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Versions: (draft-housley-cms-eddsa-signatures) 00 01 02 03 04 05

Internet-Draft                                                R. Housley
Intended status: Standards Track                          Vigil Security
Expires: 11 October 2017                                   11 April 2017


   Use of EdDSA Signatures in the Cryptographic Message Syntax (CMS)
            <draft-ietf-curdle-cms-eddsa-signatures-05.txt>

Abstract

   This document specifies the conventions for using Edwards-curve
   Digital Signature Algorithm (EdDSA) for Curve25519 and Curve448 in
   the Cryptographic Message Syntax (CMS).  For each curve, EdDSA
   defines the PureEdDSA and HashEdDSA modes.  However, the HashEdDSA
   mode is not used with the CMS.  In addition, no context string is
   used with the CMS.

Status of This Memo

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1.  Introduction

   This document specifies the conventions for using the Edwards-curve
   Digital Signature Algorithm (EdDSA) [EDDSA] for Curve25519 and
   Curve448 with the Cryptographic Message Syntax [CMS] signed-data
   content type.  For each curve, [EDDSA] defines the PureEdDSA and
   HashEdDSA modes.  However, the HashEdDSA mode is not used with the
   CMS.  In addition, no context string is used with CMS.  EdDSA with
   Curve25519 is referred to as Ed25519, and EdDSA with Curve448 is
   referred to as Ed448.  The CMS conventions for PureEdDSA with Ed25519
   and Ed448 are described in this document.

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.

2.1.  Algorithm Identifiers

   Each algorithms are identified by an object identifier, and the
   algorithm identifier may contain parameters if needed.








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   The ALGORITHM definition is repeated here for convenience:

      ALGORITHM ::= CLASS {
          &id    OBJECT IDENTIFIER UNIQUE,
          &Type  OPTIONAL }
        WITH SYNTAX {
          OID &id [PARMS &Type] }

2.2.  EdDSA Algorithm Identifiers

   The EdDSA signature algorithm is defined in [EDDSA], and the
   conventions for encoding the public key are defined in
   [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 [CURDLE-PKIX], and they are repeated here for
   convenience:

      sigAlg-Ed25519  ALGORITHM  ::=  { OID id-Ed25519 }

      sigAlg-Ed448    ALGORITHM  ::=  { OID id-Ed448 }

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

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

2.3.  Message Digest Algorithm Identifiers

   When the signer includes signed attributes, a message digest
   algorithm is used to compute the message digest on the eContent
   value.  When signing with Ed25519, the message digest algorithm MUST
   be SHA-512 [FIPS180].  Additional information on SHA-512 is available
   in RFC 6234 [RFC6234].  When signing with Ed448, the message digest
   algorithm MUST be SHAKE256 [FIPS202] with a 512-bit output value.

   Signing with Ed25519 uses SHA-512 as part of the signing operation,
   and signing with Ed448 uses SHAKE256 as part of the signing
   operation.

   For convenience, the object identifiers and parameter syntax for
   these algorithms are repeated here:

      hashAlg-SHA-512  ALGORITHM  ::=  { OID id-sha512 }

      hashAlg-SHAKE256  ALGORITHM  ::=  { OID id-shake256 }





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      hashAlg-SHAKE256-LEN  ALGORITHM  ::=  { OID id-shake256-len
                              PARMS ShakeOutputLen }

      hashalgs  OBJECT IDENTIFIER  ::=  { joint-iso-itu-t(2)
                              country(16) us(840) organization(1)
                              gov(101) csor(3) nistalgorithm(4) 2 }

      id-sha512  OBJECT IDENTIFIER  ::=  { hashAlgs 3 }

      id-shake256  OBJECT IDENTIFIER  ::=  { hashAlgs 12 }

      id-shake256-len  OBJECT IDENTIFIER  ::=  { hashAlgs 18 }

      ShakeOutputLen  ::=  INTEGER  -- Output length in bits

   When using the id-sha512 or id-shake256 algorithm identifier, the
   parameters MUST be absent.

   When using the id-shake256-len algorithm identifier, the parameters
   MUST be present, and the parameter MUST contain 512, encoded as a
   positive integer value.

2.4.  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.

   The data to be signed is processed using PureEdDSA, and then a
   private key operation generates 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.

3.  Signed-data Conventions

   The processing depends on whether the signer includes signed
   attributes.

   The inclusion of signed attributes is preferred, but the conventions
   for signed-data without signed attributes are provided for
   completeness.

3.1.  Signed-data Conventions With Signed Attributes

   The SignedData digestAlgorithms field includes the identifiers of the
   message digest algorithms used by one or more signer.  There MAY be



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   any number of elements in the collection, including zero.  When
   signing with Ed25519, the digestAlgorithm SHOULD include id-sha512,
   and if present, the algorithm parameters field MUST be absent.  When
   signing with Ed448, the digestAlgorithm SHOULD include
   id-shake256-len, and if present, the algorithm parameters field MUST
   also be present, and the parameter MUST contain 512, encoded as a
   positive integer value.

   The SignerInfo digestAlgorithm field includes the identifier of the
   message digest algorithms used by the signer.  When signing with
   Ed25519, the digestAlgorithm MUST be id-sha512, and the algorithm
   parameters field MUST be absent.  When signing with Ed448, the
   digestAlgorithm MUST be id-shake256-len, the algorithm parameters
   field MUST be present, and the parameter MUST contain 512, encoded as
   a positive integer value.

   The SignerInfo signedAttributes MUST include the message-digest
   attribute as specified in Section 11.2 of [RFC5652].  When signing
   with Ed25519, the message-digest attribute MUST contain the message
   digest computed over the eContent value using SHA-512.  When signing
   with Ed448, the message-digest attribute MUST contain the message
   digest computed over the eContent value using SHAKE256 with an output
   length of 512 bits.

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

3.2.  Signed-data Conventions Without Signed Attributes

   The SignedData digestAlgorithms field includes the identifiers of the
   message digest algorithms used by one or more signer.  There MAY be
   any number of elements in the collection, including zero.  When
   signing with Ed25519, list of identifiers MAY include id-sha512, and
   if present, the algorithm parameters field MUST be absent.  When
   signing with Ed448, list of identifiers MAY include id-shake256, and
   if present, the algorithm parameters field MUST be absent.

   The SignerInfo digestAlgorithm field includes the identifier of the
   message digest algorithms used by the signer.  When signing with
   Ed25519, the digestAlgorithm MUST be id-sha512, and the algorithm
   parameters field MUST be absent.  When signing with Ed448, the
   digestAlgorithm MUST be id-shake256, and the algorithm parameters
   field MUST be absent.




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      NOTE: Either id-sha512 or id-shake256 is used as part to the
      private key signing operation.  However, the private key signing
      operation does not take a message digest computed with one of
      these algorithms as an input.

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

4.  Implementation Considerations

   The EdDSA specification [EDDSA] includes the following warning.  It
   deserves highlighting, especially when signed-data is used without
   signed attributes and the content to be signed might be quite large:

      PureEdDSA requires two passes over the input.  Many existing APIs,
      protocols, and environments assume digital signature algorithms
      only need one pass over the input, and may have API or bandwidth
      concerns supporting anything else.

5.  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
   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.

   Unlike DSA and ECDSA, EdDSA does not require the generation of a
   random value for each signature operation.

   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



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   signature process.

6.  IANA Considerations

   This document requires no actions by IANA.

7.  Acknowledgements

   Many thanks to Jim Schaad and Daniel Migault for the careful review
   and comment on the draft document.  Thanks to Quynh Dang for
   coordinating the object identifiers assignment by NIST.

8.  Normative References

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

   [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",
              draft-ietf-curdle-pkix-02, 31 October 2016,
              Work-in-progress.

   [EDDSA]    Josefsson, S. and I. Liusvaara, "Edwards-curve Digital
              Signature Algorithm (EdDSA)", RFC 8032, January 2017.

   [FIPS180]  National Institute of Standards and Technology, U.S.
              Department of Commerce, "Secure Hash Standard", Federal
              Information Processing Standard (FIPS) 180-3, October
              2008.

   [FIPS202]  National Institute of Standards and Technology, U.S.
              Department of Commerce, "SHA-3 Standard: Permutation-Based
              Hash and Extendable-Output Functions", Federal Information
              Processing Standard (FIPS) 202, August 2015.

   [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.



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9.  Informative References

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

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011.

Author's Address

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




































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