Internet Engineering Task Force (IETF) M. Jones
Request for Comments: 8812 Microsoft
Category: Standards Track August 2020
ISSN: 2070-1721
CBOR Object Signing and Encryption (COSE) and JSON Object Signing and
Encryption (JOSE) Registrations for Web Authentication (WebAuthn)
Algorithms
Abstract
The W3C Web Authentication (WebAuthn) specification and the FIDO
Alliance FIDO2 Client to Authenticator Protocol (CTAP) specification
use CBOR Object Signing and Encryption (COSE) algorithm identifiers.
This specification registers the following algorithms (which are used
by WebAuthn and CTAP implementations) in the IANA "COSE Algorithms"
registry: RSASSA-PKCS1-v1_5 using SHA-256, SHA-384, SHA-512, and SHA-
1; and Elliptic Curve Digital Signature Algorithm (ECDSA) using the
secp256k1 curve and SHA-256. It registers the secp256k1 elliptic
curve in the IANA "COSE Elliptic Curves" registry. Also, for use
with JSON Object Signing and Encryption (JOSE), it registers the
algorithm ECDSA using the secp256k1 curve and SHA-256 in the IANA
"JSON Web Signature and Encryption Algorithms" registry and the
secp256k1 elliptic curve in the IANA "JSON Web Key Elliptic Curve"
registry.
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/rfc8812.
Copyright Notice
Copyright (c) 2020 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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction
1.1. Requirements Notation and Conventions
2. RSASSA-PKCS1-v1_5 Signature Algorithm
3. Using secp256k1 with JOSE and COSE
3.1. JOSE and COSE secp256k1 Curve Key Representations
3.2. ECDSA Signature with secp256k1 Curve
3.3. Other Uses of the secp256k1 Elliptic Curve
4. IANA Considerations
4.1. COSE Algorithms Registrations
4.2. COSE Elliptic Curves Registrations
4.3. JOSE Algorithms Registrations
4.4. JSON Web Key Elliptic Curves Registrations
5. Security Considerations
5.1. RSA Key Size Security Considerations
5.2. RSASSA-PKCS1-v1_5 with SHA-2 Security Considerations
5.3. RSASSA-PKCS1-v1_5 with SHA-1 Security Considerations
5.4. secp256k1 Security Considerations
6. References
6.1. Normative References
6.2. Informative References
Acknowledgements
Author's Address
1. Introduction
This specification defines how to use several algorithms with CBOR
Object Signing and Encryption (COSE) [RFC8152] that are used by
implementations of the W3C Web Authentication (WebAuthn) [WebAuthn]
and FIDO Alliance FIDO2 Client to Authenticator Protocol (CTAP)
[CTAP] specifications. This specification registers these algorithms
in the IANA "COSE Algorithms" registry [IANA.COSE.Algorithms] and
registers an elliptic curve in the IANA "COSE Elliptic Curves"
registry [IANA.COSE.Curves]. This specification also registers a
corresponding algorithm for use with JSON Object Signing and
Encryption (JOSE) [RFC7515] in the IANA "JSON Web Signature and
Encryption Algorithms" registry [IANA.JOSE.Algorithms] and registers
an elliptic curve in the IANA "JSON Web Key Elliptic Curve" registry
[IANA.JOSE.Curves].
1.1. Requirements Notation and Conventions
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.
2. RSASSA-PKCS1-v1_5 Signature Algorithm
The RSASSA-PKCS1-v1_5 signature algorithm is defined in [RFC8017].
The RSASSA-PKCS1-v1_5 signature algorithm is parameterized with a
hash function (h).
A key of size 2048 bits or larger MUST be used with these algorithms.
Implementations need to check that the key type is 'RSA' when
creating or verifying a signature.
The RSASSA-PKCS1-v1_5 algorithms specified in this document are in
the following table.
+=======+========+=========+===================+=============+
| Name | Value | Hash | Description | Recommended |
+=======+========+=========+===================+=============+
| RS256 | -257 | SHA-256 | RSASSA-PKCS1-v1_5 | No |
| | | | using SHA-256 | |
+-------+--------+---------+-------------------+-------------+
| RS384 | -258 | SHA-384 | RSASSA-PKCS1-v1_5 | No |
| | | | using SHA-384 | |
+-------+--------+---------+-------------------+-------------+
| RS512 | -259 | SHA-512 | RSASSA-PKCS1-v1_5 | No |
| | | | using SHA-512 | |
+-------+--------+---------+-------------------+-------------+
| RS1 | -65535 | SHA-1 | RSASSA-PKCS1-v1_5 | Deprecated |
| | | | using SHA-1 | |
+-------+--------+---------+-------------------+-------------+
Table 1: RSASSA-PKCS1-v1_5 Algorithm Values
Security considerations for use of the first three algorithms are in
Section 5.2. Security considerations for use of the last algorithm
are in Section 5.3.
Note that these algorithms are already present in the IANA "JSON Web
Signature and Encryption Algorithms" registry [IANA.JOSE.Algorithms],
and so these registrations are only for the IANA "COSE Algorithms"
registry [IANA.COSE.Algorithms].
3. Using secp256k1 with JOSE and COSE
This section defines algorithm encodings and representations enabling
the Standards for Efficient Cryptography Group (SECG) elliptic curve
secp256k1 [SEC2] to be used for JOSE [RFC7515] and COSE [RFC8152]
messages.
3.1. JOSE and COSE secp256k1 Curve Key Representations
The SECG elliptic curve secp256k1 [SEC2] is represented in a JSON Web
Key (JWK) [RFC7517] using these values:
* "kty": "EC"
* "crv": "secp256k1"
plus the values needed to represent the curve point, as defined in
Section 6.2.1 of [RFC7518]. As a compressed point encoding
representation is not defined for JWK elliptic curve points, the
uncompressed point encoding defined there MUST be used. The "x" and
"y" values represented MUST both be exactly 256 bits, with any
leading zeros preserved. Other optional values such as "alg" MAY
also be present.
It is represented in a COSE_Key [RFC8152] using these values:
* "kty" (1): "EC2" (2)
* "crv" (-1): "secp256k1" (8)
plus the values needed to represent the curve point, as defined in
Section 13.1.1 of [RFC8152]. Either the uncompressed or compressed
point encoding representations defined there can be used. The "x"
value represented MUST be exactly 256 bits, with any leading zeros
preserved. If the uncompressed representation is used, the "y" value
represented MUST likewise be exactly 256 bits, with any leading zeros
preserved; if the compressed representation is used, the "y" value is
a boolean value, as specified in Section 13.1.1 of [RFC8152]. Other
optional values such as "alg" (3) MAY also be present.
3.2. ECDSA Signature with secp256k1 Curve
The ECDSA signature algorithm is defined in [DSS]. This
specification defines the "ES256K" algorithm identifier, which is
used to specify the use of ECDSA with the secp256k1 curve and the
SHA-256 [DSS] cryptographic hash function. Implementations need to
check that the key type is "EC" for JOSE or "EC2" (2) for COSE and
that the curve of the key is secp256k1 when creating or verifying a
signature.
The ECDSA secp256k1 SHA-256 digital signature is generated as
follows:
1. Generate a digital signature of the JWS Signing Input or the COSE
Sig_structure using ECDSA secp256k1 SHA-256 with the desired
private key. The output will be the pair (R, S), where R and S
are 256-bit unsigned integers.
2. Turn R and S into octet sequences in big-endian order, with each
array being 32 octets long. The octet sequence representations
MUST NOT be shortened to omit any leading zero octets contained
in the values.
3. Concatenate the two octet sequences in the order R and then S.
(Note that many ECDSA implementations will directly produce this
concatenation as their output.)
4. The resulting 64-octet sequence is the JWS Signature or COSE
signature value.
Implementations SHOULD use a deterministic algorithm to generate the
ECDSA nonce, k, such as the algorithm defined in [RFC6979]. However,
in situations where devices are vulnerable to physical attacks,
deterministic ECDSA has been shown to be susceptible to fault
injection attacks [KUDELSKI17] [EURO-SP18]. Where this is a
possibility, implementations SHOULD implement appropriate
countermeasures. Where there are specific certification requirements
(such as FIPS approval), implementors should check whether
deterministic ECDSA is an approved nonce generation method.
The ECDSA secp256k1 SHA-256 algorithm specified in this document uses
these identifiers:
+========+=======+=======================+=============+
| Name | Value | Description | Recommended |
+========+=======+=======================+=============+
| ES256K | -47 | ECDSA using secp256k1 | No |
| | | curve and SHA-256 | |
+--------+-------+-----------------------+-------------+
Table 2: ECDSA Algorithm Values
When using a JWK or COSE_Key for this algorithm, the following checks
are made:
* The "kty" field MUST be present, and it MUST be "EC" for JOSE or
"EC2" for COSE.
* The "crv" field MUST be present, and it MUST represent the
"secp256k1" elliptic curve.
* If the "alg" field is present, it MUST represent the "ES256K"
algorithm.
* If the "key_ops" field is present, it MUST include "sign" when
creating an ECDSA signature.
* If the "key_ops" field is present, it MUST include "verify" when
verifying an ECDSA signature.
* If the JWK "use" field is present, its value MUST be "sig".
3.3. Other Uses of the secp256k1 Elliptic Curve
This specification defines how to use the secp256k1 curve for ECDSA
signatures for both JOSE and COSE implementations. While in theory
the curve could also be used for ECDH-ES key agreement, it is beyond
the scope of this specification to state whether this is or is not
advisable. Thus, whether or not to recommend its use with ECDH-ES is
left for experts to decide in future specifications.
When used for ECDSA, the secp256k1 curve MUST be used only with the
"ES256K" algorithm identifier and not any others, including not with
the COSE "ES256" identifier. Note that the "ES256K" algorithm
identifier needed to be introduced for JOSE to sign with the
secp256k1 curve because the JOSE "ES256" algorithm is defined to be
used only with the P-256 curve. The COSE treatment of how to sign
with secp256k1 is intentionally parallel to that for JOSE, where the
secp256k1 curve MUST be used with the "ES256K" algorithm identifier.
4. IANA Considerations
4.1. COSE Algorithms Registrations
IANA has registered the following values in the "COSE Algorithms"
registry [IANA.COSE.Algorithms].
Name: RS256
Value: -257
Description: RSASSA-PKCS1-v1_5 using SHA-256
Change Controller: IESG
Reference: Section 2 of RFC 8812
Recommended: No
Name: RS384
Value: -258
Description: RSASSA-PKCS1-v1_5 using SHA-384
Change Controller: IESG
Reference: Section 2 of RFC 8812
Recommended: No
Name: RS512
Value: -259
Description: RSASSA-PKCS1-v1_5 using SHA-512
Change Controller: IESG
Reference: Section 2 of RFC 8812
Recommended: No
Name: RS1
Value: -65535
Description: RSASSA-PKCS1-v1_5 using SHA-1
Change Controller: IESG
Reference: Section 2 of RFC 8812
Recommended: Deprecated
Name: ES256K
Value: -47
Description: ECDSA using secp256k1 curve and SHA-256
Change Controller: IESG
Reference: Section 3.2 of RFC 8812
Recommended: No
4.2. COSE Elliptic Curves Registrations
IANA has registered the following value in the "COSE Elliptic Curves"
registry [IANA.COSE.Curves].
Name: secp256k1
Value: 8
Key Type: EC2
Description: SECG secp256k1 curve
Change Controller: IESG
Reference: Section 3.1 of RFC 8812
Recommended: No
4.3. JOSE Algorithms Registrations
IANA has registered the following value in the "JSON Web Signature
and Encryption Algorithms" registry [IANA.JOSE.Algorithms].
Algorithm Name: ES256K
Algorithm Description: ECDSA using secp256k1 curve and SHA-256
Algorithm Usage Location(s): alg
JOSE Implementation Requirements: Optional
Change Controller: IESG
Reference: Section 3.2 of RFC 8812
Algorithm Analysis Document(s): [SEC2]
4.4. JSON Web Key Elliptic Curves Registrations
IANA has registered the following value in the "JSON Web Key Elliptic
Curve" registry [IANA.JOSE.Curves].
Curve Name: secp256k1
Curve Description: SECG secp256k1 curve
JOSE Implementation Requirements: Optional
Change Controller: IESG
Specification Document(s): Section 3.1 of RFC 8812
5. Security Considerations
5.1. RSA Key Size Security Considerations
The security considerations on key sizes for RSA algorithms from
Section 6.1 of [RFC8230] also apply to the RSA algorithms in this
specification.
5.2. RSASSA-PKCS1-v1_5 with SHA-2 Security Considerations
The security considerations on the use of RSASSA-PKCS1-v1_5 with
SHA-2 hash functions (SHA-256, SHA-384, and SHA-512) from Section 8.3
of [RFC7518] also apply to their use in this specification. For that
reason, these algorithms are registered as being "Not Recommended".
Likewise, the exponent restrictions described in Section 8.3 of
[RFC7518] also apply.
5.3. RSASSA-PKCS1-v1_5 with SHA-1 Security Considerations
The security considerations on the use of the SHA-1 hash function
from [RFC6194] apply in this specification. For that reason, the
"RS1" algorithm is registered as "Deprecated". Likewise, the
exponent restrictions described in Section 8.3 of [RFC7518] also
apply.
A COSE algorithm identifier for this algorithm is nonetheless being
registered because deployed Trusted Platform Modules (TPMs) continue
to use it; therefore, WebAuthn implementations need a COSE algorithm
identifier for "RS1" when TPM attestations using this algorithm are
being represented. New COSE applications and protocols MUST NOT use
this algorithm.
5.4. secp256k1 Security Considerations
Care should be taken that a secp256k1 key is not mistaken for a P-256
[RFC7518] key, given that their representations are the same except
for the "crv" value. As described in Section 8.1.1 of [RFC8152], we
currently do not have any way to deal with this attack except to
restrict the set of curves that can be used.
The procedures and security considerations described in the [SEC1],
[SEC2], and [DSS] specifications apply to implementations of this
specification.
Timing side-channel attacks are possible if the implementation of
scalar multiplication over the curve does not execute in constant
time.
There are theoretical weaknesses with this curve that could result in
future attacks. While these potential weaknesses are not unique to
this curve, they are the reason that this curve is registered as
"Recommended: No".
6. References
6.1. Normative References
[DSS] National Institute of Standards and Technology (NIST),
"Digital Signature Standard (DSS)", FIPS PUB 186-4,
DOI 10.6028/NIST.FIPS.186-4, July 2013,
.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
[RFC6194] Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
Considerations for the SHA-0 and SHA-1 Message-Digest
Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011,
.
[RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
2015, .
[RFC7517] Jones, M., "JSON Web Key (JWK)", RFC 7517,
DOI 10.17487/RFC7517, May 2015,
.
[RFC7518] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
DOI 10.17487/RFC7518, May 2015,
.
[RFC8017] Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
"PKCS #1: RSA Cryptography Specifications Version 2.2",
RFC 8017, DOI 10.17487/RFC8017, November 2016,
.
[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, .
[RFC8230] Jones, M., "Using RSA Algorithms with CBOR Object Signing
and Encryption (COSE) Messages", RFC 8230,
DOI 10.17487/RFC8230, September 2017,
.
[SEC1] Standards for Efficient Cryptography Group, "SEC 1:
Elliptic Curve Cryptography", Version 2.0, May 2009,
.
[SEC2] Standards for Efficient Cryptography Group, "SEC 2:
Recommended Elliptic Curve Domain Parameters",
Version 2.0, January 2010,
.
6.2. Informative References
[CTAP] Brand, C., Czeskis, A., Ehrensvärd, J., Jones, M., Kumar,
A., Lindemann, R., Powers, A., and J. Verrept, "Client to
Authenticator Protocol (CTAP)", FIDO Alliance Proposed
Standard, January 2019, .
[EURO-SP18]
Poddebniak, D., Somorovsky, J., Schinzel, S., Lochter, M.,
and P. Rösler, "Attacking Deterministic Signature Schemes
using Fault Attacks", 2018 IEEE European Symposium on
Security and Privacy (EuroS&P),
DOI 10.1109/EuroSP.2018.00031, April 2018,
.
[IANA.COSE.Algorithms]
IANA, "COSE Algorithms",
.
[IANA.COSE.Curves]
IANA, "COSE Elliptic Curves",
.
[IANA.JOSE.Algorithms]
IANA, "JSON Web Signature and Encryption Algorithms",
.
[IANA.JOSE.Curves]
IANA, "JSON Web Key Elliptic Curve",
.
[KUDELSKI17]
Romailler, Y., "How to Defeat Ed25519 and EdDSA Using
Faults", Kudelski Security Research, October 2017,
.
[RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature
Algorithm (DSA) and Elliptic Curve Digital Signature
Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
2013, .
[WebAuthn] Balfanz, D., Czeskis, A., Hodges, J., Jones, J.C., Jones,
M., Kumar, A., Liao, A., Lindemann, R., and E. Lundberg,
"Web Authentication: An API for accessing Public Key
Credentials - Level 1", W3C Recommendation, March 2019,
.
Acknowledgements
Thanks to Roman Danyliw, Linda Dunbar, Stephen Farrell, John Fontana,
Jeff Hodges, Kevin Jacobs, J.C. Jones, Benjamin Kaduk, Murray
Kucherawy, Neil Madden, John Mattsson, Matthew Miller, Tony Nadalin,
Matt Palmer, Eric Rescorla, Rich Salz, Jim Schaad, Goeran Selander,
Wendy Seltzer, Sean Turner, and Samuel Weiler for their roles in
registering these algorithm identifiers.
Author's Address
Michael B. Jones
Microsoft
Email: mbj@microsoft.com
URI: https://self-issued.info/