draft-ietf-tls-tls13-20.txt   draft-ietf-tls-tls13-21.txt 
Network Working Group E. Rescorla Network Working Group E. Rescorla
Internet-Draft RTFM, Inc. Internet-Draft RTFM, Inc.
Obsoletes: 5077, 5246 (if approved) April 28, 2017 Obsoletes: 5077, 5246 (if approved) July 03, 2017
Updates: 4492, 5705, 6066, 6961 (if Updates: 4492, 5705, 6066, 6961 (if
approved) approved)
Intended status: Standards Track Intended status: Standards Track
Expires: October 30, 2017 Expires: January 4, 2018
The Transport Layer Security (TLS) Protocol Version 1.3 The Transport Layer Security (TLS) Protocol Version 1.3
draft-ietf-tls-tls13-20 draft-ietf-tls-tls13-21
Abstract Abstract
This document specifies version 1.3 of the Transport Layer Security This document specifies version 1.3 of the Transport Layer Security
(TLS) protocol. TLS allows client/server applications to communicate (TLS) protocol. TLS allows client/server applications to communicate
over the Internet in a way that is designed to prevent eavesdropping, over the Internet in a way that is designed to prevent eavesdropping,
tampering, and message forgery. tampering, and message forgery.
Status of This Memo Status of This Memo
skipping to change at page 1, line 36 skipping to change at page 1, line 36
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on October 30, 2017. This Internet-Draft will expire on January 4, 2018.
Copyright Notice Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
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not be created outside the IETF Standards Process, except to format not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other it for publication as an RFC or to translate it into languages other
than English. than English.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Conventions and Terminology . . . . . . . . . . . . . . . 6 1.1. Conventions and Terminology . . . . . . . . . . . . . . . 6
1.2. Change Log . . . . . . . . . . . . . . . . . . . . . . . 6 1.2. Change Log . . . . . . . . . . . . . . . . . . . . . . . 6
1.3. Major Differences from TLS 1.2 . . . . . . . . . . . . . 14 1.3. Major Differences from TLS 1.2 . . . . . . . . . . . . . 14
1.4. Updates Affecting TLS 1.2 . . . . . . . . . . . . . . . . 15 1.4. Updates Affecting TLS 1.2 . . . . . . . . . . . . . . . . 16
2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 16 2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 16
2.1. Incorrect DHE Share . . . . . . . . . . . . . . . . . . . 19 2.1. Incorrect DHE Share . . . . . . . . . . . . . . . . . . . 19
2.2. Resumption and Pre-Shared Key (PSK) . . . . . . . . . . . 20 2.2. Resumption and Pre-Shared Key (PSK) . . . . . . . . . . . 20
2.3. Zero-RTT Data . . . . . . . . . . . . . . . . . . . . . . 22 2.3. Zero-RTT Data . . . . . . . . . . . . . . . . . . . . . . 22
3. Presentation Language . . . . . . . . . . . . . . . . . . . . 24 3. Presentation Language . . . . . . . . . . . . . . . . . . . . 24
3.1. Basic Block Size . . . . . . . . . . . . . . . . . . . . 24 3.1. Basic Block Size . . . . . . . . . . . . . . . . . . . . 24
3.2. Miscellaneous . . . . . . . . . . . . . . . . . . . . . . 24 3.2. Miscellaneous . . . . . . . . . . . . . . . . . . . . . . 24
3.3. Vectors . . . . . . . . . . . . . . . . . . . . . . . . . 25 3.3. Vectors . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.4. Numbers . . . . . . . . . . . . . . . . . . . . . . . . . 26 3.4. Numbers . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.5. Enumerateds . . . . . . . . . . . . . . . . . . . . . . . 26 3.5. Enumerateds . . . . . . . . . . . . . . . . . . . . . . . 26
3.6. Constructed Types . . . . . . . . . . . . . . . . . . . . 27 3.6. Constructed Types . . . . . . . . . . . . . . . . . . . . 27
3.7. Constants . . . . . . . . . . . . . . . . . . . . . . . . 27 3.7. Constants . . . . . . . . . . . . . . . . . . . . . . . . 27
3.8. Variants . . . . . . . . . . . . . . . . . . . . . . . . 28 3.8. Variants . . . . . . . . . . . . . . . . . . . . . . . . 27
4. Handshake Protocol . . . . . . . . . . . . . . . . . . . . . 29 4. Handshake Protocol . . . . . . . . . . . . . . . . . . . . . 28
4.1. Key Exchange Messages . . . . . . . . . . . . . . . . . . 30 4.1. Key Exchange Messages . . . . . . . . . . . . . . . . . . 29
4.1.1. Cryptographic Negotiation . . . . . . . . . . . . . . 30 4.1.1. Cryptographic Negotiation . . . . . . . . . . . . . . 30
4.1.2. Client Hello . . . . . . . . . . . . . . . . . . . . 31 4.1.2. Client Hello . . . . . . . . . . . . . . . . . . . . 31
4.1.3. Server Hello . . . . . . . . . . . . . . . . . . . . 34 4.1.3. Server Hello . . . . . . . . . . . . . . . . . . . . 34
4.1.4. Hello Retry Request . . . . . . . . . . . . . . . . . 36 4.1.4. Hello Retry Request . . . . . . . . . . . . . . . . . 35
4.2. Extensions . . . . . . . . . . . . . . . . . . . . . . . 37 4.2. Extensions . . . . . . . . . . . . . . . . . . . . . . . 37
4.2.1. Supported Versions . . . . . . . . . . . . . . . . . 40 4.2.1. Supported Versions . . . . . . . . . . . . . . . . . 40
4.2.2. Cookie . . . . . . . . . . . . . . . . . . . . . . . 41 4.2.2. Cookie . . . . . . . . . . . . . . . . . . . . . . . 41
4.2.3. Signature Algorithms . . . . . . . . . . . . . . . . 42 4.2.3. Signature Algorithms . . . . . . . . . . . . . . . . 42
4.2.4. Certificate Authorities . . . . . . . . . . . . . . . 45 4.2.4. Certificate Authorities . . . . . . . . . . . . . . . 45
4.2.5. Post-Handshake Client Authentication . . . . . . . . 46 4.2.5. Post-Handshake Client Authentication . . . . . . . . 47
4.2.6. Negotiated Groups . . . . . . . . . . . . . . . . . . 46 4.2.6. Negotiated Groups . . . . . . . . . . . . . . . . . . 47
4.2.7. Key Share . . . . . . . . . . . . . . . . . . . . . . 47 4.2.7. Key Share . . . . . . . . . . . . . . . . . . . . . . 48
4.2.8. Pre-Shared Key Exchange Modes . . . . . . . . . . . . 51 4.2.8. Pre-Shared Key Exchange Modes . . . . . . . . . . . . 52
4.2.9. Early Data Indication . . . . . . . . . . . . . . . . 51 4.2.9. Early Data Indication . . . . . . . . . . . . . . . . 52
4.2.10. Pre-Shared Key Extension . . . . . . . . . . . . . . 54 4.2.10. Pre-Shared Key Extension . . . . . . . . . . . . . . 55
4.3. Server Parameters . . . . . . . . . . . . . . . . . . . . 58 4.3. Server Parameters . . . . . . . . . . . . . . . . . . . . 58
4.3.1. Encrypted Extensions . . . . . . . . . . . . . . . . 58 4.3.1. Encrypted Extensions . . . . . . . . . . . . . . . . 58
4.3.2. Certificate Request . . . . . . . . . . . . . . . . . 59 4.3.2. Certificate Request . . . . . . . . . . . . . . . . . 59
4.4. Authentication Messages . . . . . . . . . . . . . . . . . 61 4.4. Authentication Messages . . . . . . . . . . . . . . . . . 59
4.4.1. The Transcript Hash . . . . . . . . . . . . . . . . . 62 4.4.1. The Transcript Hash . . . . . . . . . . . . . . . . . 61
4.4.2. Certificate . . . . . . . . . . . . . . . . . . . . . 63 4.4.2. Certificate . . . . . . . . . . . . . . . . . . . . . 62
4.4.3. Certificate Verify . . . . . . . . . . . . . . . . . 67 4.4.3. Certificate Verify . . . . . . . . . . . . . . . . . 66
4.4.4. Finished . . . . . . . . . . . . . . . . . . . . . . 69 4.4.4. Finished . . . . . . . . . . . . . . . . . . . . . . 68
4.5. End of Early Data . . . . . . . . . . . . . . . . . . . . 70 4.5. End of Early Data . . . . . . . . . . . . . . . . . . . . 70
4.6. Post-Handshake Messages . . . . . . . . . . . . . . . . . 71 4.6. Post-Handshake Messages . . . . . . . . . . . . . . . . . 70
4.6.1. New Session Ticket Message . . . . . . . . . . . . . 71 4.6.1. New Session Ticket Message . . . . . . . . . . . . . 70
4.6.2. Post-Handshake Authentication . . . . . . . . . . . . 73 4.6.2. Post-Handshake Authentication . . . . . . . . . . . . 72
4.6.3. Key and IV Update . . . . . . . . . . . . . . . . . . 73 4.6.3. Key and IV Update . . . . . . . . . . . . . . . . . . 73
5. Record Protocol . . . . . . . . . . . . . . . . . . . . . . . 74 5. Record Protocol . . . . . . . . . . . . . . . . . . . . . . . 74
5.1. Record Layer . . . . . . . . . . . . . . . . . . . . . . 75 5.1. Record Layer . . . . . . . . . . . . . . . . . . . . . . 74
5.2. Record Payload Protection . . . . . . . . . . . . . . . . 77 5.2. Record Payload Protection . . . . . . . . . . . . . . . . 76
5.3. Per-Record Nonce . . . . . . . . . . . . . . . . . . . . 79 5.3. Per-Record Nonce . . . . . . . . . . . . . . . . . . . . 78
5.4. Record Padding . . . . . . . . . . . . . . . . . . . . . 79 5.4. Record Padding . . . . . . . . . . . . . . . . . . . . . 79
5.5. Limits on Key Usage . . . . . . . . . . . . . . . . . . . 81 5.5. Limits on Key Usage . . . . . . . . . . . . . . . . . . . 80
6. Alert Protocol . . . . . . . . . . . . . . . . . . . . . . . 81 6. Alert Protocol . . . . . . . . . . . . . . . . . . . . . . . 80
6.1. Closure Alerts . . . . . . . . . . . . . . . . . . . . . 82 6.1. Closure Alerts . . . . . . . . . . . . . . . . . . . . . 82
6.2. Error Alerts . . . . . . . . . . . . . . . . . . . . . . 83 6.2. Error Alerts . . . . . . . . . . . . . . . . . . . . . . 83
7. Cryptographic Computations . . . . . . . . . . . . . . . . . 86 7. Cryptographic Computations . . . . . . . . . . . . . . . . . 85
7.1. Key Schedule . . . . . . . . . . . . . . . . . . . . . . 86 7.1. Key Schedule . . . . . . . . . . . . . . . . . . . . . . 86
7.2. Updating Traffic Keys and IVs . . . . . . . . . . . . . . 89 7.2. Updating Traffic Keys and IVs . . . . . . . . . . . . . . 88
7.3. Traffic Key Calculation . . . . . . . . . . . . . . . . . 90 7.3. Traffic Key Calculation . . . . . . . . . . . . . . . . . 89
7.4. (EC)DHE Shared Secret Calculation . . . . . . . . . . . . 90 7.4. (EC)DHE Shared Secret Calculation . . . . . . . . . . . . 90
7.4.1. Finite Field Diffie-Hellman . . . . . . . . . . . . . 90 7.4.1. Finite Field Diffie-Hellman . . . . . . . . . . . . . 90
7.4.2. Elliptic Curve Diffie-Hellman . . . . . . . . . . . . 91 7.4.2. Elliptic Curve Diffie-Hellman . . . . . . . . . . . . 90
7.5. Exporters . . . . . . . . . . . . . . . . . . . . . . . . 91 7.5. Exporters . . . . . . . . . . . . . . . . . . . . . . . . 91
8. Compliance Requirements . . . . . . . . . . . . . . . . . . . 92 8. 0-RTT and Anti-Replay . . . . . . . . . . . . . . . . . . . . 91
8.1. Mandatory-to-Implement Cipher Suites . . . . . . . . . . 92 8.1. Single-Use Tickets . . . . . . . . . . . . . . . . . . . 92
8.2. Mandatory-to-Implement Extensions . . . . . . . . . . . . 92 8.2. Client Hello Recording . . . . . . . . . . . . . . . . . 93
9. Security Considerations . . . . . . . . . . . . . . . . . . . 94 8.3. Freshness Checks . . . . . . . . . . . . . . . . . . . . 94
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 94 9. Compliance Requirements . . . . . . . . . . . . . . . . . . . 95
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 95 9.1. Mandatory-to-Implement Cipher Suites . . . . . . . . . . 95
11.1. Normative References . . . . . . . . . . . . . . . . . . 95 9.2. Mandatory-to-Implement Extensions . . . . . . . . . . . . 96
11.2. Informative References . . . . . . . . . . . . . . . . . 97 10. Security Considerations . . . . . . . . . . . . . . . . . . . 97
Appendix A. State Machine . . . . . . . . . . . . . . . . . . . 105 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 97
A.1. Client . . . . . . . . . . . . . . . . . . . . . . . . . 105 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 98
A.2. Server . . . . . . . . . . . . . . . . . . . . . . . . . 105 12.1. Normative References . . . . . . . . . . . . . . . . . . 98
Appendix B. Protocol Data Structures and Constant Values . . . . 106 12.2. Informative References . . . . . . . . . . . . . . . . . 101
B.1. Record Layer . . . . . . . . . . . . . . . . . . . . . . 106 Appendix A. State Machine . . . . . . . . . . . . . . . . . . . 108
B.2. Alert Messages . . . . . . . . . . . . . . . . . . . . . 107 A.1. Client . . . . . . . . . . . . . . . . . . . . . . . . . 108
B.3. Handshake Protocol . . . . . . . . . . . . . . . . . . . 109 A.2. Server . . . . . . . . . . . . . . . . . . . . . . . . . 109
B.3.1. Key Exchange Messages . . . . . . . . . . . . . . . . 109 Appendix B. Protocol Data Structures and Constant Values . . . . 109
B.3.2. Server Parameters Messages . . . . . . . . . . . . . 114 B.1. Record Layer . . . . . . . . . . . . . . . . . . . . . . 110
B.3.3. Authentication Messages . . . . . . . . . . . . . . . 115 B.2. Alert Messages . . . . . . . . . . . . . . . . . . . . . 110
B.3.4. Ticket Establishment . . . . . . . . . . . . . . . . 116 B.3. Handshake Protocol . . . . . . . . . . . . . . . . . . . 112
B.3.5. Updating Keys . . . . . . . . . . . . . . . . . . . . 116 B.3.1. Key Exchange Messages . . . . . . . . . . . . . . . . 112
B.4. Cipher Suites . . . . . . . . . . . . . . . . . . . . . . 117 B.3.2. Server Parameters Messages . . . . . . . . . . . . . 117
Appendix C. Implementation Notes . . . . . . . . . . . . . . . . 118 B.3.3. Authentication Messages . . . . . . . . . . . . . . . 118
C.1. API considerations for 0-RTT . . . . . . . . . . . . . . 118 B.3.4. Ticket Establishment . . . . . . . . . . . . . . . . 119
C.2. Random Number Generation and Seeding . . . . . . . . . . 118 B.3.5. Updating Keys . . . . . . . . . . . . . . . . . . . . 119
C.3. Certificates and Authentication . . . . . . . . . . . . . 118 B.4. Cipher Suites . . . . . . . . . . . . . . . . . . . . . . 120
C.4. Implementation Pitfalls . . . . . . . . . . . . . . . . . 119 Appendix C. Implementation Notes . . . . . . . . . . . . . . . . 121
C.5. Client Tracking Prevention . . . . . . . . . . . . . . . 120 C.1. Random Number Generation and Seeding . . . . . . . . . . 121
C.6. Unauthenticated Operation . . . . . . . . . . . . . . . . 120 C.2. Certificates and Authentication . . . . . . . . . . . . . 121
Appendix D. Backward Compatibility . . . . . . . . . . . . . . . 121 C.3. Implementation Pitfalls . . . . . . . . . . . . . . . . . 121
D.1. Negotiating with an older server . . . . . . . . . . . . 122 C.4. Client Tracking Prevention . . . . . . . . . . . . . . . 123
D.2. Negotiating with an older client . . . . . . . . . . . . 122 C.5. Unauthenticated Operation . . . . . . . . . . . . . . . . 123
D.3. Zero-RTT backwards compatibility . . . . . . . . . . . . 123 Appendix D. Backward Compatibility . . . . . . . . . . . . . . . 124
D.4. Backwards Compatibility Security Restrictions . . . . . . 123 D.1. Negotiating with an older server . . . . . . . . . . . . 124
Appendix E. Overview of Security Properties . . . . . . . . . . 124 D.2. Negotiating with an older client . . . . . . . . . . . . 125
E.1. Handshake . . . . . . . . . . . . . . . . . . . . . . . . 124 D.3. Zero-RTT backwards compatibility . . . . . . . . . . . . 125
E.1.1. Key Derivation and HKDF . . . . . . . . . . . . . . . 127 D.4. Backwards Compatibility Security Restrictions . . . . . . 126
E.1.2. Client Authentication . . . . . . . . . . . . . . . . 128 Appendix E. Overview of Security Properties . . . . . . . . . . 127
E.1.3. 0-RTT . . . . . . . . . . . . . . . . . . . . . . . . 128 E.1. Handshake . . . . . . . . . . . . . . . . . . . . . . . . 127
E.1.4. Post-Compromise Security . . . . . . . . . . . . . . 128 E.1.1. Key Derivation and HKDF . . . . . . . . . . . . . . . 130
E.1.5. External References . . . . . . . . . . . . . . . . . 128 E.1.2. Client Authentication . . . . . . . . . . . . . . . . 131
E.2. Record Layer . . . . . . . . . . . . . . . . . . . . . . 129 E.1.3. 0-RTT . . . . . . . . . . . . . . . . . . . . . . . . 131
E.2.1. External References . . . . . . . . . . . . . . . . . 130 E.1.4. Exporter Independence . . . . . . . . . . . . . . . . 131
E.3. Traffic Analysis . . . . . . . . . . . . . . . . . . . . 130 E.1.5. Post-Compromise Security . . . . . . . . . . . . . . 131
E.4. Side Channel Attacks . . . . . . . . . . . . . . . . . . 130 E.1.6. External References . . . . . . . . . . . . . . . . . 132
Appendix F. Working Group Information . . . . . . . . . . . . . 131 E.2. Record Layer . . . . . . . . . . . . . . . . . . . . . . 132
Appendix G. Contributors . . . . . . . . . . . . . . . . . . . . 131 E.2.1. External References . . . . . . . . . . . . . . . . . 133
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 137 E.3. Traffic Analysis . . . . . . . . . . . . . . . . . . . . 133
E.4. Side Channel Attacks . . . . . . . . . . . . . . . . . . 134
E.5. Replay Attacks on 0-RTT . . . . . . . . . . . . . . . . . 134
E.5.1. Replay and Exporters . . . . . . . . . . . . . . . . 136
Appendix F. Working Group Information . . . . . . . . . . . . . 136
Appendix G. Contributors . . . . . . . . . . . . . . . . . . . . 136
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 143
1. Introduction 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 RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH The source for this
draft is maintained in GitHub. Suggested changes should be submitted draft is maintained in GitHub. Suggested changes should be submitted
as pull requests at https://github.com/tlswg/tls13-spec. as pull requests at https://github.com/tlswg/tls13-spec.
Instructions are on that page as well. Editorial changes can be Instructions are on that page as well. Editorial changes can be
managed in GitHub, but any substantive change should be discussed on managed in GitHub, but any substantive change should be discussed on
the TLS mailing list. the TLS mailing list.
The primary goal of TLS is to provide a secure channel between two The primary goal of TLS is to provide a secure channel between two
communicating peers. Specifically, the channel should provide the communicating peers. Specifically, the channel should provide the
following properties: following properties:
- Authentication: The server side of the channel is always - Authentication: The server side of the channel is always
authenticated; the client side is optionally authenticated. authenticated; the client side is optionally authenticated.
Authentication can happen via asymmetric cryptography (e.g., RSA Authentication can happen via asymmetric cryptography (e.g., RSA
[RSA], ECDSA [ECDSA]) or a pre-shared key (PSK). [RSA], ECDSA [ECDSA], EdDSA [RFC8032]) or a pre-shared key (PSK).
- Confidentiality: Data sent over the channel is only visible to the - Confidentiality: Data sent over the channel after establishment is
endpoints. TLS does not hide the length of the data it transmits, only visible to the endpoints. TLS does not hide the length of
though endpoints are able to pad in order to obscure lengths. the data it transmits, though endpoints are able to pad TLS
records in order to obscure lengths and improve protection agains
traffic analysis techniques.
- Integrity: Data sent over the channel cannot be modified by - Integrity: Data sent over the channel after establishment cannot
attackers. be modified by attackers.
These properties should be true even in the face of an attacker who These properties should be true even in the face of an attacker who
has complete control of the network, as described in [RFC3552]. See has complete control of the network, as described in [RFC3552]. See
Appendix E for a more complete statement of the relevant security Appendix E for a more complete statement of the relevant security
properties. properties.
TLS consists of two primary components: TLS consists of two primary components:
- A handshake protocol (Section 4) that authenticates the - A handshake protocol (Section 4) that authenticates the
communicating parties, negotiates cryptographic modes and communicating parties, negotiates cryptographic modes and
skipping to change at page 5, line 52 skipping to change at page 6, line 5
TLS is application protocol independent; higher-level protocols can TLS is application protocol independent; higher-level protocols can
layer on top of TLS transparently. The TLS standard, however, does layer on top of TLS transparently. The TLS standard, however, does
not specify how protocols add security with TLS; how to initiate TLS not specify how protocols add security with TLS; how to initiate TLS
handshaking and how to interpret the authentication certificates handshaking and how to interpret the authentication certificates
exchanged are left to the judgment of the designers and implementors exchanged are left to the judgment of the designers and implementors
of protocols that run on top of TLS. of protocols that run on top of TLS.
This document defines TLS version 1.3. While TLS 1.3 is not directly This document defines TLS version 1.3. While TLS 1.3 is not directly
compatible with previous versions, all versions of TLS incorporate a compatible with previous versions, all versions of TLS incorporate a
versioning mechanism which allows clients and servers to versioning mechanism which allows clients and servers to
interoperably negotiate a common version if one is supported. interoperably negotiate a common version if one is supported by both
peers.
This document supersedes and obsoletes previous versions of TLS This document supersedes and obsoletes previous versions of TLS
including version 1.2 [RFC5246]. It also obsoletes the TLS ticket including version 1.2 [RFC5246]. It also obsoletes the TLS ticket
mechanism defined in [RFC5077] and replaces it with the mechanism mechanism defined in [RFC5077] and replaces it with the mechanism
defined in Section 2.2. Section 4.2.6 updates [RFC4492] by modifying defined in Section 2.2. Section 4.2.6 updates [RFC4492] by modifying
the protocol attributes used to negotiate Elliptic Curves. Because the protocol attributes used to negotiate Elliptic Curves. Because
TLS 1.3 changes the way keys are derived it updates [RFC5705] as TLS 1.3 changes the way keys are derived it updates [RFC5705] as
described in Section 7.5 it also changes how OCSP messages are described in Section 7.5 it also changes how OCSP messages are
carried and therefore updates [RFC6066] and obsoletes [RFC6961] as carried and therefore updates [RFC6066] and obsoletes [RFC6961] as
described in section Section 4.4.2.1. described in section Section 4.4.2.1.
skipping to change at page 6, line 49 skipping to change at page 7, line 5
server: The endpoint which did not initiate the TLS connection. server: The endpoint which did not initiate the TLS connection.
1.2. Change Log 1.2. Change Log
RFC EDITOR PLEASE DELETE THIS SECTION. RFC EDITOR PLEASE DELETE THIS SECTION.
(*) indicates changes to the wire protocol which may require (*) indicates changes to the wire protocol which may require
implementations to update. implementations to update.
- Add a per-ticket nonce so that each ticket is associated with a
different PSK (*).
- Clarify that clients should send alerts with the handshake key if
possible.
- Update state machine to show rekeying events
- Add discussion of 0-RTT and replay. Recommend that
implementations implement some anti-replay mechanism.
draft-20 draft-20
- Add "post_handshake_auth" extension to negotiate post-handshake - Add "post_handshake_auth" extension to negotiate post-handshake
authentication (*). authentication (*).
- Shorten labels for HKDF-Expand-Label so that we can fit within one - Shorten labels for HKDF-Expand-Label so that we can fit within one
compression block (*). compression block (*).
- Define how RFC 7250 works (*). - Define how RFC 7250 works (*).
skipping to change at page 8, line 5 skipping to change at page 8, line 24
- Remove spurious requirement to implement "pre_shared_key". - Remove spurious requirement to implement "pre_shared_key".
- Clarify location of "early_data" from server (it goes in EE, as - Clarify location of "early_data" from server (it goes in EE, as
indicated by the table in S 10). indicated by the table in S 10).
- Require peer public key validation - Require peer public key validation
- Add state machine diagram. - Add state machine diagram.
draft-18
- Remove unnecessary resumption_psk which is the only thing expanded - Remove unnecessary resumption_psk which is the only thing expanded
from the resumption master secret. (*). from the resumption master secret. (*).
- Fix signature_algorithms entry in extensions table. - Fix signature_algorithms entry in extensions table.
- Restate rule from RFC 6066 that you can't resume unless SNI is the - Restate rule from RFC 6066 that you can't resume unless SNI is the
same. same.
draft-17 draft-17
skipping to change at page 12, line 5 skipping to change at page 12, line 23
- Unify authentication modes. Add post-handshake client - Unify authentication modes. Add post-handshake client
authentication. authentication.
- Remove early_handshake content type. Terminate 0-RTT data with an - Remove early_handshake content type. Terminate 0-RTT data with an
alert. alert.
- Reset sequence number upon key change (as proposed by Fournet et - Reset sequence number upon key change (as proposed by Fournet et
al.) al.)
draft-10
- Remove ClientCertificateTypes field from CertificateRequest and - Remove ClientCertificateTypes field from CertificateRequest and
add extensions. add extensions.
- Merge client and server key shares into a single extension. - Merge client and server key shares into a single extension.
draft-09 draft-09
- Change to RSA-PSS signatures for handshake messages. - Change to RSA-PSS signatures for handshake messages.
- Remove support for DSA. - Remove support for DSA.
skipping to change at page 12, line 33 skipping to change at page 13, line 5
- Change HKDF labeling to include protocol version and value - Change HKDF labeling to include protocol version and value
lengths. lengths.
- Shift the final decision to abort a handshake due to incompatible - Shift the final decision to abort a handshake due to incompatible
certificates to the client rather than having servers abort early. certificates to the client rather than having servers abort early.
- Deprecate SHA-1 with signatures. - Deprecate SHA-1 with signatures.
- Add MTI algorithms. - Add MTI algorithms.
draft-08
- Remove support for weak and lesser used named curves. - Remove support for weak and lesser used named curves.
- Remove support for MD5 and SHA-224 hashes with signatures. - Remove support for MD5 and SHA-224 hashes with signatures.
- Update lists of available AEAD cipher suites and error alerts. - Update lists of available AEAD cipher suites and error alerts.
- Reduce maximum permitted record expansion for AEAD from 2048 to - Reduce maximum permitted record expansion for AEAD from 2048 to
256 octets. 256 octets.
- Require digital signatures even when a previous configuration is - Require digital signatures even when a previous configuration is
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Authenticated Encryption with Associated Data (AEAD) algorithms. Authenticated Encryption with Associated Data (AEAD) algorithms.
The ciphersuite concept has been changed to separate the The ciphersuite concept has been changed to separate the
authentication and key exchange mechanisms from the record authentication and key exchange mechanisms from the record
protection algorithm (including secret key length) and a hash to protection algorithm (including secret key length) and a hash to
be used with the key derivation function and HMAC. be used with the key derivation function and HMAC.
- A Zero-RTT mode was added, saving a round-trip at connection setup - A Zero-RTT mode was added, saving a round-trip at connection setup
for some application data, at the cost of certain security for some application data, at the cost of certain security
properties. properties.
- Static RSA and Diffie-Hellman cipher suites have been removed; all
public-key based key exchange mechanisms now provide forward
secrecy.
- All handshake messages after the ServerHello are now encrypted. - All handshake messages after the ServerHello are now encrypted.
The newly introduced EncryptedExtension message allows various The newly introduced EncryptedExtension message allows various
extensions previously sent in clear in the ServerHello to also extensions previously sent in clear in the ServerHello to also
enjoy confidentiality protection. enjoy confidentiality protection.
- The key derivation functions have been re-designed. The new - The key derivation functions have been re-designed. The new
design allows easier analysis by cryptographers due to their design allows easier analysis by cryptographers due to their
improved key separation properties. The HMAC-based Extract-and- improved key separation properties. The HMAC-based Extract-and-
Expand Key Derivation Function (HKDF) is used as an underlying Expand Key Derivation Function (HKDF) is used as an underlying
primitive. primitive.
skipping to change at page 15, line 32 skipping to change at page 16, line 5
- The TLS 1.2 version negotiation mechanism has been deprecated in - The TLS 1.2 version negotiation mechanism has been deprecated in
favor of a version list in an extension. This increases favor of a version list in an extension. This increases
compatibility with servers which incorrectly implemented version compatibility with servers which incorrectly implemented version
negotiation. negotiation.
- Session resumption with and without server-side state as well as - Session resumption with and without server-side state as well as
the PSK-based ciphersuites of earlier TLS versions have been the PSK-based ciphersuites of earlier TLS versions have been
replaced by a single new PSK exchange. replaced by a single new PSK exchange.
- Updated references to point to the updated versions of RFCs, as
appropriate (e.g., RFC 5280 rather than RFC 3280).
1.4. Updates Affecting TLS 1.2 1.4. Updates Affecting TLS 1.2
This document defines several changes that optionally affect This document defines several changes that optionally affect
implementations of TLS 1.2: implementations of TLS 1.2:
- A version downgrade protection mechanism is described in - A version downgrade protection mechanism is described in
Section 4.1.3. Section 4.1.3.
- RSASSA-PSS signature schemes are defined in Section 4.2.3. - RSASSA-PSS signature schemes are defined in Section 4.2.3.
- The "supported_versions" ClientHello extension can be used to - The "supported_versions" ClientHello extension can be used to
negotiate the version of TLS to use, in preference to the negotiate the version of TLS to use, in preference to the
legacy_version field of the ClientHello. legacy_version field of the ClientHello.
An implementation of TLS 1.3 that also supports TLS 1.2 might need to An implementation of TLS 1.3 that also supports TLS 1.2 might need to
include changes to support these changes even when TLS 1.3 is not in include changes to support these changes even when TLS 1.3 is not in
use. See the referenced sections for more details. use. See the referenced sections for more details.
2. Protocol Overview 2. Protocol Overview
The cryptographic parameters of the connection state are produced by The cryptographic parameters used by the secure channel are produced
the TLS handshake protocol, which a TLS client and server use when by the TLS handshake protocol. This sub-protocol of TLS is used by
first communicating to agree on a protocol version, select the client and server when first communicating with each other. The
cryptographic algorithms, optionally authenticate each other, and handshake protocol allows peers to negotiate a protocol version,
establish shared secret keying material. Once the handshake is select cryptographic algorithms, optionally authenticate each other,
complete, the peers use the established keys to protect application and establish shared secret keying material. Once the handshake is
layer traffic. complete, the peers use the established keys to protect the
application layer traffic.
A failure of the handshake or other protocol error triggers the A failure of the handshake or other protocol error triggers the
termination of the connection, optionally preceded by an alert termination of the connection, optionally preceded by an alert
message (Section 6). message (Section 6).
TLS supports three basic key exchange modes: TLS supports three basic key exchange modes:
- (EC)DHE (Diffie-Hellman, both the finite field and elliptic curve - (EC)DHE (Diffie-Hellman over either finite fields or elliptic
varieties), curves)
- PSK-only, and - PSK-only
- PSK with (EC)DHE - PSK with (EC)DHE
Figure 1 below shows the basic full TLS handshake: Figure 1 below shows the basic full TLS handshake:
Client Server Client Server
Key ^ ClientHello Key ^ ClientHello
Exch | + key_share* Exch | + key_share*
| + signature_algorithms* | + signature_algorithms*
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- Server Parameters: Establish other handshake parameters (whether - Server Parameters: Establish other handshake parameters (whether
the client is authenticated, application layer protocol support, the client is authenticated, application layer protocol support,
etc.). etc.).
- Authentication: Authenticate the server (and optionally the - Authentication: Authenticate the server (and optionally the
client) and provide key confirmation and handshake integrity. client) and provide key confirmation and handshake integrity.
In the Key Exchange phase, the client sends the ClientHello In the Key Exchange phase, the client sends the ClientHello
(Section 4.1.2) message, which contains a random nonce (Section 4.1.2) message, which contains a random nonce
(ClientHello.random); its offered protocol versions; a list of (ClientHello.random); its offered protocol versions; a list of
symmetric cipher/HKDF hash pairs; some set of Diffie-Hellman key symmetric cipher/HKDF hash pairs; either a set of Diffie-Hellman key
shares (in the "key_share" extension Section 4.2.7), a set of pre- shares (in the "key_share" extension Section 4.2.7), a set of pre-
shared key labels (in the "pre_shared_key" extension Section 4.2.10) shared key labels (in the "pre_shared_key" extension Section 4.2.10)
or both; and potentially some other extensions. or both; and potentially additional extensions.
The server processes the ClientHello and determines the appropriate The server processes the ClientHello and determines the appropriate
cryptographic parameters for the connection. It then responds with cryptographic parameters for the connection. It then responds with
its own ServerHello (Section 4.1.3), which indicates the negotiated its own ServerHello (Section 4.1.3), which indicates the negotiated
connection parameters. The combination of the ClientHello and the connection parameters. The combination of the ClientHello and the
ServerHello determines the shared keys. If (EC)DHE key establishment ServerHello determines the shared keys. If (EC)DHE key establishment
is in use, then the ServerHello contains a "key_share" extension with is in use, then the ServerHello contains a "key_share" extension with
the server's ephemeral Diffie-Hellman share which MUST be in the same the server's ephemeral Diffie-Hellman share which MUST be in the same
group as one of the client's shares. If PSK key establishment is in group as one of the client's shares. If PSK key establishment is in
use, then the ServerHello contains a "pre_shared_key" extension use, then the ServerHello contains a "pre_shared_key" extension
skipping to change at page 19, line 20 skipping to change at page 19, line 20
Finished: a MAC (Message Authentication Code) over the entire Finished: a MAC (Message Authentication Code) over the entire
handshake. This message provides key confirmation, binds the handshake. This message provides key confirmation, binds the
endpoint's identity to the exchanged keys, and in PSK mode also endpoint's identity to the exchanged keys, and in PSK mode also
authenticates the handshake. [Section 4.4.4] authenticates the handshake. [Section 4.4.4]
Upon receiving the server's messages, the client responds with its Upon receiving the server's messages, the client responds with its
Authentication messages, namely Certificate and CertificateVerify (if Authentication messages, namely Certificate and CertificateVerify (if
requested), and Finished. requested), and Finished.
At this point, the handshake is complete, and the client and server At this point, the handshake is complete, and the client and server
may exchange application-layer data. Application data MUST NOT be must derive the keying material required by the record layer to
sent prior to sending the Finished message. Note that while the exchange application-layer data protected through authenticated
server may send application data prior to receiving the client's encryption. Application data MUST NOT be sent prior to sending the
Authentication messages, any data sent at that point is, of course, Finished message and until the record layer starts using encryption
being sent to an unauthenticated peer. keys. Note that while the server may send application data prior to
receiving the client's Authentication messages, any data sent at that
point is, of course, being sent to an unauthenticated peer.
2.1. Incorrect DHE Share 2.1. Incorrect DHE Share
If the client has not provided a sufficient "key_share" extension If the client has not provided a sufficient "key_share" extension
(e.g., it includes only DHE or ECDHE groups unacceptable to or (e.g., it includes only DHE or ECDHE groups unacceptable to or
unsupported by the server), the server corrects the mismatch with a unsupported by the server), the server corrects the mismatch with a
HelloRetryRequest and the client needs to restart the handshake with HelloRetryRequest and the client needs to restart the handshake with
an appropriate "key_share" extension, as shown in Figure 2. If no an appropriate "key_share" extension, as shown in Figure 2. If no
common cryptographic parameters can be negotiated, the server MUST common cryptographic parameters can be negotiated, the server MUST
abort the handshake with an appropriate alert. abort the handshake with an appropriate alert.
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Note: The handshake transcript includes the initial ClientHello/ Note: The handshake transcript includes the initial ClientHello/
HelloRetryRequest exchange; it is not reset with the new ClientHello. HelloRetryRequest exchange; it is not reset with the new ClientHello.
TLS also allows several optimized variants of the basic handshake, as TLS also allows several optimized variants of the basic handshake, as
described in the following sections. described in the following sections.
2.2. Resumption and Pre-Shared Key (PSK) 2.2. Resumption and Pre-Shared Key (PSK)
Although TLS PSKs can be established out of band, PSKs can also be Although TLS PSKs can be established out of band, PSKs can also be
established in a previous connection and then reused ("session established in a previous connection and then reused ("session
resumption"). Once a handshake has completed, the server can send resumption"). Once a handshake has completed, the server can send to
the client a PSK identity that corresponds to a key derived from the the client a PSK identity that corresponds to a unique key derived
initial handshake (see Section 4.6.1). The client can then use that from the initial handshake (see Section 4.6.1). The client can then
PSK identity in future handshakes to negotiate use of the PSK. If use that PSK identity in future handshakes to negotiate the use of
the server accepts it, then the security context of the new the associated PSK. If the server accepts it, then the security
connection is tied to the original connection and the key derived context of the new connection is cryptographically tied to the
from the initial handshake is used to bootstrap the cryptographic original connection and the key derived from the initial handshake is
state instead of a full handshake. In TLS 1.2 and below, this used to bootstrap the cryptographic state instead of a full
functionality was provided by "session IDs" and "session tickets" handshake. In TLS 1.2 and below, this functionality was provided by
[RFC5077]. Both mechanisms are obsoleted in TLS 1.3. "session IDs" and "session tickets" [RFC5077]. Both mechanisms are
obsoleted in TLS 1.3.
PSKs can be used with (EC)DHE key exchange in order to provide PSKs can be used with (EC)DHE key exchange in order to provide
forward secrecy in combination with shared keys, or can be used forward secrecy in combination with shared keys, or can be used
alone, at the cost of losing forward secrecy. alone, at the cost of losing forward secrecy for the application
data.
Figure 3 shows a pair of handshakes in which the first establishes a Figure 3 shows a pair of handshakes in which the first establishes a
PSK and the second uses it: PSK and the second uses it:
Client Server Client Server
Initial Handshake: Initial Handshake:
ClientHello ClientHello
+ key_share --------> + key_share -------->
ServerHello ServerHello
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As the server is authenticating via a PSK, it does not send a As the server is authenticating via a PSK, it does not send a
Certificate or a CertificateVerify message. When a client offers Certificate or a CertificateVerify message. When a client offers
resumption via PSK, it SHOULD also supply a "key_share" extension to resumption via PSK, it SHOULD also supply a "key_share" extension to
the server to allow the server to decline resumption and fall back to the server to allow the server to decline resumption and fall back to
a full handshake, if needed. The server responds with a a full handshake, if needed. The server responds with a
"pre_shared_key" extension to negotiate use of PSK key establishment "pre_shared_key" extension to negotiate use of PSK key establishment
and can (as shown here) respond with a "key_share" extension to do and can (as shown here) respond with a "key_share" extension to do
(EC)DHE key establishment, thus providing forward secrecy. (EC)DHE key establishment, thus providing forward secrecy.
When PSKs are provisioned out of band, the PSK identity and the KDF When PSKs are provisioned out of band, the PSK identity and the KDF
to be used with the PSK MUST also be provisioned. Note: When using hash algorithm to be used with the PSK MUST also be provisioned.
an out-of-band provisioned pre-shared secret, a critical Note: When using an out-of-band provisioned pre-shared secret, a
consideration is using sufficient entropy during the key generation, critical consideration is using sufficient entropy during the key
as discussed in [RFC4086]. Deriving a shared secret from a password generation, as discussed in [RFC4086]. Deriving a shared secret from
or other low-entropy sources is not secure. A low-entropy secret, or a password or other low-entropy sources is not secure. A low-entropy
password, is subject to dictionary attacks based on the PSK binder. secret, or password, is subject to dictionary attacks based on the
The specified PSK authentication is not a strong password-based PSK binder. The specified PSK authentication is not a strong
authenticated key exchange even when used with Diffie-Hellman key password-based authenticated key exchange even when used with Diffie-
establishment. Hellman key establishment.
2.3. Zero-RTT Data 2.3. Zero-RTT Data
When clients and servers share a PSK (either obtained externally or When clients and servers share a PSK (either obtained externally or
via a previous handshake), TLS 1.3 allows clients to send data on the via a previous handshake), TLS 1.3 allows clients to send data on the
first flight ("early data"). The client uses the PSK to authenticate first flight ("early data"). The client uses the PSK to authenticate
the server and to encrypt the early data. the server and to encrypt the early data.
When clients use a PSK obtained externally to send early data, then When clients use a PSK obtained externally to send early data, then
the following additional information MUST be provisioned to both the following additional information MUST be provisioned to both
parties: parties:
- The TLS version number for use with this PSK
- The cipher suite for use with this PSK - The cipher suite for use with this PSK
- The Application-Layer Protocol Negotiation (ALPN) protocol, if any - The Application-Layer Protocol Negotiation (ALPN) protocol
is to be used [RFC7301], if any is to be used
- The Server Name Indication (SNI), if any is to be used - The Server Name Indication (SNI), if any is to be used
As shown in Figure 4, the 0-RTT data is just added to the 1-RTT As shown in Figure 4, the 0-RTT data is just added to the 1-RTT
handshake in the first flight. The rest of the handshake uses the handshake in the first flight. The rest of the handshake uses the
same messages as with a 1-RTT handshake with PSK resumption. same messages as with a 1-RTT handshake with PSK resumption.
Client Server Client Server
ClientHello ClientHello
skipping to change at page 23, line 49 skipping to change at page 23, line 49
Figure 4: Message flow for a zero round trip handshake Figure 4: Message flow for a zero round trip handshake
IMPORTANT NOTE: The security properties for 0-RTT data are weaker IMPORTANT NOTE: The security properties for 0-RTT data are weaker
than those for other kinds of TLS data. Specifically: than those for other kinds of TLS data. Specifically:
1. This data is not forward secret, as it is encrypted solely under 1. This data is not forward secret, as it is encrypted solely under
keys derived using the offered PSK. keys derived using the offered PSK.
2. There are no guarantees of non-replay between connections. 2. There are no guarantees of non-replay between connections.
Unless the server takes special measures outside those provided Protection against replay for ordinary TLS 1.3 1-RTT data is
by TLS, the server has no guarantee that the same 0-RTT data was provided via the server's Random value, but 0-RTT data does not
not transmitted on multiple 0-RTT connections (see depend on the ServerHello and therefore has weaker guarantees.
Section 4.2.10.4 for more details). This is especially relevant This is especially relevant if the data is authenticated either
if the data is authenticated either with TLS client with TLS client authentication or inside the application
authentication or inside the application layer protocol. protocol. The same warnings apply to any use of the
However, 0-RTT data cannot be duplicated within a connection early_exporter_master_secret.
(i.e., the server will not process the same data twice for the
same connection) and an attacker will not be able to make 0-RTT
data appear to be 1-RTT data (because it is protected with
different keys.)
Protocols MUST NOT use 0-RTT data without a profile that defines its
use. That profile needs to identify which messages or interactions
are safe to use with 0-RTT. In addition, to avoid accidental misuse,
implementations SHOULD NOT enable 0-RTT unless specifically
requested. Implementations SHOULD provide special functions for
0-RTT data to ensure that an application is always aware that it is
sending or receiving data that might be replayed.
The same warnings apply to any use of the
early_exporter_master_secret.
The remainder of this document provides a detailed description of 0-RTT data cannot be duplicated within a connection (i.e., the server
TLS. will not process the same data twice for the same connection) and an
attacker will not be able to make 0-RTT data appear to be 1-RTT data
(because it is protected with different keys.) Appendix E.5 contains
a description of potential attacks and Section 8 describes mechanisms
which the server can use to limit the impact of replay.
3. Presentation Language 3. Presentation Language
This document deals with the formatting of data in an external This document deals with the formatting of data in an external
representation. The following very basic and somewhat casually representation. The following very basic and somewhat casually
defined presentation syntax will be used. defined presentation syntax will be used.
3.1. Basic Block Size 3.1. Basic Block Size
The representation of all data items is explicitly specified. The The representation of all data items is explicitly specified. The
skipping to change at page 29, line 7 skipping to change at page 28, line 42
struct { struct {
VariantTag type; VariantTag type;
select (VariantRecord.type) { select (VariantRecord.type) {
case apple: V1; case apple: V1;
case orange: V2; case orange: V2;
}; };
} VariantRecord; } VariantRecord;
4. Handshake Protocol 4. Handshake Protocol
The handshake protocol is used to negotiate the secure attributes of The handshake protocol is used to negotiate the security parameters
a connection. Handshake messages are supplied to the TLS record of a connection. Handshake messages are supplied to the TLS record
layer, where they are encapsulated within one or more TLSPlaintext or layer, where they are encapsulated within one or more TLSPlaintext or
TLSCiphertext structures, which are processed and transmitted as TLSCiphertext structures, which are processed and transmitted as
specified by the current active connection state. specified by the current active connection state.
enum { enum {
client_hello(1), client_hello(1),
server_hello(2), server_hello(2),
new_session_ticket(4), new_session_ticket(4),
end_of_early_data(5), end_of_early_data(5),
hello_retry_request(6), hello_retry_request(6),
skipping to change at page 29, line 50 skipping to change at page 29, line 42
case certificate_verify: CertificateVerify; case certificate_verify: CertificateVerify;
case finished: Finished; case finished: Finished;
case new_session_ticket: NewSessionTicket; case new_session_ticket: NewSessionTicket;
case key_update: KeyUpdate; case key_update: KeyUpdate;
} body; } body;
} Handshake; } Handshake;
Protocol messages MUST be sent in the order defined in Section 4.4.1 Protocol messages MUST be sent in the order defined in Section 4.4.1
and shown in the diagrams in Section 2. A peer which receives a and shown in the diagrams in Section 2. A peer which receives a
handshake message in an unexpected order MUST abort the handshake handshake message in an unexpected order MUST abort the handshake
with an "unexpected_message" alert. However, unneeded handshake with an "unexpected_message" alert.
messages are omitted.
New handshake message types are assigned by IANA as described in New handshake message types are assigned by IANA as described in
Section 10. Section 11.
4.1. Key Exchange Messages 4.1. Key Exchange Messages
The key exchange messages are used to exchange security capabilities The key exchange messages are used to determine the security
between the client and server and to establish the traffic keys used capabilities of the client and the server and to establish shared
to protect the handshake and data. secrets including the traffic keys used to protect the rest of the
handshake and the data.
4.1.1. Cryptographic Negotiation 4.1.1. Cryptographic Negotiation
TLS cryptographic negotiation proceeds by the client offering the In TLS, the cryptographic negotiation proceeds by the client offering
following four sets of options in its ClientHello: the following four sets of options in its ClientHello:
- A list of cipher suites which indicates the AEAD algorithm/HKDF - A list of cipher suites which indicates the AEAD algorithm/HKDF
hash pairs which the client supports. hash pairs which the client supports.
- A "supported_groups" (Section 4.2.6) extension which indicates the - A "supported_groups" (Section 4.2.6) extension which indicates the
(EC)DHE groups which the client supports and a "key_share" (EC)DHE groups which the client supports and a "key_share"
(Section 4.2.7) extension which contains (EC)DHE shares for some (Section 4.2.7) extension which contains (EC)DHE shares for some
or all of these groups. or all of these groups.
- A "signature_algorithms" (Section 4.2.3) extension which indicates - A "signature_algorithms" (Section 4.2.3) extension which indicates
skipping to change at page 30, line 41 skipping to change at page 30, line 32
list of symmetric key identities known to the client and a list of symmetric key identities known to the client and a
"psk_key_exchange_modes" (Section 4.2.8) extension which indicates "psk_key_exchange_modes" (Section 4.2.8) extension which indicates
the key exchange modes that may be used with PSKs. the key exchange modes that may be used with PSKs.
If the server does not select a PSK, then the first three of these If the server does not select a PSK, then the first three of these
options are entirely orthogonal: the server independently selects a options are entirely orthogonal: the server independently selects a
cipher suite, an (EC)DHE group and key share for key establishment, cipher suite, an (EC)DHE group and key share for key establishment,
and a signature algorithm/certificate pair to authenticate itself to and a signature algorithm/certificate pair to authenticate itself to
the client. If there is no overlap between the received the client. If there is no overlap between the received
"supported_groups" and the groups supported by the server then the "supported_groups" and the groups supported by the server then the
server MUST abort the handshake. server MUST abort the handshake with a "handshake_failure" or an
"insufficient_security" alert.
If the server selects a PSK, then it MUST also select a key If the server selects a PSK, then it MUST also select a key
establishment mode from the set indicated by client's establishment mode from the set indicated by client's
"psk_key_exchange_modes" extension (at present, PSK alone or with "psk_key_exchange_modes" extension (at present, PSK alone or with
(EC)DHE). Note that if the PSK can be used without (EC)DHE then non- (EC)DHE). Note that if the PSK can be used without (EC)DHE then non-
overlap in the "supported_groups" parameters need not be fatal, as it overlap in the "supported_groups" parameters need not be fatal, as it
is in the non-PSK case discussed in the previous paragraph. is in the non-PSK case discussed in the previous paragraph.
If the server selects an (EC)DHE group and the client did not offer a If the server selects an (EC)DHE group and the client did not offer a
compatible "key_share" extension in the initial ClientHello, the compatible "key_share" extension in the initial ClientHello, the
skipping to change at page 32, line 7 skipping to change at page 31, line 47
- Including a "cookie" extension if one was provided in the - Including a "cookie" extension if one was provided in the
HelloRetryRequest. HelloRetryRequest.
- Updating the "pre_shared_key" extension if present by recomputing - Updating the "pre_shared_key" extension if present by recomputing
the "obfuscated_ticket_age" and binder values and (optionally) the "obfuscated_ticket_age" and binder values and (optionally)
removing any PSKs which are incompatible with the server's removing any PSKs which are incompatible with the server's
indicated cipher suite. indicated cipher suite.
Because TLS 1.3 forbids renegotiation, if a server has negotiated TLS Because TLS 1.3 forbids renegotiation, if a server has negotiated TLS
1.3 and receives a ClientHello at any other time, it MUST terminate 1.3 and receives a ClientHello at any other time, it MUST terminate
the connection. the connection with an "unexpected_message" alert.
If a server established a TLS connection with a previous version of If a server established a TLS connection with a previous version of
TLS and receives a TLS 1.3 ClientHello in a renegotiation, it MUST TLS and receives a TLS 1.3 ClientHello in a renegotiation, it MUST
retain the previous protocol version. In particular, it MUST NOT retain the previous protocol version. In particular, it MUST NOT
negotiate TLS 1.3. negotiate TLS 1.3.
Structure of this message: Structure of this message:
uint16 ProtocolVersion; uint16 ProtocolVersion;
opaque Random[32]; opaque Random[32];
skipping to change at page 33, line 9 skipping to change at page 32, line 47
"session resumption" feature which has been merged with Pre-Shared "session resumption" feature which has been merged with Pre-Shared
Keys in this version (see Section 2.2). This field MUST be Keys in this version (see Section 2.2). This field MUST be
ignored by a server negotiating TLS 1.3 and MUST be set as a zero ignored by a server negotiating TLS 1.3 and MUST be set as a zero
length vector (i.e., a single zero byte length field) by clients length vector (i.e., a single zero byte length field) by clients
that do not have a cached session ID set by a pre-TLS 1.3 server. that do not have a cached session ID set by a pre-TLS 1.3 server.
cipher_suites This is a list of the symmetric cipher options cipher_suites This is a list of the symmetric cipher options
supported by the client, specifically the record protection supported by the client, specifically the record protection
algorithm (including secret key length) and a hash to be used with algorithm (including secret key length) and a hash to be used with
HKDF, in descending order of client preference. If the list HKDF, in descending order of client preference. If the list
contains cipher suites the server does not recognize, support, or contains cipher suites that the server does not recognize, support
wish to use, the server MUST ignore those cipher suites, and or wish to use, the server MUST ignore those cipher suites and
process the remaining ones as usual. Values are defined in process the remaining ones as usual. Values are defined in
Appendix B.4. If the client is attempting a PSK key Appendix B.4. If the client is attempting a PSK key
establishment, it SHOULD advertise at least one cipher suite establishment, it SHOULD advertise at least one cipher suite
indicating a Hash associated with the PSK. indicating a Hash associated with the PSK.
legacy_compression_methods Versions of TLS before 1.3 supported legacy_compression_methods Versions of TLS before 1.3 supported
compression with the list of supported compression methods being compression with the list of supported compression methods being
sent in this field. For every TLS 1.3 ClientHello, this vector sent in this field. For every TLS 1.3 ClientHello, this vector
MUST contain exactly one byte set to zero, which corresponds to MUST contain exactly one byte set to zero, which corresponds to
the "null" compression method in prior versions of TLS. If a TLS the "null" compression method in prior versions of TLS. If a TLS
skipping to change at page 33, line 37 skipping to change at page 33, line 26
0x0303 and a supported_versions extension present with 0x0304 as 0x0303 and a supported_versions extension present with 0x0304 as
the highest version indicated therein. the highest version indicated therein.
extensions Clients request extended functionality from servers by extensions Clients request extended functionality from servers by
sending data in the extensions field. The actual "Extension" sending data in the extensions field. The actual "Extension"
format is defined in Section 4.2. In TLS 1.3, use of certain format is defined in Section 4.2. In TLS 1.3, use of certain
extensions is mandatory, as functionality is moved into extensions extensions is mandatory, as functionality is moved into extensions
to preserve ClientHello compatibility with previous versions of to preserve ClientHello compatibility with previous versions of
TLS. Servers MUST ignore unrecognized extensions. TLS. Servers MUST ignore unrecognized extensions.
All versions of TLS allow extensions to optionally follow the All versions of TLS allow an extensions field to optionally follow
compression_methods field as an extensions field. TLS 1.3 the compression_methods field. TLS 1.3 ClientHello messages always
ClientHello messages always contain extensions (minimally, contain extensions (minimally "supported_versions", otherwise they
"supported_versions", or they will be interpreted as TLS 1.2 will be interpreted as TLS 1.2 ClientHello messages). However, TLS
ClientHello messages), however TLS 1.3 servers might receive 1.3 servers might receive ClientHello messages without an extensions
ClientHello messages without an extensions field from prior versions field from prior versions of TLS. The presence of extensions can be
of TLS. The presence of extensions can be detected by determining detected by determining whether there are bytes following the
whether there are bytes following the compression_methods field at compression_methods field at the end of the ClientHello. Note that
the end of the ClientHello. Note that this method of detecting this method of detecting optional data differs from the normal TLS
optional data differs from the normal TLS method of having a method of having a variable-length field, but it is used for
variable-length field, but it is used for compatibility with TLS compatibility with TLS before extensions were defined. TLS 1.3
before extensions were defined. TLS 1.3 servers will need to perform servers will need to perform this check first and only attempt to
this check first and only attempt to negotiate TLS 1.3 if a negotiate TLS 1.3 if a "supported_version" extension is present. If
"supported_version" extension is present. If negotiating a version negotiating a version of TLS prior to 1.3, a server MUST check that
of TLS prior to 1.3, a server MUST check that the message either the message either contains no data after legacy_compression_methods
contains no data after legacy_compression_methods or that it contains or that it contains a valid extensions block with no data following.
a valid extensions block with no data following. If not, then it If not, then it MUST abort the handshake with a "decode_error" alert.
MUST abort the handshake with a "decode_error" alert.
In the event that a client requests additional functionality using In the event that a client requests additional functionality using
extensions, and this functionality is not supplied by the server, the extensions, and this functionality is not supplied by the server, the
client MAY abort the handshake. client MAY abort the handshake.
After sending the ClientHello message, the client waits for a After sending the ClientHello message, the client waits for a
ServerHello or HelloRetryRequest message. If early data is in use, ServerHello or HelloRetryRequest message. If early data is in use,
the client may transmit early application data Section 2.3 while the client may transmit early application data Section 2.3 while
waiting for the next handshake message. waiting for the next handshake message.
4.1.3. Server Hello 4.1.3. Server Hello
The server will send this message in response to a ClientHello The server will send this message in response to a ClientHello
message if it is able to find an acceptable set of parameters and the message to proceed with the handshake if it is able to negotiate an
ClientHello contains sufficient information to proceed with the acceptable set of handshake parameters based on the ClientHello.
handshake.
Structure of this message: Structure of this message:
struct { struct {
ProtocolVersion version; ProtocolVersion version;
Random random; Random random;
CipherSuite cipher_suite; CipherSuite cipher_suite;
Extension extensions<6..2^16-1>; Extension extensions<6..2^16-1>;
} ServerHello; } ServerHello;
skipping to change at page 34, line 43 skipping to change at page 34, line 31
connection. Servers MUST select a version from the list in connection. Servers MUST select a version from the list in
ClientHello's supported_versions extension, or otherwise negotiate ClientHello's supported_versions extension, or otherwise negotiate
TLS 1.2 or previous. A client that receives a version that was TLS 1.2 or previous. A client that receives a version that was
not offered MUST abort the handshake. For this version of the not offered MUST abort the handshake. For this version of the
specification, the version is 0x0304. (See Appendix D for details specification, the version is 0x0304. (See Appendix D for details
about backward compatibility.) about backward compatibility.)
random 32 bytes generated by a secure random number generator. See random 32 bytes generated by a secure random number generator. See
Appendix C for additional information. The last eight bytes MUST Appendix C for additional information. The last eight bytes MUST
be overwritten as described below if negotiating TLS 1.2 or TLS be overwritten as described below if negotiating TLS 1.2 or TLS
1.1. This structure is generated by the server and MUST be 1.1, but the remaining bytes MUST be random. This structure is
generated independently of the ClientHello.random. generated by the server and MUST be generated independently of the
ClientHello.random.
cipher_suite The single cipher suite selected by the server from the cipher_suite The single cipher suite selected by the server from the
list in ClientHello.cipher_suites. A client which receives a list in ClientHello.cipher_suites. A client which receives a
cipher suite that was not offered MUST abort the handshake. cipher suite that was not offered MUST abort the handshake with an
"illegal_parameter" alert.
extensions A list of extensions. The ServerHello MUST only include extensions A list of extensions. The ServerHello MUST only include
extensions which are required to establish the cryptographic extensions which are required to establish the cryptographic
context. Currently the only such extensions are "key_share" and context. Currently the only such extensions are "key_share" and
"pre_shared_key". All current TLS 1.3 ServerHello messages will "pre_shared_key". All current TLS 1.3 ServerHello messages will
contain one of these two extensions, or both when using a PSK with contain one of these two extensions, or both when using a PSK with
(EC)DHE key establishment. (EC)DHE key establishment. The remaining extensions are sent
separately in the EncryptedExtensions message.
TLS 1.3 has a downgrade protection mechanism embedded in the server's TLS 1.3 has a downgrade protection mechanism embedded in the server's
random value. TLS 1.3 servers which negotiate TLS 1.2 or below in random value. TLS 1.3 servers which negotiate TLS 1.2 or below in
response to a ClientHello MUST set the last eight bytes of their response to a ClientHello MUST set the last eight bytes of their
Random value specially. Random value specially.
If negotiating TLS 1.2, TLS 1.3 servers MUST set the last eight bytes If negotiating TLS 1.2, TLS 1.3 servers MUST set the last eight bytes
of their Random value to the bytes: of their Random value to the bytes:
44 4F 57 4E 47 52 44 01 44 4F 57 4E 47 52 44 01
If negotiating TLS 1.1 or below, TLS 1.3 servers MUST and TLS 1.2 If negotiating TLS 1.1 or below, TLS 1.3 servers MUST and TLS 1.2
servers SHOULD set the last eight bytes of their Random value to the servers SHOULD set the last eight bytes of their Random value to the
bytes: bytes:
44 4F 57 4E 47 52 44 00 44 4F 57 4E 47 52 44 00
TLS 1.3 clients receiving a TLS 1.2 or below ServerHello MUST check TLS 1.3 clients receiving a ServerHello indicating TLS 1.2 or below
that the last eight bytes are not equal to either of these values. MUST check that the last eight bytes are not equal to either of these
TLS 1.2 clients SHOULD also check that the last eight bytes are not values. TLS 1.2 clients SHOULD also check that the last eight bytes
equal to the second value if the ServerHello indicates TLS 1.1 or are not equal to the second value if the ServerHello indicates TLS
below. If a match is found, the client MUST abort the handshake with 1.1 or below. If a match is found, the client MUST abort the
an "illegal_parameter" alert. This mechanism provides limited handshake with an "illegal_parameter" alert. This mechanism provides
protection against downgrade attacks over and above that provided by limited protection against downgrade attacks over and above what is
the Finished exchange: because the ServerKeyExchange, a message provided by the Finished exchange: because the ServerKeyExchange, a
present in TLS 1.2 and below, includes a signature over both random message present in TLS 1.2 and below, includes a signature over both
values, it is not possible for an active attacker to modify the random values, it is not possible for an active attacker to modify
random values without detection as long as ephemeral ciphers are the random values without detection as long as ephemeral ciphers are
used. It does not provide downgrade protection when static RSA is used. It does not provide downgrade protection when static RSA is
used. used.
Note: This is a change from [RFC5246], so in practice many TLS 1.2 Note: This is a change from [RFC5246], so in practice many TLS 1.2
clients and servers will not behave as specified above. clients and servers will not behave as specified above.
A client that receives a TLS 1.3 ServerHello during renegotiation A legacy TLS client performing renegotiation with TLS 1.2 or prior
MUST abort the handshake with a "protocol_version" alert. Note that and which receives a TLS 1.3 ServerHello during renegotiation MUST
renegotiation is only possible when a version of TLS prior to 1.3 has abort the handshake with a "protocol_version" alert. Note that
been negotiated. renegotiation is not possible when TLS 1.3 has been negotiated.
RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH Implementations of RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH Implementations of
draft versions (see Section 4.2.1.1) of this specification SHOULD NOT draft versions (see Section 4.2.1.1) of this specification SHOULD NOT
implement this mechanism on either client and server. A pre-RFC implement this mechanism on either client and server. A pre-RFC
client connecting to RFC servers, or vice versa, will appear to client connecting to RFC servers, or vice versa, will appear to
downgrade to TLS 1.2. With the mechanism enabled, this will cause an downgrade to TLS 1.2. With the mechanism enabled, this will cause an
interoperability failure. interoperability failure.
4.1.4. Hello Retry Request 4.1.4. Hello Retry Request
skipping to change at page 37, line 34 skipping to change at page 37, line 26
server_name(0), /* RFC 6066 */ server_name(0), /* RFC 6066 */
max_fragment_length(1), /* RFC 6066 */ max_fragment_length(1), /* RFC 6066 */
status_request(5), /* RFC 6066 */ status_request(5), /* RFC 6066 */
supported_groups(10), /* RFC 4492, 7919 */ supported_groups(10), /* RFC 4492, 7919 */
signature_algorithms(13), /* RFC 5246 */ signature_algorithms(13), /* RFC 5246 */
use_srtp(14), /* RFC 5764 */ use_srtp(14), /* RFC 5764 */
heartbeat(15), /* RFC 6520 */ heartbeat(15), /* RFC 6520 */
application_layer_protocol_negotiation(16), /* RFC 7301 */ application_layer_protocol_negotiation(16), /* RFC 7301 */
signed_certificate_timestamp(18), /* RFC 6962 */ signed_certificate_timestamp(18), /* RFC 6962 */
client_certificate_type(19), /* RFC 7250 */ client_certificate_type(19), /* RFC 7250 */
server_certificate_type(20) /* RFC 7250 */ server_certificate_type(20), /* RFC 7250 */
padding(21), /* RFC 7685 */ padding(21), /* RFC 7685 */
key_share(40), /* [[this document]] */ key_share(40), /* [[this document]] */
pre_shared_key(41), /* [[this document]] */ pre_shared_key(41), /* [[this document]] */
early_data(42), /* [[this document]] */ early_data(42), /* [[this document]] */
supported_versions(43), /* [[this document]] */ supported_versions(43), /* [[this document]] */
cookie(44), /* [[this document]] */ cookie(44), /* [[this document]] */
psk_key_exchange_modes(45), /* [[this document]] */ psk_key_exchange_modes(45), /* [[this document]] */
certificate_authorities(47), /* [[this document]] */ certificate_authorities(47), /* [[this document]] */
oid_filters(48), /* [[this document]] */ oid_filters(48), /* [[this document]] */
post_handshake_auth(49), /* [[this document]] */ post_handshake_auth(49), /* [[this document]] */
skipping to change at page 38, line 9 skipping to change at page 37, line 48
} ExtensionType; } ExtensionType;
Here: Here:
- "extension_type" identifies the particular extension type. - "extension_type" identifies the particular extension type.
- "extension_data" contains information specific to the particular - "extension_data" contains information specific to the particular
extension type. extension type.
The list of extension types is maintained by IANA as described in The list of extension types is maintained by IANA as described in
Section 10. Section 11.
Extensions are generally structured in a request/response fashion, Extensions are generally structured in a request/response fashion,
though some extensions are just indications with no corresponding though some extensions are just indications with no corresponding
response. The client sends its extension requests in the ClientHello response. The client sends its extension requests in the ClientHello
message and the server sends its extension responses in the message and the server sends its extension responses in the
ServerHello, EncryptedExtensions and HelloRetryRequest messages. The ServerHello, EncryptedExtensions, HelloRetryRequest and Certificate
server sends extension requests in the CertificateRequest message messages. The server sends extension requests in the
which a client MAY respond to with a Certificate message. The server CertificateRequest message which a client MAY respond to with a
MAY also send unsolicited extensions in the NewSessionTicket, though Certificate message. The server MAY also send unsolicited extensions
the client does not respond directly to these. in the NewSessionTicket, though the client does not respond directly
to these.
Implementations MUST NOT send extension responses if the remote Implementations MUST NOT send extension responses if the remote
endpoint did not send the corresponding extension requests, with the endpoint did not send the corresponding extension requests, with the
exception of the "cookie" extension in HelloRetryRequest. Upon exception of the "cookie" extension in HelloRetryRequest. Upon
receiving such an extension, an endpoint MUST abort the handshake receiving such an extension, an endpoint MUST abort the handshake
with an "unsupported_extension" alert. with an "unsupported_extension" alert.
The table below indicates the messages where a given extension may The table below indicates the messages where a given extension may
appear, using the following notation: CH (ClientHello), SH appear, using the following notation: CH (ClientHello), SH
(ServerHello), EE (EncryptedExtensions), CT (Certificate), CR (ServerHello), EE (EncryptedExtensions), CT (Certificate), CR
skipping to change at page 40, line 8 skipping to change at page 40, line 8
| | | | | |
| post_handshake_auth [[this document]] | CH | | post_handshake_auth [[this document]] | CH |
+--------------------------------------------------+-------------+ +--------------------------------------------------+-------------+
When multiple extensions of different types are present, the When multiple extensions of different types are present, the
extensions MAY appear in any order, with the exception of extensions MAY appear in any order, with the exception of
"pre_shared_key" Section 4.2.10 which MUST be the last extension in "pre_shared_key" Section 4.2.10 which MUST be the last extension in
the ClientHello. There MUST NOT be more than one extension of the the ClientHello. There MUST NOT be more than one extension of the
same type in a given extension block. same type in a given extension block.
In TLS 1.3, unlike TLS 1.2, extensions are renegotiated with each In TLS 1.3, unlike TLS 1.2, extensions are negotiated for each
handshake even when in resumption-PSK mode. However, 0-RTT handshake even when in resumption-PSK mode. However, 0-RTT
parameters are those negotiated in the previous handshake; mismatches parameters are those negotiated in the previous handshake; mismatches
may require rejecting 0-RTT (see Section 4.2.9). may require rejecting 0-RTT (see Section 4.2.9).
There are subtle (and not so subtle) interactions that may occur in There are subtle (and not so subtle) interactions that may occur in
this protocol between new features and existing features which may this protocol between new features and existing features which may
result in a significant reduction in overall security. The following result in a significant reduction in overall security. The following
considerations should be taken into account when designing new considerations should be taken into account when designing new
extensions: extensions:
- Some cases where a server does not agree to an extension are error - Some cases where a server does not agree to an extension are error
conditions, and some are simply refusals to support particular conditions, and some are simply refusals to support particular
features. In general, error alerts should be used for the former, features. In general, error alerts should be used for the former
and a field in the server extension response for the latter. and a field in the server extension response for the latter.
- Extensions should, as far as possible, be designed to prevent any - Extensions should, as far as possible, be designed to prevent any
attack that forces use (or non-use) of a particular feature by attack that forces use (or non-use) of a particular feature by
manipulation of handshake messages. This principle should be manipulation of handshake messages. This principle should be
followed regardless of whether the feature is believed to cause a followed regardless of whether the feature is believed to cause a
security problem. Often the fact that the extension fields are security problem. Often the fact that the extension fields are
included in the inputs to the Finished message hashes will be included in the inputs to the Finished message hashes will be
sufficient, but extreme care is needed when the extension changes sufficient, but extreme care is needed when the extension changes
the meaning of messages sent in the handshake phase. Designers the meaning of messages sent in the handshake phase. Designers
skipping to change at page 40, line 48 skipping to change at page 40, line 48
struct { struct {
ProtocolVersion versions<2..254>; ProtocolVersion versions<2..254>;
} SupportedVersions; } SupportedVersions;
The "supported_versions" extension is used by the client to indicate The "supported_versions" extension is used by the client to indicate
which versions of TLS it supports. The extension contains a list of which versions of TLS it supports. The extension contains a list of
supported versions in preference order, with the most preferred supported versions in preference order, with the most preferred
version first. Implementations of this specification MUST send this version first. Implementations of this specification MUST send this
extension containing all versions of TLS which they are prepared to extension containing all versions of TLS which they are prepared to
negotiate (for this specification, that means minimally 0x0304, but negotiate (for this specification, that means minimally 0x0304, but
if previous versions of TLS are supported, they MUST be present as if previous versions of TLS are allowed to be negotiated, they MUST
well). be present as well).
If this extension is not present, servers which are compliant with If this extension is not present, servers which are compliant with
this specification MUST negotiate TLS 1.2 or prior as specified in this specification MUST negotiate TLS 1.2 or prior as specified in
[RFC5246], even if ClientHello.legacy_version is 0x0304 or later. [RFC5246], even if ClientHello.legacy_version is 0x0304 or later.
Servers MAY abort the handshake upon receiving a ClientHello with Servers MAY abort the handshake upon receiving a ClientHello with
legacy_version 0x0304 or later. legacy_version 0x0304 or later.
If this extension is present, servers MUST ignore the If this extension is present, servers MUST ignore the
ClientHello.legacy_version value and MUST use only the ClientHello.legacy_version value and MUST use only the
"supported_versions" extension to determine client preferences. "supported_versions" extension to determine client preferences.
skipping to change at page 42, line 27 skipping to change at page 42, line 27
cookies in subsequent connections. cookies in subsequent connections.
4.2.3. Signature Algorithms 4.2.3. Signature Algorithms
The client uses the "signature_algorithms" extension to indicate to The client uses the "signature_algorithms" extension to indicate to
the server which signature algorithms may be used in digital the server which signature algorithms may be used in digital
signatures. Clients which desire the server to authenticate itself signatures. Clients which desire the server to authenticate itself
via a certificate MUST send this extension. If a server is via a certificate MUST send this extension. If a server is
authenticating via a certificate and the client has not sent a authenticating via a certificate and the client has not sent a
"signature_algorithms" extension, then the server MUST abort the "signature_algorithms" extension, then the server MUST abort the
handshake with a "missing_extension" alert (see Section 8.2). handshake with a "missing_extension" alert (see Section 9.2).
The "extension_data" field of this extension in a ClientHello The "extension_data" field of this extension in a ClientHello
contains a SignatureSchemeList value: contains a SignatureSchemeList value:
enum { enum {
/* RSASSA-PKCS1-v1_5 algorithms */ /* RSASSA-PKCS1-v1_5 algorithms */
rsa_pkcs1_sha256(0x0401), rsa_pkcs1_sha256(0x0401),
rsa_pkcs1_sha384(0x0501), rsa_pkcs1_sha384(0x0501),
rsa_pkcs1_sha512(0x0601), rsa_pkcs1_sha512(0x0601),
skipping to change at page 46, line 10 skipping to change at page 46, line 10
The client MAY send the "certificate_authorities" extension in the The client MAY send the "certificate_authorities" extension in the
ClientHello message. The server MAY send it in the ClientHello message. The server MAY send it in the
CertificateRequest message. CertificateRequest message.
The "trusted_ca_keys" extension, which serves a similar purpose The "trusted_ca_keys" extension, which serves a similar purpose
[RFC6066], but is more complicated, is not used in TLS 1.3 (although [RFC6066], but is more complicated, is not used in TLS 1.3 (although
it may appear in ClientHello messages from clients which are offering it may appear in ClientHello messages from clients which are offering
prior versions of TLS). prior versions of TLS).
4.2.4.1. OID Filters
The "oid_filters" extension allows servers to provide a set of OID/
value pairs which it would like the client's certificate to match.
This extension, if provided by the server, MUST only be sent in the
CertificateRequest message.
struct {
opaque certificate_extension_oid<1..2^8-1>;
opaque certificate_extension_values<0..2^16-1>;
} OIDFilter;
struct {
OIDFilter filters<0..2^16-1>;
} OIDFilterExtension;
filters A list of certificate extension OIDs [RFC5280] with their
allowed values and represented in DER-encoded [X690] format. Some
certificate extension OIDs allow multiple values (e.g., Extended
Key Usage). If the server has included a non-empty filters list,
the client certificate included in the response MUST contain all
of the specified extension OIDs that the client recognizes. For
each extension OID recognized by the client, all of the specified
values MUST be present in the client certificate (but the
certificate MAY have other values as well). However, the client
MUST ignore and skip any unrecognized certificate extension OIDs.
If the client ignored some of the required certificate extension
OIDs and supplied a certificate that does not satisfy the request,
the server MAY at its discretion either continue the connection
without client authentication, or abort the handshake with an
"unsupported_certificate" alert.
PKIX RFCs define a variety of certificate extension OIDs and their
corresponding value types. Depending on the type, matching
certificate extension values are not necessarily bitwise-equal. It
is expected that TLS implementations will rely on their PKI libraries
to perform certificate selection using certificate extension OIDs.
This document defines matching rules for two standard certificate
extensions defined in [RFC5280]:
- The Key Usage extension in a certificate matches the request when
all key usage bits asserted in the request are also asserted in
the Key Usage certificate extension.
- The Extended Key Usage extension in a certificate matches the
request when all key purpose OIDs present in the request are also
found in the Extended Key Usage certificate extension. The
special anyExtendedKeyUsage OID MUST NOT be used in the request.
Separate specifications may define matching rules for other
certificate extensions.
4.2.5. Post-Handshake Client Authentication 4.2.5. Post-Handshake Client Authentication
The "post_handshake_auth" extension is used to indicate that a client The "post_handshake_auth" extension is used to indicate that a client
is willing to perform post-handshake authentication Section 4.6.2. is willing to perform post-handshake authentication Section 4.6.2.
Servers MUST not send a post-handshake CertificateRequest to clients Servers MUST not send a post-handshake CertificateRequest to clients
which do not offer this extension. Servers MUST NOT send this which do not offer this extension. Servers MUST NOT send this
extension. extension.
The "extension_data" field of the "post_handshake_auth" extension is The "extension_data" field of the "post_handshake_auth" extension is
zero length. zero length.
skipping to change at page 47, line 37 skipping to change at page 48, line 37
private use. private use.
Finite Field Groups (DHE) Indicates support of the corresponding Finite Field Groups (DHE) Indicates support of the corresponding
finite field group, defined in [RFC7919]. Values 0x01FC through finite field group, defined in [RFC7919]. Values 0x01FC through
0x01FF are reserved for private use. 0x01FF are reserved for private use.
Items in named_group_list are ordered according to the client's Items in named_group_list are ordered according to the client's
preferences (most preferred choice first). preferences (most preferred choice first).
As of TLS 1.3, servers are permitted to send the "supported_groups" As of TLS 1.3, servers are permitted to send the "supported_groups"
extension to the client. If the server has a group it prefers to the extension to the client. Clients MUST NOT act upon any information
ones in the "key_share" extension but is still willing to accept the found in "supported_groups" prior to successful completion of the
ClientHello, it SHOULD send "supported_groups" to update the client's handshake but MAY use the information learned from a successfully
view of its preferences; this extension SHOULD contain all groups the completed handshake to change what groups they use in their
server supports, regardless of whether they are currently supported "key_share" extension in subsequent connections. If the server has a
by the client. Clients MUST NOT act upon any information found in group it prefers to the ones in the "key_share" extension but is
"supported_groups" prior to successful completion of the handshake, still willing to accept the ClientHello, it SHOULD send
but MAY use the information learned from a successfully completed "supported_groups" to update the client's view of its preferences;
handshake to change what groups they use in their "key_share" this extension SHOULD contain all groups the server supports,
extension in subsequent connections. regardless of whether they are currently supported by the client.
4.2.7. Key Share 4.2.7. Key Share
The "key_share" extension contains the endpoint's cryptographic The "key_share" extension contains the endpoint's cryptographic
parameters. parameters.
Clients MAY send an empty client_shares vector in order to request Clients MAY send an empty client_shares vector in order to request
group selection from the server at the cost of an additional round group selection from the server at the cost of an additional round
trip. (see Section 4.1.4) trip. (see Section 4.1.4)
skipping to change at page 49, line 8 skipping to change at page 50, line 8
arise if the most preferred groups are new and unlikely to be arise if the most preferred groups are new and unlikely to be
supported in enough places to make pregenerating key shares for supported in enough places to make pregenerating key shares for
them efficient. them efficient.
selected_group The mutually supported group the server intends to selected_group The mutually supported group the server intends to
negotiate and is requesting a retried ClientHello/KeyShare for. negotiate and is requesting a retried ClientHello/KeyShare for.
server_share A single KeyShareEntry value that is in the same group server_share A single KeyShareEntry value that is in the same group
as one of the client's shares. as one of the client's shares.
Clients offer an arbitrary number of KeyShareEntry values, each Clients can offer an arbitrary number of KeyShareEntry values, each
representing a single set of key exchange parameters. For instance, representing a single set of key exchange parameters. For instance,
a client might offer shares for several elliptic curves or multiple a client might offer shares for several elliptic curves or multiple
FFDHE groups. The key_exchange values for each KeyShareEntry MUST be FFDHE groups. The key_exchange values for each KeyShareEntry MUST be
generated independently. Clients MUST NOT offer multiple generated independently. Clients MUST NOT offer multiple
KeyShareEntry values for the same group. Clients MUST NOT offer any KeyShareEntry values for the same group. Clients MUST NOT offer any
KeyShareEntry values for groups not listed in the client's KeyShareEntry values for groups not listed in the client's
"supported_groups" extension. Servers MAY check for violations of "supported_groups" extension. Servers MAY check for violations of
these rules and abort the handshake with an "illegal_parameter" alert these rules and abort the handshake with an "illegal_parameter" alert
if one is violated. if one is violated.
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psk_ke PSK-only key establishment. In this mode, the server MUST psk_ke PSK-only key establishment. In this mode, the server MUST
NOT supply a "key_share" value. NOT supply a "key_share" value.
psk_dhe_ke PSK with (EC)DHE key establishment. In this mode, the psk_dhe_ke PSK with (EC)DHE key establishment. In this mode, the
client and servers MUST supply "key_share" values as described in client and servers MUST supply "key_share" values as described in
Section 4.2.7. Section 4.2.7.
4.2.9. Early Data Indication 4.2.9. Early Data Indication
When a PSK is used, the client can send application data in its first When a PSK is used, the client can send application data in its first
flight of messages. If the client opts to do so, it MUST supply an flight of messages. If the client opts to do so, it MUST supply both
"early_data" extension as well as the "pre_shared_key" extension. the "early_data" extension as well as the "pre_shared_key" extension.
The "extension_data" field of this extension contains an The "extension_data" field of this extension contains an
"EarlyDataIndication" value. "EarlyDataIndication" value.
struct {} Empty; struct {} Empty;
struct { struct {
select (Handshake.msg_type) { select (Handshake.msg_type) {
case new_session_ticket: uint32 max_early_data_size; case new_session_ticket: uint32 max_early_data_size;
case client_hello: Empty; case client_hello: Empty;
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The parameters for the 0-RTT data (symmetric cipher suite, ALPN The parameters for the 0-RTT data (symmetric cipher suite, ALPN
protocol, etc.) are the same as those which were negotiated in the protocol, etc.) are the same as those which were negotiated in the
connection which established the PSK. The PSK used to encrypt the connection which established the PSK. The PSK used to encrypt the
early data MUST be the first PSK listed in the client's early data MUST be the first PSK listed in the client's
"pre_shared_key" extension. "pre_shared_key" extension.
For PSKs provisioned via NewSessionTicket, a server MUST validate For PSKs provisioned via NewSessionTicket, a server MUST validate
that the ticket age for the selected PSK identity (computed by that the ticket age for the selected PSK identity (computed by
subtracting ticket_age_add from PskIdentity.obfuscated_ticket_age subtracting ticket_age_add from PskIdentity.obfuscated_ticket_age
modulo 2^32) is within a small tolerance of the time since the ticket modulo 2^32) is within a small tolerance of the time since the ticket
was issued (see Section 4.2.10.4). If it is not, the server SHOULD was issued (see Section 8). If it is not, the server SHOULD proceed
proceed with the handshake but reject 0-RTT, and SHOULD NOT take any with the handshake but reject 0-RTT, and SHOULD NOT take any other
other action that assumes that this ClientHello is fresh. action that assumes that this ClientHello is fresh.
0-RTT messages sent in the first flight have the same (encrypted) 0-RTT messages sent in the first flight have the same (encrypted)
content types as their corresponding messages sent in other flights content types as their corresponding messages sent in other flights
(handshake and application_data) but are protected under different (handshake and application_data) but are protected under different
keys. After receiving the server's Finished message, if the server keys. After receiving the server's Finished message, if the server
has accepted early data, an EndOfEarlyData message will be sent to has accepted early data, an EndOfEarlyData message will be sent to
indicate the key change. This message will be encrypted with the indicate the key change. This message will be encrypted with the
0-RTT traffic keys. 0-RTT traffic keys.
A server which receives an "early_data" extension MUST behave in one A server which receives an "early_data" extension MUST behave in one
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PskIdentity identities<7..2^16-1>; PskIdentity identities<7..2^16-1>;
PskBinderEntry binders<33..2^16-1>; PskBinderEntry binders<33..2^16-1>;
case server_hello: case server_hello:
uint16 selected_identity; uint16 selected_identity;
}; };
} PreSharedKeyExtension; } PreSharedKeyExtension;
identity A label for a key. For instance, a ticket defined in identity A label for a key. For instance, a ticket defined in
Appendix B.3.4, or a label for a pre-shared key established Appendix B.3.4 or a label for a pre-shared key established
externally. externally.
obfuscated_ticket_age An obfuscated version of the age of the key. obfuscated_ticket_age An obfuscated version of the age of the key.
Section 4.2.10.1 describes how to form this value for identities Section 4.2.10.1 describes how to form this value for identities
established via the NewSessionTicket message. For identities established via the NewSessionTicket message. For identities
established externally an obfuscated_ticket_age of 0 SHOULD be established externally an obfuscated_ticket_age of 0 SHOULD be
used, and servers MUST ignore the value. used, and servers MUST ignore the value.
identities A list of the identities that the client is willing to identities A list of the identities that the client is willing to
negotiate with the server. If sent alongside the "early_data" negotiate with the server. If sent alongside the "early_data"
skipping to change at page 55, line 13 skipping to change at page 56, line 13
for 0-RTT data. for 0-RTT data.
binders A series of HMAC values, one for each PSK offered in the binders A series of HMAC values, one for each PSK offered in the
"pre_shared_keys" extension and in the same order, computed as "pre_shared_keys" extension and in the same order, computed as
described below. described below.
selected_identity The server's chosen identity expressed as a selected_identity The server's chosen identity expressed as a
(0-based) index into the identities in the client's list. (0-based) index into the identities in the client's list.
Each PSK is associated with a single Hash algorithm. For PSKs Each PSK is associated with a single Hash algorithm. For PSKs
established via the ticket mechanism (Section 4.6.1), this is the established via the ticket mechanism (Section 4.6.1), this is the KDF
Hash used for the KDF on the connection where the ticket was Hash algorithm on the connection where the ticket was established.
established. For externally established PSKs, the Hash algorithm For externally established PSKs, the Hash algorithm MUST be set when
MUST be set when the PSK is established, or default to SHA-256 if no the PSK is established, or default to SHA-256 if no such algorithm is
such algorithm is defined. The server must ensure that it selects a defined. The server must ensure that it selects a compatible PSK (if
compatible PSK (if any) and cipher suite. any) and cipher suite.
Implementor's note: the most straightforward way to implement the Implementor's note: the most straightforward way to implement the
PSK/cipher suite matching requirements is to negotiate the cipher PSK/cipher suite matching requirements is to negotiate the cipher
suite first and then exclude any incompatible PSKs. Any unknown PSKs suite first and then exclude any incompatible PSKs. Any unknown PSKs
(e.g., they are not in the PSK database or are encrypted with an (e.g., they are not in the PSK database or are encrypted with an
unknown key) SHOULD simply be ignored. If no acceptable PSKs are unknown key) SHOULD simply be ignored. If no acceptable PSKs are
found, the server SHOULD perform a non-PSK handshake if possible. found, the server SHOULD perform a non-PSK handshake if possible.
Prior to accepting PSK key establishment, the server MUST validate Prior to accepting PSK key establishment, the server MUST validate
the corresponding binder value (see Section 4.2.10.2 below). If this the corresponding binder value (see Section 4.2.10.2 below). If this
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The full ClientHello1 is included in all other handshake hash The full ClientHello1 is included in all other handshake hash
computations. Note that in the first flight, ClientHello1[truncated] computations. Note that in the first flight, ClientHello1[truncated]
is hashed directly, but in the second flight, ClientHello1 is hashed is hashed directly, but in the second flight, ClientHello1 is hashed
and then reinjected as a "handshake_hash" message, as described in and then reinjected as a "handshake_hash" message, as described in
Section 4.4.1. Section 4.4.1.
4.2.10.3. Processing Order 4.2.10.3. Processing Order
Clients are permitted to "stream" 0-RTT data until they receive the Clients are permitted to "stream" 0-RTT data until they receive the
server's Finished, only then sending the EndOfEarlyData message. In server's Finished, only then sending the EndOfEarlyData message,
order to avoid deadlocks, when accepting "early_data", servers MUST followed by the rest of the handshake. In order to avoid deadlocks,
process the client's ClientHello and then immediately send the when accepting "early_data", servers MUST process the client's
ServerHello, rather than waiting for the client's EndOfEarlyData ClientHello and then immediately send the ServerHello, rather than
message. waiting for the client's EndOfEarlyData message.
4.2.10.4. Replay Properties
As noted in Section 2.3, TLS provides a limited mechanism for replay
protection for data sent by the client in the first flight. This
mechanism is intended to ensure that attackers cannot replay
ClientHello messages at a time substantially after the original
ClientHello was sent.
To properly validate the ticket age, a server needs to store the
following values, either locally or by encoding them in the ticket:
- The time that the server generated the session ticket.
- The estimated round trip time between the client and server; this
can be estimated by measuring the time between sending the
Finished message and receiving the first message in the client's
second flight, or potentially using information from the operating
system.
- The "ticket_age_add" parameter from the NewSessionTicket message
in which the ticket was established.
The server can determine the client's view of the age of the ticket
by subtracting the ticket's "ticket_age_add value" from the
"obfuscated_ticket_age" parameter in the client's "pre_shared_key"
extension. The server can independently determine its view of the
age of the ticket by subtracting the the time the ticket was issued
from the current time. If the client and server clocks were running
at the same rate, the client's view of would be shorter than the
actual time elapsed on the server by a single round trip time. This
difference is comprised of the delay in sending the NewSessionTicket
message to the client, plus the time taken to send the ClientHello to
the server.
The mismatch between the client's and server's views of age is thus
given by:
mismatch = (client's view + RTT estimate) - (server's view)
There are several potential sources of error that make an exact
measurement of time difficult. Variations in client and server clock
rates are likely to be minimal, though potentially with gross time
corrections. Network propagation delays are the most likely causes
of a mismatch in legitimate values for elapsed time. Both the
NewSessionTicket and ClientHello messages might be retransmitted and
therefore delayed, which might be hidden by TCP. For browser clients
on the Internet, this implies that an allowance on the order of ten
seconds to account for errors in clocks and variations in
measurements is advisable; other deployment scenarios may have
different needs. Outside the selected range, the server SHOULD
reject early data and fall back to a full 1-RTT handshake. Clock
skew distributions are not symmetric, so the optimal tradeoff may
involve an asymmetric range of permissible mismatch values.
4.3. Server Parameters 4.3. Server Parameters
The next two messages from the server, EncryptedExtensions and The next two messages from the server, EncryptedExtensions and
CertificateRequest, contain information from the server that CertificateRequest, contain information from the server that
determines the rest of the handshake. These messages are encrypted determines the rest of the handshake. These messages are encrypted
with keys derived from the server_handshake_traffic_secret. with keys derived from the server_handshake_traffic_secret.
4.3.1. Encrypted Extensions 4.3.1. Encrypted Extensions
skipping to change at page 60, line 5 skipping to change at page 59, line 48
server would accept. In TLS 1.3 the former is expressed by sending server would accept. In TLS 1.3 the former is expressed by sending
the "signature_algorithms" extension. The latter is expressed by the "signature_algorithms" extension. The latter is expressed by
sending the "certificate_authorities" extension (see Section 4.2.4). sending the "certificate_authorities" extension (see Section 4.2.4).
Servers which are authenticating with a PSK MUST NOT send the Servers which are authenticating with a PSK MUST NOT send the
CertificateRequest message in the main handshake, though they MAY CertificateRequest message in the main handshake, though they MAY
send it in post-handshake authentication (see Section 4.6.2) provided send it in post-handshake authentication (see Section 4.6.2) provided
that the client has sent the "post_handshake_auth" extension (see that the client has sent the "post_handshake_auth" extension (see
Section 4.2.5). Section 4.2.5).
4.3.2.1. OID Filters
The "oid_filters" extension allows servers to provide a set of OID/
value pairs which it would like the client's certificate to match.
This extension MUST only be sent in the CertificateRequest message.
struct {
opaque certificate_extension_oid<1..2^8-1>;
opaque certificate_extension_values<0..2^16-1>;
} OIDFilter;
struct {
OIDFilter filters<0..2^16-1>;
} OIDFilterExtension;
filters A list of certificate extension OIDs [RFC5280] with their
allowed values, represented in DER-encoded [X690] format. Some
certificate extension OIDs allow multiple values (e.g., Extended
Key Usage). If the server has included a non-empty
certificate_extensions list, the client certificate included in
the response MUST contain all of the specified extension OIDs that
the client recognizes. For each extension OID recognized by the
client, all of the specified values MUST be present in the client
certificate (but the certificate MAY have other values as well).
However, the client MUST ignore and skip any unrecognized
certificate extension OIDs. If the client ignored some of the
required certificate extension OIDs and supplied a certificate
that does not satisfy the request, the server MAY at its
discretion either continue the connection without client
authentication, or abort the handshake with an
"unsupported_certificate" alert. PKIX RFCs define a variety of
certificate extension OIDs and their corresponding value types.
Depending on the type, matching certificate extension values are
not necessarily bitwise-equal. It is expected that TLS
implementations will rely on their PKI libraries to perform
certificate selection using certificate extension OIDs. This
document defines matching rules for two standard certificate
extensions defined in [RFC5280]:
o The Key Usage extension in a certificate matches the request
when all key usage bits asserted in the request are also
asserted in the Key Usage certificate extension.
o The Extended Key Usage extension in a certificate matches the
request when all key purpose OIDs present in the request are
also found in the Extended Key Usage certificate extension.
The special anyExtendedKeyUsage OID MUST NOT be used in the
request.
Separate specifications may define matching rules for other
certificate extensions.
4.4. Authentication Messages 4.4. Authentication Messages
As discussed in Section 2, TLS generally uses a common set of As discussed in Section 2, TLS generally uses a common set of
messages for authentication, key confirmation, and handshake messages for authentication, key confirmation, and handshake
integrity: Certificate, CertificateVerify, and Finished. (The integrity: Certificate, CertificateVerify, and Finished. (The
PreSharedKey binders also perform key confirmation, in a similar PreSharedKey binders also perform key confirmation, in a similar
fashion.) These three messages are always sent as the last messages fashion.) These three messages are always sent as the last messages
in their handshake flight. The Certificate and CertificateVerify in their handshake flight. The Certificate and CertificateVerify
messages are only sent under certain circumstances, as defined below. messages are only sent under certain circumstances, as defined below.
The Finished message is always sent as part of the Authentication The Finished message is always sent as part of the Authentication
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- A Handshake Context consisting of the set of messages to be - A Handshake Context consisting of the set of messages to be
included in the transcript hash. included in the transcript hash.
- A base key to be used to compute a MAC key. - A base key to be used to compute a MAC key.
Based on these inputs, the messages then contain: Based on these inputs, the messages then contain:
Certificate The certificate to be used for authentication, and any Certificate The certificate to be used for authentication, and any
supporting certificates in the chain. Note that certificate-based supporting certificates in the chain. Note that certificate-based
client authentication is not available in the 0-RTT case. client authentication is not available in 0-RTT mode.
CertificateVerify A signature over the value Transcript- CertificateVerify A signature over the value Transcript-
Hash(Handshake Context, Certificate) Hash(Handshake Context, Certificate)
Finished A MAC over the value Transcript-Hash(Handshake Context, Finished A MAC over the value Transcript-Hash(Handshake Context,
Certificate, CertificateVerify) using a MAC key derived from the Certificate, CertificateVerify) using a MAC key derived from the
base key. base key.
The following table defines the Handshake Context and MAC Base Key The following table defines the Handshake Context and MAC Base Key
for each scenario: for each scenario:
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including record layer headers. I.e., including record layer headers. I.e.,
Transcript-Hash(M1, M2, ... MN) = Hash(M1 || M2 ... MN) Transcript-Hash(M1, M2, ... MN) = Hash(M1 || M2 ... MN)
As an exception to this general rule, when the server responds to a As an exception to this general rule, when the server responds to a
ClientHello with a HelloRetryRequest, the value of ClientHello1 is ClientHello with a HelloRetryRequest, the value of ClientHello1 is
replaced with a special synthetic handshake message of handshake type replaced with a special synthetic handshake message of handshake type
"message_hash" containing Hash(ClientHello1). I.e., "message_hash" containing Hash(ClientHello1). I.e.,
Transcript-Hash(ClientHello1, HelloRetryRequest, ... MN) = Transcript-Hash(ClientHello1, HelloRetryRequest, ... MN) =
Hash(message_hash || // Handshake Type Hash(message_hash || // Handshake type
00 00 Hash.length || // Handshake message length 00 00 Hash.length || // Handshake message length
Hash(ClientHello1) || // Hash of ClientHello1 Hash(ClientHello1) || // Hash of ClientHello1
HelloRetryRequest ... MN) HelloRetryRequest ... MN)
The reason for this construction is to allow the server to do a The reason for this construction is to allow the server to do a
stateless HelloRetryRequest by storing just the hash of ClientHello1 stateless HelloRetryRequest by storing just the hash of ClientHello1
in the cookie, rather than requiring it to export the entire in the cookie, rather than requiring it to export the entire
intermediate hash state (see Section 4.2.2). intermediate hash state (see Section 4.2.2).
For concreteness, the transcript hash is always taken from the For concreteness, the transcript hash is always taken from the
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field having length 0). field having length 0).
Structure of this message: Structure of this message:
struct { struct {
select(certificate_type){ select(certificate_type){
case RawPublicKey: case RawPublicKey:
// From RFC 7250 ASN.1_subjectPublicKeyInfo // From RFC 7250 ASN.1_subjectPublicKeyInfo
opaque ASN1_subjectPublicKeyInfo<1..2^24-1>; opaque ASN1_subjectPublicKeyInfo<1..2^24-1>;
case X.509: case X509:
opaque cert_data<1..2^24-1>; opaque cert_data<1..2^24-1>;
} };
Extension extensions<0..2^16-1>; Extension extensions<0..2^16-1>;
} CertificateEntry; } CertificateEntry;
struct { struct {
opaque certificate_request_context<0..2^8-1>; opaque certificate_request_context<0..2^8-1>;
CertificateEntry certificate_list<0..2^24-1>; CertificateEntry certificate_list<0..2^24-1>;
} Certificate; } Certificate;
certificate_request_context If this message is in response to a certificate_request_context If this message is in response to a
CertificateRequest, the value of certificate_request_context in CertificateRequest, the value of certificate_request_context in
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4.4.2.2. Server Certificate Selection 4.4.2.2. Server Certificate Selection
The following rules apply to the certificates sent by the server: The following rules apply to the certificates sent by the server:
- The certificate type MUST be X.509v3 [RFC5280], unless explicitly - The certificate type MUST be X.509v3 [RFC5280], unless explicitly
negotiated otherwise (e.g., [RFC7250]). negotiated otherwise (e.g., [RFC7250]).
- The server's end-entity certificate's public key (and associated - The server's end-entity certificate's public key (and associated
restrictions) MUST be compatible with the selected authentication restrictions) MUST be compatible with the selected authentication
algorithm (currently RSA or ECDSA). algorithm (currently RSA, ECDSA, or EdDSA).
- The certificate MUST allow the key to be used for signing (i.e., - The certificate MUST allow the key to be used for signing (i.e.,
the digitalSignature bit MUST be set if the Key Usage extension is the digitalSignature bit MUST be set if the Key Usage extension is
present) with a signature scheme indicated in the client's present) with a signature scheme indicated in the client's
"signature_algorithms" extension. "signature_algorithms" extension.
- The "server_name" and "trusted_ca_keys" extensions [RFC6066] are - The "server_name" and "certificate_authorities" extensions
used to guide certificate selection. As servers MAY require the [RFC6066] are used to guide certificate selection. As servers MAY
presence of the "server_name" extension, clients SHOULD send this require the presence of the "server_name" extension, clients
extension, when applicable. SHOULD send this extension, when applicable.
All certificates provided by the server MUST be signed by a signature All certificates provided by the server MUST be signed by a signature
algorithm that appears in the "signature_algorithms" extension algorithm that appears in the "signature_algorithms" extension
provided by the client, if they are able to provide such a chain (see provided by the client, if they are able to provide such a chain (see
Section 4.2.3). Certificates that are self-signed or certificates Section 4.2.3). Certificates that are self-signed or certificates
that are expected to be trust anchors are not validated as part of that are expected to be trust anchors are not validated as part of
the chain and therefore MAY be signed with any algorithm. the chain and therefore MAY be signed with any algorithm.
If the server cannot produce a certificate chain that is signed only If the server cannot produce a certificate chain that is signed only
via the indicated supported algorithms, then it SHOULD continue the via the indicated supported algorithms, then it SHOULD continue the
handshake by sending the client a certificate chain of its choice handshake by sending the client a certificate chain of its choice
that may include algorithms that are not known to be supported by the that may include algorithms that are not known to be supported by the
client. This fallback chain MAY use the deprecated SHA-1 hash client. This fallback chain SHOULD NOT use the deprecated SHA-1 hash
algorithm only if the "signature_algorithms" extension provided by algorithm in general, but MAY do so if the "signature_algorithms"
the client permits it. If the client cannot construct an acceptable extension provided by the client permits it, and MUST NOT do so
chain using the provided certificates and decides to abort the otherwise.
handshake, then it MUST abort the handshake with an
"unsupported_certificate" alert. If the client cannot construct an acceptable chain using the provided
certificates and decides to abort the handshake, then it MUST abort
the handshake with an appropriate certificate-related alert (by
default, "unsupported_certificate"; see Section 6.2 for more).
If the server has multiple certificates, it chooses one of them based If the server has multiple certificates, it chooses one of them based
on the above-mentioned criteria (in addition to other criteria, such on the above-mentioned criteria (in addition to other criteria, such
as transport layer endpoint, local configuration and preferences). as transport layer endpoint, local configuration and preferences).
4.4.2.3. Client Certificate Selection 4.4.2.3. Client Certificate Selection
The following rules apply to certificates sent by the client: The following rules apply to certificates sent by the client:
- The certificate type MUST be X.509v3 [RFC5280], unless explicitly - The certificate type MUST be X.509v3 [RFC5280], unless explicitly
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- If the "certificate_authorities" extension in the - If the "certificate_authorities" extension in the
CertificateRequest message was present, at least one of the CertificateRequest message was present, at least one of the
certificates in the certificate chain SHOULD be issued by one of certificates in the certificate chain SHOULD be issued by one of
the listed CAs. the listed CAs.
- The certificates MUST be signed using an acceptable signature - The certificates MUST be signed using an acceptable signature
algorithm, as described in Section 4.3.2. Note that this relaxes algorithm, as described in Section 4.3.2. Note that this relaxes
the constraints on certificate-signing algorithms found in prior the constraints on certificate-signing algorithms found in prior
versions of TLS. versions of TLS.
- If the certificate_extensions list in the CertificateRequest - If the CertificateRequest message contained a non-empty
message was non-empty, the end-entity certificate MUST match the "oid_filters" extension, the end-entity certificate MUST match the
extension OIDs recognized by the client, as described in extension OIDs recognized by the client, as described in
Section 4.3.2. Section 4.2.4.1.
Note that, as with the server certificate, there are certificates Note that, as with the server certificate, there are certificates
that use algorithm combinations that cannot be currently used with that use algorithm combinations that cannot be currently used with
TLS. TLS.
4.4.2.4. Receiving a Certificate Message 4.4.2.4. Receiving a Certificate Message
In general, detailed certificate validation procedures are out of In general, detailed certificate validation procedures are out of
scope for TLS (see [RFC5280]). This section provides TLS-specific scope for TLS (see [RFC5280]). This section provides TLS-specific
requirements. requirements.
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abort the handshake with a "decode_error" alert. abort the handshake with a "decode_error" alert.
If the client does not send any certificates, the server MAY at its If the client does not send any certificates, the server MAY at its
discretion either continue the handshake without client discretion either continue the handshake without client
authentication, or abort the handshake with a "certificate_required" authentication, or abort the handshake with a "certificate_required"
alert. Also, if some aspect of the certificate chain was alert. Also, if some aspect of the certificate chain was
unacceptable (e.g., it was not signed by a known, trusted CA), the unacceptable (e.g., it was not signed by a known, trusted CA), the
server MAY at its discretion either continue the handshake server MAY at its discretion either continue the handshake
(considering the client unauthenticated) or abort the handshake. (considering the client unauthenticated) or abort the handshake.
Any endpoint receiving any certificate signed using any signature Any endpoint receiving any certificate which it would need to
algorithm using an MD5 hash MUST abort the handshake with a validate using any signature algorithm using an MD5 hash MUST abort
"bad_certificate" alert. SHA-1 is deprecated and it is RECOMMENDED the handshake with a "bad_certificate" alert. SHA-1 is deprecated
that any endpoint receiving any certificate signed using any and it is RECOMMENDED that any endpoint receiving any certificate
signature algorithm using a SHA-1 hash abort the handshake with a which it would need to validate using any signature algorithm using a
"bad_certificate" alert. All endpoints are RECOMMENDED to transition SHA-1 hash abort the handshake with a "bad_certificate" alert. For
to SHA-256 or better as soon as possible to maintain interoperability clarity, this means that endpoints MAY accept these algorithms for
with implementations currently in the process of phasing out SHA-1 certificates that are self-signed or are trust anchors.
support.
All endpoints are RECOMMENDED to transition to SHA-256 or better as
soon as possible to maintain interoperability with implementations
currently in the process of phasing out SHA-1 support.
Note that a certificate containing a key for one signature algorithm Note that a certificate containing a key for one signature algorithm
MAY be signed using a different signature algorithm (for instance, an MAY be signed using a different signature algorithm (for instance, an
RSA key signed with an ECDSA key). RSA key signed with an ECDSA key).
4.4.3. Certificate Verify 4.4.3. Certificate Verify
This message is used to provide explicit proof that an endpoint This message is used to provide explicit proof that an endpoint
possesses the private key corresponding to its certificate and also possesses the private key corresponding to its certificate. The
provides integrity for the handshake up to this point. Servers MUST CertificateVerify message also provides integrity for the handshake
send this message when authenticating via a certificate. Clients up to this point. Servers MUST send this message when authenticating
MUST send this message whenever authenticating via a certificate via a certificate. Clients MUST send this message whenever
(i.e., when the Certificate message is non-empty). When sent, this authenticating via a certificate (i.e., when the Certificate message
message MUST appear immediately after the Certificate message and is non-empty). When sent, this message MUST appear immediately after
immediately prior to the Finished message. the Certificate message and immediately prior to the Finished
message.
Structure of this message: Structure of this message:
struct { struct {
SignatureScheme algorithm; SignatureScheme algorithm;
opaque signature<0..2^16-1>; opaque signature<0..2^16-1>;
} CertificateVerify; } CertificateVerify;
The algorithm field specifies the signature algorithm used (see The algorithm field specifies the signature algorithm used (see
Section 4.2.3 for the definition of this field). The signature is a Section 4.2.3 for the definition of this field). The signature is a
skipping to change at page 69, line 18 skipping to change at page 68, line 23
unsupported algorithms (see Section 4.2.3). unsupported algorithms (see Section 4.2.3).
If sent by a client, the signature algorithm used in the signature If sent by a client, the signature algorithm used in the signature
MUST be one of those present in the supported_signature_algorithms MUST be one of those present in the supported_signature_algorithms
field of the "signature_algorithms" extension in the field of the "signature_algorithms" extension in the
CertificateRequest message. CertificateRequest message.
In addition, the signature algorithm MUST be compatible with the key In addition, the signature algorithm MUST be compatible with the key
in the sender's end-entity certificate. RSA signatures MUST use an in the sender's end-entity certificate. RSA signatures MUST use an
RSASSA-PSS algorithm, regardless of whether RSASSA-PKCS1-v1_5 RSASSA-PSS algorithm, regardless of whether RSASSA-PKCS1-v1_5
algorithms appear in "signature_algorithms". SHA-1 MUST NOT be used algorithms appear in "signature_algorithms". The SHA-1 algorithm
in any signatures in CertificateVerify. All SHA-1 signature MUST NOT be used in any signatures of CertificateVerify messages.
algorithms in this specification are defined solely for use in legacy All SHA-1 signature algorithms in this specification are defined
certificates, and are not valid for CertificateVerify signatures. solely for use in legacy certificates and are not valid for
CertificateVerify signatures.
The receiver of a CertificateVerify message MUST verify the signature The receiver of a CertificateVerify message MUST verify the signature
field. The verification process takes as input: field. The verification process takes as input:
- The content covered by the digital signature - The content covered by the digital signature
- The public key contained in the end-entity certificate found in - The public key contained in the end-entity certificate found in
the associated Certificate message. the associated Certificate message.
- The digital signature received in the signature field of the - The digital signature received in the signature field of the
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The Finished message is the final message in the authentication The Finished message is the final message in the authentication
block. It is essential for providing authentication of the handshake block. It is essential for providing authentication of the handshake
and of the computed keys. and of the computed keys.
Recipients of Finished messages MUST verify that the contents are Recipients of Finished messages MUST verify that the contents are
correct and if incorrect MUST terminate the connection with a correct and if incorrect MUST terminate the connection with a
"decrypt_error" alert. "decrypt_error" alert.
Once a side has sent its Finished message and received and validated 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 the Finished message from its peer, it may begin to send and receive
application data over the connection. Early data may be sent prior application data over the connection. There are two settings in
to the receipt of the peer's Finished message, per Section 4.2.9. which it is permitted to send data prior to receiving the peer's
Finished:
1. Clients ending 0-RTT data as described in Section 4.2.9.
2. Servers MAY send data after sending their first flight, but
because the handshake is not yet complete, they have no assurance
of either the peer's identity or of its liveness (i.e., the
ClientHello might have been replayed).
The key used to compute the finished message is computed from the The key used to compute the finished message is computed from the
Base key defined in Section 4.4 using HKDF (see Section 7.1). Base key defined in Section 4.4 using HKDF (see Section 7.1).
Specifically: Specifically:
finished_key = finished_key =
HKDF-Expand-Label(BaseKey, "finished", "", Hash.length) HKDF-Expand-Label(BaseKey, "finished", "", Hash.length)
Structure of this message: Structure of this message:
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The verify_data value is computed as follows: The verify_data value is computed as follows:
verify_data = verify_data =
HMAC(finished_key, HMAC(finished_key,
Transcript-Hash(Handshake Context, Transcript-Hash(Handshake Context,
Certificate*, CertificateVerify*)) Certificate*, CertificateVerify*))
* Only included if present. * Only included if present.
Where HMAC [RFC2104] uses the Hash algorithm for the handshake. As HMAC [RFC2104] uses the Hash algorithm for the handshake. As noted
noted above, the HMAC input can generally be implemented by a running above, the HMAC input can generally be implemented by a running hash,
hash, i.e., just the handshake hash at this point. i.e., just the handshake hash at this point.
In previous versions of TLS, the verify_data was always 12 octets In previous versions of TLS, the verify_data was always 12 octets
long. In TLS 1.3, it is the size of the HMAC output for the Hash long. In TLS 1.3, it is the size of the HMAC output for the Hash
used for the handshake. used for the handshake.
Note: Alerts and any other record types are not handshake messages Note: Alerts and any other record types are not handshake messages
and are not included in the hash computations. and are not included in the hash computations.
Any records following a 1-RTT Finished message MUST be encrypted Any records following a 1-RTT Finished message MUST be encrypted
under the appropriate application traffic key as described in under the appropriate application traffic key as described in
Section 7.2. In particular, this includes any alerts sent by the Section 7.2. In particular, this includes any alerts sent by the
server in response to client Certificate and CertificateVerify server in response to client Certificate and CertificateVerify
messages. messages.
4.5. End of Early Data 4.5. End of Early Data
struct {} EndOfEarlyData; struct {} EndOfEarlyData;
If the server sent an "early_data" extension, the client MUST send an If the server sent an "early_data" extension, the client MUST send an
EndOfEarlyData after receiving the server Finished. This indicates EndOfEarlyData message after receiving the server Finished. If the
that all 0-RTT application_data messages, if any, have been server does not send an "early_data" extension, then the client MUST
transmitted and that the following records are protected under NOT send an EndOfEarlyData message. This message indicates that all
handshake traffic keys. Servers MUST NOT send this message and 0-RTT application_data messages, if any, have been transmitted and
clients receiving it MUST terminate the connection with an that the following records are protected under handshake traffic
"unexpected_message" alert. This message is encrypted under keys keys. Servers MUST NOT send this message and clients receiving it
derived from the client_early_traffic_secret. MUST terminate the connection with an "unexpected_message" alert.
This message is encrypted under keys derived from the
client_early_traffic_secret.
4.6. Post-Handshake Messages 4.6. Post-Handshake Messages
TLS also allows other messages to be sent after the main handshake. TLS also allows other messages to be sent after the main handshake.
These messages use a handshake content type and are encrypted under These messages use a handshake content type and are encrypted under
the appropriate application traffic key. the appropriate application traffic key.
4.6.1. New Session Ticket Message 4.6.1. New Session Ticket Message
At any time after the server has received the client Finished At any time after the server has received the client Finished
message, it MAY send a NewSessionTicket message. This message message, it MAY send a NewSessionTicket message. This message
creates a pre-shared key (PSK) binding between the ticket value and creates a unique association between the ticket value and a secret
the resumption master secret. PSK derived from the resumption master secret.
The client MAY use this PSK for future handshakes by including the The client MAY use this PSK for future handshakes by including the
ticket value in the "pre_shared_key" extension in its ClientHello ticket value in the "pre_shared_key" extension in its ClientHello
(Section 4.2.10). Servers MAY send multiple tickets on a single (Section 4.2.10). Servers MAY send multiple tickets on a single
connection, either immediately after each other or after specific connection, either immediately after each other or after specific
events. For instance, the server might send a new ticket after post- events. For instance, the server might send a new ticket after post-
handshake authentication in order to encapsulate the additional handshake authentication in order to encapsulate the additional
client authentication state. Clients SHOULD attempt to use each client authentication state. Clients SHOULD attempt to use each
ticket no more than once, with more recent tickets being used first. ticket no more than once, with more recent tickets being used first.
Any ticket MUST only be resumed with a cipher suite that has the same Any ticket MUST only be resumed with a cipher suite that has the same
KDF hash as that used to establish the original connection, and only KDF hash algorithm as that used to establish the original connection,
if the client provides the same SNI value as in the original and only if the client provides the same SNI value as in the original
connection, as described in Section 3 of [RFC6066]. connection, as described in Section 3 of [RFC6066].
Note: Although the resumption master secret depends on the client's Note: Although the resumption master secret depends on the client's
second flight, servers which do not request client authentication MAY second flight, servers which do not request client authentication MAY
compute the remainder of the transcript independently and then send a compute the remainder of the transcript independently and then send a
NewSessionTicket immediately upon sending its Finished rather than NewSessionTicket immediately upon sending its Finished rather than
waiting for the client Finished. This might be appropriate in cases waiting for the client Finished. This might be appropriate in cases
where the client is expected to open multiple TLS connections in where the client is expected to open multiple TLS connections in
parallel and would benefit from the reduced overhead of a resumption parallel and would benefit from the reduced overhead of a resumption
handshake, for example. handshake, for example.
struct { struct {
uint32 ticket_lifetime; uint32 ticket_lifetime;
uint32 ticket_age_add; uint32 ticket_age_add;
opaque ticket_nonce<1..255>;
opaque ticket<1..2^16-1>; opaque ticket<1..2^16-1>;
Extension extensions<0..2^16-2>; Extension extensions<0..2^16-2>;
} NewSessionTicket; } NewSessionTicket;
ticket_lifetime Indicates the lifetime in seconds as a 32-bit ticket_lifetime Indicates the lifetime in seconds as a 32-bit
unsigned integer in network byte order from the time of ticket unsigned integer in network byte order from the time of ticket
issuance. Servers MUST NOT use any value more than 604800 seconds issuance. Servers MUST NOT use any value greater than 604800
(7 days). The value of zero indicates that the ticket should be seconds (7 days). The value of zero indicates that the ticket
discarded immediately. Clients MUST NOT cache tickets for longer should be discarded immediately. Clients MUST NOT cache tickets
than 7 days, regardless of the ticket_lifetime, and MAY delete the for longer than 7 days, regardless of the ticket_lifetime, and MAY
ticket earlier based on local policy. A server MAY treat a ticket delete the ticket earlier based on local policy. A server MAY
as valid for a shorter period of time than what is stated in the treat a ticket as valid for a shorter period of time than what is
ticket_lifetime. stated in the ticket_lifetime.
ticket_age_add A securely generated, random 32-bit value that is ticket_age_add A securely generated, random 32-bit value that is
used to obscure the age of the ticket that the client includes in used to obscure the age of the ticket that the client includes in
the "pre_shared_key" extension. The client-side ticket age is the "pre_shared_key" extension. The client-side ticket age is
added to this value modulo 2^32 to obtain the value that is added to this value modulo 2^32 to obtain the value that is
transmitted by the client. The server MUST generate a fresh value transmitted by the client. The server MUST generate a fresh value
for each ticket it sends. for each ticket it sends.
ticket_nonce A unique per-ticket value.
ticket The value of the ticket to be used as the PSK identity. The ticket The value of the ticket to be used as the PSK identity. The
ticket itself is an opaque label. It MAY either be a database ticket itself is an opaque label. It MAY either be a database
lookup key or a self-encrypted and self-authenticated value. lookup key or a self-encrypted and self-authenticated value.
Section 4 of [RFC5077] describes a recommended ticket construction Section 4 of [RFC5077] describes a recommended ticket construction
mechanism. mechanism.
extensions A set of extension values for the ticket. The extensions A set of extension values for the ticket. The
"Extension" format is defined in Section 4.2. Clients MUST ignore "Extension" format is defined in Section 4.2. Clients MUST ignore
unrecognized extensions. unrecognized extensions.
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is allowed to send when using this ticket, in bytes. Only is allowed to send when using this ticket, in bytes. Only
Application Data payload (i.e., plaintext but not padding or the Application Data payload (i.e., plaintext but not padding or the
inner content type byte) is counted. A server receiving more than inner content type byte) is counted. A server receiving more than
max_early_data_size bytes of 0-RTT data SHOULD terminate the max_early_data_size bytes of 0-RTT data SHOULD terminate the
connection with an "unexpected_message" alert. Note that servers connection with an "unexpected_message" alert. Note that servers
that reject early data due to lack of cryptographic material will that reject early data due to lack of cryptographic material will
be unable to differentiate padding from content, so clients SHOULD be unable to differentiate padding from content, so clients SHOULD
NOT depend on being able to send large quantities of padding in NOT depend on being able to send large quantities of padding in
early data records. early data records.
The PSK associated with the ticket is computed as:
HKDF-Expand-Label(resumption_master_secret,
"resumption", ticket_nonce, Hash.length)
Because the ticket_nonce value is distinct for each NewSessionTicket
message, a different PSK will be derived for each ticket.
Note that in principle it is possible to continue issuing new tickets Note that in principle it is possible to continue issuing new tickets
which indefinitely extend the lifetime of the keying material which indefinitely extend the lifetime of the keying material
originally derived from an initial non-PSK handshake (which was most originally derived from an initial non-PSK handshake (which was most
likely tied to the peer's certificate). It is RECOMMENDED that likely tied to the peer's certificate). It is RECOMMENDED that
implementations place limits on the total lifetime of such keying implementations place limits on the total lifetime of such keying
material; these limits should take into account the lifetime of the material; these limits should take into account the lifetime of the
peer's certificate, the likelihood of intervening revocation, and the peer's certificate, the likelihood of intervening revocation, and the
time since the peer's online CertificateVerify signature. time since the peer's online CertificateVerify signature.
4.6.2. Post-Handshake Authentication 4.6.2. Post-Handshake Authentication
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Both sender and receiver MUST encrypt their KeyUpdate messages with Both sender and receiver MUST encrypt their KeyUpdate messages with
the old keys. Additionally, both sides MUST enforce that a KeyUpdate the old keys. Additionally, both sides MUST enforce that a KeyUpdate
with the old key is received before accepting any messages encrypted with the old key is received before accepting any messages encrypted
with the new key. Failure to do so may allow message truncation with the new key. Failure to do so may allow message truncation
attacks. attacks.
5. Record Protocol 5. Record Protocol
The TLS record protocol takes messages to be transmitted, fragments The TLS record protocol takes messages to be transmitted, fragments
the data into manageable blocks, protects the records, and transmits the data into manageable blocks, protects the records, and transmits
the result. Received data is verified and decrypted, reassembled, the result. Received data is verified, decrypted, reassembled, and
and then delivered to higher-level clients. then delivered to higher-level clients.
TLS records are typed, which allows multiple higher-level protocols TLS records are typed, which allows multiple higher-level protocols
to be multiplexed over the same record layer. This document to be multiplexed over the same record layer. This document
specifies three content types: handshake, application data, and specifies three content types: handshake, application data, and
alert. Implementations MUST NOT send record types not defined in alert. Implementations MUST NOT send record types not defined in
this document unless negotiated by some extension. If a TLS this document unless negotiated by some extension. If a TLS
implementation receives an unexpected record type, it MUST terminate implementation receives an unexpected record type, it MUST terminate
the connection with an "unexpected_message" alert. New record the connection with an "unexpected_message" alert. New record
content type values are assigned by IANA in the TLS Content Type content type values are assigned by IANA in the TLS Content Type
Registry as described in Section 10. Registry as described in Section 11.
5.1. Record Layer 5.1. Record Layer
The record layer fragments information blocks into TLSPlaintext The record layer fragments information blocks into TLSPlaintext
records carrying data in chunks of 2^14 bytes or less. Message records carrying data in chunks of 2^14 bytes or less. Message
boundaries are handled differently depending on the underlying boundaries are handled differently depending on the underlying
ContentType. Any future content types MUST specify appropriate ContentType. Any future content types MUST specify appropriate
rules. Note that these rules are stricter than what was enforced in rules. Note that these rules are stricter than what was enforced in
TLS 1.2. TLS 1.2.
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TLSInnerPlaintext structure. TLSInnerPlaintext structure.
AEAD algorithms take as input a single key, a nonce, a plaintext, and AEAD algorithms take as input a single key, a nonce, a plaintext, and
"additional data" to be included in the authentication check, as "additional data" to be included in the authentication check, as
described in Section 2.1 of [RFC5116]. The key is either the described in Section 2.1 of [RFC5116]. The key is either the
client_write_key or the server_write_key, the nonce is derived from client_write_key or the server_write_key, the nonce is derived from
the sequence number (see Section 5.3) and the client_write_iv or the sequence number (see Section 5.3) and the client_write_iv or
server_write_iv, and the additional data input is empty (zero server_write_iv, and the additional data input is empty (zero
length). Derivation of traffic keys is defined in Section 7.3. length). Derivation of traffic keys is defined in Section 7.3.
The plaintext input to the AEAD is the the encoded TLSInnerPlaintext The plaintext input to the AEAD algorithm is the encoded
structure. TLSInnerPlaintext structure.
The AEAD output consists of the ciphertext output from the AEAD The AEAD output consists of the ciphertext output from the AEAD
encryption operation. The length of the plaintext is greater than encryption operation. The length of the plaintext is greater than
the corresponding TLSPlaintext.length due to the inclusion of the corresponding TLSPlaintext.length due to the inclusion of
TLSInnerPlaintext.type and any padding supplied by the sender. The TLSInnerPlaintext.type and any padding supplied by the sender. The
length of the AEAD output will generally be larger than the length of the AEAD output will generally be larger than the
plaintext, but by an amount that varies with the AEAD algorithm. plaintext, but by an amount that varies with the AEAD algorithm.
Since the ciphers might incorporate padding, the amount of overhead Since the ciphers might incorporate padding, the amount of overhead
could vary with different lengths of plaintext. Symbolically, could vary with different lengths of plaintext. Symbolically,
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octet for ContentType + the maximum AEAD expansion of 255 octets. octet for ContentType + the maximum AEAD expansion of 255 octets.
5.3. Per-Record Nonce 5.3. Per-Record Nonce
A 64-bit sequence number is maintained separately for reading and A 64-bit sequence number is maintained separately for reading and
writing records. Each sequence number is set to zero at the writing records. Each sequence number is set to zero at the
beginning of a connection and whenever the key is changed. beginning of a connection and whenever the key is changed.
The appropriate sequence number is incremented by one after reading The appropriate sequence number is incremented by one after reading
or writing each record. The first record transmitted under a or writing each record. The first record transmitted under a
particular set of traffic keys MUST use sequence number 0. particular traffic key MUST use sequence number 0.
Because the size of sequence numbers is 64-bit, they should not wrap. Because the size of sequence numbers is 64-bit, they should not wrap.
If a TLS implementation would need to wrap a sequence number, it MUST If a TLS implementation would need to wrap a sequence number, it MUST
either re-key (Section 4.6.3) or terminate the connection. either re-key (Section 4.6.3) or terminate the connection.
Each AEAD algorithm will specify a range of possible lengths for the Each AEAD algorithm will specify a range of possible lengths for the
per-record nonce, from N_MIN bytes to N_MAX bytes of input per-record nonce, from N_MIN bytes to N_MAX bytes of input
([RFC5116]). The length of the TLS per-record nonce (iv_length) is ([RFC5116]). The length of the TLS per-record nonce (iv_length) is
set to the larger of 8 bytes and N_MIN for the AEAD algorithm (see set to the larger of 8 bytes and N_MIN for the AEAD algorithm (see
[RFC5116] Section 4). An AEAD algorithm where N_MAX is less than 8 [RFC5116] Section 4). An AEAD algorithm where N_MAX is less than 8
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enforces all-zero padding octets, which allows for quick detection of enforces all-zero padding octets, which allows for quick detection of
padding errors. padding errors.
Implementations MUST limit their scanning to the cleartext returned Implementations MUST limit their scanning to the cleartext returned
from the AEAD decryption. If a receiving implementation does not from the AEAD decryption. If a receiving implementation does not
find a non-zero octet in the cleartext, it MUST terminate the find a non-zero octet in the cleartext, it MUST terminate the
connection with an "unexpected_message" alert. connection with an "unexpected_message" alert.
The presence of padding does not change the overall record size The presence of padding does not change the overall record size
limitations - the full encoded TLSInnerPlaintext MUST not exceed 2^14 limitations - the full encoded TLSInnerPlaintext MUST not exceed 2^14
octets. If the maximum fragment length is reduced, such as by the octets. If the maximum fragment length is reduced, as for example by
max_fragment_length extension from [RFC6066], then the reduced limit the max_fragment_length extension from [RFC6066], then the reduced
applies to the full plaintext, including the padding. limit applies to the full plaintext, including the padding.
Selecting a padding policy that suggests when and how much to pad is Selecting a padding policy that suggests when and how much to pad is
a complex topic, and is beyond the scope of this specification. If a complex topic and is beyond the scope of this specification. If
the application layer protocol atop TLS has its own padding, it may the application layer protocol on top of TLS has its own padding, it
be preferable to pad application_data TLS records within the may be preferable to pad application_data TLS records within the
application layer. Padding for encrypted handshake and alert TLS application layer. Padding for encrypted handshake and alert TLS
records must still be handled at the TLS layer, though. Later records must still be handled at the TLS layer, though. Later
documents may define padding selection algorithms, or define a documents may define padding selection algorithms or define a padding
padding policy request mechanism through TLS extensions or some other policy request mechanism through TLS extensions or some other means.
means.
5.5. Limits on Key Usage 5.5. Limits on Key Usage
There are cryptographic limits on the amount of plaintext which can There are cryptographic limits on the amount of plaintext which can
be safely encrypted under a given set of keys. [AEAD-LIMITS] be safely encrypted under a given set of keys. [AEAD-LIMITS]
provides an analysis of these limits under the assumption that the provides an analysis of these limits under the assumption that the
underlying primitive (AES or ChaCha20) has no weaknesses. underlying primitive (AES or ChaCha20) has no weaknesses.
Implementations SHOULD do a key update as described in Section 4.6.3 Implementations SHOULD do a key update as described in Section 4.6.3
prior to reaching these limits. prior to reaching these limits.
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alert of its own and close down the connection immediately, alert of its own and close down the connection immediately,
discarding any pending writes. The initiator of the close need not discarding any pending writes. The initiator of the close need not
wait for the responding "close_notify" alert before closing the read wait for the responding "close_notify" alert before closing the read
side of the connection. side of the connection.
If the application protocol using TLS provides that any data may be If the application protocol using TLS provides that any data may be
carried over the underlying transport after the TLS connection is carried over the underlying transport after the TLS connection is
closed, the TLS implementation MUST receive the responding closed, the TLS implementation MUST receive the responding
"close_notify" alert before indicating to the application layer that "close_notify" alert before indicating to the application layer that
the TLS connection has ended. If the application protocol will not the TLS connection has ended. If the application protocol will not
transfer any additional data, but will only close the underlying transfer any additional data but will only close the underlying
transport connection, then the implementation MAY choose to close the transport connection, then the implementation MAY choose to close the
transport without waiting for the responding "close_notify". No part transport without waiting for the responding "close_notify". No part
of this standard should be taken to dictate the manner in which a of this standard should be taken to dictate the manner in which a
usage profile for TLS manages its data transport, including when usage profile for TLS manages its data transport, including when
connections are opened or closed. connections are opened or closed.
Note: It is assumed that closing a connection reliably delivers Note: It is assumed that closing a connection reliably delivers
pending data before destroying the transport. pending data before destroying the transport.
6.2. Error Alerts 6.2. Error Alerts
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Whenever an implementation encounters a fatal error condition, it Whenever an implementation encounters a fatal error condition, it
SHOULD send an appropriate fatal alert and MUST close the connection SHOULD send an appropriate fatal alert and MUST close the connection
without sending or receiving any additional data. In the rest of without sending or receiving any additional data. In the rest of
this specification, when the phrases "terminate the connection" and this specification, when the phrases "terminate the connection" and
"abort the handshake" are used without a specific alert it means that "abort the handshake" are used without a specific alert it means that
the implementation SHOULD send the alert indicated by the the implementation SHOULD send the alert indicated by the
descriptions below. The phrases "terminate the connection with a X descriptions below. The phrases "terminate the connection with a X
alert" and "abort the handshake with a X alert" mean that the alert" and "abort the handshake with a X alert" mean that the
implementation MUST send alert X if it sends any alert. All alerts implementation MUST send alert X if it sends any alert. All alerts
defined in this section below, as well as all unknown alerts, are defined in this section below, as well as all unknown alerts, are
universally considered fatal as of TLS 1.3 (see Section 6). universally considered fatal as of TLS 1.3 (see Section 6). The
implementation SHOULD provide a way to facilitate logging the sending
The following error alerts are defined: and receiving of alerts.
unexpected_message An inappropriate message (e.g., the wrong unexpected_message An inappropriate message (e.g., the wrong
handshake message, premature application data, etc.) was received. handshake message, premature application data, etc.) was received.
This alert should never be observed in communication between This alert should never be observed in communication between
proper implementations. proper implementations.
bad_record_mac This alert is returned if a record is received which bad_record_mac This alert is returned if a record is received which
cannot be deprotected. Because AEAD algorithms combine decryption cannot be deprotected. Because AEAD algorithms combine decryption
and verification, and also to avoid side channel attacks, this and verification, and also to avoid side channel attacks, this
alert is used for all deprotection failures. This alert should alert is used for all deprotection failures. This alert should
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desired but no acceptable PSK identity is provided by the client. desired but no acceptable PSK identity is provided by the client.
Sending this alert is OPTIONAL; servers MAY instead choose to send Sending this alert is OPTIONAL; servers MAY instead choose to send
a "decrypt_error" alert to merely indicate an invalid PSK a "decrypt_error" alert to merely indicate an invalid PSK
identity. identity.
certificate_required Sent by servers when a client certificate is certificate_required Sent by servers when a client certificate is
desired but none was provided by the client. desired but none was provided by the client.
no_application_protocol Sent by servers when a client no_application_protocol Sent by servers when a client
"application_layer_protocol_negotiation" extension advertises "application_layer_protocol_negotiation" extension advertises
protocols that the server does not support. protocols that the server does not support (see [RFC7301]).
New Alert values are assigned by IANA as described in Section 10. New Alert values are assigned by IANA as described in Section 11.
7. Cryptographic Computations 7. Cryptographic Computations
The TLS handshake establishes one or more input secrets which are The TLS handshake establishes one or more input secrets which are
combined to create the actual working keying material, as detailed combined to create the actual working keying material, as detailed
below. The key derivation process incorporates both the input below. The key derivation process incorporates both the input
secrets and the handshake transcript. Note that because the secrets and the handshake transcript. Note that because the
handshake transcript includes the random values in the Hello handshake transcript includes the random values from the Hello
messages, any given handshake will have different traffic secrets, messages, any given handshake will have different traffic secrets,
even if the same input secrets are used, as is the case when the same even if the same input secrets are used, as is the case when the same
PSK is used for multiple connections PSK is used for multiple connections
7.1. Key Schedule 7.1. Key Schedule
The key derivation process makes use of the HKDF-Extract and HKDF- The key derivation process makes use of the HKDF-Extract and HKDF-
Expand functions as defined for HKDF [RFC5869], as well as the Expand functions as defined for HKDF [RFC5869], as well as the
functions defined below: functions defined below:
skipping to change at page 87, line 20 skipping to change at page 86, line 29
struct { struct {
uint16 length = Length; uint16 length = Length;
opaque label<7..255> = "tls13 " + Label; opaque label<7..255> = "tls13 " + Label;
opaque hash_value<0..255> = HashValue; opaque hash_value<0..255> = HashValue;
} HkdfLabel; } HkdfLabel;
Derive-Secret(Secret, Label, Messages) = Derive-Secret(Secret, Label, Messages) =
HKDF-Expand-Label(Secret, Label, HKDF-Expand-Label(Secret, Label,
Transcript-Hash(Messages), Hash.length) Transcript-Hash(Messages), Hash.length)
The Hash function and the HKDF hash are the cipher suite hash The Hash function used by Transcript-Hash and HKDF is the cipher
algorithm. Hash.length is its output length in bytes. Messages are suite hash algorithm. Hash.length is its output length in bytes.
the concatenation of the indicated handshake messages, including the Messages are the concatenation of the indicated handshake messages,
handshake message type and length fields, but not including record including the handshake message type and length fields, but not
layer headers. Note that in some cases a zero-length HashValue including record layer headers. Note that in some cases a zero-
(indicated by "") is passed to HKDF-Expand-Label. length HashValue (indicated by "") is passed to HKDF-Expand-Label.
Note: with common hash functions, any label longer than 12 characters Note: with common hash functions, any label longer than 12 characters
requires an additional iteration of the hash function to compute. requires an additional iteration of the hash function to compute.
The labels in this specification have all been chosen to fit within The labels in this specification have all been chosen to fit within
this limit. this limit.
Given a set of n InputSecrets, the final "master secret" is computed Given a set of n InputSecrets, the final "master secret" is computed
by iteratively invoking HKDF-Extract with InputSecret_1, by iteratively invoking HKDF-Extract with InputSecret_1,
InputSecret_2, etc. The initial secret is simply a string of InputSecret_2, etc. The initial secret is simply a string of
Hash.length zero bytes. Concretely, for the present version of TLS Hash.length zero bytes. Concretely, for the present version of TLS
1.3, secrets are added in the following order: 1.3, secrets are added in the following order:
- PSK (a pre-shared key established externally or a - PSK (a pre-shared key established externally or derived from the
resumption_master_secret value from a previous connection) resumption_master_secret value from a previous connection)
- (EC)DHE shared secret (Section 7.4) - (EC)DHE shared secret (Section 7.4)
This produces a full key derivation schedule shown in the diagram This produces a full key derivation schedule shown in the diagram
below. In this diagram, the following formatting conventions apply: below. In this diagram, the following formatting conventions apply:
- HKDF-Extract is drawn as taking the Salt argument from the top and - HKDF-Extract is drawn as taking the Salt argument from the top and
the IKM argument from the left. the IKM argument from the left.
- Derive-Secret's Secret argument is indicated by the incoming - Derive-Secret's Secret argument is indicated by the incoming
arrow. For instance, the Early Secret is the Secret for arrow. For instance, the Early Secret is the Secret for
generating the client_early_traffic_secret. generating the client_early_traffic_secret.
skipping to change at page 89, line 32 skipping to change at page 88, line 42
label is "ext binder" for external PSKs (those provisioned outside of label is "ext binder" for external PSKs (those provisioned outside of
TLS) and "res binder" for resumption PSKs (those provisioned as the TLS) and "res binder" for resumption PSKs (those provisioned as the
resumption master secret of a previous handshake). The different resumption master secret of a previous handshake). The different
labels prevent the substitution of one type of PSK for the other. labels prevent the substitution of one type of PSK for the other.
There are multiple potential Early Secret values depending on which There are multiple potential Early Secret values depending on which
PSK the server ultimately selects. The client will need to compute PSK the server ultimately selects. The client will need to compute
one for each potential PSK; if no PSK is selected, it will then need one for each potential PSK; if no PSK is selected, it will then need
to compute the early secret corresponding to the zero PSK. to compute the early secret corresponding to the zero PSK.
Once all the values which are to be derived from a given secret have
been computed, that secret SHOULD be erased.
7.2. Updating Traffic Keys and IVs 7.2. Updating Traffic Keys and IVs
Once the handshake is complete, it is possible for either side to Once the handshake is complete, it is possible for either side to
update its sending traffic keys using the KeyUpdate handshake message update its sending traffic keys using the KeyUpdate handshake message
defined in Section 4.6.3. The next generation of traffic keys is defined in Section 4.6.3. The next generation of traffic keys is
computed by generating client_/server_application_traffic_secret_N+1 computed by generating client_/server_application_traffic_secret_N+1
from client_/server_application_traffic_secret_N as described in this from client_/server_application_traffic_secret_N as described in this
section then re-deriving the traffic keys as described in section then re-deriving the traffic keys as described in
Section 7.3. Section 7.3.
skipping to change at page 90, line 20 skipping to change at page 89, line 33
- A secret value - A secret value
- A purpose value indicating the specific value being generated - A purpose value indicating the specific value being generated
- The length of the key - The length of the key
The traffic keying material is generated from an input traffic secret The traffic keying material is generated from an input traffic secret
value using: value using:
[sender]_write_key = HKDF-Expand-Label(Secret, "key", "", key_length) [sender]_write_key = HKDF-Expand-Label(Secret, "key", "", key_length)
[sender]_write_iv = HKDF-Expand-Label(Secret, "iv", "", iv_length) [sender]_write_iv = HKDF-Expand-Label(Secret, "iv" , "", iv_length)
[sender] denotes the sending side. The Secret value for each record [sender] denotes the sending side. The Secret value for each record
type is shown in the table below. type is shown in the table below.
+-------------------+---------------------------------------+ +-------------------+---------------------------------------+
| Record Type | Secret | | Record Type | Secret |
+-------------------+---------------------------------------+ +-------------------+---------------------------------------+
| 0-RTT Application | client_early_traffic_secret | | 0-RTT Application | client_early_traffic_secret |
| | | | | |
| Handshake | [sender]_handshake_traffic_secret | | Handshake | [sender]_handshake_traffic_secret |
skipping to change at page 90, line 46 skipping to change at page 90, line 12
Secret changes (e.g., when changing from the handshake to application Secret changes (e.g., when changing from the handshake to application
data keys or upon a key update). data keys or upon a key update).
7.4. (EC)DHE Shared Secret Calculation 7.4. (EC)DHE Shared Secret Calculation
7.4.1. Finite Field Diffie-Hellman 7.4.1. Finite Field Diffie-Hellman
For finite field groups, a conventional Diffie-Hellman computation is For finite field groups, a conventional Diffie-Hellman computation is
performed. The negotiated key (Z) is converted to a byte string by performed. The negotiated key (Z) is converted to a byte string by
encoding in big-endian and padded with zeros up to the size of the encoding in big-endian and padded with zeros up to the size of the
prime. This byte string is used as the shared secret, and is used in prime. This byte string is used as the shared secret in the key
the key schedule as specified above. schedule as specified above.
Note that this construction differs from previous versions of TLS Note that this construction differs from previous versions of TLS
which remove leading zeros. which remove leading zeros.
7.4.2. Elliptic Curve Diffie-Hellman 7.4.2. Elliptic Curve Diffie-Hellman
For secp256r1, secp384r1 and secp521r1, ECDH calculations (including For secp256r1, secp384r1 and secp521r1, ECDH calculations (including
parameter and key generation as well as the shared secret parameter and key generation as well as the shared secret
calculation) are performed according to [IEEE1363] using the ECKAS- calculation) are performed according to [IEEE1363] using the ECKAS-
DH1 scheme with the identity map as key derivation function (KDF), so DH1 scheme with the identity map as key derivation function (KDF), so
skipping to change at page 91, line 26 skipping to change at page 90, line 39
MUST NOT be truncated. MUST NOT be truncated.
(Note that this use of the identity KDF is a technicality. The (Note that this use of the identity KDF is a technicality. The
complete picture is that ECDH is employed with a non-trivial KDF complete picture is that ECDH is employed with a non-trivial KDF
because TLS does not directly use this secret for anything other than because TLS does not directly use this secret for anything other than
for computing other secrets.) for computing other secrets.)
ECDH functions are used as follows: ECDH functions are used as follows:
- The public key to put into the KeyShareEntry.key_exchange - The public key to put into the KeyShareEntry.key_exchange
structure is the result of applying the ECDH function to the structure is the result of applying the ECDH scalar multiplication
secret key of appropriate length (into scalar input) and the function to the secret key of appropriate length (into scalar
standard public basepoint (into u-coordinate point input). input) and the standard public basepoint (into u-coordinate point
input).
- The ECDH shared secret is the result of applying the ECDH function - The ECDH shared secret is the result of applying the ECDH scalar
to the secret key (into scalar input) and the peer's public key multiplication function to the secret key (into scalar input) and
(into u-coordinate point input). The output is used raw, with no the peer's public key (into u-coordinate point input). The output
processing. is used raw, with no processing.
For X25519 and X448, implementations SHOULD use the approach For X25519 and X448, implementations SHOULD use the approach
specified in [RFC7748] to calculate the Diffie-Hellman shared secret. specified in [RFC7748] to calculate the Diffie-Hellman shared secret.
Implementations MUST check whether the computed Diffie-Hellman shared Implementations MUST check whether the computed Diffie-Hellman shared
secret is the all-zero value and abort if so, as described in secret is the all-zero value and abort if so, as described in
Section 6 of [RFC7748]. If implementers use an alternative Section 6 of [RFC7748]. If implementers use an alternative
implementation of these elliptic curves, they SHOULD perform the implementation of these elliptic curves, they SHOULD perform the
additional checks specified in Section 7 of [RFC7748]. additional checks specified in Section 7 of [RFC7748].
7.5. Exporters 7.5. Exporters
skipping to change at page 92, line 11 skipping to change at page 91, line 23
remains the same. remains the same.
The exporter value is computed as: The exporter value is computed as:
HKDF-Expand-Label(Derive-Secret(Secret, label, ""), HKDF-Expand-Label(Derive-Secret(Secret, label, ""),
"exporter", Hash(context_value), key_length) "exporter", Hash(context_value), key_length)
Where Secret is either the early_exporter_master_secret or the Where Secret is either the early_exporter_master_secret or the
exporter_master_secret. Implementations MUST use the exporter_master_secret. Implementations MUST use the
exporter_master_secret unless explicitly specified by the exporter_master_secret unless explicitly specified by the
application. The early_exporter_master_secret is define for use in application. The early_exporter_master_secret is defined for use in
settings where an exporter is needed for 0-RTT data. A separate settings where an exporter is needed for 0-RTT data. A separate
interface for the early exporter is RECOMMENDED, especially on a interface for the early exporter is RECOMMENDED, especially on a
server where a single interface can make the early exporter server where a single interface can make the early exporter
inaccessible. inaccessible.
If no context is provided, the context_value is zero-length. If no context is provided, the context_value is zero-length.
Consequently, providing no context computes the same value as Consequently, providing no context computes the same value as
providing an empty context. This is a change from previous versions providing an empty context. This is a change from previous versions
of TLS where an empty context produced a different output to an of TLS where an empty context produced a different output to an
absent context. As of this document's publication, no allocated absent context. As of this document's publication, no allocated
exporter label is used both with and without a context. Future exporter label is used both with and without a context. Future
specifications MUST NOT define a use of exporters that permit both an specifications MUST NOT define a use of exporters that permit both an
empty context and no context with the same label. New uses of empty context and no context with the same label. New uses of
exporters SHOULD provide a context in all exporter computations, exporters SHOULD provide a context in all exporter computations,
though the value could be empty. though the value could be empty.
Requirements for the format of exporter labels are defined in section Requirements for the format of exporter labels are defined in section
4 of [RFC5705]. 4 of [RFC5705].
8. Compliance Requirements 8. 0-RTT and Anti-Replay
8.1. Mandatory-to-Implement Cipher Suites As noted in Section 2.3 and Appendix E.5, TLS does not provide
inherent replay protections for 0-RTT data. There are two potential
threats to be concerned with:
- Network attackers who mount a replay attack by simply duplicating
a flight of 0-RTT data.
- Network attackers who take advantage of client retry behavior to
arrange for the server to receive multiple copies of an
application message. This threat already exists to some extent
because clients that value robustness respond to network errors by
attempting to retry requests. However, 0-RTT adds an additional
dimension for any server system which does not maintain globally
consistent server state. Specifically, if a server system has
multiple zones where tickets from zone A will not be accepted in
zone B, then an attacker can duplicate a ClientHello and early
data intended for A to both A and B. At A, the data will be
accepted in 0-RTT, but at B the server will reject 0-RTT data and
instead force a full handshake. If the attacker blocks the
ServerHello from A, then the client will complete the handshake
with B and probably retry the request, leading to duplication on
the server system as a whole.
The first class of attack can be prevented by the mechanism described
in this section. Servers need not permit 0-RTT at all, but those
which do SHOULD implement either the single-use tickets or
ClientHello recording techniques described in the following two
sections.
The second class of attack cannot be prevented at the TLS layer and
MUST be dealt with by any application. Note that any application
whose clients implement any kind of retry behavior already needs to
implement some sort of anti-replay defense.
In normal operation, clients will not know which, if any, of these
mechanisms servers actually implement and therefore MUST only send
early data which they are willing to have subject to the attacks
described in Appendix E.5.
8.1. Single-Use Tickets
The simplest form of anti-replay defense is for the server to only
allow each session ticket to be used once. For instance, the server
can maintain a database of all outstanding valid tickets; deleting
each ticket from the database as it is used. If an unknown ticket is
provided, the server would then fall back to a full handshake.
If the tickets are not self-contained but rather are database keys,
and the corresponding PSKs are deleted upon use, then connections
established using one PSK enjoy forward security. This improves
security for all 0-RTT data and PSK usage when PSK is used without
(EC)DHE.
Because this mechanism requires sharing the session database between
server nodes in environments with multiple distributed servers, it
may be hard to achieve high rates of successful PSK 0-RTT connections
when compared to self-encrypted tickets. Unlike session databases,
session tickets can successfully do PSK-based session establishment
even without consistent storage, though when 0-RTT is allowed they
still require consistent storage for anti-replay of 0-RTT data, as
detailed in the following section.
8.2. Client Hello Recording
An alternative form of anti-replay is to record a unique value
derived from the ClientHello (generally either the random value or
the PSK binder) and reject duplicates. Recording all ClientHellos
causes state to grow without bound, but a server can instead record
ClientHellos within a given time window and use the
"obfuscated_ticket_age" to ensure that tickets aren't reused outside
that window.
In order to implement this, when a ClientHello is received, the
server first verifies the PSK binder as described Section 4.2.10. It
then computes the expected_arrival_time as described in the next
section and rejects 0-RTT if it is outside the recording window,
falling back to the 1-RTT handshake.
If the expected arrival time is in the window, then the server checks
to see if it has recorded a matching ClientHello. If one is found,
it either aborts the handshake with an "illegal_parameter" alert or
accepts the PSK but reject 0-RTT. If no matching ClientHello is
found, then it accepts 0-RTT and then stores the ClientHello for as
long as the expected_arrival_time is inside the window. Servers MAY
also implement data stores with false positives, such as Bloom
filters, in which case they MUST respond to apparent replay by
rejecting 0-RTT but MUST NOT abort the handshake.
The server MUST derive the storage key only from validated sections
of the ClientHello. If the ClientHello contains multiple PSK
identities, then an attacker can create multiple ClientHellos with
different binder values for the less-preferred identity on the
assumption that the server will not verify it, as recommended by
Section 4.2.10. I.e., if the client sends PSKs A and B but the
server prefers A, then the attacker can change the binder for B
without affecting the binder for A. This will cause the ClientHello
to be accepted, and may casue side effects such as replay cache
pollution, although any 0-RTT data will not be decryptable because it
will use different keys. If the validated binder or the
ClientHello.random are used as the storage key, then this attack is
not possible.
Because this mechanism does not require storing all outstanding
tickets, it may be easier to implement in distributed systems with
high rates of resumption and 0-RTT, at the cost of potentially weaker
anti-replay defense because of the difficulty of reliably storing and
retrieving the received ClientHello messages. In many such systems,
it is impractical to have globally consistent storage of all the
received ClientHellos. In this case, the best anti-replay protection
is provided by having a single storage zone be authoritative for a
given ticket and refusing 0-RTT for that ticket in any other zone.
This approach prevents simple replay by the attacker because only one
zone will accept 0-RTT data. A weaker design is to implement
separate storage for each zone but allow 0-RTT in any zone. This
approach limits the number of replays to once per zone. Application
message duplication of course remains possible with either design.
When implementations are freshly started, they SHOULD reject 0-RTT as
long as any portion of their recording window overlaps the startup
time. Otherwise, they run the risk of accepting replays which were
originally sent during that period.
Note: If the client's clock is running much faster than the server's
then a ClientHello may be received that is outside the window in the
future, in which case it might be accepted for 1-RTT, causing a
client retry, and then acceptable later for 0-RTT. This is another
variant of the second form of attack described above.
8.3. Freshness Checks
Because the ClientHello indicates the time at which the client sent
it, it is possible to efficiently determine whether a ClientHello was
likely sent reasonably recently and only accept 0-RTT for such a
ClientHello, otherwise falling back to a 1-RTT handshake. This is
necessary for the ClientHello storage mechanism described in
Section 8.2 because otherwise the server needs to store an unlimited
number of ClientHellos and is a useful optimization for single-use
tickets because it allows efficient rejection of ClientHellos which
cannot be used for 0-RTT.
In order to implement this mechanism, a server needs to store the
time that the server generated the session ticket, offset by an
estimate of the round trip time between client and server. I.e.,
adjusted_creation_time = creation_time + estimated_RTT
This value can be encoded in the ticket, thus avoiding the need to
keep state for each outstanding ticket. The server can determine the
client's view of the age of the ticket by subtracting the ticket's
"ticket_age_add value" from the "obfuscated_ticket_age" parameter in
the client's "pre_shared_key" extension. The server can determine
the "expected arrival time" of the ClientHello as:
expected_arrival_time = adjusted_creation_time + clients_ticket_age
When a new ClientHello is received, the expected_arrival_time is then
compared against the current server wall clock time and if they
differ by more than a certain amount, 0-RTT is rejected, though the
1-RTT handshake can be allowed to complete.
There are several potential sources of error that might cause
mismatches between the expected arrival time and the measured time.
Variations in client and server clock rates are likely to be minimal,
though potentially with gross time corrections. Network propagation
delays are the most likely causes of a mismatch in legitimate values
for elapsed time. Both the NewSessionTicket and ClientHello messages
might be retransmitted and therefore delayed, which might be hidden
by TCP. For clients on the Internet, this implies windows on the
order of ten seconds to account for errors in clocks and variations
in measurements; other deployment scenarios may have different needs.
Clock skew distributions are not symmetric, so the optimal tradeoff
may involve an asymmetric range of permissible mismatch values.
Note that freshness checking alone is not sufficient to prevent
replays because it does not detect them during the error window,
which, depending on bandwidth and system capacity could include
billions of replays in real-world settings. In addition, this
freshness checking is only done at the time the ClientHello is
received, and not when later early application data records are
received. After early data is accepted, records may continue to be
streamed to the server over a longer time period.
9. Compliance Requirements
9.1. Mandatory-to-Implement Cipher Suites
In the absence of an application profile standard specifying In the absence of an application profile standard specifying
otherwise, a TLS-compliant application MUST implement the otherwise, a TLS-compliant application MUST implement the
TLS_AES_128_GCM_SHA256 [GCM] cipher suite and SHOULD implement the TLS_AES_128_GCM_SHA256 [GCM] cipher suite and SHOULD implement the
TLS_AES_256_GCM_SHA384 [GCM] and TLS_CHACHA20_POLY1305_SHA256 TLS_AES_256_GCM_SHA384 [GCM] and TLS_CHACHA20_POLY1305_SHA256
[RFC7539] cipher suites. (see Appendix B.4) [RFC7539] cipher suites. (see Appendix B.4)
A TLS-compliant application MUST support digital signatures with A TLS-compliant application MUST support digital signatures with
rsa_pkcs1_sha256 (for certificates), rsa_pss_sha256 (for rsa_pkcs1_sha256 (for certificates), rsa_pss_sha256 (for
CertificateVerify and certificates), and ecdsa_secp256r1_sha256. A CertificateVerify and certificates), and ecdsa_secp256r1_sha256. A
TLS-compliant application MUST support key exchange with secp256r1 TLS-compliant application MUST support key exchange with secp256r1
(NIST P-256) and SHOULD support key exchange with X25519 [RFC7748]. (NIST P-256) and SHOULD support key exchange with X25519 [RFC7748].
8.2. Mandatory-to-Implement Extensions 9.2. Mandatory-to-Implement Extensions
In the absence of an application profile standard specifying In the absence of an application profile standard specifying
otherwise, a TLS-compliant application MUST implement the following otherwise, a TLS-compliant application MUST implement the following
TLS extensions: TLS extensions:
- Supported Versions ("supported_versions"; Section 4.2.1) - Supported Versions ("supported_versions"; Section 4.2.1)
- Cookie ("cookie"; Section 4.2.2) - Cookie ("cookie"; Section 4.2.2)
- Signature Algorithms ("signature_algorithms"; Section 4.2.3) - Signature Algorithms ("signature_algorithms"; Section 4.2.3)
skipping to change at page 93, line 24 skipping to change at page 96, line 30
- Server Name Indication ("server_name"; Section 3 of [RFC6066]) - Server Name Indication ("server_name"; Section 3 of [RFC6066])
All implementations MUST send and use these extensions when offering All implementations MUST send and use these extensions when offering
applicable features: applicable features:
- "supported_versions" is REQUIRED for all ClientHello messages. - "supported_versions" is REQUIRED for all ClientHello messages.
- "signature_algorithms" is REQUIRED for certificate authentication. - "signature_algorithms" is REQUIRED for certificate authentication.
- "supported_groups" and "key_share" are REQUIRED for DHE or ECDHE - "supported_groups" is REQUIRED for ClientHello messages using DHE
key exchange. or ECDHE key exchange.
- "key_share" is REQUIRED for DHE or ECDHE key exchange.
- "pre_shared_key" is REQUIRED for PSK key agreement. - "pre_shared_key" is REQUIRED for PSK key agreement.
A client is considered to be attempting to negotiate using this A client is considered to be attempting to negotiate using this
specification if the ClientHello contains a "supported_versions" specification if the ClientHello contains a "supported_versions"
extension 0x0304 the highest version number contained in its body. extension 0x0304 the highest version number contained in its body.
Such a ClientHello message MUST meet the following requirements: Such a ClientHello message MUST meet the following requirements:
- If not containing a "pre_shared_key" extension, it MUST contain - If not containing a "pre_shared_key" extension, it MUST contain
both a "signature_algorithms" extension and a "supported_groups" both a "signature_algorithms" extension and a "supported_groups"
skipping to change at page 94, line 5 skipping to change at page 97, line 12
requirements MUST abort the handshake with a "missing_extension" requirements MUST abort the handshake with a "missing_extension"
alert. alert.
Additionally, all implementations MUST support use of the Additionally, all implementations MUST support use of the
"server_name" extension with applications capable of using it. "server_name" extension with applications capable of using it.
Servers MAY require clients to send a valid "server_name" extension. Servers MAY require clients to send a valid "server_name" extension.
Servers requiring this extension SHOULD respond to a ClientHello Servers requiring this extension SHOULD respond to a ClientHello
lacking a "server_name" extension by terminating the connection with lacking a "server_name" extension by terminating the connection with
a "missing_extension" alert. a "missing_extension" alert.
9. Security Considerations 10. Security Considerations
Security issues are discussed throughout this memo, especially in Security issues are discussed throughout this memo, especially in
Appendix C, Appendix D, and Appendix E. Appendix C, Appendix D, and Appendix E.
10. IANA Considerations 11. IANA Considerations
This document uses several registries that were originally created in This document uses several registries that were originally created in
[RFC4346]. IANA has updated these to reference this document. The [RFC4346]. IANA has updated these to reference this document. The
registries and their allocation policies are below: registries and their allocation policies are below:
- TLS Cipher Suite Registry: Values with the first byte in the range - TLS Cipher Suite Registry: values with the first byte in the range
0-254 (decimal) are assigned via Specification Required [RFC5226]. 0-254 (decimal) are assigned via Specification Required [RFC5226].
Values with the first byte 255 (decimal) are reserved for Private Values with the first byte 255 (decimal) are reserved for Private
Use [RFC5226]. Use [RFC5226].
IANA [SHALL add/has added] the cipher suites listed in IANA [SHALL add/has added] the cipher suites listed in
Appendix B.4 to the registry. The "Value" and "Description" Appendix B.4 to the registry. The "Value" and "Description"
columns are taken from the table. The "DTLS-OK" and "Recommended" columns are taken from the table. The "DTLS-OK" and "Recommended"
columns are both marked as "Yes" for each new cipher suite. columns are both marked as "Yes" for each new cipher suite.
[[This assumes [I-D.ietf-tls-iana-registry-updates] has been [[This assumes [I-D.ietf-tls-iana-registry-updates] has been
applied.]] applied.]]
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[RFC5226]. Values with the first byte 255 (decimal) are reserved [RFC5226]. Values with the first byte 255 (decimal) are reserved
for Private Use [RFC5226]. Values with the first byte in the for Private Use [RFC5226]. Values with the first byte in the
range 0-6 or with the second byte in the range 0-3 that are not range 0-6 or with the second byte in the range 0-3 that are not
currently allocated are reserved for backwards compatibility. currently allocated are reserved for backwards compatibility.
This registry SHALL have a "Recommended" column. The registry This registry SHALL have a "Recommended" column. The registry
[shall be/ has been] initially populated with the values described [shall be/ has been] initially populated with the values described
in Section 4.2.3. The following values SHALL be marked as in Section 4.2.3. The following values SHALL be marked as
"Recommended": ecdsa_secp256r1_sha256, ecdsa_secp384r1_sha384, "Recommended": ecdsa_secp256r1_sha256, ecdsa_secp384r1_sha384,
rsa_pss_sha256, rsa_pss_sha384, rsa_pss_sha512, ed25519. rsa_pss_sha256, rsa_pss_sha384, rsa_pss_sha512, ed25519.
11. References 12. References
11.1. Normative References 12.1. Normative References
[DH] Diffie, W. and M. Hellman, "New Directions in [DH] Diffie, W. and M. Hellman, "New Directions in
Cryptography", IEEE Transactions on Information Theory, Cryptography", IEEE Transactions on Information Theory,
V.IT-22 n.6 , June 1977. V.IT-22 n.6 , June 1977.
[GCM] Dworkin, M., "Recommendation for Block Cipher Modes of [GCM] Dworkin, M., "Recommendation for Block Cipher Modes of
Operation: Galois/Counter Mode (GCM) and GMAC", Operation: Galois/Counter Mode (GCM) and GMAC",
NIST Special Publication 800-38D, November 2007. NIST Special Publication 800-38D, November 2007.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997, DOI 10.17487/RFC2104, February 1997,
<http://www.rfc-editor.org/info/rfc2104>. <http://www.rfc-editor.org/info/rfc2104>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <http://www.rfc-editor.org/info/rfc2119>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226, IANA Considerations Section in RFCs", RFC 5226,
DOI 10.17487/RFC5226, May 2008, DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>. <http://www.rfc-editor.org/info/rfc5226>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>. <http://www.rfc-editor.org/info/rfc5280>.
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport [RFC5705] Rescorla, E., "Keying Material Exporters for Transport
skipping to change at page 97, line 5 skipping to change at page 100, line 14
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate [RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013, Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
<http://www.rfc-editor.org/info/rfc6962>. <http://www.rfc-editor.org/info/rfc6962>.
[RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature [RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature
Algorithm (DSA) and Elliptic Curve Digital Signature Algorithm (DSA) and Elliptic Curve Digital Signature
Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
2013, <http://www.rfc-editor.org/info/rfc6979>. 2013, <http://www.rfc-editor.org/info/rfc6979>.
[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <http://www.rfc-editor.org/info/rfc7301>.
[RFC7507] Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher [RFC7507] Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
Suite Value (SCSV) for Preventing Protocol Downgrade Suite Value (SCSV) for Preventing Protocol Downgrade
Attacks", RFC 7507, DOI 10.17487/RFC7507, April 2015, Attacks", RFC 7507, DOI 10.17487/RFC7507, April 2015,
<http://www.rfc-editor.org/info/rfc7507>. <http://www.rfc-editor.org/info/rfc7507>.
[RFC7539] Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF [RFC7539] Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
Protocols", RFC 7539, DOI 10.17487/RFC7539, May 2015, Protocols", RFC 7539, DOI 10.17487/RFC7539, May 2015,
<http://www.rfc-editor.org/info/rfc7539>. <http://www.rfc-editor.org/info/rfc7539>.
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves [RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
skipping to change at page 97, line 46 skipping to change at page 101, line 14
[X690] ITU-T, "Information technology - ASN.1 encoding Rules: [X690] ITU-T, "Information technology - ASN.1 encoding Rules:
Specification of Basic Encoding Rules (BER), Canonical Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules Encoding Rules (CER) and Distinguished Encoding Rules
(DER)", ISO/IEC 8825-1:2002, 2002. (DER)", ISO/IEC 8825-1:2002, 2002.
[X962] ANSI, "Public Key Cryptography For The Financial Services [X962] ANSI, "Public Key Cryptography For The Financial Services
Industry: The Elliptic Curve Digital Signature Algorithm Industry: The Elliptic Curve Digital Signature Algorithm
(ECDSA)", ANSI X9.62, 1998. (ECDSA)", ANSI X9.62, 1998.
11.2. Informative References 12.2. Informative References
[AEAD-LIMITS] [AEAD-LIMITS]
Luykx, A. and K. Paterson, "Limits on Authenticated Luykx, A. and K. Paterson, "Limits on Authenticated
Encryption Use in TLS", 2016, Encryption Use in TLS", 2016,
<http://www.isg.rhul.ac.uk/~kp/TLS-AEbounds.pdf>. <http://www.isg.rhul.ac.uk/~kp/TLS-AEbounds.pdf>.
[BBFKZG16] [BBFKZG16]
Bhargavan, K., Brzuska, C., Fournet, C., Kohlweiss, M., Bhargavan, K., Brzuska, C., Fournet, C., Kohlweiss, M.,
Zanella-Beguelin, S., and M. Green, "Downgrade Resilience Zanella-Beguelin, S., and M. Green, "Downgrade Resilience
in Key-Exchange Protocols", Proceedings of IEEE Symposium in Key-Exchange Protocols", Proceedings of IEEE Symposium
skipping to change at page 100, line 23 skipping to change at page 103, line 37
Proceedings of USENIX Workshop on Offensive Technologies , Proceedings of USENIX Workshop on Offensive Technologies ,
2015. 2015.
[I-D.ietf-tls-iana-registry-updates] [I-D.ietf-tls-iana-registry-updates]
Salowey, J. and S. Turner, "D/TLS IANA Registry Updates", Salowey, J. and S. Turner, "D/TLS IANA Registry Updates",
draft-ietf-tls-iana-registry-updates-01 (work in draft-ietf-tls-iana-registry-updates-01 (work in
progress), April 2017. progress), April 2017.
[I-D.ietf-tls-tls13-vectors] [I-D.ietf-tls-tls13-vectors]
Thomson, M., "Example Handshake Traces for TLS 1.3", Thomson, M., "Example Handshake Traces for TLS 1.3",
draft-ietf-tls-tls13-vectors-00 (work in progress), draft-ietf-tls-tls13-vectors-01 (work in progress), June
January 2017. 2017.
[IEEE1363] [IEEE1363]
IEEE, "Standard Specifications for Public Key IEEE, "Standard Specifications for Public Key
Cryptography", IEEE 1363 , 2000. Cryptography", IEEE 1363 , 2000.
[KEYAGREEMENT] [KEYAGREEMENT]
Barker, E., Lily Chen, ., Roginsky, A., and M. Smid, Barker, E., Lily Chen, ., Roginsky, A., and M. Smid,
"Recommendation for Pair-Wise Key Establishment Schemes "Recommendation for Pair-Wise Key Establishment Schemes
Using Discrete Logarithm Cryptography", NIST Special Using Discrete Logarithm Cryptography", NIST Special
Publication 800-38D, May 2013. Publication 800-38D, May 2013.
skipping to change at page 101, line 5 skipping to change at page 104, line 19
[KW16] Krawczyk, H. and H. Wee, "The OPTLS Protocol and TLS 1.3", [KW16] Krawczyk, H. and H. Wee, "The OPTLS Protocol and TLS 1.3",
Proceedings of Euro S"P 2016 , 2016, Proceedings of Euro S"P 2016 , 2016,
<https://eprint.iacr.org/2015/978>. <https://eprint.iacr.org/2015/978>.
[LXZFH16] Li, X., Xu, J., Feng, D., Zhang, Z., and H. Hu, "Multiple [LXZFH16] Li, X., Xu, J., Feng, D., Zhang, Z., and H. Hu, "Multiple
Handshakes Security of TLS 1.3 Candidates", Proceedings of Handshakes Security of TLS 1.3 Candidates", Proceedings of
IEEE Symposium on Security and Privacy (Oakland) 2016 , IEEE Symposium on Security and Privacy (Oakland) 2016 ,
2016, <http://ieeexplore.ieee.org/document/7546519/>. 2016, <http://ieeexplore.ieee.org/document/7546519/>.
[Mac17] MacCarthaigh, C., "Security Review of TLS1.3 0-RTT", 2017,
<https://github.com/tlswg/tls13-spec/issues/1001>.
[PSK-FINISHED] [PSK-FINISHED]
Cremers, C., Horvat, M., van der Merwe, T., and S. Scott, Cremers, C., Horvat, M., van der Merwe, T., and S. Scott,
"Revision 10: possible attack if client authentication is "Revision 10: possible attack if client authentication is
allowed during PSK", 2015, <https://www.ietf.org/mail- allowed during PSK", 2015, <https://www.ietf.org/mail-
archive/web/tls/current/msg18215.html>. archive/web/tls/current/msg18215.html>.
[REKEY] Abdalla, M. and M. Bellare, "Increasing the Lifetime of a [REKEY] Abdalla, M. and M. Bellare, "Increasing the Lifetime of a
Key: A Comparative Analysis of the Security of Re-keying Key: A Comparative Analysis of the Security of Re-keying
Techniques", ASIACRYPT2000 , October 2000. Techniques", ASIACRYPT2000 , October 2000.
skipping to change at page 103, line 5 skipping to change at page 106, line 22
Protocol (HTTP/1.1): Message Syntax and Routing", Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014, RFC 7230, DOI 10.17487/RFC7230, June 2014,
<http://www.rfc-editor.org/info/rfc7230>. <http://www.rfc-editor.org/info/rfc7230>.
[RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J., [RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
Weiler, S., and T. Kivinen, "Using Raw Public Keys in Weiler, S., and T. Kivinen, "Using Raw Public Keys in
Transport Layer Security (TLS) and Datagram Transport Transport Layer Security (TLS) and Datagram Transport
Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250, Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
June 2014, <http://www.rfc-editor.org/info/rfc7250>. June 2014, <http://www.rfc-editor.org/info/rfc7250>.
[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <http://www.rfc-editor.org/info/rfc7301>.
[RFC7465] Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465, [RFC7465] Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465,
DOI 10.17487/RFC7465, February 2015, DOI 10.17487/RFC7465, February 2015,
<http://www.rfc-editor.org/info/rfc7465>. <http://www.rfc-editor.org/info/rfc7465>.
[RFC7568] Barnes, R., Thomson, M., Pironti, A., and A. Langley, [RFC7568] Barnes, R., Thomson, M., Pironti, A., and A. Langley,
"Deprecating Secure Sockets Layer Version 3.0", RFC 7568, "Deprecating Secure Sockets Layer Version 3.0", RFC 7568,
DOI 10.17487/RFC7568, June 2015, DOI 10.17487/RFC7568, June 2015,
<http://www.rfc-editor.org/info/rfc7568>. <http://www.rfc-editor.org/info/rfc7568>.
[RFC7627] Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A., [RFC7627] Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
skipping to change at page 104, line 11 skipping to change at page 107, line 25
[SSL3] Freier, A., Karlton, P., and P. Kocher, "The SSL 3.0 [SSL3] Freier, A., Karlton, P., and P. Kocher, "The SSL 3.0
Protocol", November 1996. Protocol", November 1996.
[TIMING] Boneh, D. and D. Brumley, "Remote timing attacks are [TIMING] Boneh, D. and D. Brumley, "Remote timing attacks are
practical", USENIX Security Symposium, 2003. practical", USENIX Security Symposium, 2003.
[X501] "Information Technology - Open Systems Interconnection - [X501] "Information Technology - Open Systems Interconnection -
The Directory: Models", ITU-T X.501, 1993. The Directory: Models", ITU-T X.501, 1993.
11.3. URIs 12.3. URIs
[1] mailto:tls@ietf.org [1] mailto:tls@ietf.org
Appendix A. State Machine Appendix A. State Machine
This section provides a summary of the legal state transitions for This section provides a summary of the legal state transitions for
the client and server handshakes. State names (in all capitals, the client and server handshakes. State names (in all capitals,
e.g., START) have no formal meaning but are provided for ease of e.g., START) have no formal meaning but are provided for ease of
comprehension. Messages which are sent only sometimes are indicated comprehension. Actions which are taken only in certain circumstances
in []. are indicated in []. The notation "K_{send,recv} = foo" means "set
the send/recv key to the given key".
A.1. Client A.1. Client
START <----+ START <----+
Send ClientHello | | Recv HelloRetryRequest Send ClientHello | | Recv HelloRetryRequest
/ v | [K_send = early data] | |
| WAIT_SH ---+ v |
Can | | Recv ServerHello / WAIT_SH ----+
send | V | | Recv ServerHello
early | WAIT_EE | | K_recv = handshake
data | | Recv EncryptedExtensions Can | V
| +--------+--------+ send | WAIT_EE
early | | Recv EncryptedExtensions
data | +--------+--------+
| Using | | Using certificate | Using | | Using certificate
| PSK | v | PSK | v
| | WAIT_CERT_CR | | WAIT_CERT_CR
| | Recv | | Recv CertificateRequest | | Recv | | Recv CertificateRequest
| | Certificate | v | | Certificate | v
| | | WAIT_CERT | | | WAIT_CERT
| | | | Recv Certificate | | | | Recv Certificate
| | v v | | v v
| | WAIT_CV | | WAIT_CV
| | | Recv CertificateVerify | | | Recv CertificateVerify
| +> WAIT_FINISHED <+ | +> WAIT_FINISHED <+
| | Recv Finished | | Recv Finished
\ | \ | [Send EndOfEarlyData]
| [Send EndOfEarlyData] | K_send = handshake
| [Send Certificate [+ CertificateVerify]] | [Send Certificate [+ CertificateVerify]]
| Send Finished Can send | Send Finished
Can send v app data --> | K_send = K_recv = application
app data --> CONNECTED after here v
after CONNECTED
here
Note that with the transitions as shown above, clients may send
alerts that derive from post-ServerHello messages in the clear or
with the early data keys. If clients need to send such alerts, they
SHOULD first rekey to the handshake keys if possible.
A.2. Server A.2. Server
START <-----+ START <-----+
Recv ClientHello | | Send HelloRetryRequest Recv ClientHello | | Send HelloRetryRequest
v | v |
RECVD_CH ----+ RECVD_CH ----+
| Select parameters | Select parameters
v v
NEGOTIATED NEGOTIATED
| Send ServerHello | Send ServerHello
| K_send = handshake
| Send EncryptedExtensions | Send EncryptedExtensions
| [Send CertificateRequest] | [Send CertificateRequest]
Can send | [Send Certificate + CertificateVerify] Can send | [Send Certificate + CertificateVerify]
app data --> | Send Finished app data | Send Finished
after +--------+--------+ after --> | K_send = application
here No 0-RTT | | 0-RTT here +--------+--------+
| v No 0-RTT | | 0-RTT
| WAIT_EOED <---+ K_recv = handshake | | K_recv = early_data
[Skip decrypt errors] | WAIT_EOED <---+
| Recv | | | Recv | Recv | | | Recv
| EndOfEarlyData | | | early data | EndOfEarlyData | | | Early data
| | +-----+ | K_recv = | +-----+
| handshake |
| |
+> WAIT_FLIGHT2 <-+ +> WAIT_FLIGHT2 <-+
| |
+--------+--------+ +--------+--------+
No auth | | Client auth No auth | | Client auth
| | | |
| v | v
| WAIT_CERT | WAIT_CERT
| Recv | | Recv Certificate | Recv | | Recv Certificate
| empty | v | empty | v
| Certificate | WAIT_CV | Certificate | WAIT_CV
| | | Recv | | | Recv
| v | CertificateVerify | v | CertificateVerify
+-> WAIT_FINISHED <---+ +-> WAIT_FINISHED <---+
| Recv Finished | Recv Finished
| K_recv = application
v v
CONNECTED CONNECTED
Appendix B. Protocol Data Structures and Constant Values Appendix B. Protocol Data Structures and Constant Values
This section describes protocol types and constants. Values listed This section describes protocol types and constants. Values listed
as _RESERVED were used in previous versions of TLS and are listed as _RESERVED were used in previous versions of TLS and are listed
here for completeness. TLS 1.3 implementations MUST NOT send them here for completeness. TLS 1.3 implementations MUST NOT send them
but might receive them from older TLS implementations. but might receive them from older TLS implementations.
skipping to change at page 110, line 42 skipping to change at page 113, line 42
server_name(0), /* RFC 6066 */ server_name(0), /* RFC 6066 */
max_fragment_length(1), /* RFC 6066 */ max_fragment_length(1), /* RFC 6066 */
status_request(5), /* RFC 6066 */ status_request(5), /* RFC 6066 */
supported_groups(10), /* RFC 4492, 7919 */ supported_groups(10), /* RFC 4492, 7919 */
signature_algorithms(13), /* RFC 5246 */ signature_algorithms(13), /* RFC 5246 */
use_srtp(14), /* RFC 5764 */ use_srtp(14), /* RFC 5764 */
heartbeat(15), /* RFC 6520 */ heartbeat(15), /* RFC 6520 */
application_layer_protocol_negotiation(16), /* RFC 7301 */ application_layer_protocol_negotiation(16), /* RFC 7301 */
signed_certificate_timestamp(18), /* RFC 6962 */ signed_certificate_timestamp(18), /* RFC 6962 */
client_certificate_type(19), /* RFC 7250 */ client_certificate_type(19), /* RFC 7250 */
server_certificate_type(20) /* RFC 7250 */ server_certificate_type(20), /* RFC 7250 */
padding(21), /* RFC 7685 */ padding(21), /* RFC 7685 */
key_share(40), /* [[this document]] */ key_share(40), /* [[this document]] */
pre_shared_key(41), /* [[this document]] */ pre_shared_key(41), /* [[this document]] */
early_data(42), /* [[this document]] */ early_data(42), /* [[this document]] */
supported_versions(43), /* [[this document]] */ supported_versions(43), /* [[this document]] */
cookie(44), /* [[this document]] */ cookie(44), /* [[this document]] */
psk_key_exchange_modes(45), /* [[this document]] */ psk_key_exchange_modes(45), /* [[this document]] */
certificate_authorities(47), /* [[this document]] */ certificate_authorities(47), /* [[this document]] */
oid_filters(48), /* [[this document]] */ oid_filters(48), /* [[this document]] */
post_handshake_auth(49), /* [[this document]] */ post_handshake_auth(49), /* [[this document]] */
skipping to change at page 115, line 11 skipping to change at page 118, line 11
configured TLS implementations. configured TLS implementations.
B.3.2. Server Parameters Messages B.3.2. Server Parameters Messages
opaque DistinguishedName<1..2^16-1>; opaque DistinguishedName<1..2^16-1>;
struct { struct {
DistinguishedName authorities<3..2^16-1>; DistinguishedName authorities<3..2^16-1>;
} CertificateAuthoritiesExtension; } CertificateAuthoritiesExtension;
struct { struct {
Extension extensions<0..2^16-1>;
} EncryptedExtensions;
struct {
opaque certificate_request_context<0..2^8-1>;
Extension extensions<2..2^16-1>;
} CertificateRequest;
struct {
opaque certificate_extension_oid<1..2^8-1>; opaque certificate_extension_oid<1..2^8-1>;
opaque certificate_extension_values<0..2^16-1>; opaque certificate_extension_values<0..2^16-1>;
} OIDFilter; } OIDFilter;
struct { struct {
OIDFilter filters<0..2^16-1>; OIDFilter filters<0..2^16-1>;
} OIDFilterExtension; } OIDFilterExtension;
struct {
Extension extensions<0..2^16-1>;
} EncryptedExtensions;
struct {
opaque certificate_request_context<0..2^8-1>;
Extension extensions<2..2^16-1>;
} CertificateRequest;
B.3.3. Authentication Messages B.3.3. Authentication Messages
struct { struct {
select(certificate_type){ select(certificate_type){
case RawPublicKey: case RawPublicKey:
// From RFC 7250 ASN.1_subjectPublicKeyInfo // From RFC 7250 ASN.1_subjectPublicKeyInfo
opaque ASN1_subjectPublicKeyInfo<1..2^24-1>; opaque ASN1_subjectPublicKeyInfo<1..2^24-1>;
case X.509: case X509:
opaque cert_data<1..2^24-1>; opaque cert_data<1..2^24-1>;
} };
Extension extensions<0..2^16-1>; Extension extensions<0..2^16-1>;
} CertificateEntry; } CertificateEntry;
struct { struct {
opaque certificate_request_context<0..2^8-1>; opaque certificate_request_context<0..2^8-1>;
CertificateEntry certificate_list<0..2^24-1>; CertificateEntry certificate_list<0..2^24-1>;
} Certificate; } Certificate;
struct { struct {
SignatureScheme algorithm; SignatureScheme algorithm;
skipping to change at page 116, line 35 skipping to change at page 119, line 35
struct { struct {
opaque verify_data[Hash.length]; opaque verify_data[Hash.length];
} Finished; } Finished;
B.3.4. Ticket Establishment B.3.4. Ticket Establishment
struct { struct {
uint32 ticket_lifetime; uint32 ticket_lifetime;
uint32 ticket_age_add; uint32 ticket_age_add;
opaque ticket_nonce<1..255>;
opaque ticket<1..2^16-1>; opaque ticket<1..2^16-1>;
Extension extensions<0..2^16-2>; Extension extensions<0..2^16-2>;
} NewSessionTicket; } NewSessionTicket;
B.3.5. Updating Keys B.3.5. Updating Keys
struct {} EndOfEarlyData; struct {} EndOfEarlyData;
enum { enum {
update_not_requested(0), update_requested(1), (255) update_not_requested(0), update_requested(1), (255)
skipping to change at page 118, line 6 skipping to change at page 121, line 6
is defined in [RFC6655]. The corresponding hash algorithms are is defined in [RFC6655]. The corresponding hash algorithms are
defined in [SHS]. defined in [SHS].
Although TLS 1.3 uses the same cipher suite space as previous Although TLS 1.3 uses the same cipher suite space as previous
versions of TLS, TLS 1.3 cipher suites are defined differently, only versions of TLS, TLS 1.3 cipher suites are defined differently, only
specifying the symmetric ciphers, and cannot be used for TLS 1.2. specifying the symmetric ciphers, and cannot be used for TLS 1.2.
Similarly, TLS 1.2 and lower cipher suites cannot be used with TLS Similarly, TLS 1.2 and lower cipher suites cannot be used with TLS
1.3. 1.3.
New cipher suite values are assigned by IANA as described in New cipher suite values are assigned by IANA as described in
Section 10. Section 11.
Appendix C. Implementation Notes Appendix C. Implementation Notes
The TLS protocol cannot prevent many common security mistakes. This The TLS protocol cannot prevent many common security mistakes. This
section provides several recommendations to assist implementors. section provides several recommendations to assist implementors.
[I-D.ietf-tls-tls13-vectors] provides test vectors for TLS 1.3 [I-D.ietf-tls-tls13-vectors] provides test vectors for TLS 1.3
handshakes. handshakes.
C.1. API considerations for 0-RTT C.1. Random Number Generation and Seeding
0-RTT data has very different security properties from data
transmitted after a completed handshake: it can be replayed.
Implementations SHOULD provide different functions for reading and
writing 0-RTT data and data transmitted after the handshake, and
SHOULD NOT automatically resend 0-RTT data if it is rejected by the
server.
C.2. Random Number Generation and Seeding
TLS requires a cryptographically secure pseudorandom number generator TLS requires a cryptographically secure pseudorandom number generator
(PRNG). In most cases, the operating system provides an appropriate (CSPRNG). In most cases, the operating system provides an
facility such as /dev/urandom, which should be used absent other appropriate facility such as /dev/urandom, which should be used
(performance) concerns. It is generally preferable to use an absent other (performance) concerns. It is RECOMMENDED to use an
existing PRNG implementation in preference to crafting a new one, and existing CSPRNG implementation in preference to crafting a new one.
many adequate cryptographic libraries are already available under Many adequate cryptographic libraries are already available under
favorable license terms. Should those prove unsatisfactory, favorable license terms. Should those prove unsatisfactory,
[RFC4086] provides guidance on the generation of random values. [RFC4086] provides guidance on the generation of random values.
C.3. Certificates and Authentication C.2. Certificates and Authentication
Implementations are responsible for verifying the integrity of Implementations are responsible for verifying the integrity of
certificates and should generally support certificate revocation certificates and should generally support certificate revocation
messages. Absent a specific indication from an application profile, messages. Absent a specific indication from an application profile,
Certificates should always be verified to ensure proper signing by a Certificates should always be verified to ensure proper signing by a
trusted Certificate Authority (CA). The selection and addition of trusted Certificate Authority (CA). The selection and addition of
trust anchors should be done very carefully. Users should be able to trust anchors should be done very carefully. Users should be able to
view information about the certificate and trust anchor. view information about the certificate and trust anchor.
Applications SHOULD also enforce minimum and maximum key sizes. For Applications SHOULD also enforce minimum and maximum key sizes. For
example, certification paths containing keys or signatures weaker example, certification paths containing keys or signatures weaker
than 2048-bit RSA or 224-bit ECDSA are not appropriate for secure than 2048-bit RSA or 224-bit ECDSA are not appropriate for secure
applications. applications.
C.4. Implementation Pitfalls C.3. Implementation Pitfalls
Implementation experience has shown that certain parts of earlier TLS Implementation experience has shown that certain parts of earlier TLS
specifications are not easy to understand, and have been a source of specifications are not easy to understand and have been a source of
interoperability and security problems. Many of these areas have interoperability and security problems. Many of these areas have
been clarified in this document, but this appendix contains a short been clarified in this document but this appendix contains a short
list of the most important things that require special attention from list of the most important things that require special attention from
implementors. implementors.
TLS protocol issues: TLS protocol issues:
- Do you correctly handle handshake messages that are fragmented to - Do you correctly handle handshake messages that are fragmented to
multiple TLS records (see Section 5.1)? Including corner cases multiple TLS records (see Section 5.1)? Including corner cases
like a ClientHello that is split to several small fragments? Do like a ClientHello that is split to several small fragments? Do
you fragment handshake messages that exceed the maximum fragment you fragment handshake messages that exceed the maximum fragment
size? In particular, the Certificate and CertificateRequest size? In particular, the Certificate and CertificateRequest
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- What countermeasures do you use to prevent timing attacks - What countermeasures do you use to prevent timing attacks
[TIMING]? [TIMING]?
- When using Diffie-Hellman key exchange, do you correctly preserve - When using Diffie-Hellman key exchange, do you correctly preserve
leading zero bytes in the negotiated key (see Section 7.4.1)? leading zero bytes in the negotiated key (see Section 7.4.1)?
- Does your TLS client check that the Diffie-Hellman parameters sent - Does your TLS client check that the Diffie-Hellman parameters sent
by the server are acceptable, (see Section 4.2.7.1)? by the server are acceptable, (see Section 4.2.7.1)?
- Do you use a strong and, most importantly, properly seeded random - Do you use a strong and, most importantly, properly seeded random
number generator (see Appendix C.2) when generating Diffie-Hellman number generator (see Appendix C.1) when generating Diffie-Hellman
private values, the ECDSA "k" parameter, and other security- private values, the ECDSA "k" parameter, and other security-
critical values? It is RECOMMENDED that implementations implement critical values? It is RECOMMENDED that implementations implement
"deterministic ECDSA" as specified in [RFC6979]. "deterministic ECDSA" as specified in [RFC6979].
- Do you zero-pad Diffie-Hellman public key values to the group size - Do you zero-pad Diffie-Hellman public key values to the group size
(see Section 4.2.7.1)? (see Section 4.2.7.1)?
- Do you verify signatures after making them to protect against RSA- - Do you verify signatures after making them to protect against RSA-
CRT key leaks? [FW15] CRT key leaks? [FW15]
C.5. Client Tracking Prevention C.4. Client Tracking Prevention
Clients SHOULD NOT reuse a ticket for multiple connections. Reuse of Clients SHOULD NOT reuse a ticket for multiple connections. Reuse of
a ticket allows passive observers to correlate different connections. a ticket allows passive observers to correlate different connections.
Servers that issue tickets SHOULD offer at least as many tickets as Servers that issue tickets SHOULD offer at least as many tickets as
the number of connections that a client might use; for example, a web the number of connections that a client might use; for example, a web
browser using HTTP/1.1 [RFC7230] might open six connections to a browser using HTTP/1.1 [RFC7230] might open six connections to a
server. Servers SHOULD issue new tickets with every connection. server. Servers SHOULD issue new tickets with every connection.
This ensures that clients are always able to use a new ticket when This ensures that clients are always able to use a new ticket when
creating a new connection. creating a new connection.
C.6. Unauthenticated Operation C.5. Unauthenticated Operation
Previous versions of TLS offered explicitly unauthenticated cipher Previous versions of TLS offered explicitly unauthenticated cipher
suites based on anonymous Diffie-Hellman. These modes have been suites based on anonymous Diffie-Hellman. These modes have been
deprecated in TLS 1.3. However, it is still possible to negotiate deprecated in TLS 1.3. However, it is still possible to negotiate
parameters that do not provide verifiable server authentication by parameters that do not provide verifiable server authentication by
several methods, including: several methods, including:
- Raw public keys [RFC7250]. - Raw public keys [RFC7250].
- Using a public key contained in a certificate but without - Using a public key contained in a certificate but without
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attempts to repair this error SHOULD NOT send a TLS 1.2 ClientHello, attempts to repair this error SHOULD NOT send a TLS 1.2 ClientHello,
but instead send a TLS 1.3 ClientHello without 0-RTT data. but instead send a TLS 1.3 ClientHello without 0-RTT data.
To avoid this error condition, multi-server deployments SHOULD ensure To avoid this error condition, multi-server deployments SHOULD ensure
a uniform and stable deployment of TLS 1.3 without 0-RTT prior to a uniform and stable deployment of TLS 1.3 without 0-RTT prior to
enabling 0-RTT. enabling 0-RTT.
D.4. Backwards Compatibility Security Restrictions D.4. Backwards Compatibility Security Restrictions
Implementations negotiating use of older versions of TLS SHOULD Implementations negotiating use of older versions of TLS SHOULD
prefer forward secure and AEAD cipher suites, when available. prefer forward secret and AEAD cipher suites, when available.
The security of RC4 cipher suites is considered insufficient for the The security of RC4 cipher suites is considered insufficient for the
reasons cited in [RFC7465]. Implementations MUST NOT offer or reasons cited in [RFC7465]. Implementations MUST NOT offer or
negotiate RC4 cipher suites for any version of TLS for any reason. negotiate RC4 cipher suites for any version of TLS for any reason.
Old versions of TLS permitted the use of very low strength ciphers. Old versions of TLS permitted the use of very low strength ciphers.
Ciphers with a strength less than 112 bits MUST NOT be offered or Ciphers with a strength less than 112 bits MUST NOT be offered or
negotiated for any version of TLS for any reason. negotiated for any version of TLS for any reason.
The security of SSL 3.0 [SSL3] is considered insufficient for the The security of SSL 3.0 [SSL3] is considered insufficient for the
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completion of the handshake, each side outputs its view of the completion of the handshake, each side outputs its view of the
following values: following values:
- A set of "session keys" (the various secrets derived from the - A set of "session keys" (the various secrets derived from the
master secret) from which can be derived a set of working keys. master secret) from which can be derived a set of working keys.
- A set of cryptographic parameters (algorithms, etc.) - A set of cryptographic parameters (algorithms, etc.)
- The identities of the communicating parties. - The identities of the communicating parties.
We assume that the attacker has complete control of the network in We assume the attacker to be an active network attacker, which means
between the parties [RFC3552]. Even under these conditions, the it has complete control over the network used to communicate between
handshake should provide the properties listed below. Note that the parties [RFC3552]. Even under these conditions, the handshake
these properties are not necessarily independent, but reflect the should provide the properties listed below. Note that these
protocol consumers' needs. properties are not necessarily independent, but reflect the protocol
consumers' needs.
Establishing the same session keys. The handshake needs to output Establishing the same session keys. The handshake needs to output
the same set of session keys on both sides of the handshake, the same set of session keys on both sides of the handshake,
provided that it completes successfully on each endpoint (See provided that it completes successfully on each endpoint (See
[CK01]; defn 1, part 1). [CK01]; defn 1, part 1).
Secrecy of the session keys. The shared session keys should be known Secrecy of the session keys. The shared session keys should be known
only to the communicating parties, not to the attacker (See only to the communicating parties and not to the attacker (See
[CK01]; defn 1, part 2). Note that in a unilaterally [CK01]; defn 1, part 2). Note that in a unilaterally
authenticated connection, the attacker can establish its own authenticated connection, the attacker can establish its own
session keys with the server, but those session keys are distinct session keys with the server, but those session keys are distinct
from those established by the client. from those established by the client.
Peer Authentication. The client's view of the peer identity should Peer Authentication. The client's view of the peer identity should
reflect the server's identity. If the client is authenticated, reflect the server's identity. If the client is authenticated,
the server's view of the peer identity should match the client's the server's view of the peer identity should match the client's
identity. identity.
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Downgrade protection. The cryptographic parameters should be the Downgrade protection. The cryptographic parameters should be the
same on both sides and should be the same as if the peers had been same on both sides and should be the same as if the peers had been
communicating in the absence of an attack (See [BBFKZG16]; defns 8 communicating in the absence of an attack (See [BBFKZG16]; defns 8
and 9}). and 9}).
Forward secret with respect to long-term keys If the long-term Forward secret with respect to long-term keys If the long-term
keying material (in this case the signature keys in certificate- keying material (in this case the signature keys in certificate-
based authentication modes or the external/resumption PSK in PSK based authentication modes or the external/resumption PSK in PSK
with (EC)DHE modes) is compromised after the handshake is with (EC)DHE modes) is compromised after the handshake is
complete, this does not compromise the security of the session key complete, this does not compromise the security of the session key
(See [DOW92]). The forward secrecy property is not satisfied when (See [DOW92]), as long as the session key itself has been erased.
PSK is used in the "psk_ke" PskKeyExchangeMode. The forward secrecy property is not satisfied when PSK is used in
the "psk_ke" PskKeyExchangeMode.
Key Compromise Impersonation (KCI) resistance In a mutually- Key Compromise Impersonation (KCI) resistance In a mutually-
authenticated connection with certificates, peer authentication authenticated connection with certificates, peer authentication
should hold even if the local long-term secret was compromised should hold even if the local long-term secret was compromised
before the connection was established (see [HGFS15]). For before the connection was established (see [HGFS15]). For
example, if a client's signature key is compromised, it should not example, if a client's signature key is compromised, it should not
be possible to impersonate arbitrary servers to that client in be possible to impersonate arbitrary servers to that client in
subsequent handshakes. subsequent handshakes.
Protection of endpoint identities. The server's identity Protection of endpoint identities. The server's identity
(certificate) should be protected against passive attackers. The (certificate) should be protected against passive attackers. The
client's identity should be protected against both passive and client's identity should be protected against both passive and
active attackers. active attackers.
Informally, the signature-based modes of TLS 1.3 provide for the Informally, the signature-based modes of TLS 1.3 provide for the
establishment of a unique, secret, shared, key established by an establishment of a unique, secret, shared key established by an
(EC)DHE key exchange and authenticated by the server's signature over (EC)DHE key exchange and authenticated by the server's signature over
the handshake transcript, as well as tied to the server's identity by the handshake transcript, as well as tied to the server's identity by
a MAC. If the client is authenticated by a certificate, it also a MAC. If the client is authenticated by a certificate, it also
signs over the handshake transcript and provides a MAC tied to both signs over the handshake transcript and provides a MAC tied to both
identities. [SIGMA] describes the analysis of this type of key identities. [SIGMA] describes the design and analysis of this type
exchange protocol. If fresh (EC)DHE keys are used for each of key exchange protocol. If fresh (EC)DHE keys are used for each
connection, then the output keys are forward secret. connection, then the output keys are forward secret.
The external PSK and resumption PSK bootstrap from a long-term shared The external PSK and resumption PSK bootstrap from a long-term shared
secret into a unique per-connection set of short-term session keys. secret into a unique per-connection set of short-term session keys.
This secret may have been established in a previous handshake. If This secret may have been established in a previous handshake. If
PSK with (EC)DHE key establishment is used, these session keys will PSK with (EC)DHE key establishment is used, these session keys will
also be forward secret. The resumption PSK has been designed so that also be forward secret. The resumption PSK has been designed so that
the resumption master secret computed by connection N and needed to the resumption master secret computed by connection N and needed to
form connection N+1 is separate from the traffic keys used by form connection N+1 is separate from the traffic keys used by
connection N, thus providing forward secrecy between the connections. connection N, thus providing forward secrecy between the connections.
In addition, if multiple tickets are established on the same
connection, they are associated with different keys, so compromise of
the PSK associated with one ticket does not lead to the compromise of
connections established with PSKs associated with other tickets.
This property is most interesting if tickets are stored in a database
(and so can be deleted) rather than if they are self-encrypted.
The PSK binder value forms a binding between a PSK and the current The PSK binder value forms a binding between a PSK and the current
handshake, as well as between the session where the PSK was handshake, as well as between the session where the PSK was
established and the session where it was used. This binding established and the session where it was used. This binding
transitively includes the original handshake transcript, because that transitively includes the original handshake transcript, because that
transcript is digested into the values which produce the Resumption transcript is digested into the values which produce the Resumption
Master Secret. This requires that both the KDF used to produce the Master Secret. This requires that both the KDF used to produce the
resumption master secret and the MAC used to compute the binder be resumption master secret and the MAC used to compute the binder be
collision resistant. See Appendix E.1.1 for more on this. Note: The collision resistant. See Appendix E.1.1 for more on this. Note: The
binder does not cover the binder values from other PSKs, though they binder does not cover the binder values from other PSKs, though they
are included in the Finished MAC. are included in the Finished MAC.
Note: TLS does not currently permit the server to send a Note: TLS does not currently permit the server to send a
certificate_request message in non-certificate-based handshakes certificate_request message in non-certificate-based handshakes
(e.g., PSK). If this restriction were to be relaxed in future, the (e.g., PSK). If this restriction were to be relaxed in future, the
client's signature would not cover the server's certificate directly. client's signature would not cover the server's certificate directly.
However, if the PSK was established through a NewSessionTicket, the However, if the PSK was established through a NewSessionTicket, the
client's signature would transitively cover the server's certificate client's signature would transitively cover the server's certificate
through the PSK binder. [PSK-FINISHED] describes a concrete attack through the PSK binder. [PSK-FINISHED] describes a concrete attack
on constructions that do not bind to the server's certificate. It is on constructions that do not bind to the server's certificate (see
unsafe to use certificate-based client authentication when the client also [Kraw16]). It is unsafe to use certificate-based client
might potentially share the same PSK/key-id pair with two different authentication when the client might potentially share the same PSK/
endpoints. Implementations MUST NOT combine external PSKs with key-id pair with two different endpoints. Implementations MUST NOT
certificate-based authentication of either the client or the server. combine external PSKs with certificate-based authentication of either
the client or the server.
If an exporter is used, then it produces values which are unique and If an exporter is used, then it produces values which are unique and
secret (because they are generated from a unique session key). secret (because they are generated from a unique session key).
Exporters computed with different labels and contexts are Exporters computed with different labels and contexts are
computationally independent, so it is not feasible to compute one computationally independent, so it is not feasible to compute one
from another or the session secret from the exported value. Note: from another or the session secret from the exported value. Note:
exporters can produce arbitrary-length values. If exporters are to exporters can produce arbitrary-length values. If exporters are to
be used as channel bindings, the exported value MUST be large enough be used as channel bindings, the exported value MUST be large enough
to provide collision resistance. The exporters provided in TLS 1.3 to provide collision resistance. The exporters provided in TLS 1.3
are derived from the same handshake contexts as the early traffic are derived from the same handshake contexts as the early traffic
keys and the application traffic keys respectively, and thus have keys and the application traffic keys respectively, and thus have
similar security properties. Note that they do not include the similar security properties. Note that they do not include the
client's certificate; future applications which wish to bind to the client's certificate; future applications which wish to bind to the
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are derived from the same handshake contexts as the early traffic are derived from the same handshake contexts as the early traffic
keys and the application traffic keys respectively, and thus have keys and the application traffic keys respectively, and thus have
similar security properties. Note that they do not include the similar security properties. Note that they do not include the
client's certificate; future applications which wish to bind to the client's certificate; future applications which wish to bind to the
client's certificate may need to define a new exporter that includes client's certificate may need to define a new exporter that includes
the full handshake transcript. the full handshake transcript.
For all handshake modes, the Finished MAC (and where present, the For all handshake modes, the Finished MAC (and where present, the
signature), prevents downgrade attacks. In addition, the use of signature), prevents downgrade attacks. In addition, the use of
certain bytes in the random nonces as described in Section 4.1.3 certain bytes in the random nonces as described in Section 4.1.3
allows the detection of downgrade to previous TLS versions. allows the detection of downgrade to previous TLS versions. See
[BBFKZG16] for more detail on TLS 1.3 and downgrade.
As soon as the client and the server have exchanged enough As soon as the client and the server have exchanged enough
information to establish shared keys, the remainder of the handshake information to establish shared keys, the remainder of the handshake
is encrypted, thus providing protection against passive attackers. is encrypted, thus providing protection against passive attackers,
Because the server authenticates before the client, the client can even if the computed shared key is not authenticated. Because the
ensure that it only reveals its identity to an authenticated server. server authenticates before the client, the client can ensure that if
Note that implementations must use the provided record padding it authenticates to the server, it only reveals its identity to an
mechanism during the handshake to avoid leaking information about the authenticated server. Note that implementations must use the
identities due to length. The client's proposed PSK identities are provided record padding mechanism during the handshake to avoid
not encrypted, nor is the one that the server selects. leaking information about the identities due to length. The client's
proposed PSK identities are not encrypted, nor is the one that the
server selects.
E.1.1. Key Derivation and HKDF E.1.1. Key Derivation and HKDF
Key derivation in TLS 1.3 uses the HKDF function defined in [RFC5869] Key derivation in TLS 1.3 uses the HKDF function defined in [RFC5869]
and its two components, HKDF-Extract and HKDF-Expand. The full and its two components, HKDF-Extract and HKDF-Expand. The full
rationale for the HKDF construction can be found in [Kraw10] and the rationale for the HKDF construction can be found in [Kraw10] and the
rationale for the way it is used in TLS 1.3 in [KW16]. Throughout rationale for the way it is used in TLS 1.3 in [KW16]. Throughout
this document, each application of HKDF-Extract is followed by one or this document, each application of HKDF-Extract is followed by one or
more invocations of HKDF-Expand. This ordering should always be more invocations of HKDF-Expand. This ordering should always be
followed (including in future revisions of this document), in followed (including in future revisions of this document), in
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be provisioned by the application layer. See [CHHSV17] for details. be provisioned by the application layer. See [CHHSV17] for details.
In addition, the analysis of post-handshake authentication from In addition, the analysis of post-handshake authentication from
[Kraw16] shows that the client identified by the certificate sent in [Kraw16] shows that the client identified by the certificate sent in
the post-handshake phase possesses the traffic key. This party is the post-handshake phase possesses the traffic key. This party is
therefore the client that participated in the original handshake or therefore the client that participated in the original handshake or
one to whom the original client delegated the traffic key (assuming one to whom the original client delegated the traffic key (assuming
that the traffic key has not been compromised). that the traffic key has not been compromised).
E.1.3. 0-RTT E.1.3. 0-RTT
The 0-RTT mode of operation generally provides the same security The 0-RTT mode of operation generally provides similar security
properties as 1-RTT data, with the two exceptions that the 0-RTT properties as 1-RTT data, with the two exceptions that the 0-RTT
encryption keys do not provide full forward secrecy and that the encryption keys do not provide full forward secrecy and that the
server is not able to guarantee full uniqueness of the handshake server is not able to guarantee uniqueness of the handshake (non-
(non-replayability) without keeping potentially undue amounts of replayability) without keeping potentially undue amounts of state.
state. See Section 4.2.9 for one mechanism to limit the exposure to See Section 4.2.9 for one mechanism to limit the exposure to replay.
replay.
E.1.4. Post-Compromise Security E.1.4. Exporter Independence
The exporter_master_secret and early_exporter_master_secret are
derived to be independent of the traffic keys and therefore do not
represent a threat to the security of traffic encrypted with those
keys. However, because these secrets can be used to compute any
exporter value, they SHOULD be erased as soon as possible. If the
total set of exporter labels is known, then implementations SHOULD
pre-compute the inner Derive-Secret stage of the exporter computation
for all those labels, then erase the [early_]exporter_master_secret,
followed by each inner values as soon as it is known that it will not
be needed again.
E.1.5. Post-Compromise Security
TLS does not provide security for handshakes which take place after TLS does not provide security for handshakes which take place after
the peer's long-term secret (signature key or external PSK) is the peer's long-term secret (signature key or external PSK) is
compromised. It therefore does not provide post-compromise security compromised. It therefore does not provide post-compromise security
[CCG16], sometimes also referred to as backwards or future security. [CCG16], sometimes also referred to as backwards or future secrecy.
This is in contrast to KCI resistance, which describes the security This is in contrast to KCI resistance, which describes the security
guarantees that a party has after its own long-term secret has been guarantees that a party has after its own long-term secret has been
compromised. compromised.
E.1.5. External References E.1.6. External References
The reader should refer to the following references for analysis of The reader should refer to the following references for analysis of
the TLS handshake: [DFGS15] [CHSV16] [DFGS16] [KW16] [Kraw16] the TLS handshake: [DFGS15] [CHSV16] [DFGS16] [KW16] [Kraw16]
[FGSW16] [LXZFH16] [FG17] [BBK17]. [FGSW16] [LXZFH16] [FG17] [BBK17].
E.2. Record Layer E.2. Record Layer
The record layer depends on the handshake producing strong traffic The record layer depends on the handshake producing strong traffic
secrets which can be used to derive bidirectional encryption keys and secrets which can be used to derive bidirectional encryption keys and
nonces. Assuming that is true, and the keys are used for no more nonces. Assuming that is true, and the keys are used for no more
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Order protection/non-replayability An attacker should not be able to Order protection/non-replayability An attacker should not be able to
cause the receiver to accept a record which it has already cause the receiver to accept a record which it has already
accepted or cause the receiver to accept record N+1 without having accepted or cause the receiver to accept record N+1 without having
first processed record N. first processed record N.
Length concealment. Given a record with a given external length, the Length concealment. Given a record with a given external length, the
attacker should not be able to determine the amount of the record attacker should not be able to determine the amount of the record
that is content versus padding. that is content versus padding.
Forward security after key change. If the traffic key update Forward secrecy after key change. If the traffic key update
mechanism described in Section 4.6.3 has been used and the mechanism described in Section 4.6.3 has been used and the
previous generation key is deleted, an attacker who compromises previous generation key is deleted, an attacker who compromises
the endpoint should not be able to decrypt traffic encrypted with the endpoint should not be able to decrypt traffic encrypted with
the old key. the old key.
Informally, TLS 1.3 provides these properties by AEAD-protecting the Informally, TLS 1.3 provides these properties by AEAD-protecting the
plaintext with a strong key. AEAD encryption [RFC5116] provides plaintext with a strong key. AEAD encryption [RFC5116] provides
confidentiality and integrity for the data. Non-replayability is confidentiality and integrity for the data. Non-replayability is
provided by using a separate nonce for each record, with the nonce provided by using a separate nonce for each record, with the nonce
being derived from the record sequence number (Section 5.3), with the being derived from the record sequence number (Section 5.3), with the
sequence number being maintained independently at both sides thus sequence number being maintained independently at both sides thus
records which are delivered out of order result in AEAD deprotection records which are delivered out of order result in AEAD deprotection
failures. failures. In order to prevent mass cryptanalysis when the same
plaintext is repeatedly encrypted by different users under the same
key (as is commonly the case for HTTP), the nonce is formed by mixing
the sequence number with a secret per-connection initialization
vector derived along with the traffic keys. See [BT16] for analysis
of this construction.
The re-keying technique in TLS 1.3 (see Section 7.2) follows the The re-keying technique in TLS 1.3 (see Section 7.2) follows the
construction of the serial generator in [REKEY], which shows that re- construction of the serial generator in [REKEY], which shows that re-
keying can allow keys to be used for a larger number of encryptions keying can allow keys to be used for a larger number of encryptions
than without re-keying. This relies on the security of the HKDF- than without re-keying. This relies on the security of the HKDF-
Expand-Label function as a pseudorandom function (PRF). In addition, Expand-Label function as a pseudorandom function (PRF). In addition,
as long as this function is truly one way, it is not possible to as long as this function is truly one way, it is not possible to
compute traffic keys from prior to a key change (forward secrecy). compute traffic keys from prior to a key change (forward secrecy).
TLS does not provide security for data which is communicated on a TLS does not provide security for data which is communicated on a
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errors; a new connection will have different cryptographic errors; a new connection will have different cryptographic
material, preventing attacks against the cryptographic primitives material, preventing attacks against the cryptographic primitives
that require multiple trials. that require multiple trials.
Information leakage through side channels can occur at layers above Information leakage through side channels can occur at layers above
TLS, in application protocols and the applications that use them. TLS, in application protocols and the applications that use them.
Resistance to side-channel attacks depends on applications and Resistance to side-channel attacks depends on applications and
application protocols separately ensuring that confidential application protocols separately ensuring that confidential
information is not inadvertently leaked. information is not inadvertently leaked.
E.5. Replay Attacks on 0-RTT
Replayable 0-RTT data presents a number of security threats to TLS-
using applications, unless those applications are specifically
engineered to be safe under replay (minimally, this means idempotent,
but in many cases may also require other stronger conditions, such as
constant-time response). Potential attacks include:
- Duplication of actions which cause side effects (e.g., purchasing
an item or transferring money) to be duplicated, thus harming the
site or the user.
- Attackers can store and replay 0-RTT messages in order to re-order
them with respect to other messages (e.g., moving a delete to
after a create).
- Exploiting cache timing behavior to discover the content of 0-RTT
messages by replaying a 0-RTT message to a different cache node
and then using a separate connection to measure request latency,
to see if the two requests address the same resource.
If data can be replayed a large number of times, additional attacks
become possible, such as making repeated measurements of the the
speed of cryptographic operations. In addition, they may be able to
overload rate-limiting systems. For further description of these
attacks, see [Mac17].
Ultimately, servers have the responsibility to protect themselves
against attacks employing 0-RTT data replication. The mechanisms
described in Section 8 are intended to prevent replay at the TLS
layer do not provide complete protection against receiving multiple
copies of client data. TLS 1.3 falls back to the 1-RTT handshake
when the server does not have any information about the client, e.g.,
because it is in a different cluster which does not share state or
because the ticket has been deleted as described in Section 8.1. If
the application layer protocol retransmits data in this setting, then
it is possible for an attacker to induce message duplication by
sending the ClientHello to both the original cluster (which processes
the data immediately) and another cluster which will fall back to
1-RTT and process the data upon application layer replay. The scale
of this attack is limited by the client's willingness to retry
transactions and therefore only allows a limited amount of
duplication, with each copy appearing as a new connection at the
server.
If implemented correctly, the mechanisms described in Section 8.1 and
Section 8.2 prevent a replayed ClientHello and its associated 0-RTT
data from being accepted multiple times by any cluster with
consistent state; for servers which limit the use of 0-RTT to one
cluster for a single ticket, then a given ClientHello and its
associated 0-RTT data will only be accepted once. However, if state
is not completely consistent, then an attacker might be able to have
multiple copies of the data be accepted during the replication
window. Because clients do not know the exact details of server
behavior, they MUST NOT send messages in early data which are not
safe to have replayed and which they would not be willing to retry
across multiple 1-RTT connections.
Application protocols MUST NOT use 0-RTT data without a profile that
defines its use. That profile needs to identify which messages or
interactions are safe to use with 0-RTT and how to handle the
situation when the server rejects 0-RTT and falls back to 1-RTT.
In addition, to avoid accidental misuse, TLS implementations MUST NOT
enable 0-RTT (either sending or accepting) unless specifically
requested by the application and MUST NOT automatically resend 0-RTT
data if it is rejected by the server unless instructed by the
application. Server-side applications may wish to implement special
processing for 0-RTT data for some kinds of application traffic
(e.g., abort the connection, request that data be resent at the
application layer, or delay processing until the handshake
completes). In order to allow applications to implement this kind of
processing, TLS implementations MUST provide a way for the
application to determine if the handshake has completed.
E.5.1. Replay and Exporters
Replays of the ClientHello produce the same early exporter, thus
requiring additional care by applications which use these exporters.
In particular, if these exporters are used as an authentication
channel binding (e.g., by signing the output of the exporter) an
attacker who compromises the PSK can transplant authenticators
between connections without compromising the authentication key.
In addition, the early exporter SHOULD NOT be used to generate
server-to-client encryption keys because that would entail the reuse
of those keys. This parallels the use of the early application
traffic keys only in the client-to-server direction.
Appendix F. Working Group Information Appendix F. Working Group Information
The discussion list for the IETF TLS working group is located at the The discussion list for the IETF TLS working group is located at the
e-mail address tls@ietf.org [1]. Information on the group and e-mail address tls@ietf.org [1]. Information on the group and
information on how to subscribe to the list is at information on how to subscribe to the list is at
https://www.ietf.org/mailman/listinfo/tls https://www.ietf.org/mailman/listinfo/tls
Archives of the list can be found at: https://www.ietf.org/mail- Archives of the list can be found at: https://www.ietf.org/mail-
archive/web/tls/current/index.html archive/web/tls/current/index.html
skipping to change at page 132, line 7 skipping to change at page 137, line 14
- Steven M. Bellovin - Steven M. Bellovin
Columbia University Columbia University
smb@cs.columbia.edu smb@cs.columbia.edu
- David Benjamin - David Benjamin
Google Google
davidben@google.com davidben@google.com
- Benjamin Beurdouche - Benjamin Beurdouche
INRIA & Microsoft Research - Joint Center INRIA & Microsoft Research
benjamin.beurdouche@ens.fr benjamin.beurdouche@ens.fr
- Karthikeyan Bhargavan (co-author of [RFC7627]) - Karthikeyan Bhargavan (co-author of [RFC7627])
INRIA INRIA
karthikeyan.bhargavan@inria.fr karthikeyan.bhargavan@inria.fr
- Simon Blake-Wilson (co-author of [RFC4492]) - Simon Blake-Wilson (co-author of [RFC4492])
BCI BCI
sblakewilson@bcisse.com sblakewilson@bcisse.com
- Nelson Bolyard (co-author of [RFC4492]) - Nelson Bolyard (co-author of [RFC4492])
Sun Microsystems, Inc. Sun Microsystems, Inc.
nelson@bolyard.com nelson@bolyard.com
- Ran Canetti - Ran Canetti
IBM IBM
canetti@watson.ibm.com canetti@watson.ibm.com
- Matt Caswell
OpenSSL
matt@openssl.org
- Pete Chown - Pete Chown
Skygate Technology Ltd Skygate Technology Ltd
pc@skygate.co.uk pc@skygate.co.uk
- Katriel Cohn-Gordon - Katriel Cohn-Gordon
University of Oxford University of Oxford
me@katriel.co.uk me@katriel.co.uk
- Cas Cremers - Cas Cremers
University of Oxford University of Oxford
skipping to change at page 134, line 36 skipping to change at page 139, line 45
- Leon Klingele - Leon Klingele
Independent Independent
mail@leonklingele.de mail@leonklingele.de
- Paul Kocher (co-author of SSL 3.0) - Paul Kocher (co-author of SSL 3.0)
Cryptography Research Cryptography Research
paul@cryptography.com paul@cryptography.com
- Hugo Krawczyk - Hugo Krawczyk
IBM IBM
hugo@ee.technion.ac.il hugokraw@us.ibm.com
- Adam Langley (co-author of [RFC7627]) - Adam Langley (co-author of [RFC7627])
Google Google
agl@google.com agl@google.com
- Olivier Levillain - Olivier Levillain
ANSSI ANSSI
olivier.levillain@ssi.gouv.fr olivier.levillain@ssi.gouv.fr
- Xiaoyin Liu - Xiaoyin Liu
skipping to change at page 135, line 9 skipping to change at page 140, line 19
xiaoyin.l@outlook.com xiaoyin.l@outlook.com
- Ilari Liusvaara - Ilari Liusvaara
Independent Independent
ilariliusvaara@welho.com ilariliusvaara@welho.com
- Atul Luykx - Atul Luykx
K.U. Leuven K.U. Leuven
atul.luykx@kuleuven.be atul.luykx@kuleuven.be
- Colm MacCarthaigh
Amazon Web Services
colm@allcosts.net
- Carl Mehner - Carl Mehner
USAA USAA
carl.mehner@usaa.com carl.mehner@usaa.com
- Jan Mikkelsen - Jan Mikkelsen
Transactionware Transactionware
janm@transactionware.com janm@transactionware.com
- Bodo Moeller (co-author of [RFC4492]) - Bodo Moeller (co-author of [RFC4492])
Google Google
skipping to change at page 136, line 35 skipping to change at page 141, line 49
rsalz@akamai.com rsalz@akamai.com
- Sam Scott - Sam Scott
Royal Holloway, University of London Royal Holloway, University of London
me@samjs.co.uk me@samjs.co.uk
- Dan Simon - Dan Simon
Microsoft, Inc. Microsoft, Inc.
dansimon@microsoft.com dansimon@microsoft.com
- Brian Smith
Independent
brian@briansmith.org
- Brian Sniffen - Brian Sniffen
Akamai Technologies Akamai Technologies
ietf@bts.evenmere.org ietf@bts.evenmere.org
- Nick Sullivan - Nick Sullivan
Cloudflare Inc. Cloudflare Inc.
nick@cloudflare.com nick@cloudflare.com
- Bjoern Tackmann - Bjoern Tackmann
University of California, San Diego University of California, San Diego
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