XChaCha: eXtended-nonce ChaCha and AEAD_XChaCha20_Poly1305
draft-irtf-cfrg-xchacha-01

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Last updated 2019-07-22
Replaces draft-arciszewski-xchacha
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(No Working Group)                                        S. Arciszewski
Internet-Draft                            Paragon Initiative Enterprises
Intended status: Informational                             July 23, 2019
Expires: January 24, 2020

       XChaCha: eXtended-nonce ChaCha and AEAD_XChaCha20_Poly1305
                       draft-irtf-cfrg-xchacha-01

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 helps for long running
   TLS connections.  This also 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

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   This Internet-Draft will expire on January 24, 2020.

Arciszewski             Expires January 24, 2020                [Page 1]
Internet-Draft                                                 July 2019

Copyright Notice

   Copyright (c) 2019 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
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notation and Conventions  . . . . . . . . . . . . . . . .   3
   2.  AEAD_XChaCha20_Poly1305 . . . . . . . . . . . . . . . . . . .   3
     2.1.  Motivation for XChaCha20-Poly1305 . . . . . . . . . . . .   4
     2.2.  HChaCha20 . . . . . . . . . . . . . . . . . . . . . . . .   4
       2.2.1.  Test Vector for the HChaCha20 Block Function  . . . .   5
     2.3.  XChaCha20 . . . . . . . . . . . . . . . . . . . . . . . .   6
       2.3.1.  XChaCha20 Pseudocode  . . . . . . . . . . . . . . . .   6
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
     3.1.  Proving XChaCha20 to be Secure  . . . . . . . . . . . . .   6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     5.2.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Appendix A.  Additional Test Vectors  . . . . . . . . . . . . . .   9
     A.1.  Example and Test Vector for AEAD_XCHACHA20_POLY1305 . . .   9
     A.2.  Example and Test Vectors for XChaCha20  . . . . . . . . .  11
       A.2.1.  With Block Counter = 0  . . . . . . . . . . . . . . .  11
       A.2.2.  With Block Counter = 1  . . . . . . . . . . . . . . .  13
     A.3.  Developer-Friendly Test Vectors . . . . . . . . . . . . .  15
       A.3.1.  AEAD_XCHACHA20_POLY1305 . . . . . . . . . . . . . . .  15
       A.3.2.  XChaCha20 . . . . . . . . . . . . . . . . . . . . . .  16
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  18

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].

Arciszewski             Expires January 24, 2020                [Page 2]
Internet-Draft                                                 July 2019

   Several nonce misuse resistant cipher constructions have been
   proposed over the years, including AES-SIV ([RFC5297]), AES-GCM-SIV
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