Network Working Group E. Rescorla
Internet-Draft RTFM, Inc.
Obsoletes: 3268, 4346, 4366, 5246 (if March 09, 2015
approved)
Updates: 4492 (if approved)
Intended status: Standards Track
Expires: September 10, 2015
The Transport Layer Security (TLS) Protocol Version 1.3
draft-ietf-tls-tls13-05
Abstract
This document specifies Version 1.3 of the Transport Layer Security
(TLS) protocol. The TLS protocol provides communications security
over the Internet. The protocol allows client/server applications to
communicate in a way that is designed to prevent eavesdropping,
tampering, or message forgery.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 10, 2015.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements Terminology . . . . . . . . . . . . . . . . 5
1.2. Major Differences from TLS 1.2 . . . . . . . . . . . . . 5
2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Goals of This Document . . . . . . . . . . . . . . . . . . . 7
4. Presentation Language . . . . . . . . . . . . . . . . . . . . 7
4.1. Basic Block Size . . . . . . . . . . . . . . . . . . . . 7
4.2. Miscellaneous . . . . . . . . . . . . . . . . . . . . . . 8
4.3. Vectors . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.4. Numbers . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.5. Enumerateds . . . . . . . . . . . . . . . . . . . . . . . 9
4.6. Constructed Types . . . . . . . . . . . . . . . . . . . . 10
4.6.1. Variants . . . . . . . . . . . . . . . . . . . . . . 11
4.7. Cryptographic Attributes . . . . . . . . . . . . . . . . 12
4.8. Constants . . . . . . . . . . . . . . . . . . . . . . . . 14
5. The Pseudorandom Function . . . . . . . . . . . . . . . . . . 15
6. The TLS Record Protocol . . . . . . . . . . . . . . . . . . . 16
6.1. Connection States . . . . . . . . . . . . . . . . . . . . 16
6.2. Record Layer . . . . . . . . . . . . . . . . . . . . . . 19
6.2.1. Fragmentation . . . . . . . . . . . . . . . . . . . . 19
6.2.2. Record Payload Protection . . . . . . . . . . . . . . 20
6.3. Key Calculation . . . . . . . . . . . . . . . . . . . . . 22
7. The TLS Handshaking Protocols . . . . . . . . . . . . . . . . 23
7.1. Alert Protocol . . . . . . . . . . . . . . . . . . . . . 24
7.1.1. Closure Alerts . . . . . . . . . . . . . . . . . . . 25
7.1.2. Error Alerts . . . . . . . . . . . . . . . . . . . . 25
7.2. Handshake Protocol Overview . . . . . . . . . . . . . . . 29
7.3. Handshake Protocol . . . . . . . . . . . . . . . . . . . 33
7.3.1. Hello Messages . . . . . . . . . . . . . . . . . . . 34
7.3.2. Client Key Share Message . . . . . . . . . . . . . . 37
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7.3.3. Server Key Share Message . . . . . . . . . . . . . . 48
7.3.4. Encrypted Extensions . . . . . . . . . . . . . . . . 49
7.3.5. Server Certificate . . . . . . . . . . . . . . . . . 49
7.3.6. Certificate Request . . . . . . . . . . . . . . . . . 52
7.3.7. Server Certificate Verify . . . . . . . . . . . . . . 53
7.3.8. Server Finished . . . . . . . . . . . . . . . . . . . 55
7.3.9. Client Certificate . . . . . . . . . . . . . . . . . 56
7.3.10. Client Certificate Verify . . . . . . . . . . . . . . 58
8. Cryptographic Computations . . . . . . . . . . . . . . . . . 58
8.1. Computing the Master Secret . . . . . . . . . . . . . . . 59
8.1.1. The Session Hash . . . . . . . . . . . . . . . . . . 60
8.1.2. Diffie-Hellman . . . . . . . . . . . . . . . . . . . 61
8.1.3. Elliptic Curve Diffie-Hellman . . . . . . . . . . . . 61
9. Mandatory Cipher Suites . . . . . . . . . . . . . . . . . . . 61
10. Application Data Protocol . . . . . . . . . . . . . . . . . . 61
11. Security Considerations . . . . . . . . . . . . . . . . . . . 62
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 62
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 63
13.1. Normative References . . . . . . . . . . . . . . . . . . 63
13.2. Informative References . . . . . . . . . . . . . . . . . 65
13.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 68
Appendix A. Protocol Data Structures and Constant Values . . . . 69
A.1. Record Layer . . . . . . . . . . . . . . . . . . . . . . 69
A.2. Alert Messages . . . . . . . . . . . . . . . . . . . . . 69
A.3. Handshake Protocol . . . . . . . . . . . . . . . . . . . 70
A.3.1. Hello Messages . . . . . . . . . . . . . . . . . . . 71
A.3.2. Key Exchange Messages . . . . . . . . . . . . . . . . 74
A.3.3. Authentication Messages . . . . . . . . . . . . . . . 74
A.3.4. Handshake Finalization Messages . . . . . . . . . . . 75
A.4. The Cipher Suite . . . . . . . . . . . . . . . . . . . . 75
A.5. The Security Parameters . . . . . . . . . . . . . . . . . 77
A.6. Changes to RFC 4492 . . . . . . . . . . . . . . . . . . . 77
Appendix B. Glossary . . . . . . . . . . . . . . . . . . . . . . 78
Appendix C. Cipher Suite Definitions . . . . . . . . . . . . . . 81
Appendix D. Implementation Notes . . . . . . . . . . . . . . . . 81
D.1. Random Number Generation and Seeding . . . . . . . . . . 81
D.2. Certificates and Authentication . . . . . . . . . . . . . 82
D.3. Cipher Suites . . . . . . . . . . . . . . . . . . . . . . 82
D.4. Implementation Pitfalls . . . . . . . . . . . . . . . . . 82
Appendix E. Backward Compatibility . . . . . . . . . . . . . . . 83
E.1. Compatibility with prior versions . . . . . . . . . . . . 83
E.2. Compatibility with SSL . . . . . . . . . . . . . . . . . 85
Appendix F. Security Analysis . . . . . . . . . . . . . . . . . 85
F.1. Handshake Protocol . . . . . . . . . . . . . . . . . . . 85
F.1.1. Authentication and Key Exchange . . . . . . . . . . . 86
F.1.2. Version Rollback Attacks . . . . . . . . . . . . . . 87
F.1.3. Detecting Attacks Against the Handshake Protocol . . 87
F.1.4. Resuming Sessions . . . . . . . . . . . . . . . . . . 87
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F.2. Protecting Application Data . . . . . . . . . . . . . . . 88
F.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 88
F.4. Final Notes . . . . . . . . . . . . . . . . . . . . . . . 89
Appendix G. Working Group Information . . . . . . . . . . . . . 89
Appendix H. Contributors . . . . . . . . . . . . . . . . . . . . 89
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 applications. The 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], etc.). 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 Record Protocol can also be used
without encryption, i.e., in integrity-only modes.
- The connection is reliable. Messages include an authentication
tag which protects them against modification.
- The Record Protocol can operate in an insecure mode but is
generally only used in this mode while another protocol is using
the Record Protocol as a transport for negotiating security
parameters.
The TLS Record Protocol is used for encapsulation of various higher-
level protocols. One such encapsulated protocol, the TLS Handshake
Protocol, allows the server and client to authenticate each other and
to negotiate an encryption algorithm and cryptographic keys before
the application protocol transmits or receives its first byte of
data. The TLS Handshake Protocol provides connection security that
has three basic properties:
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- The peer's identity can be authenticated using asymmetric, or
public key, cryptography (e.g., RSA [RSA], DSA [DSS], etc.). 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. Requirements 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].
1.2. Major Differences from TLS 1.2
draft-05
- Prohibit SSL negotiation for backwards compatibility.
- Fix which MS is used for exporters.
draft-04
- Modify key computations to include session hash.
- Remove ChangeCipherSpec
- Renumber the new handshake messages to be somewhat more consistent
with existing convention and to remove a duplicate registration.
- Remove renegotiation.
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- Update format of signatures with context.
- 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.
- Reworked handshake to provide 1-RTT mode.
- Remove custom DHE groups.
- Removed support for compression.
- Removed support for static RSA and DH key exchange.
- Removed support for non-AEAD ciphers
2. Goals
The goals of the TLS protocol, in order of priority, are as follows:
1. Cryptographic security: TLS should be used to establish a secure
connection between two parties.
2. Interoperability: Independent programmers should be able to
develop applications utilizing TLS that can successfully exchange
cryptographic parameters without knowledge of one another's code.
3. Extensibility: TLS seeks to provide a framework into which new
public key and record protection methods can be incorporated as
necessary. This will also accomplish two sub-goals: preventing
the need to create a new protocol (and risking the introduction
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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 are based on the SSL 3.0
Protocol Specification as published by Netscape. The differences
between this protocol and SSL 3.0 are not dramatic, but they 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.
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
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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
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.
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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).
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.
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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.
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.
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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. 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 6.1).
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 7.3.2.5.1 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.
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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-PKCS1-v1_5 signature scheme defined in [RFC3447].
As discussed in [RFC3447], the DigestInfo MUST be DER-encoded [X680]
[X690]. For hash algorithms without parameters (which includes SHA-
1), the DigestInfo.AlgorithmIdentifier.parameters field MUST be NULL,
but implementations MUST accept both without parameters and with NULL
parameters. Note that earlier versions of TLS used a different RSA
signature scheme that did not include a DigestInfo encoding.
In DSA, the 20 bytes of the SHA-1 hash are run directly through the
Digital Signing Algorithm with no additional hashing. This produces
two values, r and s. The DSA signature is an opaque vector, as
above, the contents of which are the DER encoding of:
Dss-Sig-Value ::= SEQUENCE {
r INTEGER,
s INTEGER
}
Note: In current terminology, DSA refers to the Digital Signature
Algorithm and DSS refers to the NIST standard. In the original SSL
and TLS specs, "DSS" was used universally. This document uses "DSA"
to refer to the algorithm, "DSS" to refer to the standard, and it
uses "DSS" in the code point definitions for historical continuity.
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 default hash function is SHA-1 [SHS].
However, an alternative hash function, such as one of the new SHA
hash functions specified in FIPS 180-2 may be used instead if the
certificate containing the EC public key explicitly requires use of
another hash function. (The mechanism for specifying the required
hash function has not been standardized, but this provision
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anticipates such standardization and obviates the need to update this
document in response. Future PKIX RFCs may choose, for example, to
specify the hash function to be used with a public key in the
parameters field of subjectPublicKeyInfo.) [[OPEN ISSUE: This needs
updating per 4492-bis https://github.com/tlswg/tls13-spec/issues/59]]
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.
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.
4.8. 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:
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struct {
uint8 f1;
uint8 f2;
} Example1;
Example1 ex1 = {1, 4}; /* assigns f1 = 1, f2 = 4 */
5. The Pseudorandom Function
A construction is required to do expansion of secrets into blocks of
data for the purposes of key generation or validation. This
pseudorandom function (PRF) takes as input a secret, a seed, and an
identifying label and produces an output of arbitrary length.
In this section, we define one PRF, based on HMAC [RFC2104]. This
PRF with the SHA-256 hash function is used for all cipher suites
defined in this document and in TLS documents published prior to this
document when TLS 1.2 is negotiated. New cipher suites MUST
explicitly specify a PRF and, in general, SHOULD use the TLS PRF with
SHA-256 or a stronger standard hash function.
First, we define a data expansion function, P_hash(secret, data),
that uses a single hash function to expand a secret and seed into an
arbitrary quantity of output:
P_hash(secret, seed) = HMAC_hash(secret, A(1) + seed) +
HMAC_hash(secret, A(2) + seed) +
HMAC_hash(secret, A(3) + seed) + ...
where + indicates concatenation.
A() is defined as:
A(0) = seed
A(i) = HMAC_hash(secret, A(i-1))
P_hash can be iterated as many times as necessary to produce the
required quantity of data. For example, if P_SHA256 is being used to
create 80 bytes of data, it will have to be iterated three times
(through A(3)), creating 96 bytes of output data; the last 16 bytes
of the final iteration will then be discarded, leaving 80 bytes of
output data.
TLS's PRF is created by applying P_hash to the secret as:
PRF(secret, label, seed) = P_<hash>(secret, label + seed)
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The label is an ASCII string. It should be included in the exact
form it is given without a length byte or trailing null character.
For example, the label "slithy toves" would be processed by hashing
the following bytes:
73 6C 69 74 68 79 20 74 6F 76 65 73
6. 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 Record Protocol takes messages to be transmitted, fragments the
data into manageable blocks, protects the records, and transmits the
result. Received data is decrypted and verified, reassembled, and
then delivered to higher-level clients.
Three protocols that use the record protocol are described in this
document: the 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
record protocol. New record content type values are assigned by IANA
in the TLS Content Type Registry as described in Section 12.
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 type and length of a record are not protected
by encryption. If this information is itself sensitive, application
designers may wish to take steps (padding, cover traffic) to minimize
information leakage.
6.1. Connection States
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
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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.
PRF algorithm
An algorithm used to generate keys from the master secret (see
Section 5 and Section 6.3).
record protection algorithm
The algorithm to be used for record protection. This algorithm
must be of the AEAD type and thus provides integrity and
confidentiality as a single primitive. It is possible to have
AEAD algorithms which do not provide any confidentiality and
Section 6.2.2 defines a special NULL_NULL AEAD algorithm for use
in the initial handshake). This specification includes the key
size of this algorithm and the lengths of explicit and implicit
initialization vectors (or nonces).
handshake master secret
A 48-byte secret shared between the two peers in the connection
and used to generate keys for protecting the handshake.
master secret
A 48-byte secret shared between the two peers in the connection
and used to generate keys for protecting application 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:
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enum { server, client } ConnectionEnd;
enum { tls_prf_sha256 } PRFAlgorithm;
enum { aes_gcm } RecordProtAlgorithm;
/* The algorithms specified in PRFAlgorithm and
RecordProtAlgorithm may be added to. */
struct {
ConnectionEnd entity;
PRFAlgorithm prf_algorithm;
RecordProtAlgorithm record_prot_algorithm;
uint8 enc_key_length;
uint8 block_length;
uint8 fixed_iv_length;
uint8 record_iv_length;
opaque hs_master_secret[48];
opaque master_secret[48];
opaque client_random[32];
opaque server_random[32];
} SecurityParameters;
The record layer will use the security parameters to generate the
following four items (some of which are not required by all ciphers,
and are thus empty):
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 6.3
Once the security parameters have been set and the keys have been
generated, the connection states can be instantiated by making them
the current states. These current states MUST be updated for each
record processed. Each connection state includes the following
elements:
cipher state
The current state of the encryption algorithm. This will consist
of the scheduled key for that connection.
sequence number
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Each connection state contains a sequence number, which is
maintained separately for read and write states. The sequence
number MUST be set to zero whenever a connection state is made the
active state. Sequence numbers are of type uint64 and may 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.
6.2. Record Layer
The TLS record layer receives uninterpreted data from higher layers
in non-empty blocks of arbitrary size.
6.2.1. 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).
struct {
uint8 major;
uint8 minor;
} ProtocolVersion;
ProtocolVersion version = { 3, 4 }; /* TLS v1.3*/
enum {
reserved(20), alert(21), handshake(22),
application_data(23), (255)
} ContentType;
struct {
ContentType type;
ProtocolVersion version;
uint16 length;
opaque fragment[TLSPlaintext.length];
} TLSPlaintext;
type
The higher-level protocol used to process the enclosed fragment.
version
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The version of the protocol being employed. 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. (See Appendix A.1.) Note that a client that
supports multiple versions of TLS may not know what version will
be employed before it receives the ServerHello. See Appendix E
for discussion about what record layer version number should be
employed for ClientHello.
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.
Implementations MUST NOT send zero-length fragments of Handshake or
Alert types. Zero-length fragments of Application data MAY be sent
as they are potentially useful as a traffic analysis countermeasure.
6.2.2. 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 modelled 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 type;
ProtocolVersion version;
uint16 length;
opaque nonce_explicit[SecurityParameters.record_iv_length];
aead-ciphered struct {
opaque content[TLSPlaintext.length];
} fragment;
} TLSCiphertext;
type
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The type field is identical to TLSPlaintext.type.
version
The version field is identical to TLSPlaintext.version.
length
The length (in bytes) of the following TLSCiphertext.fragment.
The length MUST NOT exceed 2^14 + 2048.
fragment
The AEAD encrypted form of TLSPlaintext.fragment.
Each AEAD cipher suite MUST specify how the nonce supplied to the
AEAD operation is constructed, and what is the length of the
TLSCiphertext.nonce_explicit part. In many cases, it is appropriate
to use the partially implicit nonce technique described in
Section 3.2.1 of [RFC5116]; with record_iv_length being the length of
the explicit part. In this case, the implicit part SHOULD be derived
from key_block as client_write_iv and server_write_iv (as described
in Section 6.3), and the explicit part is included in
GenericAEAEDCipher.nonce_explicit.
The plaintext is the TLSPlaintext.fragment.
The additional authenticated data, which we denote as
additional_data, is defined as follows:
additional_data = seq_num + TLSPlaintext.type +
TLSPlaintext.version
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 will generally be larger than
TLSPlaintext.length, but by an amount that varies with the AEAD
cipher. Since the ciphers might incorporate padding, the amount of
overhead could vary with different TLSPlaintext.length values. Each
AEAD cipher MUST NOT produce an expansion of greater than 1024 bytes.
Symbolically,
AEADEncrypted = AEAD-Encrypt(write_key, nonce, plaintext,
additional_data)
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[[OPEN ISSUE: Reduce these values? https://github.com/tlswg/tls13-
spec/issues/55]]
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:
TLSPlaintext.fragment = AEAD-Decrypt(write_key, nonce,
AEADEncrypted,
additional_data)
If the decryption fails, a fatal bad_record_mac alert MUST be
generated.
As a special case, we define the NULL_NULL AEAD cipher which is
simply the identity operation and thus provides no security. This
cipher MUST ONLY be used with the initial TLS_NULL_WITH_NULL_NULL
cipher suite.
6.3. Key Calculation
[[OPEN ISSUE: This needs to be revised. 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 master secret is expanded into a sequence of secure bytes, which
is then split to a client write encryption key and a server write
encryption key. Each of these is generated from the byte sequence in
that order. Unused values are empty. Some ciphers may additionally
require a client write IV and a server write IV.
When keys are generated, the current master secret (MS) is used as an
entropy source. For handshake records, this means the
hs_master_secret. For application data records, this means the
regular master_secret.
To generate the key material, compute
key_block = PRF(MS,
"key expansion",
SecurityParameters.server_random +
SecurityParameters.client_random);
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where MS is the relevant master secret. The PRF is computed enough
times to generate the necessary amount of data for the key_block,
which is then partitioned as follows:
client_write_key[SecurityParameters.enc_key_length]
server_write_key[SecurityParameters.enc_key_length]
client_write_IV[SecurityParameters.fixed_iv_length]
server_write_IV[SecurityParameters.fixed_iv_length]
Currently, the client_write_IV and server_write_IV are only generated
for implicit nonce techniques as described in Section 3.2.1 of
[RFC5116].
7. 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.
The Handshake Protocol is responsible for negotiating a session,
which consists of the following items:
session identifier
An arbitrary byte sequence chosen by the server to identify an
active or resumable session state.
peer certificate
X509v3 [RFC3280] certificate of the peer. This element of the
state may be null.
cipher spec
Specifies the authentication and key establishment algorithms, the
pseudorandom function (PRF) used to generate keying material, and
the record protection algorithm (See Appendix A.5 for formal
definition.)
resumption premaster secret
48-byte secret shared between the client and server.
is resumable
A flag indicating whether the session can be used to initiate new
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 through the resumption
feature of the TLS Handshake Protocol.
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7.1. 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.
enum { warning(1), fatal(2), (255) } AlertLevel;
enum {
close_notify(0),
unexpected_message(10),
bad_record_mac(20),
decryption_failed_RESERVED(21),
record_overflow(22),
decompression_failure_RESERVED(30),
handshake_failure(40),
no_certificate_RESERVED(41),
bad_certificate(42),
unsupported_certificate(43),
certificate_revoked(44),
certificate_expired(45),
certificate_unknown(46),
illegal_parameter(47),
unknown_ca(48),
access_denied(49),
decode_error(50),
decrypt_error(51),
export_restriction_RESERVED(60),
protocol_version(70),
insufficient_security(71),
internal_error(80),
user_canceled(90),
no_renegotiation(100),
unsupported_extension(110),
(255)
} AlertDescription;
struct {
AlertLevel level;
AlertDescription description;
} Alert;
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7.1.1. Closure Alerts
The client and the server must share knowledge that the connection is
ending in order to avoid a truncation attack. Either party may
initiate the exchange of closing messages.
close_notify
This message notifies the recipient that the sender will not send
any more messages on this connection. Note that as of TLS 1.1,
failure to properly close a connection no longer requires that a
session not be resumed. This is a change from TLS 1.0 to conform
with widespread implementation practice.
Either party MAY initiate a close by sending a close_notify alert.
Any data received after a closure alert is ignored.
Unless some other fatal alert has been transmitted, each party is
required to send a close_notify alert before closing the write side
of the connection. The other party MUST respond with a close_notify
alert of its own and close down the connection immediately,
discarding any pending writes. It is not required for the initiator
of the close to 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.
7.1.2. Error Alerts
Error handling in the TLS Handshake protocol is very simple. When an
error is detected, the detecting party sends a message to the other
party. 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.
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Whenever an implementation encounters a condition which is defined as
a fatal alert, it MUST send the appropriate alert prior to closing
the connection. For all errors where an alert level is not
explicitly specified, the sending party MAY determine at its
discretion whether to treat this as a fatal error or not. If the
implementation chooses to send an alert but intends to close the
connection immediately afterwards, it MUST send that alert at the
fatal alert level.
If an alert with a level of warning is sent and received, generally
the connection can continue normally. If the receiving party decides
not to proceed with the connection (e.g., after having received a
no_renegotiation alert that it is not willing to accept), it SHOULD
send a fatal alert to terminate the connection. Given this, the
sending party 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 peer 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 message is used for all deprotection failures.
This message is always fatal and should never be observed in
communication between proper implementations (except when messages
were corrupted in the network).
decryption_failed_RESERVED
This alert was used in some earlier versions of TLS, and may have
permitted certain attacks against the CBC mode [CBCATT]. It MUST
NOT be sent by compliant implementations.
record_overflow
A TLSCiphertext record was received that had a length more than
2^14+2048 bytes, or a record decrypted to a TLSPlaintext record
with more than 2^14 bytes. This message is always fatal and
should never be observed in communication between proper
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implementations (except when messages were corrupted in the
network).
decompression_failure
This alert was used in previous versions of TLS. TLS 1.3 does not
include compression and TLS 1.3 implementations MUST NOT send this
alert when in TLS 1.3 mode.
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 is a fatal error.
no_certificate_RESERVED
This alert was used in SSLv3 but not any version of TLS. It MUST
NOT be sent by compliant implementations.
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 message 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
message is always fatal.
access_denied
A valid certificate was received, but when access control was
applied, the sender decided not to proceed with negotiation. This
message is always fatal.
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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
message 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 message is always fatal.
export_restriction_RESERVED
This alert was used in some earlier versions of TLS. It MUST NOT
be sent by compliant implementations.
protocol_version
The protocol version the client has attempted to negotiate is
recognized but not supported. (For example, old protocol versions
might be avoided for security reasons.) This message 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 message 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 message is always fatal.
user_canceled
This handshake is being canceled for some reason unrelated to a
protocol failure. If the 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 message is generally a warning.
no_renegotiation
Sent by the client in response to a hello request or by the server
in response to a client hello after initial handshaking. Versions
of TLS prior to TLS 1.3 supported renegotiation of a previously
established connection; TLS 1.3 removes this feature. This
message is always fatal.
unsupported_extension
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sent by clients that receive an extended server hello containing
an extension that they did not put in the corresponding client
hello. This message is always fatal.
New Alert values are assigned by IANA as described in Section 12.
7.2. 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 use public-key encryption
techniques to generate shared secrets.
The TLS Handshake Protocol involves the following steps:
- Exchange hello messages to agree on a protocol version,
algorithms, exchange random values, and check for session
resumption.
- Exchange the necessary cryptographic parameters to allow the
client and server to agree on a premaster secret.
- Exchange certificates and cryptographic information to allow the
client and server to authenticate themselves.
- Generate a master secret from the premaster secret and exchanged
random values.
- Provide security parameters to the record layer.
- Allow the client and server to verify that their peer has
calculated the same security parameters and that the handshake
occurred without tampering by an attacker.
Note that higher layers should not be overly reliant on whether TLS
always negotiates the strongest possible connection between two
peers. 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. The 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
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level of security: if you negotiate AES-GCM [GCM] with a 1024-bit DHE
key exchange with a host whose certificate you have verified, you can
expect to be that secure.
These goals are achieved by the handshake protocol, which can be
summarized as follows: The client sends a ClientHello message which
contains a random nonce (ClientHello.random), its preferences for
Protocol Version, Cipher Suite, and a variety of extensions. In the
same flight, it sends a ClientKeyShare message which contains its
share of the parameters for key agreement for some set of expected
server parameters (DHE/ECDHE groups, etc.).
If the client has provided a ClientKeyShare with an appropriate set
of keying material, the server responds to the ClientHello with a
ServerHello message. The ServerHello contains the server's nonce
(ServerHello.random), the server's choice of the Protocol Version,
Session ID and Cipher Suite, and the server's response to the
extensions the client offered.
The server can then generate its own keying material share and send a
ServerKeyShare message which contains its share of the parameters for
the key agreement. The server can now compute the shared secret (the
premaster secret). At this point, the server starts encrypting all
remaining handshake traffic with the negotiated cipher suite using a
key derived from the premaster secret (via the "handshake master
secret"). The remainder of the server's handshake messages will be
encrypted using that key.
Following these messages, the server will send an EncryptedExtensions
message which contains a response to any client's extensions which
are not necessary to establish the Cipher Suite. The server will
then send its certificate in a Certificate message if it is to be
authenticated. The server may optionally request a certificate from
the client by sending a CertificateRequest message at this point.
Finally, if the server is authenticated, it will send a
CertificateVerify message which provides a signature over the entire
handshake up to this point. This serves both to authenticate the
server and to establish the integrity of the negotiation. Finally,
the server sends a Finished message which includes an integrity check
over the handshake keyed by the shared secret and demonstrates that
the server and client have agreed upon the same keys. [[TODO: If the
server is not requesting client authentication, it MAY start sending
application data following the Finished, though the server has no way
of knowing who will be receiving the data. Add this.]]
Once the client receives the ServerKeyShare, it can also compute the
premaster secret and decrypt the server's remaining handshake
messages. The client generates its own sending keys based on the
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premaster secret and will encrypt the remainder of its handshake
messages using those keys and the newly established cipher suite. If
the server has sent a CertificateRequest message, the client MUST
send the Certificate message, though it may contain zero
certificates. If the client has sent a certificate, a digitally-
signed CertificateVerify message is sent to explicitly verify
possession of the private key in the certificate. Finally, the
client sends the Finished message.
At this point, the handshake is complete, and the client and server
may exchange application layer data, which is protected using a new
set of keys derived from both the premaster secret and the handshake
transcript (see [I-D.ietf-tls-session-hash] for the security
rationale for this.)
Application data MUST NOT be sent prior to the Finished message.
[[TODO: can we make this clearer and more clearly match the text
above about server-side False Start.]] Client Server
ClientHello
ClientKeyShare -------->
ServerHello
ServerKeyShare
{EncryptedExtensions*}
{Certificate*}
{CertificateRequest*}
{CertificateVerify*}
<-------- {Finished}
{Certificate*}
{CertificateVerify*}
{Finished} -------->
[Application Data] <-------> [Application Data]
Figure 1. Message flow for a full handshake
* Indicates optional or situation-dependent messages that are not
always sent.
{} Indicates messages protected using keys derived from the handshake
master secret.
[] Indicates messages protected using keys derived from the master
secret.
If the client has not provided an appropriate ClientKeyShare (e.g. it
includes only DHE or ECDHE groups unacceptable or unsupported by the
server), the server corrects the mismatch with a HelloRetryRequest
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and the client will need to restart the handshake with an appropriate
ClientKeyShare, as shown in Figure 2:
Client Server
ClientHello
ClientKeyShare -------->
<-------- HelloRetryRequest
ClientHello
ClientKeyShare -------->
ServerHello
ServerKeyShare
{EncryptedExtensions*}
{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 fatal alert.
When the client and server decide to resume a previous session or
duplicate an existing session (instead of negotiating new security
parameters), the message flow is as follows:
The client sends a ClientHello using the Session ID of the session to
be resumed. The server then checks its session cache for a match.
If a match is found, and the server is willing to re-establish the
connection under the specified session state, it will send a
ServerHello with the same Session ID value. At this point, both
client and server MUST proceed directly to sending Finished messages,
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which are protected using handshake keys as described above, computed
using resumption premaster secret created in the first handshake as
the premaster secret. Once the re-establishment is complete, the
client and server MAY begin to exchange application layer data, which
is protected using the application secrets (See flow chart below.)
If a Session ID match is not found, the server generates a new
session ID, and the TLS client and server perform a full handshake.
Client Server
ClientHello
ClientKeyExhange -------->
ServerHello
<-------- {Finished}
{Finished} -------->
[Application Data] <-------> [Application Data]
Figure 3. Message flow for an abbreviated handshake
The contents and significance of each message will be presented in
detail in the following sections.
7.3. Handshake Protocol
The TLS Handshake Protocol is one of the defined higher-level clients
of the TLS Record Protocol. This protocol is used to negotiate the
secure attributes of a session. Handshake messages are supplied to
the TLS record layer, where they are encapsulated within one or more
TLSPlaintext structures, which are processed and transmitted as
specified by the current active session state.
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enum {
reserved(0), client_hello(1), server_hello(2),
client_key_share(5), hello_retry_request(6),
server_key_share(7), certificate(11), reserved(12),
certificate_request(13), certificate_verify(15),
reserved(16), finished(20), (255)
} HandshakeType;
struct {
HandshakeType msg_type; /* handshake type */
uint24 length; /* bytes in message */
select (HandshakeType) {
case client_hello: ClientHello;
case client_key_share: ClientKeyShare;
case server_hello: ServerHello;
case hello_retry_request: HelloRetryRequest;
case server_key_share: ServerKeyShare;
case certificate: Certificate;
case certificate_request: CertificateRequest;
case certificate_verify: CertificateVerify;
case finished: Finished;
} body;
} Handshake;
The handshake protocol messages are presented below in the order they
MUST be sent; sending handshake messages in an unexpected order
results in a fatal error. Unneeded handshake messages can be
omitted, however.
New handshake message types are assigned by IANA as described in
Section 12.
7.3.1. Hello Messages
The hello phase messages are used to exchange security enhancement
capabilities between the client and server. When a new session
begins, the record layer's connection state AEAD algorithm is
initialized to NULL_NULL.
7.3.1.1. 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 ClientKeyShare. In that case, the client MUST
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send the same ClientHello (without modification) along with the
new ClientKeyShare. If a server receives a ClientHello at any
other time, it MUST send a fatal no_renegotiation alert.
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.
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.
Note: The ClientHello message includes a variable-length session
identifier. If not empty, the value identifies a session between the
same client and server whose security parameters the client wishes to
reuse. The session identifier MAY be from an earlier connection,
this connection, or from another currently active connection. The
second option is useful if the client only wishes to update the
random structures and derived values of a connection, and the third
option makes it possible to establish several independent secure
connections without repeating the full handshake protocol. These
independent connections may occur sequentially or simultaneously; a
SessionID becomes valid when the handshake negotiating it completes
with the exchange of Finished messages and persists until it is
removed due to aging or because a fatal error was encountered on a
connection associated with the session. The actual contents of the
SessionID are defined by the server.
opaque SessionID<0..32>;
Warning: Because the SessionID is transmitted without confidentiality
or integrity protection, servers MUST NOT place confidential
information in session identifiers or let the contents of fake
session identifiers cause any breach of security. (Note that the
content of the handshake as a whole, including the SessionID, is
protected by the Finished messages exchanged at the end of the
handshake.)
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
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preference (favorite choice first). Each cipher suite defines a key
exchange algorithm, a record protection algorithm (including secret
key length) and a PRF. 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.
uint8 CipherSuite[2]; /* Cryptographic suite selector */
enum { null(0), (255) } CompressionMethod;
struct {
ProtocolVersion client_version;
Random random;
SessionID session_id;
CipherSuite cipher_suites<2..2^16-2>;
CompressionMethod compression_methods<1..2^8-1>;
select (extensions_present) {
case false:
struct {};
case true:
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 E for details about backward compatibility).
random
A client-generated random structure.
session_id
The ID of a session the client wishes to use for this connection.
This field is empty if no session_id is available, or if the
client wishes to generate new security parameters.
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cipher_suites
This is a list of the cryptographic options supported by the
client, with the client's first preference first. If the
session_id field is not empty (implying a session resumption
request), this vector MUST include at least the cipher_suite from
that session. 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
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 7.3.2.5.
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.
7.3.2. Client Key Share Message
When this message will be sent:
This message is always sent by the client. It MUST immediately
follow the ClientHello message. In backward compatibility mode
(see Section XXX) it will be included in the EarlyData extension
(Section 7.3.2.5.3) in the ClientHello.
Meaning of this message:
This message contains the client's cryptographic parameters for
zero or more key establishment methods.
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Structure of this message:
struct {
NamedGroup group;
opaque key_exchange<1..2^16-1>;
} ClientKeyShareOffer;
group The named group for the key share offer. This identifies the
specific key exchange method that the ClientKeyShareOffer
describes. Finite Field Diffie-Hellman parameters are described
in Section 7.3.2.1; Elliptic Curve Diffie-Hellman parameters are
described in Section 7.3.2.2.
key_exchange Key exchange information. The contents of this field
are determined by the value of NamedGroup entry and its
corresponding definition.
struct {
ClientKeyShareOffer offers<0..2^16-1>;
} ClientKeyShare;
offers
A list of ClientKeyShareOffer values.
Clients may offer an arbitrary number of ClientKeyShareOffer values,
each representing a single set of key agreement parameters; for
instance a client might offer shares for several elliptic curves or
multiple integer DH groups. The shares for each ClientKeyShareOffer
MUST by generated independently. Clients MUST NOT offer multiple
ClientKeyShareOffers for the same parameters. It is explicitly
permitted to send an empty ClientKeyShare message, as this is used to
elicit the server's parameters if the client has no useful
information. [TODO: Recommendation about what the client offers.
Presumably which integer DH groups and which curves.] [TODO: Work
out how this interacts with PSK and SRP.]
7.3.2.1. Diffie-Hellman Parameters
Diffie-Hellman parameters for both clients and servers are encoded in
the opaque key_exchange field of the ClientKeyShareOffer or
ServerKeyShare structures. The opaque value contains the Diffie-
Hellman public value (dh_Y = g^X mod p), encoded as a big-endian
integer.
opaque dh_Y<1..2^16-1>;
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7.3.2.2. ECHDE Parameters
ECDHE parameters for both clients and servers are encoded in the
opaque key_exchange field of the ClientKeyShareOffer or
ServerKeyShare structures. 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.]]
7.3.2.3. 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 ClientKeyShare message was acceptable. If the
client proposed groups are not acceptable by the server, it will
respond with an insufficient_security fatal alert.
Structure of this message:
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struct {
ProtocolVersion server_version;
Random random;
SessionID session_id;
CipherSuite cipher_suite;
select (extensions_present) {
case false:
struct {};
case true:
Extension extensions<0..2^16-1>;
};
} ServerHello;
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 client hello and the highest supported by the server. For
this version of the specification, the version is 3.4. (See
Appendix E for details about backward compatibility.)
random
This structure is generated by the server and MUST be generated
independently of the ClientHello.random.
session_id
This is the identity of the session corresponding to this
connection. If the ClientHello.session_id was non-empty, the
server will look in its session cache for a match. If a match is
found and the server is willing to establish the new connection
using the specified session state, the server will respond with
the same value as was supplied by the client. This indicates a
resumed session and dictates that the parties must proceed
directly to the Finished messages. Otherwise, this field will
contain a different value identifying the new session. The server
may return an empty session_id to indicate that the session will
not be cached and therefore cannot be resumed. If a session is
resumed, it must be resumed using the same cipher suite it was
originally negotiated with. Note that there is no requirement
that the server resume any session even if it had formerly
provided a session_id. Clients MUST be prepared to do a full
negotiation -- including negotiating new cipher suites -- during
any handshake.
cipher_suite
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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.
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 7.3.4
message. The ServerHello MUST only include extensions which are
required to establish the cryptographic context.
7.3.2.4. HelloRetryRequest
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
but the client's ClientKeyShare message 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?]]
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/ClientKeyShare pair.
Upon receipt of a HelloRetryRequest, the client MUST send a new
ClientHello/ClientKeyShare pair to the server. The ClientKeyShare
MUST contain both the groups in the original ClientKeyShare as well
as a ClientKeyShareOffer consistent with the "selected_group" field.
I.e., it MUST be a superset of the previous ClientKeyShareOffer.
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Upon re-sending the ClientHello/ClientKeyShare and receiving the
server's ServerHello/ServerKeyShare, the client MUST verify that the
selected ciphersuite and NamedGroup match that supplied in the
HelloRetryRequest.
7.3.2.5. Hello Extensions
The extension format is:
struct {
ExtensionType extension_type;
opaque extension_data<0..2^16-1>;
} Extension;
enum {
signature_algorithms(13), early_data(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 a companion document
[TLSEXT]. The list of extension types is maintained by IANA as
described in Section 12.
An extension type MUST NOT appear in the ServerHello unless the same
extension type appeared in the corresponding ClientHello. If a
client receives an extension type in ServerHello 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.
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Finally, note that extensions can be sent both when starting a new
session and when requesting session resumption. Indeed, a client
that requests session resumption does not in general know whether the
server will accept this request, and therefore it SHOULD send the
same extensions as it would send if it were not attempting
resumption.
In general, the specification of each extension type needs to
describe the effect of the extension both during full handshake and
session resumption. Most current TLS extensions are relevant only
when a session is initiated: when an older session is resumed, the
server does not process these extensions in Client Hello, and does
not include them in Server Hello. However, some extensions may
specify different behavior during session 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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7.3.2.5.1. 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. The "extension_data" field of this extension
contains a "supported_signature_algorithms" value.
enum {
none(0), md5(1), sha1(2), sha224(3), sha256(4), sha384(5),
sha512(6), (255)
} HashAlgorithm;
enum { anonymous(0), rsa(1), dsa(2), ecdsa(3), (255) }
SignatureAlgorithm;
struct {
HashAlgorithm hash;
SignatureAlgorithm signature;
} SignatureAndHashAlgorithm;
SignatureAndHashAlgorithm
supported_signature_algorithms<2..2^16-2>;
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., DSA with SHA-1, but not SHA-
256), algorithms here are listed in pairs.
hash
This field indicates the hash algorithm which may be used. The
values indicate support for unhashed data, MD5 [RFC1321], SHA-1,
SHA-224, 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.
signature
This field indicates the signature algorithm that may be used.
The values indicate anonymous signatures, RSASSA-PKCS1-v1_5
[RFC3447] and DSA [DSS], and ECDSA [ECDSA], respectively. The
"anonymous" value is meaningless in this context but used in
Section 7.3.3. It MUST NOT appear in this extension.
The semantics of this extension are somewhat complicated because the
cipher suite indicates permissible signature algorithms but not hash
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algorithms. Section 7.3.5 and Section 7.3.3 describe the appropriate
rules.
If the client supports only the default hash and signature algorithms
(listed in this section), it MAY omit the signature_algorithms
extension. If the client does not support the default algorithms, or
supports other hash and signature algorithms (and it is willing to
use them for verifying messages sent by the server, i.e., server
certificates and server key share), it MUST send the
signature_algorithms extension, listing the algorithms it is willing
to accept.
If the client does not send the signature_algorithms extension, the
server MUST do the following:
- If the negotiated key exchange algorithm is one of (DHE_RSA,
ECDHE_RSA), behave as if client had sent the value {sha1,rsa}.
- If the negotiated key exchange algorithm is DHE_DSS, behave as if
the client had sent the value {sha1,dsa}.
- If the negotiated key exchange algorithm is ECDHE_ECDSA, behave as
if the client had sent value {sha1,ecdsa}.
Note: this is a change from TLS 1.1 where there are no explicit
rules, but as a practical matter one can assume that the peer
supports MD5 and SHA-1.
Note: this extension is not meaningful for TLS versions prior to 1.2.
Clients MUST NOT offer it if they are offering prior versions.
However, even if clients do offer it, the rules specified in [TLSEXT]
require servers to ignore extensions they do not understand.
Servers MUST NOT send this extension. TLS servers MUST support
receiving this extension.
When performing session resumption, this extension is not included in
Server Hello, and the server ignores the extension in Client Hello
(if present).
7.3.2.5.2. 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].
The "extension_data" field of this extension SHALL contain a
"NamedGroupList" value:
enum {
// Elliptic Curve Groups.
sect163k1 (1), sect163r1 (2), sect163r2 (3),
sect193r1 (4), sect193r2 (5), sect233k1 (6),
sect233r1 (7), sect239k1 (8), sect283k1 (9),
sect283r1 (10), sect409k1 (11), sect409r1 (12),
sect571k1 (13), sect571r1 (14), secp160k1 (15),
secp160r1 (16), secp160r2 (17), secp192k1 (18),
secp192r1 (19), secp224k1 (20), secp224r1 (21),
secp256k1 (22), secp256r1 (23), secp384r1 (24),
secp521r1 (25),
// Finite Field Groups.
ffdhe2048(256), ffdhe3072(257), ffdhe4096(258),
ffdhe8192(259),
// Reserved Code Points.
reserved (0xFE00..0xFEFF),
reserved(0xFF01),
reserved(0xFF02),
(0xFFFF)
} NamedGroup;
struct {
NamedGroup named_group_list<1..2^16-1>
} NamedGroupList;
sect163k1, etc
Indicates support of the corresponding named curve The named
curves defined here are those specified in SEC 2 [13]. Note that
many of these curves are also recommended in ANSI X9.62 [X962] and
FIPS 186-2 [DSS]. Values 0xFE00 through 0xFEFF are reserved for
private use. Values 0xFF01 and 0xFF02 were used in previous
versions of TLS but MUST NOT be offered by TLS 1.3
implementations. [[OPEN ISSUE: Triage curve list.]]
ffdhe2432, etc
Indicates support of the corresponding finite field group, defined
in [I-D.ietf-tls-negotiated-ff-dhe]
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Items in named_curve_list are ordered according to the client's
preferences (favorite choice first).
As an example, a client that only supports secp192r1 (aka NIST P-192;
value 19 = 0x0013) and secp224r1 (aka NIST P-224; value 21 = 0x0015)
and prefers to use secp192r1 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 13 00 15
The client MUST supply a "named_groups" extension containing at least
one group for each key exchange algorithm (currently DHE and ECDHE)
for which it offers a cipher suite. If the client does not supply a
"named_groups" extension with a compatible group, 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.
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 the ServerKeyExchange message. The server
must consider the supported groups in both cases.
[[TODO: IANA Considerations.]]
7.3.2.5.3. Early Data Extension
TLS versions before 1.3 have a strict message ordering and do not
permit additional messages to follow the ClientHello. The EarlyData
extension allows TLS messages which would otherwise be sent as
separate records to be instead inserted in the ClientHello. The
extension simply contains the TLS records which would otherwise have
been included in the client's first flight.
struct {
TLSCipherText messages<5 .. 2^24-1>;
} EarlyDataExtension;
Extra messages for the client's first flight MAY either be
transmitted standalone or sent as EarlyData. However, when a client
does not know whether TLS 1.3 can be negotiated - e.g., because the
server may support a prior version of TLS or because of network
intermediaries - it SHOULD use the EarlyData extension. If the
EarlyData extension is used, then clients MUST NOT send any messages
other than the ClientHello in their initial flight.
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Any data included in EarlyData is not integrated into the handshake
hashes directly. E.g., if the ClientKeyShare is included in
EarlyData, then the handshake hashes consist of ClientHello +
ServerHello, etc. However, because the ClientKeyShare is in a
ClientHello extension, it is still hashed transitively. This
procedure guarantees that the Finished message covers these messages
even if they are ultimately ignored by the server (e.g., because it
is sent to a TLS 1.2 server). TLS 1.3 servers MUST understand
messages sent in EarlyData, and aside from hashing them differently,
MUST treat them as if they had been sent immediately after the
ClientHello.
Servers MUST NOT send the EarlyData extension. Negotiating TLS 1.3
serves as acknowledgement that it was processed as described above.
[[OPEN ISSUE: This is a fairly general mechanism which is possibly
overkill in the 1-RTT case, where it would potentially be more
attractive to just have a "ClientKeyShare" extension. However, for
the 0-RTT case we will want to send the Certificate,
CertificateVerify, and application data, so a more general extension
seems appropriate at least until we have determined we don't need it
for 0-RTT.]]
7.3.3. Server Key Share Message
When this message will be sent:
This message will be sent immediately after the ServerHello
message if the client has provided a ClientKeyShare message which
is compatible with the selected cipher suite and group parameters.
Meaning of this message:
This message conveys cryptographic information to allow the client
to compute the premaster secret: a Diffie-Hellman public key with
which the client can complete a key exchange (with the result
being the premaster secret) or a public key for some other
algorithm.
Structure of this message:
struct {
NamedGroup group;
opaque key_exchange<1..2^16-1>;
} ServerKeyShare;
group The named group for the key share offer. This identifies the
selected key exchange method from the ClientKeyShare message
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(Section 7.3.2), identifying which value from the
ClientKeyShareOffer the server has accepted as is responding to.
key_exchange Key exchange information. The contents of this field
are determined by the value of NamedGroup entry and its
corresponding definition.
7.3.4. Encrypted Extensions
When this message will be sent:
If this message is sent, it MUST be sent immediately after the
server's ServerKeyShare.
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.
7.3.5. 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 DH_anon). This message will always immediately follow
either the EncryptedExtensions message if one is sent or the
ServerKeyShare message.
Meaning of this message:
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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:
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 MUST directly certify the one preceding it. Because
certificate validation requires that root keys be distributed
independently, the self-signed certificate that specifies the root
certificate authority MAY be omitted from the chain, under the
assumption that the remote end must already possess it in order to
validate it in any case.
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, unless explicitly negotiated
otherwise (e.g., [RFC5081]).
- The 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 (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 key exchange message.
Note: ECDHE_RSA is defined in [RFC4492].
DHE_DSS DSA public key; the certificate MUST allow the
key to be used for signing with the hash
algorithm that will be employed in the server
key exchange message.
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 key exchange message. 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 [TLSEXT] are
used to guide certificate selection.
If the client provided a "signature_algorithms" extension, then all
certificates provided by the server MUST be signed by a hash/
signature algorithm pair that appears in that extension. Note that
this implies 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 DSA key). This is a departure
from TLS 1.1, which required that the algorithms be the same.
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,
etc.). If the server has a single certificate, it SHOULD attempt to
validate that it meets these criteria.
Note that there are certificates that use algorithms and/or algorithm
combinations that cannot be currently used with TLS. For example, a
certificate with RSASSA-PSS signature key (id-RSASSA-PSS OID in
SubjectPublicKeyInfo) cannot be used because TLS defines no
corresponding signature algorithm.
As cipher suites that specify new key exchange methods are specified
for the TLS protocol, they will imply the certificate format and the
required encoded keying information.
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7.3.6. 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:
enum {
rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
rsa_ephemeral_dh_RESERVED(5), dss_ephemeral_dh_RESERVED(6),
fortezza_dms_RESERVED(20), (255)
} ClientCertificateType;
opaque DistinguishedName<1..2^16-1>;
struct {
ClientCertificateType certificate_types<1..2^8-1>;
SignatureAndHashAlgorithm
supported_signature_algorithms<2..2^16-2>;
DistinguishedName certificate_authorities<0..2^16-1>;
} CertificateRequest;
certificate_types
A list of the types of certificate types that the client may
offer.
rsa_sign a certificate containing an RSA key
dss_sign a certificate containing a DSA key
rsa_fixed_dh a certificate containing a static DH key.
dss_fixed_dh a certificate containing a static DH key
supported_signature_algorithms
A list of the hash/signature algorithm pairs that the server is
able to verify, listed in descending order of preference.
certificate_authorities
A list of the distinguished names [X501] of acceptable
certificate_authorities, represented in DER-encoded format. 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 of the appropriate ClientCertificateType,
unless there is some external arrangement to the contrary.
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The interaction of the certificate_types and
supported_signature_algorithms fields is somewhat complicated.
certificate_types has been present in TLS since SSLv3, but was
somewhat underspecified. Much of its functionality is superseded by
supported_signature_algorithms. The following rules apply:
- Any certificates provided by the client MUST be signed using a
hash/signature algorithm pair found in
supported_signature_algorithms.
- The end-entity certificate provided by the client MUST contain a
key that is compatible with certificate_types. If the key is a
signature key, it MUST be usable with some hash/signature
algorithm pair in supported_signature_algorithms.
- For historical reasons, the names of some client certificate types
include the algorithm used to sign the certificate. For example,
in earlier versions of TLS, rsa_fixed_dh meant a certificate
signed with RSA and containing a static DH key. In TLS 1.2, this
functionality has been obsoleted by the
supported_signature_algorithms, and the certificate type no longer
restricts the algorithm used to sign the certificate. For
example, if the server sends dss_fixed_dh certificate type and
{{sha1, dsa}, {sha1, rsa}} signature types, the client MAY reply
with a certificate containing a static DH key, signed with RSA-
SHA1.
New ClientCertificateType values are assigned by IANA as described in
Section 12.
Note: Values listed as RESERVED may not be used. They were used in
SSLv3.
Note: It is a fatal handshake_failure alert for an anonymous server
to request client authentication.
7.3.7. 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.
certificate and also provides integrity for the handshake up to
this point. This message is only 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:
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struct {
digitally-signed struct {
opaque handshake_messages_hash[hash_length];
}
} CertificateVerify;
Here handshake_messages_hash is a digest of all handshake messages
sent or received, starting at ClientHello and up to, 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 7.3) exchanged
thus far. For the PRF defined in Section 5, the digest MUST be
the Hash used as the basis for the PRF. Any cipher suite which
defines a different PRF MUST also define the Hash to use in this
computation. Note that this is the same running hash that is used
in the Finished message Section 7.3.8.
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.
If the client has offered the "signature_algorithms" extension,
the signature algorithm and hash algorithm MUST be a pair listed
in that extension. Note that there is a possibility for
inconsistencies here. For instance, the client might offer
DHE_DSS key exchange but omit any DSA 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.
Because DSA signatures do not contain any secure indication of
hash algorithm, there is a risk of hash substitution if multiple
hashes may be used with any key. Currently, DSA [DSS] may only be
used with SHA-1. Future revisions of DSS [DSS-3] are expected to
allow the use of other digest algorithms with DSA, as well as
guidance as to which digest algorithms should be used with each
key size. In addition, future revisions of [RFC3280] may specify
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mechanisms for certificates to indicate which digest algorithms
are to be used with DSA. [[TODO: Update this to deal with DSS-3
and DSS-4. https://github.com/tlswg/tls13-spec/issues/59]]
7.3.8. Server Finished
When this message will be sent:
The Server's Finished message is the final message sent by the
server and indicates that the key exchange and authentication
processes were successful.
Meaning of this message:
Recipients of Finished messages MUST verify that the contents are
correct. Once a side has sent its Finished message and received
and validated the Finished message from its peer, it may begin to
send and receive application data over the connection. This data
will be protected under keys derived from the hs_master_secret
(see Section 8.
Structure of this message:
struct {
opaque verify_data[verify_data_length];
} Finished;
verify_data
PRF(hs_master_secret, finished_label, Hash(handshake_messages))
[0..verify_data_length-1];
finished_label
For Finished messages sent by the client, the string "client
finished". For Finished messages sent by the server, the string
"server finished".
Hash denotes a Hash of the handshake messages. For the PRF
defined in Section 5, the Hash MUST be the Hash used as the basis
for the PRF. Any cipher suite which defines a different PRF MUST
also define the Hash to use in the Finished computation.
In previous versions of TLS, the verify_data was always 12 octets
long. In the current version of TLS, it depends on the cipher
suite. Any cipher suite which does not explicitly specify
verify_data_length has a verify_data_length equal to 12. This
includes all existing cipher suites. Note that this
representation has the same encoding as with previous versions.
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Future cipher suites MAY specify other lengths but such length
MUST be at least 12 bytes.
handshake_messages
All of the data from all messages in this handshake (not including
any HelloRequest messages) up to, but not including, this message.
This is only data visible at the handshake layer and does not
include record layer headers. This is the concatenation of all
the Handshake structures as defined in Section 7.3, exchanged thus
far.
The value handshake_messages includes all handshake messages starting
at ClientHello up to, but not including, this Finished message. This
may be different from handshake_messages in Section 7.3.7 or
Section 7.3.10. Also, the handshake_messages for the Finished
message sent by the client will be different from that for the
Finished message sent by the server, because the one that is sent
second will include the prior one.
Note: Alerts and any other record types are not handshake messages
and are not included in the hash computations. Also, HelloRequest
messages are omitted from handshake hashes.
7.3.9. 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.
Client certificates are sent using the Certificate structure
defined in Section 7.3.5.
Meaning of this message:
This message conveys the client's certificate chain to the server;
the server will use it when verifying the CertificateVerify
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message (when the client authentication is based on signing) or
calculating the premaster secret (for non-ephemeral Diffie-
Hellman). 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, unless explicitly negotiated
otherwise (e.g., [RFC5081]).
- The end-entity certificate's public key (and associated
restrictions) has to be compatible with the certificate types
listed in CertificateRequest:
Client Cert. Type Certificate Key Type
rsa_sign RSA public key; the certificate MUST allow the
key to be used for signing with the signature
scheme and hash algorithm that will be
employed in the certificate verify message.
dss_sign DSA public key; the certificate MUST allow the
key to be used for signing with the hash
algorithm that will be employed in the
certificate verify message.
ecdsa_sign 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
certificate verify message; the public key
MUST use a curve and point format supported by
the server.
rsa_fixed_dh Diffie-Hellman public key; MUST use the same
dss_fixed_dh parameters as server's key.
rsa_fixed_ecdh ECDH-capable public key; MUST use the
ecdsa_fixed_ecdh same curve as the server's key, and MUST use a
point format supported by the server.
- 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.
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- The certificates MUST be signed using an acceptable hash/
signature algorithm pair, as described in Section 7.3.6. Note
that this relaxes the constraints on certificate-signing
algorithms found in prior versions of TLS.
Note that, as with the server certificate, there are certificates
that use algorithms/algorithm combinations that cannot be currently
used with TLS.
7.3.10. 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 7.3.7, except that the context string is "TLS 1.3, client
CertificateVerify".
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.
Because DSA signatures do not contain any secure indication of
hash algorithm, there is a risk of hash substitution if multiple
hashes may be used with any key. Currently, DSA [DSS] may only be
used with SHA-1. Future revisions of DSS [DSS-3] are expected to
allow the use of other digest algorithms with DSA, as well as
guidance as to which digest algorithms should be used with each
key size. In addition, future revisions of [RFC3280] may specify
mechanisms for certificates to indicate which digest algorithms
are to be used with DSA.
8. 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
agreement, and record protection algorithms are determined by the
cipher_suite selected by the server and revealed in the ServerHello
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message. The random values are exchanged in the hello messages. All
that remains is to calculate the master secret.
8.1. Computing the Master Secret
The pre_master_secret is used to generate a series of master secret
values, as shown in the following diagram and described below.
Premaster Secret <---------+
| |
PRF |
| |
v |
Handshake <-PRF- Handshake |
Traffic Keys Master Secret |
| | Via
| | Session
+----------+----------+ | Cache
| | |
PRF PRF |
| | |
v v |
Application <-PRF- Master Resumption |
Traffic Keys Secret Premaster --+
Secret
First, as soon as the ClientKeyShare and ServerKeyShare messages have
been exchanged, the client and server each use the unauthenticated
key shares to generate a master secret which is used for the
protection of the remaining handshake records. Specifically, they
generate:
hs_master_secret = PRF(pre_master_secret, "handshake master secret",
session_hash)
[0..47];
During resumption, the premaster secret is initialized with the
"resumption premaster secret", rather than using the values from the
ClientKeyShare/ServerKeyShare exchange.
This master secret value is used to generate the record protection
keys used for the handshake, as described in Section 6.3.
Once the hs_master_secret has been computed, the premaster secret
SHOULD be deleted from memory.
Once the last non-Finished message has been sent, the client and
server then compute the master secret which will be used for the
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remainder of the session. It is also used with TLS Exporters
[RFC5705].
master_secret = PRF(hs_master_secret, "extended master secret",
session_hash)
[0..47];
If the server does not request client authentication, the master
secret can be computed at the time that the server sends its
Finished, thus allowing the server to send traffic on its first
flight (see [TODO] for security considerations on this practice.) If
the server requests client authentication, this secret can be
computed after the client's Certificate and CertificateVerify have
been sent, or, if the client refuses client authentication, after the
client's empty Certificate message has been sent.
For full handshakes, each side also derives a new secret which will
be used as the premaster_secret for future resumptions of the newly
established session. This is computed as:
resumption_premaster_secret = PRF(hs_master_secret,
"resumption premaster secret",
session_hash)
[0..47];
The session_hash value is a running hash of the handshake as defined
in Section 8.1.1. Thus, the hs_master_secret is generated using a
different session_hash from the other two secrets.
All master secrets are always exactly 48 bytes in length. The length
of the premaster secret will vary depending on key exchange method.
8.1.1. The Session Hash
When a handshake takes place, we define
session_hash = Hash(handshake_messages)
where "handshake_messages" refers to all handshake messages sent or
received, starting at client hello 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.
For concreteness, at the point where the handshake master secret is
derived, the session hash includes the ClientHello, ClientKeyShare,
ServerHello, and ServerKeyShare, and HelloRetryRequest (if any)
(though see [https://github.com/tlswg/tls13-spec/issues/104]). At
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the point where the master secret is derived, it includes every
handshake message, with the exception of the Finished messages. 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.
8.1.2. Diffie-Hellman
A conventional Diffie-Hellman computation is performed. The
negotiated key (Z) is used as the pre_master_secret, and is converted
into the master_secret, as specified above. Leading bytes of Z that
contain all zero bits are stripped before it is used as the
pre_master_secret.
8.1.3. 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 premaster 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 the premaster secret for anything
other than for computing the master secret.)
9. Mandatory Cipher Suites
In the absence of an application profile standard specifying
otherwise, a TLS-compliant application MUST implement the cipher
suite TODO:Needs to be selected [1]. (See Appendix A.4 for the
definition).
10. 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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11. Security Considerations
Security issues are discussed throughout this memo, especially in
Appendices D, E, and F.
12. IANA Considerations
[[TODO: Update https://github.com/tlswg/tls13-spec/issues/62]]
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 ClientCertificateType Identifiers Registry: 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 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 7.3.2.5).
This document also uses two registries originally created in
[RFC4492]. IANA [should update/has updated] it to reference this
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document. The registries and their allocation policies are listed
below.
- TLS NamedCurve registry: Future values are allocated via IETF
Consensus [RFC2434].
- 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 7.3.2.5.1. 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 7.3.2.5.1. 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].
13. References
13.1. Normative References
[AES] National Institute of Standards and Technology,
"Specification for the Advanced Encryption Standard
(AES)", NIST FIPS 197, November 2001.
[DSS] National Institute of Standards and Technology, U.S.
Department of Commerce, "Digital Signature Standard", NIST
FIPS PUB 186-2, 2000.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February
1997.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
[RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
August 2008.
[SCH] Schneier, B., "Applied Cryptography: Protocols,
Algorithms, and Source Code in C, 2nd ed.", 1996.
[SHS] National Institute of Standards and Technology, U.S.
Department of Commerce, "Secure Hash Standard", NIST FIPS
PUB 180-2, August 2002.
[TRIPLEDES]
National Institute of Standards and Technology,
"Recommendation for the Triple Data Encryption Algorithm
(TDEA) Block Cipher", NIST Special Publication 800-67, May
2004.
[X680] ITU-T, "Information technology - Abstract Syntax Notation
One (ASN.1): Specification of basic notation", ISO/IEC
8824-1:2002, 2002.
[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.
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13.2. Informative References
[CBCATT] Moeller, B., "Security of CBC Ciphersuites in SSL/TLS:
Problems and Countermeasures", May 2004,
<http://www.openssl.org/~bodo/tls-cbc.txt>.
[CCM] "NIST Special Publication 800-38C: The CCM Mode for
Authentication and Confidentiality", May 2004,
<http://csrc.nist.gov/publications/nistpubs/800-38C/
SP800-38C.pdf>.
[DES] "Data Encryption Standard (DES)", NIST FIPS PUB 46-3,
October 1999.
[DSS-3] National Institute of Standards and Technology, U.S.,
"Digital Signature Standard", NIST FIPS PUB 186-3 Draft,
2006.
[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.
[ENCAUTH] Krawczyk, H., "The Order of Encryption and Authentication
for Protecting Communications (Or: How Secure is SSL?)",
2001.
[FI06] "Bleichenbacher's RSA signature forgery based on
implementation error", August 2006, <http://www.imc.org/
ietf-openpgp/mail-archive/msg14307.html>.
[GCM] Dworkin, M., "Recommendation for Block Cipher Modes of
Operation: Galois/Counter Mode (GCM) and GMAC", NIST
Special Publication 800-38D, November 2007.
[I-D.ietf-tls-negotiated-ff-dhe]
Gillmor, D., "Negotiated Finite Field Diffie-Hellman
Ephemeral Parameters for TLS", draft-ietf-tls-negotiated-
ff-dhe-07 (work in progress), March 2015.
[I-D.ietf-tls-session-hash]
Bhargavan, K., Delignat-Lavaud, A., Pironti, A., Langley,
A., and M. Ray, "Transport Layer Security (TLS) Session
Hash and Extended Master Secret Extension", draft-ietf-
tls-session-hash-03 (work in progress), November 2014.
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[I-D.ietf-tls-sslv3-diediedie]
Barnes, R., Thomson, M., Pironti, A., and A. Langley,
"Deprecating Secure Sockets Layer Version 3.0", draft-
ietf-tls-sslv3-diediedie-02 (work in progress), March
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, September 1981.
[RFC1948] Bellovin, S., "Defending Against Sequence Number Attacks",
RFC 1948, May 1996.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[RFC2785] Zuccherato, R., "Methods for Avoiding the "Small-Subgroup"
Attacks on the Diffie-Hellman Key Agreement Method for S/
MIME", RFC 2785, March 2000.
[RFC3268] Chown, P., "Advanced Encryption Standard (AES)
Ciphersuites for Transport Layer Security (TLS)", RFC
3268, June 2002.
[RFC3526] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
Diffie-Hellman groups for Internet Key Exchange (IKE)",
RFC 3526, May 2003.
[RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For
Public Keys Used For Exchanging Symmetric Keys", BCP 86,
RFC 3766, April 2004.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302, December
2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
4303, December 2005.
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[RFC4307] Schiller, J., "Cryptographic Algorithms for Use in the
Internet Key Exchange Version 2 (IKEv2)", RFC 4307,
December 2005.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
[RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 4366, April 2006.
[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, May 2006.
[RFC4506] Eisler, M., "XDR: External Data Representation Standard",
STD 67, RFC 4506, May 2006.
[RFC5081] Mavrogiannopoulos, N., "Using OpenPGP Keys for Transport
Layer Security (TLS) Authentication", RFC 5081, November
2007.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, January 2008.
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, March 2010.
[RFC6176] Turner, S. and T. Polk, "Prohibiting Secure Sockets Layer
(SSL) Version 2.0", RFC 6176, March 2011.
[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.
[TLSEXT] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", February 2008.
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[X501] "Information Technology - Open Systems Interconnection -
The Directory: Models", ITU-T X.501, 1993.
13.3. URIs
[1] https://github.com/tlswg/tls13-spec/issues/32
[2] mailto:tls@ietf.org
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Appendix A. Protocol Data Structures and Constant Values
This section describes protocol types and constants.
A.1. Record Layer
struct {
uint8 major;
uint8 minor;
} ProtocolVersion;
ProtocolVersion version = { 3, 4 }; /* TLS v1.3*/
enum {
reserved(20), alert(21), handshake(22),
application_data(23), (255)
} ContentType;
struct {
ContentType type;
ProtocolVersion version;
uint16 length;
opaque fragment[TLSPlaintext.length];
} TLSPlaintext;
struct {
ContentType type;
ProtocolVersion version;
uint16 length;
opaque nonce_explicit[SecurityParameters.record_iv_length];
aead-ciphered struct {
opaque content[TLSPlaintext.length];
} fragment;
} TLSCiphertext;
A.2. Alert Messages
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enum { warning(1), fatal(2), (255) } AlertLevel;
enum {
close_notify(0),
unexpected_message(10),
bad_record_mac(20),
decryption_failed_RESERVED(21),
record_overflow(22),
decompression_failure_RESERVED(30),
handshake_failure(40),
no_certificate_RESERVED(41),
bad_certificate(42),
unsupported_certificate(43),
certificate_revoked(44),
certificate_expired(45),
certificate_unknown(46),
illegal_parameter(47),
unknown_ca(48),
access_denied(49),
decode_error(50),
decrypt_error(51),
export_restriction_RESERVED(60),
protocol_version(70),
insufficient_security(71),
internal_error(80),
user_canceled(90),
no_renegotiation(100),
unsupported_extension(110),
(255)
} AlertDescription;
struct {
AlertLevel level;
AlertDescription description;
} Alert;
A.3. Handshake Protocol
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enum {
reserved(0), client_hello(1), server_hello(2),
client_key_share(5), hello_retry_request(6),
server_key_share(7), certificate(11), reserved(12),
certificate_request(13), certificate_verify(15),
reserved(16), finished(20), (255)
} HandshakeType;
struct {
HandshakeType msg_type; /* handshake type */
uint24 length; /* bytes in message */
select (HandshakeType) {
case client_hello: ClientHello;
case client_key_share: ClientKeyShare;
case server_hello: ServerHello;
case hello_retry_request: HelloRetryRequest;
case server_key_share: ServerKeyShare;
case certificate: Certificate;
case certificate_request: CertificateRequest;
case certificate_verify: CertificateVerify;
case finished: Finished;
} body;
} Handshake;
A.3.1. Hello Messages
opaque SessionID<0..32>;
uint8 CipherSuite[2]; /* Cryptographic suite selector */
enum { null(0), (255) } CompressionMethod;
struct {
ProtocolVersion client_version;
Random random;
SessionID session_id;
CipherSuite cipher_suites<2..2^16-2>;
CompressionMethod compression_methods<1..2^8-1>;
select (extensions_present) {
case false:
struct {};
case true:
Extension extensions<0..2^16-1>;
};
} ClientHello;
struct {
ProtocolVersion server_version;
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Random random;
SessionID session_id;
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 {
signature_algorithms(13), early_data(TBD), (65535)
} ExtensionType;
struct {
TLSCipherText messages<5 .. 2^24-1>;
} EarlyDataExtension;
struct {
Extension extensions<0..2^16-1>;
} EncryptedExtensions;
A.3.1.1. Signature Algorithm Extension
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enum {
none(0), md5(1), sha1(2), sha224(3), sha256(4), sha384(5),
sha512(6), (255)
} HashAlgorithm;
enum { anonymous(0), rsa(1), dsa(2), ecdsa(3), (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.
sect163k1 (1), sect163r1 (2), sect163r2 (3),
sect193r1 (4), sect193r2 (5), sect233k1 (6),
sect233r1 (7), sect239k1 (8), sect283k1 (9),
sect283r1 (10), sect409k1 (11), sect409r1 (12),
sect571k1 (13), sect571r1 (14), secp160k1 (15),
secp160r1 (16), secp160r2 (17), secp192k1 (18),
secp192r1 (19), secp224k1 (20), secp224r1 (21),
secp256k1 (22), secp256r1 (23), secp384r1 (24),
secp521r1 (25),
// Finite Field Groups.
ffdhe2048(256), ffdhe3072(257), ffdhe4096(258),
ffdhe8192(259),
// Reserved Code Points.
reserved (0xFE00..0xFEFF),
reserved(0xFF01),
reserved(0xFF02),
(0xFFFF)
} NamedGroup;
struct {
NamedGroup named_group_list<1..2^16-1>
} NamedGroupList;
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A.3.2. Key Exchange Messages
struct {
NamedGroup group;
opaque key_exchange<1..2^16-1>;
} ClientKeyShareOffer;
struct {
ClientKeyShareOffer offers<0..2^16-1>;
} ClientKeyShare;
opaque dh_Y<1..2^16-1>;
opaque point <1..2^8-1>;
struct {
NamedGroup group;
opaque key_exchange<1..2^16-1>;
} ServerKeyShare;
A.3.3. Authentication Messages
opaque ASN1Cert<1..2^24-1>;
struct {
ASN1Cert certificate_list<0..2^24-1>;
} Certificate;
enum {
rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
rsa_ephemeral_dh_RESERVED(5), dss_ephemeral_dh_RESERVED(6),
fortezza_dms_RESERVED(20), (255)
} ClientCertificateType;
opaque DistinguishedName<1..2^16-1>;
struct {
ClientCertificateType certificate_types<1..2^8-1>;
SignatureAndHashAlgorithm
supported_signature_algorithms<2..2^16-2>;
DistinguishedName certificate_authorities<0..2^16-1>;
} CertificateRequest;
struct {
digitally-signed struct {
opaque handshake_messages_hash[hash_length];
}
} CertificateVerify;
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A.3.4. Handshake Finalization Messages
struct {
opaque verify_data[verify_data_length];
} Finished;
A.4. The Cipher Suite
The following values define the cipher suite codes used in the
ClientHello and ServerHello messages.
A cipher suite defines a cipher specification supported in TLS
Version 1.2.
TLS_NULL_WITH_NULL_NULL is specified and is the initial state of a
TLS connection during the first handshake on that channel, but MUST
NOT be negotiated, as it provides no more protection than an
unsecured connection.
CipherSuite TLS_NULL_WITH_NULL_NULL = { 0x00,0x00 };
The following cipher suite definitions, defined in {{RFC5288}, are
used for server-authenticated (and optionally client-authenticated)
Diffie-Hellman. DHE denotes ephemeral Diffie-Hellman, where the
Diffie-Hellman parameters are signed by a signature-capable
certificate, which has been signed by the CA. The signing algorithm
used by the server is specified after the DHE component of the
CipherSuite name. The server can request any signature-capable
certificate from the client for client authentication.
CipherSuite TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 = {0x00,0x9E}
CipherSuite TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 = {0x00,0x9F}
CipherSuite TLS_DHE_DSS_WITH_AES_128_GCM_SHA256 = {0x00,0xA2}
CipherSuite TLS_DHE_DSS_WITH_AES_256_GCM_SHA384 = {0x00,0xA3}
The following cipher suite definitions, defined in {{RFC5289}, are
used for server-authenticated (and optionally client-authenticated)
Elliptic Curve Diffie-Hellman. ECDHE denotes ephemeral Diffie-
Hellman, where the Diffie-Hellman parameters are signed by a
signature-capable certificate, which has been signed by the CA. The
signing algorithm used by the server is specified after the DHE
component of the CipherSuite name. The server can request any
signature-capable certificate from the client for client
authentication.
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CipherSuite TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 = {0xC0,0x2B};
CipherSuite TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 = {0xC0,0x2C};
CipherSuite TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 = {0xC0,0x2F};
CipherSuite TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 = {0xC0,0x30};
The following ciphers, defined in [RFC5288], are used for completely
anonymous Diffie-Hellman communications in which neither party is
authenticated. Note that this mode is vulnerable to man-in-the-
middle attacks. Using this mode therefore is of limited use: These
cipher suites MUST NOT be used by TLS 1.2 implementations unless the
application layer has specifically requested to allow anonymous key
exchange. (Anonymous key exchange may sometimes be acceptable, for
example, to support opportunistic encryption when no set-up for
authentication is in place, or when TLS is used as part of more
complex security protocols that have other means to ensure
authentication.)
CipherSuite TLS_DH_anon_WITH_AES_128_GCM_SHA256 = {0x00,0xA6}
CipherSuite TLS_DH_anon_WITH_AES_256_GCM_SHA384 = {0x00,0xA7}
[[TODO: Add all the defined AEAD ciphers. This currently only lists
GCM. https://github.com/tlswg/tls13-spec/issues/53]] Note that using
non-anonymous key exchange without actually verifying the key
exchange is essentially equivalent to anonymous key exchange, and the
same precautions apply. While non-anonymous key exchange will
generally involve a higher computational and communicational cost
than anonymous key exchange, it may be in the interest of
interoperability not to disable non-anonymous key exchange when the
application layer is allowing anonymous key exchange.
The PRFs SHALL be as follows:
o For cipher suites ending with _SHA256, the PRF is the TLS PRF with
SHA-256 as the hash function.
o For cipher suites ending with _SHA384, the PRF is the TLS PRF with
SHA-384 as the hash function.
New cipher suite values are been assigned by IANA as described in
Section 12.
Note: The cipher suite values { 0x00, 0x1C } and { 0x00, 0x1D } are
reserved to avoid collision with Fortezza-based cipher suites in SSL
3.
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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_prf_sha256 } PRFAlgorithm;
enum { aes_gcm } RecordProtAlgorithm;
/* The algorithms specified in PRFAlgorithm and
RecordProtAlgorithm may be added to. */
struct {
ConnectionEnd entity;
PRFAlgorithm prf_algorithm;
RecordProtAlgorithm record_prot_algorithm;
uint8 enc_key_length;
uint8 block_length;
uint8 fixed_iv_length;
uint8 record_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
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 [RFC3280].
As described in Section 7.3.5 and Section 7.3.9, the restrictions on
the signature algorithms used to sign certificates are no longer tied
to the cipher suite (when used by the server) or the
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ClientCertificateType (when used by the client). 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. Glossary
Advanced Encryption Standard (AES)
AES [AES] is a widely used symmetric encryption algorithm. AES is
a block cipher with a 128-, 192-, or 256-bit keys and a 16-byte
block size. TLS currently only supports the 128- and 256-bit key
sizes.
application protocol
An application protocol is a protocol that normally layers
directly on top of the transport layer (e.g., TCP/IP). Examples
include HTTP, TELNET, FTP, and SMTP.
asymmetric cipher
See public key cryptography.
authenticated encryption with additional data (AEAD)
A symmetric encryption algorithm that simultaneously provides
confidentiality and message integrity.
authentication
Authentication is the ability of one entity to determine the
identity of another entity.
certificate
As part of the X.509 protocol (a.k.a. ISO Authentication
framework), certificates are assigned by a trusted Certificate
Authority and provide a strong binding between a party's identity
or some other attributes and its public key.
client
The application entity that initiates a TLS connection to a
server. This may or may not imply that the client initiated the
underlying transport connection. The primary operational
difference between the server and client is that the server is
generally authenticated, while the client is only optionally
authenticated.
client write key
The key used to protect data written by the client.
connection
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A connection is a transport (in the OSI layering model definition)
that provides a suitable type of service. For TLS, such
connections are peer-to-peer relationships. The connections are
transient. Every connection is associated with one session.
Digital Signature Standard (DSS)
A standard for digital signing, including the Digital Signing
Algorithm, approved by the National Institute of Standards and
Technology, defined in NIST FIPS PUB 186-2, "Digital Signature
Standard", published January 2000 by the U.S. Department of
Commerce [DSS]. A significant update [DSS-3] has been drafted and
was published in March 2006.
digital signatures
Digital signatures utilize public key cryptography and one-way
hash functions to produce a signature of the data that can be
authenticated, and is difficult to forge or repudiate.
handshake
An initial negotiation between client and server that establishes
the parameters of their transactions.
Initialization Vector (IV)
Some AEAD ciphers require an initialization vector to allow the
cipher to safely protect multiple chunks of data with the same
keying material. The size of the IV depends on the cipher suite.
Message Authentication Code (MAC)
A Message Authentication Code is a one-way hash computed from a
message and some secret data. It is difficult to forge without
knowing the secret data. Its purpose is to detect if the message
has been altered.
master secret
Secure secret data used for generating keys and IVs.
MD5
MD5 [RFC1321] is a hashing function that converts an arbitrarily
long data stream into a hash of fixed size (16 bytes). Due to
significant progress in cryptanalysis, at the time of publication
of this document, MD5 no longer can be considered a 'secure'
hashing function.
public key cryptography
A class of cryptographic techniques employing two-key ciphers.
Messages encrypted with the public key can only be decrypted with
the associated private key. Conversely, messages signed with the
private key can be verified with the public key.
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one-way hash function
A one-way transformation that converts an arbitrary amount of data
into a fixed-length hash. It is computationally hard to reverse
the transformation or to find collisions. MD5 and SHA are
examples of one-way hash functions.
RSA
A very widely used public key algorithm that can be used for
either encryption or digital signing. [RSA]
server
The server is the application entity that responds to requests for
connections from clients. See also "client".
session
A TLS session is an association between a client and a server.
Sessions are created by the handshake protocol. Sessions define a
set of cryptographic security parameters that can be shared among
multiple connections. Sessions are used to avoid the expensive
negotiation of new security parameters for each connection.
session identifier
A session identifier is a value generated by a server that
identifies a particular session.
server write key
The key used to protect data written by the server.
SHA
The Secure Hash Algorithm [SHS] is defined in FIPS PUB 180-2. It
produces a 20-byte output. Note that all references to SHA
(without a numerical suffix) actually use the modified SHA-1
algorithm.
SHA-256
The 256-bit Secure Hash Algorithm is defined in FIPS PUB 180-2.
It produces a 32-byte output.
SSL
Netscape's Secure Socket Layer protocol [SSL3]. TLS is based on
SSL Version 3.0.
Transport Layer Security (TLS)
This protocol; also, the Transport Layer Security working group of
the Internet Engineering Task Force (IETF). See "Working Group
Information" at the end of this document (see page 99).
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Appendix C. Cipher Suite Definitions
Cipher Suite Key Record
Exchange Protection PRF
TLS_NULL_WITH_NULL_NULL NULL NULL_NULL N/A
TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 DHE_RSA AES_128_GCM SHA256
TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 DHE_RSA AES_256_GCM SHA384
TLS_DHE_DSS_WITH_AES_128_GCM_SHA256 DHE_DSS AES_128_GCM SHA256
TLS_DHE_DSS_WITH_AES_256_GCM_SHA384 DHE_DSS AES_256_GCM SHA384
TLS_DH_anon_WITH_AES_128_GCM_SHA256 DH_anon AES_128_GCM SHA256
TLS_DH_anon_WITH_AES_256_GCM_SHA384 DH_anon AES_128_GCM SHA384
Key Implicit IV Explicit IV
Cipher Material Size Size
------------ -------- ---------- -----------
NULL 0 0 0
AES_128_GCM 16 4 8
AES_256_GCM 32 4 8
Key Material
The number of bytes from the key_block that are used for
generating the write keys.
Implicit IV Size
The amount of data to be generated for the per-connection part of
the initialization vector. This is equal to
SecurityParameters.fixed_iv_length).
Explicit IV Size
The amount of data needed to be generated for the per-record part
of the initialization vector. This is equal to
SecurityParameters.record_iv_length).
Appendix D. Implementation Notes
The TLS protocol cannot prevent many common security mistakes. This
section provides several recommendations to assist implementors.
D.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-1, are acceptable,
but cannot provide more security than the size of the random number
generator state.
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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's 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.
D.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.
D.3. Cipher Suites
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. For instance, anonymous
Diffie-Hellman is strongly discouraged because it cannot prevent man-
in-the-middle attacks. Applications should also enforce minimum and
maximum key sizes. For example, certificate chains containing 512-
bit RSA keys or signatures are not appropriate for high-security
applications.
D.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 6.2.1)? 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.
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- Do you ignore the TLS record layer version number in all TLS
records before ServerHello (see Appendix E.1)?
- 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 7.3.9)?
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 4.7)? 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 8.1.2)?
- Does your TLS client check that the Diffie-Hellman parameters sent
by the server are acceptable (see Appendix F.1.1.2)?
- Do you use a strong and, most importantly, properly seeded random
number generator (see Appendix D.1) Diffie-Hellman private values,
the DSA "k" parameter, and other security-critical values?
Appendix E. Backward Compatibility
E.1. Compatibility with prior versions
[[TODO: Revise backward compatibility section for TLS 1.3.
https://github.com/tlswg/tls13-spec/issues/54]] Since there are
various versions of TLS (1.0, 1.1, 1.2, 1.3, and any future versions)
and SSL (2.0 and 3.0), means are needed to negotiate the specific
protocol version to use. The TLS protocol provides a built-in
mechanism for version negotiation so as not to bother other protocol
components with the complexities of version selection.
TLS versions 1.0, 1.1, and 1.2, and SSL 3.0 are very similar, and use
compatible ClientHello messages; thus, supporting all of them is
relatively easy. Similarly, servers can easily 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.
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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.
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.
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 server supports (or is
willing to use) only versions greater than client_version, it MUST
send a "protocol_version" alert message and close the connection.
Whenever a client already knows the highest protocol version known to
a server (for example, when resuming a session), it SHOULD initiate
the connection in that native protocol.
Note: some server implementations are known to implement version
negotiation incorrectly. For example, there are buggy TLS 1.0
servers that simply close the connection when the client offers a
version newer than TLS 1.0. Also, it is known that some servers will
refuse the connection if any TLS extensions are included in
ClientHello. Interoperability with such buggy servers is a complex
topic beyond the scope of this document, and may require multiple
connection attempts by the client.
Earlier versions of the TLS specification were not fully clear on
what the record layer version number (TLSPlaintext.version) should
contain when sending ClientHello (i.e., before it is known which
version of the protocol will be employed). Thus, TLS servers
compliant with this specification MUST accept any value {03,XX} as
the record layer version number for ClientHello.
TLS clients that wish to negotiate with older servers MAY send any
value {03,XX} as the record layer version number. Typical values
would be {03,00}, the lowest version number supported by the client,
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and the value of ClientHello.client_version. No single value will
guarantee interoperability with all old servers, but this is a
complex topic beyond the scope of this document.
E.2. Compatibility with SSL
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 [I-D.ietf-tls-sslv3-diediedie], and MUST NOT be
negotiated for any reason.
Implementations MUST NOT send a ClientHello.version or
ServerHello.version set to { 3, 0 } or less. Any endpoint receiving
a Hello message with ClientHello.version or ServerHello.version set
to { 3, 0 } MUST respond with a "protocol_version" alert message and
close the connection.
Appendix F. Security Analysis
The TLS protocol is designed to establish a secure connection between
a client and a server communicating over an insecure channel. This
document makes several traditional assumptions, including that
attackers have substantial computational resources and cannot obtain
secret information from sources outside the protocol. Attackers are
assumed to have the ability to capture, modify, delete, replay, and
otherwise tamper with messages sent over the communication channel.
This appendix outlines how TLS has been designed to resist a variety
of attacks.
F.1. Handshake Protocol
The 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
handshake protocol can also optionally authenticate parties who have
certificates signed by a trusted certificate authority.
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F.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.
The general goal of the key exchange process is to create a
pre_master_secret known to the communicating parties and not to
attackers. The pre_master_secret will be used to generate the
master_secret (see Section 8.1). The master_secret is required to
generate the Finished messages and record protection keys (see
Section 7.3.8 and Section 6.3). By sending a correct Finished
message, parties thus prove that they know the correct
pre_master_secret.
F.1.1.1. Anonymous Key Exchange
Completely anonymous sessions can be established using Diffie-Hellman
for key exchange. The server's public parameters are contained in
the server key share message, and the client's are sent in the client
key share message. Eavesdroppers who do not know the private values
should not be able to find the Diffie-Hellman result (i.e., the
pre_master_secret).
Warning: Completely anonymous connections only provide protection
against passive eavesdropping. Unless an independent tamper-proof
channel is used to verify that the Finished messages were not
replaced by an attacker, server authentication is required in
environments where active man-in-the-middle attacks are a concern.
F.1.1.2. Diffie-Hellman Key Exchange with Authentication
When Diffie-Hellman key exchange is used, the client and server use
the client key exchange and server key exchange messages 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.
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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.
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.
F.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.
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.
F.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 master_secret, the attacker cannot repair the Finished
messages, so the attack will be discovered.
F.1.4. Resuming Sessions
When a connection is established by resuming a session, new
ClientHello.random and ServerHello.random values are hashed with the
session's master_secret. Provided that the master_secret has not
been compromised and that the secure hash operations used to produce
the record protection kayes are secure, the connection should be
secure and effectively independent from previous connections.
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Attackers cannot use known keys to compromise the master_secret
without breaking the secure hash operations.
Sessions cannot be resumed unless both the client and server agree.
If either party suspects that the session may have been compromised,
or that certificates may have expired or been revoked, it should
force a full handshake. An upper limit of 24 hours is suggested for
session ID lifetimes, since an attacker who obtains a master_secret
may be able to impersonate the compromised party until the
corresponding session ID is retired. Applications that may be run in
relatively insecure environments should not write session IDs to
stable storage.
F.2. Protecting Application Data
The master_secret is hashed with the ClientHello.random and
ServerHello.random to produce unique record protection secrets for
each connection.
Outgoing data is protected using an AEAD algorithm before
transmission. The authentication data includes the sequence number,
message type, message length, and the message contents. The message
type field is necessary to ensure that messages intended for one TLS
record layer client are not redirected to another. The sequence
number ensures that attempts to delete or reorder messages will be
detected. Since sequence numbers are 64 bits long, they should never
overflow. Messages from one party cannot be inserted into the
other's output, since they use independent keys.
F.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].
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F.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 G. Working Group Information
The discussion list for the IETF TLS working group is located at the
e-mail address tls@ietf.org [2]. 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: http://www.ietf.org/mail-
archive/web/tls/current/index.html
Appendix H. Contributors
Christopher Allen (co-editor of TLS 1.0)
Alacrity Ventures
ChristopherA@AlacrityManagement.com
Martin Abadi
University of California, Santa Cruz
abadi@cs.ucsc.edu
Karthikeyan Bhargavan (co-author of [I-D.ietf-tls-session-hash])
INRIA
karthikeyan.bhargavan@inria.fr
Steven M. Bellovin
Columbia University
smb@cs.columbia.edu
Simon Blake-Wilson (co-author of RFC4492)
BCI
sblakewilson@bcisse.com
Nelson Bolyard
Sun Microsystems, Inc.
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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 [I-D.ietf-tls-session-hash])
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
taher@securify.com
Securify
Pasi Eronen
pasi.eronen@nokia.com
Nokia
Anil Gangolli
anil@busybuddha.org
Vipul Gupta (co-author of RFC4492)
Sun Microsystems Laboratories
vipul.gupta@sun.com
Kipp Hickman
Chris Hawk (co-author of RFC4492)
Corriente Networks LLC
chris@corriente.net
Alfred Hoenes
David Hopwood
Independent Consultant
david.hopwood@blueyonder.co.uk
Daniel Kahn Gillmor
ACLU
dkg@fifthhorseman.net
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Phil Karlton (co-author of SSLv3)
Paul Kocher (co-author of SSLv3)
Cryptography Research
paul@cryptography.com
Hugo Krawczyk
IBM
hugo@ee.technion.ac.il
Adam Langley (co-author of [I-D.ietf-tls-session-hash])
Google
agl@google.com
Ilari Liusvaara
ilari.liusvaara@elisanet.fi
Jan Mikkelsen
Transactionware
janm@transactionware.com
Bodo Moeller (co-author of RFC4492)
Google
bodo@openssl.org
Magnus Nystrom
RSA Security
magnus@rsasecurity.com
Alfredo Pironti (co-author of [I-D.ietf-tls-session-hash])
INRIA
alfredo.pironti@inria.fr
Marsh Ray (co-author of [I-D.ietf-tls-session-hash])
Microsoft
maray@microsoft.com
Robert Relyea
Netscape Communications
relyea@netscape.com
Jim Roskind
Netscape Communications
jar@netscape.com
Michael Sabin
Dan Simon
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Microsoft, Inc.
dansimon@microsoft.com
Martin Thomson
Mozilla
mt@mozilla.com
Tom Weinstein
Tim Wright
Vodafone
timothy.wright@vodafone.com
Author's Address
Eric Rescorla
RTFM, Inc.
EMail: ekr@rtfm.com
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