    Internet Engineering Task Force (IETF)                        A. Langley
    Request for Comments: 7905                                      W. Chang
    Updates: 5246, 6347                                         Google, Inc.
    Category: Standards Track                           N. Mavrogiannopoulos
    ISSN: 2070-1721                                                  Red Hat
                                                             J. Strombergson
                                                          Secworks Sweden AB
                                                                S. Josefsson
                                                                      SJD AB
                                                                   June 2016

       ChaCha20-Poly1305 Cipher Suites for Transport Layer Security (TLS)

    Abstract

       This document describes the use of the ChaCha stream cipher and
       Poly1305 authenticator in the Transport Layer Security (TLS) and
       Datagram Transport Layer Security (DTLS) protocols.

       This document updates RFCs 5246 and 6347.

    Status of This Memo

       This is an Internet Standards Track document.

       This document is a product of the Internet Engineering Task Force
       (IETF).  It represents the consensus of the IETF community.  It has
       received public review and has been approved for publication by the
       Internet Engineering Steering Group (IESG).  Further information on
       Internet Standards is available in Section 2 of RFC 7841.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       http://www.rfc-editor.org/info/rfc7905.

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    RFC 7905                 ChaCha-Poly1305 for TLS               June 2016

    Copyright Notice

       Copyright (c) 2016 IETF Trust and the persons identified as the
       document authors.  All rights reserved.

       This document is subject to BCP 78 and the IETF Trust's Legal
       Provisions Relating to IETF Documents
       (http://trustee.ietf.org/license-info) in effect on the date of
       publication of this document.  Please review these documents
       carefully, as they describe your rights and restrictions with respect
       to this document.  Code Components extracted from this document must
       include Simplified BSD License text as described in Section 4.e of
       the Trust Legal Provisions and are provided without warranty as
       described in the Simplified BSD License.

    Table of Contents

       1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
       2.  ChaCha20 Cipher Suites  . . . . . . . . . . . . . . . . . . .   4
       3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
       4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
       5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
         5.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
         5.2.  Informative References  . . . . . . . . . . . . . . . . .   6
       Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   8
       Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

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    RFC 7905                 ChaCha-Poly1305 for TLS               June 2016

    1.  Introduction

       This document describes the use of the ChaCha stream cipher and
       Poly1305 authenticator in version 1.2 or later of the Transport Layer
       Security (TLS) protocol [RFC5246] as well as version 1.2 or later of
       the Datagram Transport Layer Security (DTLS) protocol [RFC6347].

       ChaCha [CHACHA] is a stream cipher developed by D. J. Bernstein in
       2008.  It is a refinement of Salsa20, which is one of the selected
       ciphers in the eSTREAM portfolio [ESTREAM], and it was used as the
       core of the SHA-3 finalist, BLAKE.

       The variant of ChaCha used in this document has 20 rounds, a 96-bit
       nonce, and a 256-bit key; it is referred to as "ChaCha20".  This is
       the conservative variant (with respect to security) of the ChaCha
       family and is described in [RFC7539].

       Poly1305 [POLY1305] is a Wegman-Carter, one-time authenticator
       designed by D. J. Bernstein.  Poly1305 takes a 256-bit, one-time key
       and a message, and it produces a 16-byte tag that authenticates the
       message such that an attacker has a negligible chance of producing a
       valid tag for an inauthentic message.  It is described in [RFC7539].

       ChaCha and Poly1305 have both been designed for high performance in
       software implementations.  They typically admit a compact
       implementation that uses few resources and inexpensive operations,
       which makes them suitable on a wide range of architectures.  They
       have also been designed to minimize leakage of information through
       side-channels.

       Recent attacks [CBC-ATTACK] have indicated problems with the CBC-mode
       cipher suites in TLS and DTLS, as well as issues with the only
       supported stream cipher (RC4) [RC4-ATTACK].  While the existing
       Authenticated Encryption with Associated Data (AEAD) cipher suites
       (based on AES-GCM) address some of these issues, there are concerns
       about their performance and ease of software implementation.

       Therefore, a new stream cipher to replace RC4 and address all the
       previous issues is needed.  It is the purpose of this document to
       describe a secure stream cipher for both TLS and DTLS that is
       comparable to RC4 in speed on a wide range of platforms and can be
       implemented easily without being vulnerable to software side-channel
       attacks.

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    RFC 7905                 ChaCha-Poly1305 for TLS               June 2016

    2.  ChaCha20 Cipher Suites

       The ChaCha20 and Poly1305 primitives are built into an AEAD algorithm
       [RFC5116], AEAD_CHACHA20_POLY1305, as described in [RFC7539].  This
       AEAD is incorporated into TLS and DTLS as specified in
       Section 6.2.3.3 of [RFC5246].

       AEAD_CHACHA20_POLY1305 requires a 96-bit nonce, which is formed as
       follows:

       1.  The 64-bit record sequence number is serialized as an 8-byte,
           big-endian value and padded on the left with four 0x00 bytes.

       2.  The padded sequence number is XORed with the client_write_IV
           (when the client is sending) or server_write_IV (when the server
           is sending).

       In DTLS, the 64-bit seq_num is the 16-bit epoch concatenated with the
       48-bit sequence_number.

       This nonce construction is different from the one used with AES-GCM
       in TLS 1.2 but matches the scheme expected to be used in TLS 1.3.
       The nonce is constructed from the record sequence number and the
       shared secret, both of which are known to the recipient.  The
       advantage is that no per-record, explicit nonce need be transmitted,
       which saves eight bytes per record and prevents implementations from
       mistakenly using a random nonce.  Thus, in the terms of [RFC5246],
       SecurityParameters.fixed_iv_length is twelve bytes and
       SecurityParameters.record_iv_length is zero bytes.

       The following cipher suites are defined:

       TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256   = {0xCC, 0xA8}
       TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256 = {0xCC, 0xA9}
       TLS_DHE_RSA_WITH_CHACHA20_POLY1305_SHA256     = {0xCC, 0xAA}

       TLS_PSK_WITH_CHACHA20_POLY1305_SHA256         = {0xCC, 0xAB}
       TLS_ECDHE_PSK_WITH_CHACHA20_POLY1305_SHA256   = {0xCC, 0xAC}
       TLS_DHE_PSK_WITH_CHACHA20_POLY1305_SHA256     = {0xCC, 0xAD}
       TLS_RSA_PSK_WITH_CHACHA20_POLY1305_SHA256     = {0xCC, 0xAE}

       The DHE_RSA, ECDHE_RSA, ECDHE_ECDSA, PSK, ECDHE_PSK, DHE_PSK, and
       RSA_PSK key exchanges for these cipher suites are unaltered; thus,
       they are performed as defined in [RFC5246], [RFC4492], and [RFC5489].

       The pseudorandom function (PRF) for all the cipher suites defined in
       this document is the TLS PRF with SHA-256 [FIPS180-4] as the hash
       function.

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    RFC 7905                 ChaCha-Poly1305 for TLS               June 2016

    3.  IANA Considerations

       IANA has added the following entries in the TLS Cipher Suite
       Registry:

       TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256   = {0xCC, 0xA8}
       TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256 = {0xCC, 0xA9}
       TLS_DHE_RSA_WITH_CHACHA20_POLY1305_SHA256     = {0xCC, 0xAA}

       TLS_PSK_WITH_CHACHA20_POLY1305_SHA256         = {0xCC, 0xAB}
       TLS_ECDHE_PSK_WITH_CHACHA20_POLY1305_SHA256   = {0xCC, 0xAC}
       TLS_DHE_PSK_WITH_CHACHA20_POLY1305_SHA256     = {0xCC, 0xAD}
       TLS_RSA_PSK_WITH_CHACHA20_POLY1305_SHA256     = {0xCC, 0xAE}

    4.  Security Considerations

       ChaCha20 follows the same basic principle as Salsa20 [SALSA20SPEC], a
       cipher with significant security review [SALSA20-SECURITY] [ESTREAM].
       At the time of writing this document, there are no known significant
       security problems with either cipher, and ChaCha20 is shown to be
       more resistant in certain attacks than Salsa20 [SALSA20-ATTACK].
       Furthermore, ChaCha20 was used as the core of the BLAKE hash
       function, a SHA3 finalist, which has received considerable
       cryptanalytic attention [NIST-SHA3].

       Poly1305 is designed to ensure that forged messages are rejected with
       a probability of 1-(n/2^107), where n is the maximum length of the
       input to Poly1305.  In the case of (D)TLS, this means a maximum
       forgery probability of about 1 in 2^93.

       The cipher suites described in this document require that a nonce
       never be repeated under the same key.  The design presented ensures
       this by using the TLS sequence number, which is unique and does not
       wrap [RFC5246].

       It should be noted that AEADs, such as ChaCha20-Poly1305, are not
       intended to hide the lengths of plaintexts.  When this document
       speaks of side-channel attacks, it is not considering traffic
       analysis, but rather timing and cache side-channels.  Traffic
       analysis, while a valid concern, is outside the scope of the AEAD and
       is being addressed elsewhere in future versions of TLS.

       Otherwise, this document should not introduce any additional security
       considerations other than those that follow from the use of the
       AEAD_CHACHA20_POLY1305 construction, thus the reader is directed to
       the Security Considerations section of [RFC7539].

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    RFC 7905                 ChaCha-Poly1305 for TLS               June 2016

    5.  References

    5.1.  Normative References

       [FIPS180-4]
                  National Institute of Standards and Technology, "Secure
                  Hash Standard (SHS)", FIPS PUB 180-4,
                  DOI 10.6028/NIST.FIPS180-4, August 2015,
                  <http://nvlpubs.nist.gov/nistpubs/FIPS/
                  NIST.FIPS.180-4.pdf>.

       [RFC4492]  Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
                  Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
                  for Transport Layer Security (TLS)", RFC 4492,
                  DOI 10.17487/RFC4492, May 2006,
                  <http://www.rfc-editor.org/info/rfc4492>.

       [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
                  (TLS) Protocol Version 1.2", RFC 5246,
                  DOI 10.17487/RFC5246, August 2008,
                  <http://www.rfc-editor.org/info/rfc5246>.

       [RFC5489]  Badra, M. and I. Hajjeh, "ECDHE_PSK Cipher Suites for
                  Transport Layer Security (TLS)", RFC 5489,
                  DOI 10.17487/RFC5489, March 2009,
                  <http://www.rfc-editor.org/info/rfc5489>.

       [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
                  Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
                  January 2012, <http://www.rfc-editor.org/info/rfc6347>.

       [RFC7539]  Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
                  Protocols", RFC 7539, DOI 10.17487/RFC7539, May 2015,
                  <http://www.rfc-editor.org/info/rfc7539>.

    5.2.  Informative References

       [CBC-ATTACK]
                  AlFardan, N. and K. Paterson, "Lucky Thirteen: Breaking
                  the TLS and DTLS Record Protocols", IEEE Symposium
                  on Security and Privacy, 2013,
                  <http://www.ieee-security.org/TC/SP2013/papers/
                  4977a526.pdf>.

       [CHACHA]   Bernstein, D., "ChaCha, a variant of Salsa20", January
                  2008, <http://cr.yp.to/chacha/chacha-20080128.pdf>.

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    RFC 7905                 ChaCha-Poly1305 for TLS               June 2016

       [ESTREAM]  Babbage, S., DeCanniere, C., Cantenaut, A., Cid, C.,
                  Gilbert, H., Johansson, T., Parker, M., Preneel, B.,
                  Rijmen, V., and M. Robshaw, "The eSTREAM Portfolio
                  (rev. 1)", September 2008,
                  <http://www.ecrypt.eu.org/stream/finallist.html>.

       [NIST-SHA3]
                  Chang, S., Perlner, R., Burr, W., Turan, M., Kelsey, J.,
                  Paul, S., and L. Bassham, "Third-Round Report of the SHA-3
                  Cryptographic Hash Algorithm Competition",
                  DOI 10.6028/NIST.IR.7896, November 2012,
                  <http://dx.doi.org/10.6028/NIST.IR.7896>.

       [POLY1305] Bernstein, D., "The Poly1305-AES message-authentication
                  code", FSE '05 Proceedings of the 12th international
                  conference on Fast Software Encryption Pages 32-49,
                  DOI 10.1007/11502760_3, February 2005,
                  <http://cr.yp.to/mac/poly1305-20050329.pdf>.

       [RC4-ATTACK]
                  Isobe, T., Ohigashi, T., Watanabe, Y., and M. Morii, "Full
                  Plaintext Recovery Attack on Broadcast RC4", International
                  Workshop on Fast Software Encryption FSE,
                  DOI 10.1007/978-3-662-43933-3_10, 2013,
                  <http://www.iacr.org/archive/
                  fse2013/84240167/84240167.pdf>.

       [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
                  Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
                  <http://www.rfc-editor.org/info/rfc5116>.

       [SALSA20-ATTACK]
                  Aumasson, J-P., Fischer, S., Khazaei, S., Meier, W., and
                  C. Rechberger, "New Features of Latin Dances: Analysis of
                  Salsa, ChaCha, and Rumba",
                  DOI 10.1007/978-3-540-71039-4_30, 2007,
                  <http://eprint.iacr.org/2007/472.pdf>.

       [SALSA20-SECURITY]
                  Bernstein, D., "Salsa20 security", April 2005,
                  <http://cr.yp.to/snuffle/security.pdf>.

       [SALSA20SPEC]
                  Bernstein, D., "Salsa20 specification", April 2005,
                  <http://cr.yp.to/snuffle/spec.pdf>.

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    RFC 7905                 ChaCha-Poly1305 for TLS               June 2016

    Acknowledgements

       The authors would like to thank Zooko Wilcox-O'Hearn, Samuel Neves,
       and Colm MacCarthaigh for their suggestions and guidance.

    Authors' Addresses

       Adam Langley
       Google, Inc.

       Email: agl@google.com

       Wan-Teh Chang
       Google, Inc.

       Email: wtc@google.com

       Nikos Mavrogiannopoulos
       Red Hat

       Email: nmav@redhat.com

       Joachim Strombergson
       Secworks Sweden AB

       Email: joachim@secworks.se
       URI:   http://secworks.se/

       Simon Josefsson
       SJD AB

       Email: simon@josefsson.org
       URI:   http://josefsson.org/

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