COSE Working Group M. Jones
Internet-Draft Microsoft
Intended status: Standards Track May 13, 2020
Expires: November 14, 2020

COSE and JOSE Registrations for WebAuthn Algorithms
draft-ietf-cose-webauthn-algorithms-06

Abstract

The W3C Web Authentication (WebAuthn) specification and the FIDO Alliance Client to Authenticator Protocol (CTAP) specification use CBOR Object Signing and Encryption (COSE) algorithm identifiers. This specification registers the following algorithms in the IANA "COSE Algorithms" registry, which are used by WebAuthn and CTAP implementations: RSASSA-PKCS1-v1_5 using SHA-256, SHA-384, SHA-512, and SHA-1, and 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 Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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Table of Contents

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.

RSASSA-PKCS1-v1_5 Algorithm Values
Name Value Hash Description Recommended
RS256 TBD (temporary assignment -257 already in place) SHA-256 RSASSA-PKCS1-v1_5 using SHA-256 No
RS384 TBD (temporary assignment -258 already in place) SHA-384 RSASSA-PKCS1-v1_5 using SHA-384 No
RS512 TBD (temporary assignment -259 already in place) SHA-512 RSASSA-PKCS1-v1_5 using SHA-512 No
RS1 TBD (temporary assignment -65535 already in place) SHA-1 RSASSA-PKCS1-v1_5 using SHA-1 Deprecated

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 Standards for Efficient Cryptography Group (SECG) elliptic curve secp256k1 [SEC2] is represented in a JSON Web Key (JWK) [RFC7517] using these values:

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:

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 MUST be 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 be 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 [RFC6979]. However, in situations where devices are vulnerable to physical attacks, deterministic ECDSA has been shown to be susceptible to fault injection attacks [Kudelski17] [EuroSP18]. 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:

ECDSA Algorithm Values
JOSE Alg Name COSE Alg Value Description Recommended
ES256K TBD (requested assignment -47) ECDSA using secp256k1 curve and SHA-256 Yes

Implementation of this algorithm is recommended because of its widespread use in decentralized systems and those that chose it over the NIST curves.

When using a JWK or COSE_Key for this algorithm, the following checks are made:

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 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

This section registers the following values in the IANA "COSE Algorithms" registry [IANA.COSE.Algorithms].

4.2. COSE Elliptic Curves Registrations

This section registers the following value in the IANA "COSE Elliptic Curves" registry [IANA.COSE.Curves].

4.3. JOSE Algorithms Registrations

This section registers the following value in the IANA "JSON Web Signature and Encryption Algorithms" registry [IANA.JOSE.Algorithms].

4.4. JSON Web Key Elliptic Curves Registrations

This section registers the following value in the IANA "JSON Web Key Elliptic Curve" registry [IANA.JOSE.Curves].

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 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".

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 TPMs continue to use it, and therefore WebAuthn implementations need a COSE algorithm identifier for "RS1" when TPM attestations using this algorithm are being represented. New COSE applications 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.

The procedures and security considerations described in the [SEC1], [SEC2], and [DSS] specifications apply to implementations of this specification.

6. References

6.1. Normative References

[DSS] National Institute of Standards and Technology (NIST), "Digital Signature Standard (DSS)", FIPS PUB 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., 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.
[EuroSP18] Poddebniak, D., Somorovsky, J., Schinzel, S., Lochter, M. and P. Rösler, "Attacking Deterministic Signature Schemes using Fault Attacks", IEEE European Symposium on Security and Privacy (EuroS&P) 2018, 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", 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., Jones, M., Kumar, A., Liao, A., Lindemann, R. and E. Lundberg, "Web Authentication: An API for accessing Public Key Credentials - Level 1", World Wide Web Consortium (W3C) Recommendation, March 2019.

Acknowledgements

Thanks to Stephen Farrell, John Fontana, Jeff Hodges, Kevin Jacobs, J.C. Jones, Benjamin Kaduk, Murray Kucherawy, Neil Madden, John Mattsson, Tony Nadalin, Matt Palmer, Jim Schaad, Göran Selander, Wendy Seltzer, Sean Turner, and Samuel Weiler for their roles in registering these algorithm identifiers.

Document History

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Author's Address

Michael B. Jones Microsoft EMail: mbj@microsoft.com URI: http://self-issued.info/