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) 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. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 8 March 2017. Copyright Notice Copyright (c) 2016 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 (http://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 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 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 Housley Using EdDSA Signatures with CMS [Page 3] Internet-Draft 8 September 2016 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. Housley Using EdDSA Signatures with CMS [Page 4] Internet-Draft 8 September 2016 Author Address Russ Housley 918 Spring Knoll Drive Herndon, VA 20170 USA housley@vigilsec.com Housley Using EdDSA Signatures with CMS [Page 5]