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RFC 7905

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

Updates:    5246    6347


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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.

[Page 2] 
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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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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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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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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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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   [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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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/