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Versions: 00 01 02 03 draft-irtf-cfrg-xchacha

(No Working Group)                                        S. Arciszewski
Internet-Draft                            Paragon Initiative Enterprises
Intended status: Informational                         September 7, 2018
Expires: March 11, 2019


       XChaCha: eXtended-nonce ChaCha and AEAD_XChaCha20_Poly1305
                      draft-arciszewski-xchacha-00

Abstract

   The eXtended-nonce ChaCha cipher construction (XChaCha) allows for
   ChaCha-based ciphersuites to accept a 192-bit nonce with similar
   guarantees to the original construction, except with a much lower
   probability of nonce misuse occurring.  This enables XChaCha
   constructions to be stateless, while retaining the same security
   assumptions as ChaCha.

   This document defines XChaCha20, which uses HChaCha20 to convert the
   key and part of the nonce into a subkey, which is in turn used with
   the remainder of the nonce with ChaCha20 to generate a pseudorandom
   keystream (e.g. for message encryption).

   This document also defines AEAD_XChaCha20_Poly1305, a variant of
   [RFC7539] that utilizes the XChaCha20 construction in place of
   ChaCha20.

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
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   This Internet-Draft will expire on March 11, 2019.








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Copyright Notice

   Copyright (c) 2018 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
   (https://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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notation and Conventions  . . . . . . . . . . . . . . . .   3
   2.  AEAD_XChaCha20_Poly1305 . . . . . . . . . . . . . . . . . . .   3
     2.1.  Motivation for XChaCha20-Poly1305 . . . . . . . . . . . .   3
     2.2.  HChaCha20 . . . . . . . . . . . . . . . . . . . . . . . .   4
       2.2.1.  Test Vector for the HChaCha20 Block Function  . . . .   4
     2.3.  XChaCha20 . . . . . . . . . . . . . . . . . . . . . . . .   5
       2.3.1.  XChaCha20 Pseudocode  . . . . . . . . . . . . . . . .   6
   3.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     3.2.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   Appendix A.  Additional Test Vectors  . . . . . . . . . . . . . .   7
     A.1.  Example and Test Vector for AEAD_XCHACHA20_POLY1305 . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   AEAD constructions (Authenticated Encryption with Associated Data)
   allow for message confidentiality to be assured even in the presence
   of adaptive chosen-ciphertext attacks, but they're known to be
   brittle to nonce-misuse conditions [1].

   Several nonce misuse resistant cipher constructions have been
   proposed over the years, including AES-SIV ([RFC5297]), AES-GCM-SIV
   [2], and several CAESAR candidates [3].

   However, a more straightforward strategy can prevent nonce misuse
   conditions in environments where a large number of messages are
   encrypted.  Simply use a large enough nonce such that applications
   can generate them randomly for each message and the probability of a
   collision remains low.



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   To this end, we propose a solution that is already implemented in
   many software projects that extends the nonce of ChaCha20 to 192 bits
   and uses it to build an AEAD construction.

1.1.  Notation and Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  AEAD_XChaCha20_Poly1305

   XChaCha20-Poly1305 is a variant of the ChaCha20-Poly1305 AEAD
   construction as defined in [RFC7539] that uses a 192-bit nonce
   instead of a 96-bit nonce.

   The algorithm for XChaCha20-Poly1305 is as follows:

   1.  Calculate a subkey from the first 16 bytes of the nonce and the
       key, using HChaCha20 (Section 2.2).

   2.  Use the subkey and remaining 8 bytes of the nonce (prefixed with
       4 NUL bytes) with AEAD_CHACHA20_POLY1305 from [RFC7539] as
       normal.  The definition for XChaCha20 is given in Section 2.3.

   XChaCha20-Poly1305 implementations already exist in WireGuard [4],
   libsodium [5], Monocypher [6], xsecretbox [7], and in Go's crypto/
   chacha20poly1305 [8] library.

   Similarly, Google's HPolyC [9] implements XChaCha12-Poly1305.

2.1.  Motivation for XChaCha20-Poly1305

   The nonce used by the original ChaCha20-Poly1305 is too short to
   safely use with random strings for long-lived keys.
   XChaCha20-Poly1305 does not have this restriction.

   By generating a subkey from a 128-bit nonce and the key, a reuse of
   only the latter 64 bits of the nonce isn't security-affecting, since
   the key (and thus, keystream) will be different.  Additionally a re-
   use of only the first 128 bits of the nonce isn't security-affecting,
   as the nonce derived from the latter 64 bits is different.

   Assuming a secure random number generator, random 192-bit nonces
   should experience a single collision (with probability 50%) after
   roughly 2^96 messages (approximately 7.2998163e+28).  A more
   conservative threshold (2^-32 chance of collision) still allows for
   2^64 messages to be sent under a single key.



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   Therefore, with XChaCha20-Poly1305, users can safely generate a
   random 192-bit nonce for each message and not worry about nonce-reuse
   vulnerabilities.

   As long as ChaCha20-Poly1305 is a secure AEAD cipher and ChaCha is a
   secure pseudorandom function (PRF), XChaCha20-Poly1305 is secure.

2.2.  HChaCha20

   *HChaCha20* is an intermediary step towards XChaCha20 based on the
   construction and security proof used to create XSalsa20 [10], an
   extended-nonce Salsa20 variant used in NaCl [11].

   HChaCha20 is initialized the same way as the ChaCha cipher, except
   that HChaCha20 uses a 128-bit nonce and has no counter.

   Consider the two figures below, where each non-whitespace character
   represents one nibble of information about the ChaCha states (all
   numbers little-endian):

                  cccccccc  cccccccc  cccccccc  cccccccc
                  kkkkkkkk  kkkkkkkk  kkkkkkkk  kkkkkkkk
                  kkkkkkkk  kkkkkkkk  kkkkkkkk  kkkkkkkk
                  bbbbbbbb  nnnnnnnn  nnnnnnnn  nnnnnnnn

           ChaCha20 State: c=constant k=key b=blockcount n=nonce

                  cccccccc  cccccccc  cccccccc  cccccccc
                  kkkkkkkk  kkkkkkkk  kkkkkkkk  kkkkkkkk
                  kkkkkkkk  kkkkkkkk  kkkkkkkk  kkkkkkkk
                  nnnnnnnn  nnnnnnnn  nnnnnnnn  nnnnnnnn

                 HChaCha20 State: c=constant k=key n=nonce

   After initialization, proceed through the ChaCha rounds as usual.

   Once the 20 ChaCha rounds have been completed, the first 128 bits and
   last 128 bits of the ChaCha state (both little-endian) are
   concatenated, and this 256-bit subkey is returned.

2.2.1.  Test Vector for the HChaCha20 Block Function

   o  Key = 00:01:02:03:04:05:06:07:08:09:0a:0b:0c:0d:0e:0f:10:11:12:13:
      14:15:16:17:18:19:1a:1b:1c:1d:1e:1f.  The key is a sequence of
      octets with no particular structure before we copy it into the
      HChaCha state.

   o  Nonce = (00:00:00:09:00:00:00:4a:00:00:00:00:31:41:59:27)



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   After setting up the HChaCha state, it looks like this:

                    61707865 3320646e 79622d32 6b206574
                    03020100 07060504 0b0a0908 0f0e0d0c
                    13121110 17161514 1b1a1918 1f1e1d1c
                    09000000 4a000000 00000000 27594131

                     ChaCha state with the key setup.

   After running 20 rounds (10 column rounds interleaved with 10
   "diagonal rounds"), the HChaCha state looks like this:

                    82413b42 27b27bfe d30e4250 8a877d73
                    4864a70a f3cd5479 37cd6a84 ad583c7b
                    8355e377 127ce783 2d6a07e0 e5d06cbc
                    a0f9e4d5 8a74a853 c12ec413 26d3ecdc

                       HChaCha state after 20 rounds

   HChaCha20 will then return only the first and last rows, resulting in
   the following 256-bit key:

                    82413b4 227b27bfe d30e4250 8a877d73
                    a0f9e4d 58a74a853 c12ec413 26d3ecdc

                        Resultant HChaCha20 subkey

2.3.  XChaCha20

   XChaCha20 can be constructed from an existing ChaCha20 implementation
   and HChaCha20.  All one needs to do is:

   1.  Pass the key and the first 16 bytes of the 24-byte nonce to
       HChaCha20 to obtain the subkey.

   2.  Use the subkey and remaining 8 byte nonce with ChaCha20 as normal
       (prefixed by 4 NUL bytes, since [RFC7539] specifies a 12-byte
       nonce).

   XChaCha20 is a stream cipher and offers no integrity guarantees
   without being combined with a MAC algorithm (e.g.  Poly1305).

   The same HChaCha20 subkey derivation can also be used in the context
   of an AEAD_ChaCha20_Poly1305 implementation to create
   AEAD_XChaCha20_Poly1305, as described in Section 2.






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2.3.1.  XChaCha20 Pseudocode

       xchacha20_encrypt(key, nonce, plaintext):
           subkey = hchacha20(key, nonce[0:15])
           return chacha20_encrypt(subkey, nonce[16:23], plaintext)

3.  References

3.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5297]  Harkins, D., "Synthetic Initialization Vector (SIV)
              Authenticated Encryption Using the Advanced Encryption
              Standard (AES)", RFC 5297, DOI 10.17487/RFC5297, October
              2008, <https://www.rfc-editor.org/info/rfc5297>.

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

3.2.  URIs

   [1] https://cryptologie.net/article/361/breaking-https-aes-gcm-or-a-
       part-of-it/

   [2] https://eprint.iacr.org/2017/168.pdf

   [3] https://competitions.cr.yp.to/caesar-submissions.html

   [4] https://www.wireguard.com

   [5] https://download.libsodium.org/doc/secret-key_cryptography/
       xchacha20-poly1305_construction.html

   [6] https://github.com/LoupVaillant/Monocypher

   [7] https://github.com/jedisct1/xsecretbox

   [8] https://godoc.org/golang.org/x/crypto/chacha20poly1305#NewX

   [9] https://github.com/google/hpolyc

   [10] https://cr.yp.to/snuffle/xsalsa-20110204.pdf




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   [11] https://nacl.cr.yp.to

Appendix A.  Additional Test Vectors

A.1.  Example and Test Vector for AEAD_XCHACHA20_POLY1305

   Plaintext:

  000  4c 61 64 69 65 73 20 61 6e 64 20 47 65 6e 74 6c  Ladies and Gentl
  016  65 6d 65 6e 20 6f 66 20 74 68 65 20 63 6c 61 73  emen of the clas
  032  73 20 6f 66 20 27 39 39 3a 20 49 66 20 49 20 63  s of '99: If I c
  048  6f 75 6c 64 20 6f 66 66 65 72 20 79 6f 75 20 6f  ould offer you o
  064  6e 6c 79 20 6f 6e 65 20 74 69 70 20 66 6f 72 20  nly one tip for
  080  74 68 65 20 66 75 74 75 72 65 2c 20 73 75 6e 73  the future, suns
  096  63 72 65 65 6e 20 77 6f 75 6c 64 20 62 65 20 69  creen would be i
  112  74 2e                                            t.

   AAD:

    000  50 51 52 53 c0 c1 c2 c3 c4 c5 c6 c7              PQRS........

   Key:

  000  80 81 82 83 84 85 86 87 88 89 8a 8b 8c 8d 8e 8f  ................
  016  90 91 92 93 94 95 96 97 98 99 9a 9b 9c 9d 9e 9f  ................

   IV:

  000  40 41 42 43 44 45 46 47 48 49 4a fb 4c 4d 4e 4f  @ABCDEFGHIJKLMNO
  016  50 51 52 53 54 55 56 57                          PQRSTUVW

   32-bit fixed-common part:

        000  00 00 00 00                                      ....

   Poly1305 Key:

  000  7b 19 1f 80 f3 61 f0 99 09 4f 6f 4b 8f b9 7d f8  {....a...OoK..}.
  016  47 cc 68 73 a8 f2 b1 90 dd 73 80 71 83 f9 07 d5  G.hs.....s.q....












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   Ciphertext:

  000  bd 6d 17 9d 3e 83 d4 3b 95 76 57 94 93 c0 e9 39  .m..>..;.vW....9
  016  57 2a 17 00 25 2b fa cc be d2 90 2c 21 39 6c bb  W*..%+.....,!9l.
  032  73 1c 7f 1b 0b 4a a6 44 0b f3 a8 2f 4e da 7e 39  s....J.D.../N.~9
  048  ae 64 c6 70 8c 54 c2 16 cb 96 b7 2e 12 13 b4 52  .d.p.T.........R
  064  2f 8c 9b a4 0d b5 d9 45 b1 1b 69 b9 82 c1 bb 9e  /......E..i.....
  080  3f 3f ac 2b c3 69 48 8f 76 b2 38 35 65 d3 ff f9  ??.+.iH.v.85e...
  096  21 f9 66 4c 97 63 7d a9 76 88 12 f6 15 c6 8b 13  !.fL.c}.v.......
  112  b5 2e                                            ..

   Tag:
   c0:87:59:24:c1:c7:98:79:47:de:af:d8:78:0a:cf:49

Author's Address

   Scott Arciszewski
   Paragon Initiative Enterprises
   United States

   Email: security@paragonie.com






























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