Internet Engineering Task Force (IETF)                          M. Jones
Request for Comments: 8812                                     Microsoft
Category: Standards Track                                    August 2020
ISSN: 2070-1721

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
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   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/