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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