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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
 Provisions Relating to IETF Documents
 (https://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 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,
            <https://doi.org/10.6028/NIST.FIPS.186-4>.
 [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>.
 [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,
            <https://www.rfc-editor.org/info/rfc6194>.
 [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
            Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
            2015, <https://www.rfc-editor.org/info/rfc7515>.
 [RFC7517]  Jones, M., "JSON Web Key (JWK)", RFC 7517,
            DOI 10.17487/RFC7517, May 2015,
            <https://www.rfc-editor.org/info/rfc7517>.
 [RFC7518]  Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
            DOI 10.17487/RFC7518, May 2015,
            <https://www.rfc-editor.org/info/rfc7518>.
 [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,
            <https://www.rfc-editor.org/info/rfc8017>.
 [RFC8152]  Schaad, J., "CBOR Object Signing and Encryption (COSE)",
            RFC 8152, DOI 10.17487/RFC8152, July 2017,
            <https://www.rfc-editor.org/info/rfc8152>.
 [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>.
 [RFC8230]  Jones, M., "Using RSA Algorithms with CBOR Object Signing
            and Encryption (COSE) Messages", RFC 8230,
            DOI 10.17487/RFC8230, September 2017,
            <https://www.rfc-editor.org/info/rfc8230>.
 [SEC1]     Standards for Efficient Cryptography Group, "SEC 1:
            Elliptic Curve Cryptography", Version 2.0, May 2009,
            <https://www.secg.org/sec1-v2.pdf>.
 [SEC2]     Standards for Efficient Cryptography Group, "SEC 2:
            Recommended Elliptic Curve Domain Parameters",
            Version 2.0, January 2010,
            <https://www.secg.org/sec2-v2.pdf>.

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, <https://fidoalliance.org/specs/
            fido-v2.0-ps-20190130/fido-client-to-authenticator-
            protocol-v2.0-ps-20190130.html>.
 [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,
            <https://ieeexplore.ieee.org/document/8406609>.
 [IANA.COSE.Algorithms]
            IANA, "COSE Algorithms",
            <https://www.iana.org/assignments/cose>.
 [IANA.COSE.Curves]
            IANA, "COSE Elliptic Curves",
            <https://www.iana.org/assignments/cose>.
 [IANA.JOSE.Algorithms]
            IANA, "JSON Web Signature and Encryption Algorithms",
            <https://www.iana.org/assignments/jose>.
 [IANA.JOSE.Curves]
            IANA, "JSON Web Key Elliptic Curve",
            <https://www.iana.org/assignments/jose>.
 [KUDELSKI17]
            Romailler, Y., "How to Defeat Ed25519 and EdDSA Using
            Faults", Kudelski Security Research, October 2017,
            <https://research.kudelskisecurity.com/2017/10/04/
            defeating-eddsa-with-faults/>.
 [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, <https://www.rfc-editor.org/info/rfc6979>.
 [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,
            <https://www.w3.org/TR/2019/REC-webauthn-1-20190304/>.

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/
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