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

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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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.

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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
   signature values.  When used to identify signature algorithms, the
   AlgorithmIdentifier parameters field MUST be absent.

   An EdDSA private key operation is produces the opaque signature
   value, ENC(R) || ENC(S), as described in Section 3.3 of [EDDSA].  The
   resulting octet string is carried in the signature field of
   SignerInfo.

3.  Signed-data Conventions

   The digestAlgorithms field SHOULD contain the one-way hash function
   used to compute the message digest on the eContent value.

   If signedAttributes are present, the same one-way hash function
   SHOULD be used to compute the message digest on both the eContent and
   the signedAttributes.

   The signatureAlgorithm field MUST contain either id-Ed25519 or id-
   Ed448, depending on the elliptic curve that was used by the signer.
   The algorithm parameters field MUST be absent.

   The signature field contains the octet string  resulting from the
   EdDSA private key signing operation.

4.  Security Considerations

   Implementations must protect the EdDSA private key.  Compromise of
   the EdDSA private key may result in the ability to forge signatures.

   The generation of EdDSA private key relies on random numbers.  The
   use of inadequate pseudo-random number generators (PRNGs) to generate

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   these values can result in little or no security.  An attacker may
   find it much easier to reproduce the PRNG environment that produced
   the keys, searching the resulting small set of possibilities, rather
   than brute force searching the whole key space.  The generation of
   quality random numbers is difficult.  RFC 4086 [RANDOM] offers
   important guidance in this area.

   Using the same private key for different algorithms has the potential
   of allowing an attacker to get extra information about the private
   key.  For this reason, the same private key SHOULD NOT be used with
   more than one EdDSA set of parameters.  For example, do not use the
   same private key with PureEdDSA and HashEdDSA.

   When computing signatures, the same hash function should be used for
   all operations.  This reduces the number of failure points in the
   signature process.

5.  Normative References

   [CMS]      Housley, R., "Cryptographic Message Syntax (CMS)", RFC
              5652, September 2009.

   [EDDSA]    Josefsson, S. and I. Liusvaara, "Edwards-curve Digital
              Signature Algorithm (EdDSA)", draft-irtf-cfrg-eddsa-00,
              (work in progress), October 2015.

   [ID.curdle-pkix]
              Josefsson, S., and J. Schaad, "Algorithm Identifiers for
              Ed25519, Ed25519ph, Ed448, Ed448ph, X25519 and X448 for
              use in the Internet X.509 Public Key Infrastructure",
              15 August 2016, Work-in-progress.

   [STDWORDS] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [X680]     ITU-T, "Information technology -- Abstract Syntax Notation
              One (ASN.1): Specification of basic notation", ITU-T
              Recommendation X.680, 2015.

   [X690]     ITU-T, "Information technology -- ASN.1 encoding rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER)", ITU-T Recommendation X.690, 2015.

6.  Informative References

   [RANDOM]   Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", RFC 4086, June 2005.

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Author Address

   Russ Housley
   918 Spring Knoll Drive
   Herndon, VA 20170
   USA
   housley@vigilsec.com

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