A single KeyShareEntry value for the negotiated cipher suite.
Servers MUST NOT send a KeyShareEntry value for a group not
offered by the client.
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
integer DH groups. The key_exchange values for each KeyShareEntry
MUST by generated independently. Clients MUST NOT offer multiple
KeyShareEntry values for the same parameters. Clients MAY omit this
extension from the ClientHello, and in response to this, servers MUST
send a HelloRetryRequest requesting use of one of the groups the
client offered support for in its "supported_groups" extension. If
no common supported group is available, the server MUST produce a
fatal "handshake_failure" alert. (see Section 6.3.1.3)
[[TODO: Recommendation about what the client offers. Presumably
which integer DH groups and which curves.]]
6.3.2.3.1. 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 (dh_Y = g^X mod p), encoded as a big-endian integer.
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opaque dh_Y<1..2^16-1>;
6.3.2.3.2. 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.
The opaque value conveys the Elliptic Curve Diffie-Hellman public
value (ecdh_Y) represented as a byte string ECPoint.point.
opaque point <1..2^8-1>;
point
This is the byte string representation of an elliptic curve point
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.
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.
[[OPEN ISSUE: We will need to adjust the compressed/uncompressed
point issue if we have new curves that don't need point compression.
This depends on the CFRG's recommendations. The expectation is that
future curves will come with defined point formats and that existing
curves conform to X9.62.]]
6.3.2.4. 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:
psk_identity identity;
}
} PreSharedKeyExtension;
identity
An opaque label for the pre-shared key.
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
PreSharedKeyExtension with the identity that it selected. The client
MUST verify that the server has selected one of the identities that
the client supplied. If any other identity is returned, the client
MUST generate a fatal "unknown_psk_identity" alert and close the
connection.
6.3.2.5. Early Data Indication
In cases where TLS clients have previously interacted with the server
and the server has supplied a ServerConfiguration Section 6.3.6, the
client can send application data and its Certificate/
CertificateVerify messages (if client authentication is required).
If the client opts to do so, it MUST supply an Early Data Indication
extension.
The "extension_data" field of this extension contains an
"EarlyDataIndication" value:
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enum { client_authentication(1), early_data(2),
client_authentication_and_data(3), (255) } EarlyDataType;
struct {
select (Role) {
case client:
opaque configuration_id<1..2^16-1>;
CipherSuite cipher_suite;
Extension extensions<0..2^16-1>;
opaque context<0..255>;
EarlyDataType type;
case server:
struct {};
}
} EarlyDataIndication;
configuration_id
The label for the configuration in question.
cipher_suite
The cipher suite which the client is using to encrypt the early
data.
extensions
The extensions required to define the cryptographic configuration
for the clients early data (see below for details).
context
An optional context value that can be used for anti-replay (see
below).
type
The type of early data that is being sent. "client_authentication"
means that only handshake data is being sent. "early_data" means
that only data is being sent. "client_authentication_and_data"
means that both are being sent.
The client specifies the cryptographic configuration for the 0-RTT
data using the "configuration", "cipher_suite", and "extensions"
values. For configurations received in-band (in a previous TLS
connection) the client MUST:
- Send the same cryptographic determining parameters
(Section Section 6.3.2.5.1) with the previous connection. If a
0-RTT handshake is being used with a PSK that was negotiated via a
non-PSK handshake, then the client MUST use the same symmetric
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cipher parameters as were negotiated on that handshake but with a
PSK cipher suite.
- Indicate the same parameters as the server indicated in that
connection.
If TLS client authentication is being used, then either
"early_handshake" or "early_handshake_and_data" MUST be indicated in
order to send the client authentication data on the first flight. In
either case, the client Certificate and CertificateVerify (assuming
that the Certificate is non-empty) MUST be sent on the first flight.
A server which receives an initial flight with only "early_data" and
which expects certificate-based client authentication MUST NOT accept
early data.
In order to allow servers to readily distinguish between messages
sent in the first flight and in the second flight (in cases where the
server does not accept the EarlyDataIndication extension), the client
MUST send the handshake messages as content type "early_handshake".
A server which does not accept the extension proceeds by skipping all
records after the ClientHello and until the next client message of
type "handshake". [[OPEN ISSUE: This needs replacement when we add
encrypted content types.]]
A server which receives an EarlyDataIndication 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.
- 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.
Prior to accepting the EarlyDataIndication extension, the server MUST
perform the following checks:
- The configuration_id matches a known server configuration.
- The client's cryptographic determining parameters match the
parameters that the server has negotiated based on the rest of the
ClientHello.
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).
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[[TODO: How does the client behave if the indication is rejected.]]
[[OPEN ISSUE: This just specifies the signaling for 0-RTT but not the
the 0-RTT cryptographic transforms, including:
- What is in the handshake hash (including potentially some
speculative data from the server).
- What is signed in the client's CertificateVerify.
- Whether we really want the Finished to not include the server's
data at all.
What's here now needs a lot of cleanup before it is clear and
correct.]]
6.3.2.5.1. Cryptographic Determining Parameters
In order to allow the server to decrypt 0-RTT data, the client needs
to provide enough information to allow the server to decrypt the
traffic without negotiation. This is accomplished by having the
client indicate the "cryptographic determining parameters" in its
ClientHello, which are necessary to decrypt the client's packets.
This includes the following values:
- The cipher suite identifier.
- If PSK is being used, the server's version of the PreSharedKey
extension (indicating the PSK the client is using).
[[TODO: Are there other extensions we need? I've gone over the list
and I don't see any, but...]] [[TODO: This should be the same list as
what you need for !EncryptedExtensions. Consolidate this list.]]
6.3.2.5.2. Replay Properties
As noted in Section 6.2.2, TLS does not provide any inter-connection
mechanism for replay protection for data sent by the client in the
first flight. As a special case, implementations where the server
configuration, is delivered out of band (as has been proposed for
DTLS-SRTP [RFC5763]), MAY use a unique server configuration
identifier for each connection, thus preventing replay.
Implementations are responsible for ensuring uniqueness of the
identifier in this case.
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6.3.3. Encrypted Extensions
When this message will be sent:
If this message is sent, it MUST be sent immediately after the
ServerHello message. This is the first message that is encrypted
under keys derived from ES.
Meaning of this message:
The EncryptedExtensions message simply 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. [[OPEN
ISSUE: Should we just produce a canonical list of what goes where
and have it be an error to have it in the wrong place? That seems
simpler. Perhaps have a whitelist of which extensions can be
unencrypted and everything else MUST be encrypted.]]
Structure of this message:
struct {
Extension extensions<0..2^16-1>;
} EncryptedExtensions;
extensions
A list of extensions.
6.3.4. Server 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). This message will always immediately follow the
EncryptedExtensions message.
Meaning of this message:
This message conveys the server's certificate chain to the client.
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 {
ASN1Cert certificate_list<0..2^24-1>;
} Certificate;
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 same message type and structure will be used for the client's
response to a certificate request message. Note that a client MAY
send no certificates if it does not have an appropriate certificate
to send in response to the server's authentication request.
Note: PKCS #7 [PKCS7] is not used as the format for the certificate
vector because PKCS #6 [PKCS6] extended certificates are not used.
Also, PKCS #7 defines a SET rather than a SEQUENCE, making the task
of parsing the list more difficult.
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 RSA public key; the certificate MUST allow the
ECDHE_RSA key to be used for signing (i.e., the
digitalSignature bit MUST be set if the key
usage extension is present) with the signature
scheme and hash algorithm that will be employed
in the server's KeyShare extension.
Note: ECDHE_RSA is defined in [RFC4492].
ECDHE_ECDSA ECDSA-capable public key; the certificate MUST
allow the key to be used for signing with the
hash algorithm that will be employed in the
server's KeyShare extension. The public key
MUST use a curve and point format supported by
the client, as described in [RFC4492].
- 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 hash/
signature algorithm pair that appears in the "signature_algorithms"
extension provided by the client, if they are able to provide such a
chain (see Section 6.3.2.1). If the server cannot produce a
certificate chain that is signed only via the indicated supported
pairs, 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. 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.
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.
As SHA-1 and SHA-224 are deprecated, support for them is NOT
RECOMMENDED. Endpoints that reject chains due to use of a deprecated
hash MUST send a fatal "bad_certificate" alert message before closing
the connection. All servers 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.
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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 a ECDSA key).
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).
If the server has a single certificate, it SHOULD attempt to validate
that it meets these criteria.
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.
6.3.5. 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 immediately follow the server's Certificate
message.
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 {
SignatureAndHashAlgorithm
supported_signature_algorithms<2..2^16-2>;
DistinguishedName certificate_authorities<0..2^16-1>;
CertificateExtension certificate_extensions<0..2^16-1>;
} CertificateRequest;
supported_signature_algorithms
A list of the hash/signature algorithm pairs that the server is
able to verify, listed in descending order of preference. Any
certificates provided by the client MUST be signed using a hash/
signature algorithm pair 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 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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6.3.6. Server Configuration
When this message will be sent:
This message is used to provide a server configuration which the
client can use in future to skip handshake negotiation and
(optionally) to allow 0-RTT handshakes. The ServerConfiguration
message is sent as the last message before the CertificateVerify.
Structure of this Message:
enum { (65535) } ConfigurationExtensionType;
struct {
ConfigurationExtensionType extension_type;
opaque extension_data<0..2^16-1>;
} ConfigurationExtension;
struct {
opaque configuration_id<1..2^16-1>;
uint32 expiration_date;
NamedGroup group;
opaque server_key<1..2^16-1>;
EarlyDataType early_data_type;
ConfigurationExtension extensions<0..2^16-1>;
} ServerConfiguration;
configuration_id
The configuration identifier to be used in 0-RTT mode.
group
The group for the long-term DH key that is being established for
this configuration.
expiration_date
The last time when this configuration is expected to be valid (in
seconds since the Unix epoch). Servers MUST NOT use any value
more than 604800 seconds (7 days) in the future. Clients MUST NOT
cache configurations for longer than 7 days, regardless of the
expiration_date. [[OPEN ISSUE: Is this the right value? The idea
is just to minimize exposure.]]
server_key
The long-term DH key that is being established for this
configuration.
early_data_type
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The type of 0-RTT handshake that this configuration is to be used
for (see Section 6.3.2.5). If "client_authentication" or
"client_authentication_and_data", then the client should select
the certificate for future handshakes based on the
CertificateRequest parameters supplied in this handshake. The
server MUST NOT send either of these two options unless it also
requested a certificate on this handshake. [[OPEN ISSUE: Should
we relax this?]]
extensions
This field is a placeholder for future extensions to the
ServerConfiguration format.
The semantics of this message are to establish a shared state between
the client and server for use with the "known_configuration"
extension with the key specified in key and with the handshake
parameters negotiated by this handshake.
When the ServerConfiguration message is sent, the server MUST also
send a Certificate message and a CertificateVerify message, even if
the "known_configuration" extension was used for this handshake, thus
requiring a signature over the configuration before it can be used by
the client. Clients MUST NOT rely on the ServerConfiguration message
until successfully receiving and processing the server's Certificate,
CertificateVerify, and Finished. If there is a failure in processing
those messages, the client MUST discard the ServerConfiguration.
6.3.7. Server Certificate Verify
When this message will be sent:
This message is used to provide explicit proof that the server
possesses the private key corresponding to its certificate and
also provides integrity for the handshake up to this point. This
message is sent when the server is authenticated via a
certificate. When sent, it MUST be the last server handshake
message prior to the Finished.
Structure of this message:
struct {
digitally-signed struct {
opaque handshake_hash[hash_length];
};
} CertificateVerify;
Where handshake_hash is as described in Section 7.2.1 and includes
the messages sent or received, starting at ClientHello and up to,
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but not including, this message, including the type and length
fields of the handshake messages. This is a digest of the
concatenation of all the Handshake structures (as defined in
Section 6.3) exchanged thus far. The digest MUST be the Hash used
as the basis for HKDF.
The context string for the signature is "TLS 1.3, server
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
implementation need only maintain one running hash per hash type
for CertificateVerify, Finished and other messages.
The signature algorithm and hash algorithm MUST be a pair offered
in the client's "signature_algorithms" extension unless no valid
certificate chain can be produced without unsupported algorithms
(see Section 6.3.2.1). Note that there is a possibility for
inconsistencies here. For instance, the client might offer
ECDHE_ECDSA key exchange but omit any ECDSA pairs 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.
In addition, the hash and signature algorithms MUST be compatible
with the key in the server's end-entity certificate. RSA keys MAY
be used with any permitted hash algorithm, subject to restrictions
in the certificate, if any. RSA signatures MUST be based on
RSASSA-PSS, regardless of whether RSASSA-PKCS-v1_5 appears in
"signature_algorithms". SHA-1 MUST NOT be used in any signatures
in CertificateVerify, regardless of whether SHA-1 appears in
"signature_algorithms".
6.3.8. Server Finished
When this message will be sent:
The Server's Finished message is the final message sent by the
server and is essential for providing authentication of the server
side of the handshake and computed keys.
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
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send and receive application data over the connection. This data
will be protected under keys derived from the ephemeral secret
(see Section 7).
Structure of this message:
struct {
opaque verify_data[verify_data_length];
} Finished;
The verify_data value is computed as follows:
verify_data
HMAC(finished_secret, finished_label + '\0' + handshake_hash)
where HMAC [RFC2104] uses the Hash algorithm for the handshake.
See Section 7.2.1 for the definition of handshake_hash.
finished_label
For Finished messages sent by the client, the string "client
finished". For Finished messages sent by the server, the string
"server finished".
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. Also, HelloRequest
messages and the Finished message are omitted from handshake hashes.
6.3.9. Client Certificate
When this message will be sent:
This message is the first handshake message the client can send
after receiving the server's Finished. This message is only sent
if the server requests a certificate. If no suitable certificate
is available, the client MUST send a certificate message
containing no certificates. That is, the certificate_list
structure has a length of zero. 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.
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Client certificates are sent using the Certificate structure
defined in Section 6.3.4.
Meaning of this message:
This message conveys the client's certificate chain to the server;
the server will use it when verifying the CertificateVerify
message (when the client authentication is based on signing). The
certificate MUST be appropriate for the negotiated cipher suite's
key exchange algorithm, and any negotiated extensions.
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 6.3.5. 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 6.3.5.
Note that, as with the server certificate, there are certificates
that use algorithms/algorithm combinations that cannot be currently
used with TLS.
6.3.10. Client Certificate Verify
When this message will be sent:
This message is used to provide explicit verification of a client
certificate. This message is only sent following a client
certificate that has signing capability (i.e., all certificates
except those containing fixed Diffie-Hellman parameters). When
sent, it MUST immediately follow the client's Certificate message.
The contents of the message are computed as described in
Section 6.3.7, except that the context string is "TLS 1.3, client
CertificateVerify".
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The hash and signature algorithms used in the signature MUST be
one of those present in the supported_signature_algorithms field
of the CertificateRequest message. In addition, the hash and
signature algorithms MUST be compatible with the key in the
client's end-entity certificate. RSA keys MAY be used with any
permitted hash algorithm, subject to restrictions in the
certificate, if any. RSA signatures MUST be based on RSASSA-PSS,
regardless of whether RSASSA-PKCS-v1_5 appears in
"signature_algorithms". SHA-1 MUST NOT be used in any signatures
in CertificateVerify, regardless of whether SHA-1 appears in
"signature_algorithms".
6.3.11. New Session Ticket Message
After the server has received the client Finished message, it MAY
send a NewSessionTicket message. This message MUST be sent before
the server sends any application data traffic, and is encrypted under
the application traffic key. This message creates a pre-shared key
(PSK) binding between the resumption master secret and the ticket
label. The client MAY use this PSK for future handshakes by
including it in the "pre_shared_key" extension in its ClientHello
(Section 6.3.2.4) and supplying a suitable PSK cipher suite.
struct {
uint32 ticket_lifetime_hint;
opaque ticket<0..2^16-1>;
} NewSessionTicket;
ticket_lifetime_hint
Indicates the lifetime in seconds as a 32-bit unsigned integer in
network byte order from the time of ticket issuance. A value of
zero is reserved to indicate that the lifetime of the ticket is
unspecified.
ticket
The value of the ticket to be used as the PSK identifier.
The ticket lifetime hint is informative only. A client SHOULD delete
the ticket and associated state when the time expires. It MAY delete
the ticket earlier based on local policy. A server MAY treat a
ticket as valid for a shorter or longer period of time than what is
stated in the ticket_lifetime_hint.
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.
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[[TODO: Should we require that tickets be bound to the existing
symmetric cipher suite. See the TODO above about early_data and
PSK.??]
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 secret keying material which is then
used to protect traffic. This keying material is derived from the
two input secret values: Static Secret (SS) and Ephemeral Secret
(ES).
The exact source of each of these secrets depends on the operational
mode (DHE, ECDHE, PSK, etc.) and is summarized in the table below:
Key Exchange Static Secret (SS) Ephemeral Secret (ES)
------------ ------------------ ---------------------
(EC)DHE Client ephemeral Client ephemeral
(full handshake) w/ server ephemeral w/ server ephemeral
(EC)DHE Client ephemeral Client ephemeral
(w/ 0-RTT) w/ server static w/ server ephemeral
PSK Pre-Shared Key Pre-shared key
PSK + (EC)DHE Pre-Shared Key Client ephemeral
w/ server ephemeral
These shared secret values are used to generate cryptographic keys as
shown below.
The derivation process is as follows, where L denotes the length of
the underlying hash function for HKDF [RFC5869]. SS and ES denote
the sources from the table above. Whilst SS and ES may be the same
in some cases, the extracted xSS and xES will not.
HKDF-Expand-Label(Secret, Label, HashValue, Length) =
HKDF-Expand(Secret, HkdfLabel, Length)
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Where HkdfLabel is specified as:
struct HkdfLabel {
uint16 length;
opaque hash_value<0..255>;
opaque label<9..255>;
};
Where:
- HkdfLabel.length is Length
- HkdfLabel.hash_value is HashValue.
- HkdfLabel.label is "TLS 1.3, " + Label
1. xSS = HKDF-Extract(0, SS). Note that HKDF-Extract always
produces a value the same length as the underlying hash
function.
2. xES = HKDF-Extract(0, ES)
3. mSS = HKDF-Expand-Label(xSS, "expanded static secret",
handshake_hash, L)
4. mES = HKDF-Expand-Label(xES, "expanded ephemeral secret",
handshake_hash, L)
5. master_secret = HKDF-Extract(mSS, mES)
6. finished_secret = HKDF-Expand-Label(xSS,
"finished secret",
handshake_hash, L)
Where handshake_hash includes all the messages in the
client's first flight and the server's flight, excluding
the Finished messages (which are never included in the
hashes).
5. resumption_secret = HKDF-Expand-Label(master_secret,
"resumption master secret"
session_hash, L)
Where session_hash is as defined in {{the-handshake-hash}}.
6. exporter_secret = HKDF-Expand-Label(master_secret,
"exporter master secret",
session_hash, L)
Where session_hash is the session hash as defined in
{{the-handshake-hash}} (i.e., the entire handshake except
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for Finished).
The traffic keys are computed from xSS, xES, and the master_secret as
described in Section 7.2 below.
Note: although the steps above are phrased as individual HKDF-Extract
and HKDF-Expand operations, because each HKDF-Expand operation is
paired with an HKDF-Extract, it is possible to implement this key
schedule with a black-box HKDF API, albeit at some loss of efficiency
as some HKDF-Extract operations will be repeated.
7.2. Traffic Key Calculation
[[OPEN ISSUE: This needs to be revised. Most likely we'll extract
each key component separately. See https://github.com/tlswg/tls13-
spec/issues/5]]
The Record Protocol requires an algorithm to generate keys required
by the current connection state (see Appendix A.5) from the security
parameters provided by the handshake protocol.
The traffic key computation takes four input values and returns a key
block of sufficient size to produce the needed traffic keys:
- A secret value
- A string label that indicates the purpose of keys being generated.
- The current handshake hash.
- The total length in octets of the key block.
The keying material is computed using:
key_block = HKDF-Expand-Label(Secret, Label,
handshake_hash,
total_length)
The key_block is partitioned as follows:
client_write_key[SecurityParameters.enc_key_length]
server_write_key[SecurityParameters.enc_key_length]
client_write_IV[SecurityParameters.iv_length]
server_write_IV[SecurityParameters.iv_length]
The following table describes the inputs to the key calculation for
each class of traffic keys:
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Record Type Secret Label Handshake Hash
----------- ------ ----- ---------------
Early data xSS "early data key expansion" ClientHello
Handshake xES "handshake key expansion" ClientHello...
ServerHello
Application master "application data key expansion" All handshake
secret messages but
Finished
(session_hash)
7.2.1. The Handshake Hash
handshake_hash = Hash(
Hash(handshake_messages) ||
Hash(configuration)
)
handshake_messages
All handshake messages sent or received, starting at ClientHello
up to the present time, with the exception of the Finished
message, including the type and length fields of the handshake
messages. This is the concatenation of all the exchanged
Handshake structures in plaintext form (even if they were
encrypted on the wire).
configuration
When 0-RTT is in use (Section 6.3.2.5) this contains the
concatenation of the ServerConfiguration and Certificate messages
from the handshake where the configuration was established
(including the type and length fields). Note that this requires
the client and server to memorize these values.
This final value of the handshake hash is referred to as the "session
hash" because it contains all the handshake messages required to
establish the session. Note that if client authentication is not
used, then the session hash is complete at the point when the server
has sent its first flight. Otherwise, it is only complete when the
client has sent its first flight, as it covers the client's
Certificate and CertificateVerify.
7.2.2. Diffie-Hellman
A conventional Diffie-Hellman computation is performed. The
negotiated key (Z) is used as the shared secret, and is used in the
key schedule as specified above. Leading bytes of Z that contain all
zero bits are stripped before it is used as the input to HKDF.
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7.2.3. Elliptic Curve Diffie-Hellman
All ECDH calculations (including parameter and key generation as well
as the shared secret calculation) are performed according to [6]
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.)
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 [I-D.irtf-cfrg-curves].
A TLS-compliant application SHOULD implement the following cipher
suites:
TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384
TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305
TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305
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 6.3.2.1)
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- Negotiated Groups ("supported_groups"; Section 6.3.2.2)
- Key Share ("key_share"; Section 6.3.2.3)
- Pre-Shared Key Extension ("pre_shared_key"; Section 6.3.2.4)
- Server Name Indication ("server_name"; Section 3 of [RFC6066])
All implementations MUST send and use these extensions when offering
applicable cipher suites:
- "signature_algorithms" is REQUIRED for certificate authenticated
cipher suites
- "supported_groups" and "key_share" are REQUIRED for DHE or ECDHE
cipher suites
- "pre_shared_key" is REQUIRED for PSK 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.
Some of these extensions exist only for the client to provide
additional data to the server in a backwards-compatible way and thus
have no meaning when sent from a server. The client-only extensions
defined in this document are: "Signature Algorithms" & "Negotiated
Groups". Servers MUST NOT send these extensions. Clients receiving
any of these extensions 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.
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10. Security Considerations
Security issues are discussed throughout this memo, especially in
Appendices B, C, and D.
11. IANA Considerations
[[TODO: Update https://github.com/tlswg/tls13-spec/issues/62]]
[[TODO: Rename "RSA" in TLS SignatureAlgorithm Registry to RSASSA-
PKCS1-v1_5 ]]
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 (unchanged from [RFC4346])
are listed below.
- TLS Cipher Suite Registry: Future values with the first byte in
the range 0-191 (decimal) inclusive are assigned via Standards
Action [RFC2434]. Values with the first byte in the range 192-254
(decimal) are assigned via Specification Required [RFC2434].
Values with the first byte 255 (decimal) are reserved for Private
Use [RFC2434].
- 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 (unchanged from [RFC4366]) is listed below:
- TLS ExtensionType Registry: Future values are allocated via IETF
Consensus [RFC2434]. IANA has updated this registry to include
the "signature_algorithms" extension and its corresponding value
(see Section 6.3.2).
This document also uses two registries originally created in
[RFC4492]. IANA [should update/has updated] it to reference this
document. The registries and their allocation policies are listed
below.
- TLS NamedCurve registry: Future values are allocated via IETF
Consensus [RFC2434].
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- TLS ECPointFormat Registry: Future values are allocated via IETF
Consensus [RFC2434].
In addition, this document defines two new registries to be
maintained by IANA:
- TLS SignatureAlgorithm Registry: The registry has been initially
populated with the values described in Section 6.3.2.1. Future
values in the range 0-63 (decimal) inclusive are assigned via
Standards Action [RFC2434]. Values in the range 64-223 (decimal)
inclusive are assigned via Specification Required [RFC2434].
Values from 224-255 (decimal) inclusive are reserved for Private
Use [RFC2434].
- TLS HashAlgorithm Registry: The registry has been initially
populated with the values described in Section 6.3.2.1. Future
values in the range 0-63 (decimal) inclusive are assigned via
Standards Action [RFC2434]. Values in the range 64-223 (decimal)
inclusive are assigned via Specification Required [RFC2434].
Values from 224-255 (decimal) inclusive are reserved for Private
Use [RFC2434].
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.
[I-D.ietf-tls-chacha20-poly1305]
Langley, A., Chang, W., Mavrogiannopoulos, N.,
Strombergson, J., and S. Josefsson, "The ChaCha20-Poly1305
AEAD Cipher for Transport Layer Security", draft-ietf-tls-
chacha20-poly1305-00 (work in progress), June 2015.
[I-D.irtf-cfrg-curves]
Langley, A. and M. Hamburg, "Elliptic Curves for
Security", draft-irtf-cfrg-curves-08 (work in progress),
September 2015.
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[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>.
[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>.
[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>.
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[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>.
[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>.
[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] "Bleichenbacher's RSA signature forgery based on
implementation error", August 2006, <http://www.imc.org/
ietf-openpgp/mail-archive/msg14307.html>.
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[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-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.
[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.
[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>.
[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>.
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[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>.
[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>.
[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>.
[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>.
[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>.
[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>.
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[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>.
[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>.
[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.
[SSL2] Netscape Communications Corp., "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 may 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),
early_handshake(25),
(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),
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;
A.3. Handshake Protocol
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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),
server_configuration(17),
finished(20),
(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 server_configuration:ServerConfiguration;
case certificate: Certificate;
case certificate_request: CertificateRequest;
case certificate_verify: CertificateVerify;
case finished: Finished;
case session_ticket: NewSessionTicket;
} body;
} Handshake;
A.3.1. Hello Messages
uint8 CipherSuite[2]; /* Cryptographic suite selector */
enum { null(0), (255) } CompressionMethod;
struct {
ProtocolVersion client_version = { 3, 4 }; /* TLS v1.3 */
Random random;
SessionID session_id;
CipherSuite cipher_suites<2..2^16-2>;
CompressionMethod compression_methods<1..2^8-1>;
Extension extensions<0..2^16-1>;
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} ClientHello;
struct {
ProtocolVersion server_version;
Random random;
CipherSuite cipher_suite;
select (extensions_present) {
case false:
struct {};
case true:
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),
early_data(TBD),
pre_shared_key(TBD),
key_share(TBD),
(65535)
} ExtensionType;
opaque psk_identity<0..2^16-1>;
struct {
select (Role) {
case client:
psk_identity identities<2..2^16-1>;
case server:
psk_identity identity;
}
} PreSharedKeyExtension;
enum { client_authentication(1), early_data(2),
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client_authentication_and_data(3), (255) } EarlyDataType;
struct {
select (Role) {
case client:
opaque configuration_id<1..2^16-1>;
CipherSuite cipher_suite;
Extension extensions<0..2^16-1>;
opaque context<0..255>;
EarlyDataType type;
case server:
struct {};
}
} EarlyDataIndication;
struct {
Extension extensions<0..2^16-1>;
} EncryptedExtensions;
enum { (65535) } ConfigurationExtensionType;
struct {
ConfigurationExtensionType extension_type;
opaque extension_data<0..2^16-1>;
} ConfigurationExtension;
struct {
opaque configuration_id<1..2^16-1>;
uint32 expiration_date;
NamedGroup group;
opaque server_key<1..2^16-1>;
EarlyDataType early_data_type;
ConfigurationExtension extensions<0..2^16-1>;
} ServerConfiguration;
A.3.1.1. Signature Algorithm Extension
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enum {
none(0),
md5_RESERVED(1),
sha1(2),
sha224_RESERVED(3),
sha256(4), sha384(5), sha512(6),
(255)
} HashAlgorithm;
enum {
anonymous_RESERVED(0),
rsa(1),
dsa(2),
ecdsa(3),
rsapss(4),
(255)
} SignatureAlgorithm;
struct {
HashAlgorithm hash;
SignatureAlgorithm signature;
} SignatureAndHashAlgorithm;
SignatureAndHashAlgorithm
supported_signature_algorithms<2..2^16-2>;
A.3.1.2. Named Group Extension
enum {
// Elliptic Curve Groups.
obsolete_RESERVED (1..22),
secp256r1 (23), secp384r1 (24), secp521r1 (25),
// Finite Field Groups.
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;
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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.3.2. Key Exchange Messages
struct {
NamedGroup group;
opaque key_exchange<1..2^16-1>;
} KeyShareEntry;
struct {
select (role) {
case client:
KeyShareEntry client_shares<4..2^16-1>;
case server:
KeyShareEntry server_share;
}
} KeyShare;
opaque dh_Y<1..2^16-1>;
opaque point <1..2^8-1>;
A.3.3. Authentication Messages
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opaque ASN1Cert<1..2^24-1>;
struct {
ASN1Cert certificate_list<0..2^24-1>;
} Certificate;
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 {
SignatureAndHashAlgorithm
supported_signature_algorithms<2..2^16-2>;
DistinguishedName certificate_authorities<0..2^16-1>;
CertificateExtension certificate_extensions<0..2^16-1>;
} CertificateRequest;
struct {
digitally-signed struct {
opaque handshake_hash[hash_length];
};
} CertificateVerify;
A.3.4. Handshake Finalization Messages
struct {
opaque verify_data[verify_data_length];
} Finished;
A.3.5. Ticket Establishment
struct {
uint32 ticket_lifetime_hint;
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:
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CipherSuite TLS_KEA_SIGN_WITH_CIPHER_HASH = VALUE;
Component Contents
TLS The string "TLS"
KEA The key exchange algorithm
SIGN The signature algorithm
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
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:
Cipher Suite Name Value Specification
TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 {0x00,0x9E} [RFC5288]
TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 {0x00,0x9F} [RFC5288]
TLS_DHE_RSA_WITH_AES_128_CCM {0xC0,0x9E} [RFC6655]
TLS_DHE_RSA_WITH_AES_256_CCM {0xC0,0x9F} [RFC6655]
TLS_DHE_RSA_WITH_AES_128_CCM_8 {0xC0,0xA2} [RFC6655]
TLS_DHE_RSA_WITH_AES_256_CCM_8 {0xC0,0xA3} [RFC6655]
TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305 {TBD,TBD} [I-D.ietf-tls-chacha20-poly1305]
TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305 {TBD,TBD} [I-D.ietf-tls-chacha20-poly1305]
TLS_DHE_RSA_WITH_CHACHA20_POLY1305 {TBD,TBD} [I-D.ietf-tls-chacha20-poly1305]
[[TODO: CHACHA20_POLY1305 cipher suite IDs are TBD.]]
The following is a list of non-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_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 {0xC0,0x2B} [RFC5289]
TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 {0xC0,0x2C} [RFC5289]
TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 {0xC0,0x2F} [RFC5289]
TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 {0xC0,0x30} [RFC5289]
TLS_ECDHE_ECDSA_WITH_AES_128_CCM {0xC0,0xAC} [RFC7251]
TLS_ECDHE_ECDSA_WITH_AES_256_CCM {0xC0,0xAD} [RFC7251]
TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 {0xC0,0xAE} [RFC7251]
TLS_ECDHE_ECDSA_WITH_AES_256_CCM_8 {0xC0,0xAF} [RFC7251]
TLS_DHE_RSA_WITH_ARIA_128_GCM_SHA256 {0xC0,0x52} [RFC6209]
TLS_DHE_RSA_WITH_ARIA_256_GCM_SHA384 {0xC0,0x53} [RFC6209]
TLS_ECDHE_ECDSA_WITH_ARIA_128_GCM_SHA256 {0xC0,0x5C} [RFC6209]
TLS_ECDHE_ECDSA_WITH_ARIA_256_GCM_SHA384 {0xC0,0x5D} [RFC6209]
TLS_ECDHE_RSA_WITH_ARIA_128_GCM_SHA256 {0xC0,0x60} [RFC6209]
TLS_ECDHE_RSA_WITH_ARIA_256_GCM_SHA384 {0xC0,0x61} [RFC6209]
TLS_DHE_RSA_WITH_CAMELLIA_128_GCM_SHA256 {0xC0,0x7C} [RFC6367]
TLS_DHE_RSA_WITH_CAMELLIA_256_GCM_SHA384 {0xC0,0x7D} [RFC6367]
TLS_ECDHE_ECDSA_WITH_CAMELLIA_128_GCM_SHA256 {0xC0,0x86} [RFC6367]
TLS_ECDHE_ECDSA_WITH_CAMELLIA_256_GCM_SHA384 {0xC0,0x87} [RFC6367]
TLS_ECDHE_RSA_WITH_CAMELLIA_128_GCM_SHA256 {0xC0,0x8A} [RFC6367]
TLS_ECDHE_RSA_WITH_CAMELLIA_256_GCM_SHA384 {0xC0,0x8B} [RFC6367]
ECDHE AES GCM is not yet standards track, however it is already
widely deployed.
Note: In the case of the CCM mode of AES, two variations exist:
"CCM_8" which uses an 8-bit authentication tag and "CCM" which uses a
16-bit authentication tag. Both use the default hash, SHA-256.
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 base 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 serveral 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.
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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:
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 a "signature_algorithm" field to the digitally-
signed element in order to identify the signature and digest
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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, 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.
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.
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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, certificate
chains containing keys or signatures weaker than 2048-bit RSA or
224-bit ECDSA are not appropriate for secure applications. See also
Appendix C.3.
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
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 Algorithms extension) is completely
removed from all possible configurations that support TLS 1.3 or
later, and that attempts to use these obsolete capabilities fail
correctly? (see Appendix C)
- Do you handle TLS extensions in ClientHello correctly, including
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 6.3.9)?
- When processing the plaintext fragment produced by AEAD-Decrypt
and scanning from the end for the ContentType, do you avoid
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scanning past the start of the cleartext in the event that the
peer has sent a malformed plaintext of all-zeros?
Cryptographic details:
- What countermeasures do you use to prevent timing attacks against
RSA signing operations [TIMING]?
- When verifying RSA signatures, do you accept both NULL and missing
parameters (see Section 4.9)? 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 strip
leading zero bytes from the negotiated key (see Section 7.2.2)?
- 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?
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
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 certificate chains based on the expectations in this
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document, even when handling prior TLS versions' handshakes. (see
Section 6.3.4)
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.
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.
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.
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C.3. 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.
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
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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.
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 6.3.8 and Section 7.2). 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 KeyShare 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 the ephemeral DHE keys.
Peers SHOULD validate each other's public key Y (dh_Ys offered by the
server or DH_Yc offered by the client) by ensuring that 1 < Y < p-1.
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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.
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.3.)
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
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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.
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 his 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
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Appendix F. Contributors
Martin Abadi
University of California, Santa Cruz
abadi@cs.ucsc.edu
Christopher Allen (co-editor of TLS 1.0)
Alacrity Ventures
ChristopherA@AlacrityManagement.com
Steven M. Bellovin
Columbia University
smb@cs.columbia.edu
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
Sun Microsystems, Inc.
nelson@bolyard.com (co-author of [RFC4492])
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
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Pasi Eronen
Nokia
pasi.eronen@nokia.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
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
Ilari Liusvaara
ilari.liusvaara@elisanet.fi
Jan Mikkelsen
Transactionware
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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
Bjoern Tackmann
University of California, San Diego
btackmann@eng.ucsd.edu
Martin Thomson
Mozilla
mt@mozilla.com
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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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