Using ChaCha20-Poly1305 Authenticated Encryption in the Cryptographic Message Syntax (CMS)
Internet-Draft R. Housley
Intended status: Standards Track Vigil Security
Expires: 28 April 2017 28 October 2016
Using ChaCha20-Poly1305 Authenticated Encryption
in the Cryptographic Message Syntax (CMS)
This document describes the conventions for using ChaCha20-Poly1305
Authenticated Encryption in the Cryptographic Message Syntax (CMS).
ChaCha20-Poly1305 is an authenticated encryption algorithm
constructed of the ChaCha stream cipher and Poly1305 authenticator.
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This document specifies the conventions for using the
ChaCha20-Poly1305 Authenticated Encryption as the content-
authenticated-encryption algorithm with the Cryptographic Message
Syntax (CMS) [CMS] authenticated-enveloped-data content type
ChaCha [CHACHA] is a stream cipher developed by D. J. Bernstein in
2008. It is a refinement of Salsa20, which is one of the ciphers in
the eSTREAM portfolio [ESTREAM].
ChaCha20 is the 20-round variant of ChaCha; it requires a 256-bit key
and a 96-bit nonce. ChaCha20 is described in [FORIETF].
Poly1305 [POLY1305] is a Wegman-Carter, one-time authenticator
designed by D. J. Bernstein. Poly1305 produces a 16-byte
authentication tag; it requires a 256-bit, single-use key. Poly1305
is also described in [FORIETF].
ChaCha20 and Poly1305 have been designed for high performance
software implementations. They can typically be implemented with few
resources and inexpensive operations, making them suitable on a wide
range of systems. They have also been designed to minimize leakage
of information through side channels.
1.1. The ChaCha20 and Poly1305 AEAD Construction
ChaCha20 and Poly1305 have been combined to create an Authenticated
Encryption with Associated Data (AEAD) algorithm [AEAD]. This AEAD
algorithm is often referred to as AEAD_CHACHA20_POLY1305, and it is
AEAD_CHACHA20_POLY1305 accepts four inputs: a 256-bit key, a 96-bit
nonce, an arbitrary length plaintext, and an arbitrary length
additional authenticated data (AAD). As the name implies, a nonce
value cannot be used securely more than once with the same key.
A high-level summary of AEAD_CHACHA20_POLY1305 authenticated
encryption processing is:
1) A 256-bit Poly1305 one-time key is generated from the input
256-bit key and nonce using the procedure described in
Section 2.6 of [FORIETF].
2) The ChaCha20 encryption function is used to encrypt the
plaintext, using the same 256-bit key and nonce, and with the
initial counter set to 1.
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3) The Poly1305 function is used with the Poly1305 key from
step 1, and a buffer constructed as a concatenation of the AAD,
padding1, the ciphertext, padding2, the length of the AAD in
octets, and the length of the ciphertext in octets. The
padding fields contain up to 15 octets, with all bits set to
zero, and the padding brings the total length of the buffer so
far to an integral multiple of 16. If the buffer length was
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