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IETF RFC 7905
Last modified on Thursday, June 23rd, 2016
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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/RFC 7905.
Langley, et al. Standards Track PAGE 1 
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
Langley, et al. Standards Track PAGE 2
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 [RFC 5246] as well as version 1.2 or later of
the Datagram Transport Layer Security (DTLS) protocol [RFC 6347].
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 [RFC 7539].
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 [RFC 7539].
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.
Langley, et al. Standards Track PAGE 3
RFC 7905 ChaCha-Poly1305 for TLS June 2016
2. ChaCha20 Cipher Suites
The ChaCha20 and Poly1305 primitives are built into an AEAD algorithm
[RFC 5116], AEAD_CHACHA20_POLY1305, as described in [RFC 7539]. This
AEAD is incorporated into TLS and DTLS as specified in
Section 6.2.3.3 of [RFC 5246].
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 [RFC 5246],
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 [RFC 5246], [RFC 4492], and [RFC 5489].
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.
Langley, et al. Standards Track PAGE 4
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 [RFC 5246].
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 [RFC 7539].
Langley, et al. Standards Track PAGE 5
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>.
[RFC 4492] 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/RFC 4492, May 2006,
<http://www.rfc-editor.org/info/RFC 4492>.
[RFC 5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC 5246, August 2008,
<http://www.rfc-editor.org/info/RFC 5246>.
[RFC 5489] Badra, M. and I. Hajjeh, "ECDHE_PSK Cipher Suites for
Transport Layer Security (TLS)", RFC 5489,
DOI 10.17487/RFC 5489, March 2009,
<http://www.rfc-editor.org/info/RFC 5489>.
[RFC 6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC 6347,
January 2012, <http://www.rfc-editor.org/info/RFC 6347>.
[RFC 7539] Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
Protocols", RFC 7539, DOI 10.17487/RFC 7539, May 2015,
<http://www.rfc-editor.org/info/RFC 7539>.
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>.
Langley, et al. Standards Track PAGE 6
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>.
[RFC 5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC 5116, January 2008,
<http://www.rfc-editor.org/info/RFC 5116>.
[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>.
Langley, et al. Standards Track PAGE 7
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/
Langley, et al. Standards Track PAGE 8
RFC TOTAL SIZE: 15575 bytes
PUBLICATION DATE: Thursday, June 23rd, 2016
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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