Use of EdDSA Signatures in the Cryptographic Message Syntax (CMS)
draft-ietf-curdle-cms-eddsa-signatures-00

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Internet-Draft                                                R. Housley
Intended status: Standards Track                          Vigil Security
Expires: 8 March 2017                                   8 September 2016

   Use of EdDSA Signatures in the Cryptographic Message Syntax (CMS)

            <draft-ietf-curdle-cms-eddsa-signatures-00.txt>

Abstract

   This document describes the conventions for using Edwards-curve
   Digital Signature Algorithm (EdDSA) in the Cryptographic Message
   Syntax (CMS).  The conventions for Ed25519 and Ed448 are described,
   but Ed25519ph and Ed448ph are not used with the CMS.

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Housley              Using EdDSA Signatures with CMS            [Page 1]
Internet-Draft                                          8 September 2016

1.  Introduction

   This document specifies the conventions for using the Edwards-curve
   Digital Signature Algorithm (EdDSA) [EDDSA] with the Cryptographic
   Message Syntax [CMS] signed-data content type.  For each curve,
   [EDDSA] defines two modes, the PureEdDSA mode without pre-hashing,
   and the HashEdDSA mode with pre-hashing.  The CMS conventions for two
   PureEdDSA curves (Ed25519 and Ed448) are described in this document,
   but HashEdDSA is not used with the CMS.

1.1.  Terminology

   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 [STDWORDS].

1.2.  ASN.1

   CMS values are generated using ASN.1 [X680], which uses the Basic
   Encoding Rules (BER) and the Distinguished Encoding Rules (DER)
   [X690].

2.  EdDSA Signature Algorithm

   The Edwards-curve Digital Signature Algorithm (EdDSA) [EDDSA] is a
   variant of Schnorr's signature system with (possibly twisted) Edwards
   curves.  Ed25519 is intended to operate at around the 128-bit
   security level, and Ed448 at around the 224-bit security level.

   One of the parameters of the EdDSA algorithm is the "prehash"
   function.  This may be the identity function, resulting in an
   algorithm called PureEdDSA, or a collision-resistant hash function,
   resulting in an algorithm called HashEdDSA.  In most situations the
   CMS SignedData includes signed attributes, including the message
   digest of the content.  Since HashEdDSA offers no benefit when signed
   attributes are present, only PureEdDSA is used with the CMS.

   A message digest is computed over the data to be signed using
   PureEdDSA, and then a private key operation is performed to generate
   the signature value.  As described in Section 3.3 of [EDDSA], the
   signature value is the opaque value ENC(R) || ENC(S).  As described
   in Section 5.3 of [CMS], the signature value is ASN.1 encoded as an
   OCTET STRING and included in the signature field of SignerInfo.

Housley              Using EdDSA Signatures with CMS            [Page 2]
Internet-Draft                                          8 September 2016

2.1.  EdDSA Algorithm Identifiers

   The EdDSA signature algorithm is defined in [EDDSA], and the
   conventions for encoding the public key are defined in [ID.curdle-
   pkix].

   The id-Ed25519 and id-Ed448 object identifiers are used to identify
   EdDSA public keys in certificates.  The object identifiers are
   specified in [ID.curdle-pkix], and they are repeated here for
   convenience:

      id-Ed25519  OBJECT IDENTIFIER ::= { 1 3 101 112 }
      id-Ed448    OBJECT IDENTIFIER ::= { 1 3 101 113 }

2.2.  EdDSA Signatures

   The id-Ed25519 and id-Ed448 object identifiers are also used for
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