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Internet Engineering Task Force (IETF) R. Housley Request for Comments: 8419 Vigil Security Category: Standards Track August 2018 ISSN: 2070-1721

Use of Edwards-Curve Digital Signature Algorithm (EdDSA) Signatures
             in the Cryptographic Message Syntax (CMS)

Abstract

 This document specifies the conventions for using the 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

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc8419.

Copyright Notice

 Copyright (c) 2018 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
 Provisions Relating to IETF Documents
 (https://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Housley Standards Track [Page 1] RFC 8419 Using EdDSA Signatures with CMS August 2018

Table of Contents

 1. Introduction ....................................................2
    1.1. Terminology ................................................2
    1.2. ASN.1 ......................................................2
 2. EdDSA Signature Algorithm .......................................3
    2.1. Algorithm Identifiers ......................................3
    2.2. EdDSA Algorithm Identifiers ................................3
    2.3. Message Digest Algorithm Identifiers .......................4
    2.4. EdDSA Signatures ...........................................4
 3. Signed-data Conventions .........................................5
    3.1. Signed-data Conventions with Signed Attributes .............5
    3.2. Signed-data Conventions without Signed Attributes ..........6
 4. Implementation Considerations ...................................6
 5. Security Considerations .........................................6
 6. IANA Considerations .............................................7
 7. References ......................................................7
    7.1. Normative References .......................................7
    7.2. Informative References .....................................8
 Acknowledgments ....................................................9
 Author's Address ...................................................9

1. Introduction

 This document specifies the conventions for using the Edwards-curve
 Digital Signature Algorithm (EdDSA) [RFC8032] for curve25519
 [CURVE25519] and curve448 [CURVE448] with the Cryptographic Message
 Syntax (CMS) [RFC5652] signed-data content type.  For each curve,
 [RFC8032] 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", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
 capitals, as shown here.

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].

Housley Standards Track [Page 2] RFC 8419 Using EdDSA Signatures with CMS August 2018

2. EdDSA Signature Algorithm

 The Edwards-curve Digital Signature Algorithm (EdDSA) [RFC8032] 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; Ed448 is intended to operate 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 algorithm is identified by an object identifier, and the
 algorithm identifier may contain parameters if needed.
 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 [RFC8032], and the
 conventions for encoding the public key are defined in [RFC8410].
 The id-Ed25519 and id-Ed448 object identifiers are used to identify
 EdDSA public keys in certificates.  The object identifiers are
 specified in [RFC8410], 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 }

Housley Standards Track [Page 3] RFC 8419 Using EdDSA Signatures with CMS August 2018

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 [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 }
    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.

Housley Standards Track [Page 4] RFC 8419 Using EdDSA Signatures with CMS August 2018

 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 [RFC8032], the signature value is the opaque value
 ENC(R) || ENC(S), where || represents concatenation.  As described in
 Section 5.3 of [RFC5652], 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
 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.

Housley Standards Track [Page 5] RFC 8419 Using EdDSA Signatures with CMS August 2018

 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, the list of identifiers MAY include id-sha512,
 and if present, the algorithm parameters field MUST be absent.  When
 signing with Ed448, the 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.
    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 [RFC8032] 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.

Housley Standards Track [Page 6] RFC 8419 Using EdDSA Signatures with CMS August 2018

 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 Elliptic Curve Digital Signature Algorithm (ECDSA),
 EdDSA does not require the generation of a random value for each
 signature operation.
 Using the same private key with different algorithms has the
 potential to leak extra information about the private key to an
 attacker.  For this reason, the same private key SHOULD NOT be used
 with more than one set of EdDSA parameters, although it appears that
 there are no security concerns when using the same private key with
 PureEdDSA and HashEdDSA [RFC8032].
 When computing signatures, the same hash function SHOULD be used for
 all operations.  This reduces the number of failure points in the
 signature process.

6. IANA Considerations

 This document has no IANA actions.

7. References

7.1. Normative References

 [FIPS180]    National Institute of Standards and Technology, "Secure
              Hash Standard (SHS)", FIPS PUB 180-4,
              DOI 10.6028/NIST.FIPS.180-4, August 2015.
 [FIPS202]    National Institute of Standards and Technology, "SHA-3
              Standard: Permutation-Based Hash and Extendable-Output
              Functions", FIPS PUB 202, DOI 10.6028/NIST.FIPS.202,
              August 2015.
 [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

Housley Standards Track [Page 7] RFC 8419 Using EdDSA Signatures with CMS August 2018

 [RFC5652]    Housley, R., "Cryptographic Message Syntax (CMS)",
              STD 70, RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <https://www.rfc-editor.org/info/rfc5652>.
 [RFC8032]    Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/info/rfc8032>.
 [RFC8174]    Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.
 [RFC8410]    Josefsson, S. and J. Schaad, "Algorithm Identifiers for
              Ed25519, Ed448, X25519, and X448 for Use in the Internet
              X.509 Public Key Infrastructure", RFC 8410,
              DOI 10.17487/RFC8410, August 2018,
              <https://www.rfc-editor.org/info/rfc8410>.
 [X680]       ITU-T, "Information technology -- Abstract Syntax
              Notation One (ASN.1): Specification of basic notation",
              ITU-T Recommendation X.680, ISO/IEC 8824-1, August 2015,
              <https://www.itu.int/rec/T-REC-X.680/en>.
 [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, ISO/IEC 8825-1,
              August 2015, <https://www.itu.int/rec/T-REC-X.690/en>.

7.2. Informative References

 [CURVE25519] Bernstein, D., "Curve25519: new Diffie-Hellman speed
              records", DOI 10.1007/11745853_14, February 2006,
              <http://cr.yp.to/ecdh.html>.
 [CURVE448]   Hamburg, M., "Ed448-Goldilocks, a new elliptic curve",
              June 2015, <http://eprint.iacr.org/2015/625>.
 [RANDOM]     Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106,
              RFC 4086, DOI 10.17487/RFC4086, June 2005,
              <https://www.rfc-editor.org/info/rfc4086>.
 [RFC6234]    Eastlake 3rd, D. and T. Hansen, "US Secure Hash
              Algorithms (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <https://www.rfc-editor.org/info/rfc6234>.

Housley Standards Track [Page 8] RFC 8419 Using EdDSA Signatures with CMS August 2018

Acknowledgements

 Many thanks to Jim Schaad, Daniel Migault, and Adam Roach for the
 careful review and comments.  Thanks to Quynh Dang for coordinating
 the object identifiers assignment by NIST.

Author's Address

 Russ Housley
 918 Spring Knoll Drive
 Herndon, VA  20170
 United States of America
 Email: housley@vigilsec.com

Housley Standards Track [Page 9]

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