Skip to main content

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

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7905.
Authors Adam Langley , Wan-Teh Chang , Nikos Mavrogiannopoulos , Joachim Strombergson , Simon Josefsson
Last updated 2018-12-20 (Latest revision 2015-12-16)
Replaces draft-mavrogiannopoulos-chacha-tls
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Sean Turner
Shepherd write-up Show Last changed 2016-03-10
IESG IESG state Became RFC 7905 (Proposed Standard)
Action Holders
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Stephen Farrell
Send notices to "Sean Turner" <>
IANA IANA review state IANA OK - Actions Needed
IANA action state RFC-Ed-Ack
Network Working Group                                         A. Langley
Internet-Draft                                                  W. Chang
Updates: 5246, 6347 (if approved)                             Google Inc
Intended status: Standards Track                    N. Mavrogiannopoulos
Expires: June 18, 2016                                           Red Hat
                                                         J. Strombergson
                                                      Secworks Sweden AB
                                                            S. Josefsson
                                                                  SJD AB
                                                       December 16, 2015

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


   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.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 18, 2016.

Copyright Notice

   Copyright (c) 2015 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
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect

Langley, et al.           Expires June 18, 2016                 [Page 1]
Internet-Draft                 chacha-tls                  December 2015

   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  ChaCha20 Cipher Suites  . . . . . . . . . . . . . . . . . . .   3
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

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

   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 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, and will be 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 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 also described in

   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.

Langley, et al.           Expires June 18, 2016                 [Page 2]
Internet-Draft                 chacha-tls                  December 2015

   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 AEAD
   cipher suites (based on AES-GCM) address some of these issues, there
   are concerns about their performance and ease of software

   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

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 of [RFC5246].

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

   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 seq_num.

   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 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.

Langley, et al.           Expires June 18, 2016                 [Page 3]
Internet-Draft                 chacha-tls                  December 2015

     TLS_DHE_RSA_WITH_CHACHA20_POLY1305_SHA256     = {0xTBD, 0xTBD}

     TLS_PSK_WITH_CHACHA20_POLY1305_SHA256         = {0xTBD, 0xTBD}
     TLS_DHE_PSK_WITH_CHACHA20_POLY1305_SHA256     = {0xTBD, 0xTBD}
     TLS_RSA_PSK_WITH_CHACHA20_POLY1305_SHA256     = {0xTBD, 0xTBD}

   RSA_PSK key exchanges for these cipher suites are unaltered and thus
   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 as the hash function.

3.  IANA Considerations

   IANA is requested to add the following entries in the TLS Cipher
   Suite Registry:

  TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256   = {0xTBD, 0xTBD} {0xCC, 0xA8}
  TLS_DHE_RSA_WITH_CHACHA20_POLY1305_SHA256     = {0xTBD, 0xTBD} {0xCC, 0xAA}

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

   The cipher suite numbers listed in the second column are numbers used
   for cipher suite interoperability testing and it's suggested that
   IANA use these values for assignment.

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, that 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

Langley, et al.           Expires June 18, 2016                 [Page 4]
Internet-Draft                 chacha-tls                  December 2015

   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 is
   never repeated under the same key.  The design presented ensures this
   by using the TLS sequence number, which is unique and does not wrap

   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].

5.  Acknowledgements

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

6.  References

6.1.  Normative References

   [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,

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [RFC5489]  Badra, M. and I. Hajjeh, "ECDHE_PSK Cipher Suites for
              Transport Layer Security (TLS)", RFC 5489,
              DOI 10.17487/RFC5489, March 2009,

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <>.

Langley, et al.           Expires June 18, 2016                 [Page 5]
Internet-Draft                 chacha-tls                  December 2015

   [RFC7539]  Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
              Protocols", RFC 7539, DOI 10.17487/RFC7539, May 2015,

6.2.  Informative References

   [CHACHA]   Bernstein, D., "ChaCha, a variant of Salsa20", January
              2008, <>.

              Bernstein, D., "The Poly1305-AES message-authentication
              code.", March 2005,

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,

              Bernstein, D., "Salsa20 specification", April 2005,

              Bernstein, D., "Salsa20 security", April 2005,

   [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,

              AlFardan, N. and K. Paterson, "Lucky Thirteen: Breaking
              the TLS and DTLS Record Protocols", IEEE Symposium on
              Security and Privacy , 2013.

              Isobe, T., Ohigashi, T., Watanabe, Y., and M. Morii, "Full
              Plaintext Recovery Attack on Broadcast RC4", International
              Workshop on Fast Software Encryption , 2013.

              Aumasson, J-P., Fischer, S., Khazaei, S., Meier, W., and
              C. Rechberger, "New Features of Latin Dances: Analysis of
              Salsa, ChaCha, and Rumba", 2007,

Langley, et al.           Expires June 18, 2016                 [Page 6]
Internet-Draft                 chacha-tls                  December 2015

              Chang, S., Burr, W., Kelsey, J., Paul, S., and L. Bassham,
              "Third-Round Report of the SHA-3 Cryptographic Hash
              Algorithm Competition", 2012,

Authors' Addresses

   Adam Langley
   Google Inc


   Wan-Teh Chang
   Google Inc


   Nikos Mavrogiannopoulos
   Red Hat


   Joachim Strombergson
   Secworks Sweden AB


   Simon Josefsson


Langley, et al.           Expires June 18, 2016                 [Page 7]