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)
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.
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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.
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 220.127.116.11 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 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.
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].
[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>.
[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>.
[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>.