COSE Working Group | M. Jones |
Internet-Draft | Microsoft |
Intended status: Standards Track | June 11, 2020 |
Expires: December 13, 2020 |
COSE and JOSE Registrations for WebAuthn Algorithms
draft-ietf-cose-webauthn-algorithms-08
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.
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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].
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.
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 | 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].
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.
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 is a boolean value, as specified in Section 13.1.1 of [RFC8152]. Other optional values such as alg (3) MAY also be present.
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:
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:
JOSE Alg Name | COSE Alg Value | Description | Recommended |
---|---|---|---|
ES256K | TBD (requested assignment -47) | ECDSA using secp256k1 curve and SHA-256 | No |
When using a JWK or COSE_Key for this algorithm, the following checks are made:
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.
This section registers the following values in the IANA "COSE Algorithms" registry [IANA.COSE.Algorithms].
This section registers the following value in the IANA "COSE Elliptic Curves" registry [IANA.COSE.Curves].
This section registers the following value in the IANA "JSON Web Signature and Encryption Algorithms" registry [IANA.JOSE.Algorithms].
This section registers the following value in the IANA "JSON Web Key Elliptic Curve" registry [IANA.JOSE.Curves].
The security considerations on key sizes for RSA algorithms from Section 6.1 of [RFC8230] also apply to the RSA algorithms in this specification.
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.
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 and protocols MUST NOT use this algorithm.
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".
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, Göran Selander, Wendy Seltzer, Sean Turner, and Samuel Weiler for their roles in registering these algorithm identifiers.
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