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
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
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This Internet-Draft will expire on December 13, 2020.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Notation and Conventions . . . . . . . . . . 3
2. RSASSA-PKCS1-v1_5 Signature Algorithm . . . . . . . . . . . . 3
3. Using secp256k1 with JOSE and COSE . . . . . . . . . . . . . 4
3.1. JOSE and COSE secp256k1 Curve Key Representations . . . . 5
3.2. ECDSA Signature with secp256k1 Curve . . . . . . . . . . 5
3.3. Other Uses of the secp256k1 Elliptic Curve . . . . . . . 7
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
4.1. COSE Algorithms Registrations . . . . . . . . . . . . . . 7
4.2. COSE Elliptic Curves Registrations . . . . . . . . . . . 8
4.3. JOSE Algorithms Registrations . . . . . . . . . . . . . . 8
4.4. JSON Web Key Elliptic Curves Registrations . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
5.1. RSA Key Size Security Considerations . . . . . . . . . . 9
5.2. RSASSA-PKCS1-v1_5 with SHA-2 Security Considerations . . 9
5.3. RSASSA-PKCS1-v1_5 with SHA-1 Security Considerations . . 9
5.4. secp256k1 Security Considerations . . . . . . . . . . . . 9
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Normative References . . . . . . . . . . . . . . . . . . 10
6.2. Informative References . . . . . . . . . . . . . . . . . 11
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 12
Document History . . . . . . . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14
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
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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.
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+-------+---------------+---------+-------------------+-------------+
| Name | Value | Hash | Description | Recommended |
+-------+---------------+---------+-------------------+-------------+
| RS256 | TBD | SHA-256 | RSASSA-PKCS1-v1_5 | No |
| | (temporary | | using SHA-256 | |
| | assignment | | | |
| | -257 already | | | |
| | in place) | | | |
| RS384 | TBD | SHA-384 | RSASSA-PKCS1-v1_5 | No |
| | (temporary | | using SHA-384 | |
| | assignment | | | |
| | -258 already | | | |
| | in place) | | | |
| RS512 | TBD | SHA-512 | RSASSA-PKCS1-v1_5 | No |
| | (temporary | | using SHA-512 | |
| | assignment | | | |
| | -259 already | | | |
| | in place) | | | |
| RS1 | TBD | SHA-1 | RSASSA-PKCS1-v1_5 | Deprecated |
| | (temporary | | using SHA-1 | |
| | assignment | | | |
| | -65535 | | | |
| | already in | | | |
| | place) | | | |
+-------+---------------+---------+-------------------+-------------+
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.
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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:
o "kty": "EC"
o "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:
o "kty" (1): "EC2" (2)
o "crv" (-1): "secp256k1" (TBD - requested assignment 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
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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:
+----------+-------------------+----------------------+-------------+
| JOSE Alg | COSE Alg Value | Description | Recommended |
| Name | | | |
+----------+-------------------+----------------------+-------------+
| ES256K | TBD (requested | ECDSA using | No |
| | assignment -47) | secp256k1 curve and | |
| | | SHA-256 | |
+----------+-------------------+----------------------+-------------+
Table 2: ECDSA Algorithm Values
When using a JWK or COSE_Key for this algorithm, the following checks
are made:
o The "kty" field MUST be present and it MUST be "EC" for JOSE or
"EC2" for COSE.
o The "crv" field MUST be present and it MUST represent the
"secp256k1" elliptic curve.
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o If the "alg" field is present, it MUST represent the "ES256K"
algorithm.
o If the "key_ops" field is present, it MUST include "sign" when
creating an ECDSA signature.
o If the "key_ops" field is present, it MUST include "verify" when
verifying an ECDSA signature.
o 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 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].
o Name: RS256
o Value: TBD (temporary assignment -257 already in place)
o Description: RSASSA-PKCS1-v1_5 using SHA-256
o Reference: Section 2 of this document
o Recommended: No
o Name: RS384
o Value: TBD (temporary assignment -258 already in place)
o Description: RSASSA-PKCS1-v1_5 using SHA-384
o Reference: Section 2 of this document
o Recommended: No
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o Name: RS512
o Value: TBD (temporary assignment -259 already in place)
o Description: RSASSA-PKCS1-v1_5 using SHA-512
o Reference: Section 2 of this document
o Recommended: No
o Name: RS1
o Value: TBD (temporary assignment -65535 already in place)
o Description: RSASSA-PKCS1-v1_5 using SHA-1
o Reference: Section 2 of this document
o Recommended: Deprecated
o Name: ES256K
o Value: TBD (requested assignment -47)
o Description: ECDSA using secp256k1 curve and SHA-256
o Reference: Section 3.2 of this document
o Recommended: No
4.2. COSE Elliptic Curves Registrations
This section registers the following value in the IANA "COSE Elliptic
Curves" registry [IANA.COSE.Curves].
o Name: secp256k1
o Value: TBD (requested assignment 8)
o Key Type: EC2
o Description: SECG secp256k1 curve
o Change Controller: IESG
o Reference: Section 3.1 of [[ this specification ]]
o Recommended: No
4.3. JOSE Algorithms Registrations
This section registers the following value in the IANA "JSON Web
Signature and Encryption Algorithms" registry [IANA.JOSE.Algorithms].
o Algorithm Name: ES256K
o Algorithm Description: ECDSA using secp256k1 curve and SHA-256
o Algorithm Usage Locations: alg
o JOSE Implementation Requirements: Optional
o Change Controller: IESG
o Reference: Section 3.2 of [[ this specification ]]
o Algorithm Analysis Document(s): [SEC2]
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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].
o Curve Name: secp256k1
o Curve Description: SECG secp256k1 curve
o JOSE Implementation Requirements: Optional
o Change Controller: IESG
o Specification Document(s): Section 3.1 of [[ this specification ]]
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 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.
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
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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, 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,
.
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[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,
.
[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., Ehrensvaerd, 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. Roesler, "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",
.
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[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 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.
Document History
[[ to be removed by the RFC Editor before publication as an RFC ]]
-08
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o Addressed IESG review comments by Benjamin Kaduk and Roman
Danyliw, primarily completing the edits to register secp256k1 and
ES256K as "Recommended: No" for COSE. Some additional security
considerations were also added.
-07
o Addressed editorial SecDir review comment by Linda Dunbar about
SHA-2 algorithms.
o Addressed IETF last call comments by Jim Schaad, Rich Salz, and
Eric Rescorla, now registering secp256k1 and ES256K as
"Recommended: No" for COSE.
-06
o Addressed Area Director review comment by Murray Kucherawy (which
requested an editorial correction).
o Changed requested assignment for ES256K from -46 to -47, due to an
assignment conflict.
-05
o Removed unused reference to RFC 7049.
-04
o Added explanatory comments on design decisions made that were
discussed on the mailing list that Jim Schaad requested be added
to the draft.
-03
o Addressed review of -02 by Jim Schaad.
-02
o Addressed working group last call comments. Thanks to J.C.
Jones, Kevin Jacobs, Jim Schaad, Neil Madden, and Benjamin Kaduk
for their useful feedback.
-01
o Changed the JOSE curve identifier from "P-256K" to "secp256k1".
o Specified that secp256k1 signing is done using the SHA-256 hash
function.
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-00
o Created the initial working group draft from draft-jones-cose-
additional-algorithms-00, changing only the title, date, and
history entry.
Author's Address
Michael B. Jones
Microsoft
Email: mbj@microsoft.com
URI: http://self-issued.info/
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