Internet Engineering Task Force (IETF) R. Housley
Request for Comments: 8419 Vigil Security
Category: Standards Track July 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 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.
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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 Signed-data Conventions .........................................5
3.1. Signed-Data Signed-data Conventions with Signed Attributes .............5
3.2. Signed-Data 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
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].
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 }
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.
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 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 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.
The SignerInfo signature field contains the octet string resulting
from the EdDSA private key signing operation.
3.2. Signed-Data 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.
The generation of EdDSA private key relies on random numbers. The
use of inadequate pseudorandom 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>.
[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, July 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>.
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