Microsoft

mbj@microsoft.com https://self-issued.info/

Security COSE Working Group Cryptography Digital Signature Encryption W3C World Wide Web Consortium WebAuthn Web Authentication FIDO Alliance FIDO FIDO2 CTAP CTAP2 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.

Introduction This specification defines how to use several algorithms with CBOR Object Signing and Encryption (COSE) that are used by implementations of the W3C Web Authentication (WebAuthn) and FIDO Alliance FIDO2 Client to Authenticator Protocol (CTAP) specifications. This specification registers these algorithms in the IANA "COSE Algorithms" registry and registers an elliptic curve in the IANA "COSE Elliptic Curves" registry . This specification also registers a corresponding algorithm for use with JSON Object Signing and Encryption (JOSE) in the IANA "JSON Web Signature and Encryption Algorithms" registry and registers an elliptic curve in the IANA "JSON Web Key Elliptic Curve" registry .

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 when, and only when, they appear in all capitals, as shown here.

RSASSA-PKCS1-v1_5 Signature Algorithm The RSASSA-PKCS1-v1_5 signature algorithm is defined in . 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	-257	SHA-256	RSASSA-PKCS1-v1_5 using SHA-256	No
RS384	-258	SHA-384	RSASSA-PKCS1-v1_5 using SHA-384	No
RS512	-259	SHA-512	RSASSA-PKCS1-v1_5 using SHA-512	No
RS1	-65535	SHA-1	RSASSA-PKCS1-v1_5 using SHA-1	Deprecated

Security considerations for use of the first three algorithms are in . Security considerations for use of the last algorithm are in . Note that these algorithms are already present in the IANA "JSON Web Signature and Encryption Algorithms" registry , and so these registrations are only for the IANA "COSE Algorithms" registry .

Using secp256k1 with JOSE and COSE This section defines algorithm encodings and representations enabling the Standards for Efficient Cryptography Group (SECG) elliptic curve secp256k1 to be used for JOSE and COSE messages.

JOSE and COSE secp256k1 Curve Key Representations The SECG elliptic curve secp256k1 is represented in a JSON Web Key (JWK) using these values:

• kty: EC
• crv: secp256k1

plus the values needed to represent the curve point, as defined in . 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 using these values:

• kty (1): EC2 (2)
• crv (-1): secp256k1 (8)

plus the values needed to represent the curve point, as defined in . 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 . Other optional values such as alg (3) MAY also be present.

ECDSA Signature with secp256k1 Curve The ECDSA signature algorithm is defined in . This specification defines the ES256K algorithm identifier, which is used to specify the use of ECDSA with the secp256k1 curve and the SHA-256 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 . However, in situations where devices are vulnerable to physical attacks, deterministic ECDSA has been shown to be susceptible to fault injection attacks . 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

Name	Value	Description	Recommended
ES256K	-47	ECDSA using secp256k1 curve and SHA-256	No

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.

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.

IANA Considerations

COSE Algorithms Registrations IANA has registered the following values in the "COSE Algorithms" registry .

Name:

RS256

Value:

-257

Description:

RSASSA-PKCS1-v1_5 using SHA-256

Change Controller:

IESG

Reference:

of RFC 8812

Recommended:

No

Name:

RS384

Value:

-258

Description:

RSASSA-PKCS1-v1_5 using SHA-384

Change Controller:

IESG

Reference:

of RFC 8812

Recommended:

No

Name:

RS512

Value:

-259

Description:

RSASSA-PKCS1-v1_5 using SHA-512

Change Controller:

IESG

Reference:

of RFC 8812

Recommended:

No

Name:

RS1

Value:

-65535

Description:

RSASSA-PKCS1-v1_5 using SHA-1

Change Controller:

IESG

Reference:

of RFC 8812

Recommended:

Deprecated

Name:

ES256K

Value:

-47

Description:

ECDSA using secp256k1 curve and SHA-256

Change Controller:

IESG

Reference:

of RFC 8812

Recommended:

No

COSE Elliptic Curves Registrations IANA has registered the following value in the "COSE Elliptic Curves" registry .

Name:

secp256k1

Value:

8

Key Type:

EC2

Description:

SECG secp256k1 curve

Change Controller:

IESG

Reference:

of RFC 8812

Recommended:

No

JOSE Algorithms Registrations IANA has registered the following value in the "JSON Web Signature and Encryption Algorithms" registry .

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:

of RFC 8812

Algorithm Analysis Document(s):

JSON Web Key Elliptic Curves Registrations IANA has registered the following value in the "JSON Web Key Elliptic Curve" registry .

Curve Name:

secp256k1

Curve Description:

SECG secp256k1 curve

JOSE Implementation Requirements:

Optional

Change Controller:

IESG

Specification Document(s):

of RFC 8812

Security Considerations

RSA Key Size Security Considerations The security considerations on key sizes for RSA algorithms from also apply to the RSA algorithms in this specification.

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 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 also apply.

RSASSA-PKCS1-v1_5 with SHA-1 Security Considerations The security considerations on the use of the SHA-1 hash function from apply in this specification. For that reason, the "RS1" algorithm is registered as "Deprecated". Likewise, the exponent restrictions described in 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.

secp256k1 Security Considerations Care should be taken that a secp256k1 key is not mistaken for a P-256 key, given that their representations are the same except for the crv value. As described in , 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 , , and 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".

References Normative References National Institute of Standards and Technology (NIST) Standards for Efficient Cryptography Group Standards for Efficient Cryptography Group Informative References Google

balfanz@google.com

Google

aczeskis@google.com

Google

Jeff.Hodges@paypal.com

Mozilla

jc@mozilla.com

Microsoft

mbj@microsoft.com http://self-issued.info/

Microsoft

akshayku@microsoft.com

Microsoft

huliao@microsoft.com

Nok Nok Labs

rolf@noknok.com

Yubico

emil@yubico.com

W3C Recommendation Google

cbrand@google.com

Google

aczeskis@google.com

Yubico

jakob@yubico.com

Microsoft

mbj@microsoft.com http://self-issued.info/

Microsoft

akshayku@microsoft.com

Nok Nok Labs

rolf@noknok.com

FIDO Alliance

adam@fidoalliance.org

OneSpan

johan.verrept@onespan.com

FIDO Alliance Proposed Standard IANA IANA IANA IANA Kudelski Security Research Münster University of Applied Sciences Ruhr University Bochum Münster University of Applied Sciences Federal Office for Information Security Ruhr University Bochum 2018 IEEE European Symposium on Security and Privacy (EuroS&P)

Acknowledgements Thanks to , , , , , , , , , , , , , , , , , , , , and for their roles in registering these algorithm identifiers.