Internet Engineering Task Force (IETF) K. Isobe
Request for Comments: 9679 SECOM CO., LTD.
Category: Standards Track H. Tschofenig
ISSN: 2070-1721 H-BRS
O. Steele
Transmute
October 2024
CBOR Object Signing and Encryption (COSE) Key Thumbprint
Abstract
This specification defines a method for computing a hash value over a
CBOR Object Signing and Encryption (COSE) Key. It specifies which
fields within the COSE Key structure are included in the
cryptographic hash computation, the process for creating a canonical
representation of these fields, and how to hash the resulting byte
sequence. The resulting hash value, referred to as a "thumbprint",
can be used to identify or select the corresponding key.
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/rfc9679.
Copyright Notice
Copyright (c) 2024 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
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to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
2. Terminology
3. COSE Key Thumbprint
4. Required COSE Key Parameters
4.1. Octet Key Pair (OKP)
4.2. Elliptic Curve Keys with X- and Y-Coordinate Pairs Y-Coordinates
4.3. RSA Public Keys
4.4. Symmetric Keys
4.5. HSS-LMS Keys
4.6. Others
5. Miscellaneous Considerations
5.1. Why Not Include Optional COSE Key Parameters?
5.2. Selection of Hash Function
5.3. Thumbprints of Keys Not in COSE Key Format
5.4. Relationship to Digests of X.509 Values
5.5. Confirmation Method
5.6. COSE Key Thumbprint URIs
6. Example
7. Security Considerations
8. IANA Considerations
9. References
9.1. Normative References
9.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
This specification defines a method for applying a cryptographic hash
function to a CBOR Object Signing and Encryption (COSE) Key structure
[RFC9052], resulting in a hash value known as a "thumbprint". To
achieve this, the document specifies which fields in the COSE Key
structure are included in the hash computation, the process for
creating a canonical form of these fields, and how to hash the
resulting byte sequence. One of the primary use cases for this
thumbprint is as a naming scheme for identifying or selecting the
key, such as by using the COSE Key Thumbprint value as a "kid" (key
ID). Another key use case involves key derivation functions that use
the thumbprints of public keys from the endpoints, along with other
application context, to derive a symmetric key.
This specification outlines how thumbprints of COSE Keys are
generated for both asymmetric and symmetric keys (see Sections 3 and
4). Additionally, it introduces a new CBOR Web Token (CWT)
confirmation method, which has been added to the IANA "CWT
Confirmation Methods" registry established by [RFC8747]. For further
details on the use of a confirmation claim in a CWT with a proof-of-
possession key, refer to Section 3.1 of [RFC8747].
2. Terminology
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.
3. COSE Key Thumbprint
The thumbprint of a COSE Key MUST be computed as follows:
1. Construct a COSE_Key structure (see Section 7 of [RFC9052])
containing only the required parameters representing the key as
described in Section 4 of this document.
2. Apply the deterministic encoding described in Section 4.2.1 of
[RFC8949] to the representation constructed in step 1.
3. Hash the bytes produced in step 2 with a cryptographic hash
function H. For example, SHA-256 [RFC6234] may be used as a hash
function.
The details of this computation are further described in subsequent
sections.
The SHA-256 hash algorithm MUST be supported; other algorithms MAY be
supported.
4. Required COSE Key Parameters
Only the required parameters of a key's representation are used when
computing its COSE Key Thumbprint value. This section summarizes the
required parameters.
The "kty" (label: 1) element MUST be present for all key types, and
the integer value specified in the IANA "COSE Key Types" registry
MUST be used. The tstr data type is not used with the "kty" element.
Many COSE Key parameters are specific to the chosen key type. The
following subsections list the required parameters for commonly used
key types.
4.1. Octet Key Pair (OKP)
The required parameters for elliptic curve public keys that use the
Octet Key Pair (OKP) key type, such as X25519, are:
* "kty" (label: 1, data type: int, value: 1)
* "crv" (label: -1, value: int)
* "x" (label: -2, value: bstr)
Further details are described in Section 7.1 of [RFC9053].
4.2. Elliptic Curve Keys with X- and Y-Coordinate Pairs Y-Coordinates
The required parameters for elliptic curve public keys that use the
"EC2" (two coordinate elliptic curve)
EC2 key type, such as NIST P-256, are:
* "kty" (label: 1, data type: int, value: 2)
* "crv" (label: -1, data type: int)
* "x" (label: -2, data type: bstr)
* "y" (label: -3, data type: bstr)
Further details are described in Section 7.1 of [RFC9053].
Note: [RFC9052] supports both compressed and uncompressed point
representations. For interoperability, implementations adhering to
this specification MUST use the uncompressed point representation.
Therefore, the y-coordinate is expressed as a bstr. If an
implementation uses the compressed point representation, it MUST
first convert it to the uncompressed form for the purpose of
thumbprint calculation.
4.3. RSA Public Keys
The required parameters for an RSA public key are:
* "kty" (label: 1, data type: int, value: 3)
* "n" (label: -1, data type: bstr)
* "e" (label: -2, data type: bstr)
4.4. Symmetric Keys
The required parameters for a symmetric key are:
* "kty" (label: 1, data type: int, value: 4)
* "k" (label: -1, data type: bstr)
4.5. HSS-LMS Keys
The required parameters for HSS-LMS keys are:
* "kty" (label: 1, data type: int, value: 5)
* "pub" (label: -1, data type: bstr)
4.6. Others
As other key type values are defined, their defining specifications
should be similarly consulted to determine which parameters, in
addition to the "kty" element, are required.
5. Miscellaneous Considerations
5.1. Why Not Include Optional COSE Key Parameters?
Optional parameters of COSE Keys are intentionally not included in
the COSE Key Thumbprint computation so that their absence or presence
in the COSE Key does not alter the resulting value. The COSE Key
Thumbprint is a digest of the ordered essential parameters required needed to
represent the key as a COSE Key, rather than any additional data that
might accompany the key. with all other parameters excluded.
By excluding optional parameters, the COSE Key Thumbprint
consistently refers to the key itself, not to a key with additional
attributes. Different application contexts may include various
optional attributes in the COSE Key structure. If these optional
parameters were included in the thumbprint calculation, the resulting
values could differ for the same key depending on the attributes
present. Including only the required parameters ensures that the
COSE Key Thumbprint remains consistent for a given key, regardless of
any additional attributes.
Different kinds of thumbprints could be defined by other
specifications that might include some or all additional COSE Key
parameters, if use cases arise where such different kinds of
thumbprints would be useful.
5.2. Selection of Hash Function
A specific hash function must be chosen by an application to compute
the hash value of the hash input. For instance, SHA-256 [RFC6234]
may be used as the hash function. While SHA-256 is a good default
choice at the time of writing, the preferred hash function may evolve
as the cryptographic landscape develops.
In many cases, only the party that generates the key needs to be
aware of the hash function used. For example, the key producer might
use the thumbprint value as a "kid" (key ID). In such scenarios, the
consumer of the "kid" treats it as an opaque value solely for key
selection.
However, when multiple parties are involved in reproducing and
comparing the COSE Key Thumbprint, it is crucial that they know and
use the same hash function to ensure consistent results.
5.3. Thumbprints of Keys Not in COSE Key Format
Keys that are in other formats can be represented as COSE Keys. Any
party in possession of a key that is represented as a COSE Key can
use the COSE Key Thumbprint.
5.4. Relationship to Digests of X.509 Values
COSE Key Thumbprint values are computed on the COSE Key object
required to represent a key, rather than all
containing only essential parameters of in a COSE Key
that the key is represented in. specific order. Thus, they
are more analogous to applications that use digests of X.509 Subject
Public Key Info (SPKI) values, which are defined in Section 4.1.2.7
of [RFC5280], than to applications that use digests of complete
certificate values, as the "x5t" (X.509 certificate SHA-1 thumbprint)
[RFC9360] value defined for X.509 certificate objects does. While
logically equivalent to a digest of the SPKI representation of the
key, a COSE Key Thumbprint is computed over the CBOR representation
of that key rather than over an ASN.1 representation of it.
5.5. Confirmation Method
[RFC8747] introduces confirmation methods for use with CWTs with the
addition of the "cnf" claim. CWTs are defined in [RFC8392]. This
specification adds a new confirmation method based on COSE Key
Thumbprints.
The proof-of-possession key is identified using the "ckt" claim, the
COSE Key Thumbprint claim. This claim contains the value of the COSE
Key Thumbprint encoded as a binary string. Instead of communicating
the actual COSE Key, only the thumbprint is conveyed. This approach
assumes that the recipient is able to obtain the identified COSE Key
using the thumbprint contained in the "ckt" claim. In this approach,
the issuer of a CWT declares that the presenter possesses a
particular key and that the recipient can cryptographically confirm
the presenter's proof of possession of the key by including a "ckt"
claim in the CWT.
The following example demonstrates the use of the "ckt" claim in a
CWT as part of the confirmation method (with line breaks inserted for
editorial reasons):
{
/iss/ 1 : "coaps://as.example.com",
/aud/ 3 : "coaps://resource.example.org",
/exp/ 4 : 1361398824,
/cnf/ 8 : {
/ckt/ 5 : h'496bd8afadf307e5b08c64b0421bf9dc
01528a344a43bda88fadd1669da253ec'
}
}
Section 8 registers the "ckt" claim and the confirmation method. The
"ckt" claim is expected to be used in the "cnf" claim.
5.6. COSE Key Thumbprint URIs
This specification defines Uniform Resource Identifiers (URIs) to
represent a COSE Key Thumbprint value. The design follows the work
of JSON Web Key (JWK) Thumbprint URIs, as specified in [RFC9278].
This enables COSE Key Thumbprints to be used, for example, as key
identifiers in contexts requiring URIs. This specification defines a
URI prefix indicating that the portion of the URI following the
prefix is a COSE Key Thumbprint.
The following URI prefix is defined to indicate that the portion of
the URI following the prefix is a COSE Key Thumbprint:
urn:ietf:params:oauth:ckt
To make the hash algorithm being used explicit in a URI, the prefix
is followed by a hash algorithm identifier and a COSE Key Thumbprint
value, each separated by a colon character to form a URI representing
a COSE Key Thumbprint.
Hash algorithm identifiers used in COSE Key Thumbprint URIs MUST be
values from the "Hash Name String" column in the IANA "Named
Information Hash Algorithm Registry" [IANA.Hash.Algorithms]. COSE
Key Thumbprint URIs with hash algorithm identifiers not found in this
registry are not considered valid, and applications MUST detect and
handle this error, should it occur.
Since the URN is encoded as a string, the output of the COSE Key
Thumbprint computation described in Section 3 MUST be base64url
encoded without padding.
[RFC7515] specifies base64url encoding as follows:
| Base64 encoding using the URL- and filename-safe character set
| defined in Section 5 of RFC 4648 [RFC4648], with all trailing '='
| characters omitted (as permitted by Section 3.2 of [RFC7515]) and
| without the inclusion of any line breaks, whitespace, or other
| additional characters. Note that the base64url encoding of the
| empty octet sequence is the empty string. (See Appendix C of
| [RFC7515] for notes on implementing base64url encoding without
| padding.)
The base64url encoding of the thumbprint shown in Section 6 is shown
below (with a line break added for readability purposes).
SWvYr63zB-WwjGSwQhv53AFSijRKQ72oj63RZp2iU-w
The full example of a COSE Key Thumbprint URI is shown below (with a
line break added for readability).
urn:ietf:params:oauth:ckt:sha-256:
SWvYr63zB-WwjGSwQhv53AFSijRKQ72oj63RZp2iU-w
6. Example
This section demonstrates the COSE Key Thumbprint computation for the
following example COSE Key containing an Elliptic Curve Cryptography
(ECC) public key.
For better readability, the example is first presented in CBOR
diagnostic format (with the long line broken for display purposes
only).
{
/ kty set to EC2 = Elliptic Curve Keys /
1:2,
/ crv set to P-256 /
-1:1,
/ public key: x-coordinate /
-2:h'65eda5a12577c2bae829437fe338701a10aaa375e1bb5b5de108de439c0
8551d',
/ public key: y-coordinate /
-3:h'1e52ed75701163f7f9e40ddf9f341b3dc9ba860af7e0ca7ca7e9eecd008
4d19c',
/ kid is bstr, not used in COSE Key Thumbprint /
2:h'1decade2facade3'
}
The example above corresponds to the following CBOR encoding (with
link breaks added for display purposes only):
A50102200121582065EDA5A12577C2BAE829437FE338701A10AAA375E1BB5B5DE108D
E439C08551D2258201E52ED75701163F7F9E40DDF9F341B3DC9BA860AF7E0CA7CA7E9
EECD0084D19C0258246D65726961646F632E6272616E64796275636B406275636B6C6
16E642E6578616D706C65
Not all of the parameters from the example above are used in the COSE
Key Thumbprint computation because the required parameters of an
elliptic curve public key are (as listed in Section 4.2) "kty",
"crv", "x", and "y".
The resulting COSE Key structure, in CBOR diagnostic format with line
breaks added for better readability, with the minimum parameters in
the correct order are:
{
1:2,
-1:1,
-2:h'65eda5a12577c2bae829437fe338701a
10aaa375e1bb5b5de108de439c08551d',
-3:h'1e52ed75701163f7f9e40ddf9f341b3d
c9ba860af7e0ca7ca7e9eecd0084d19c'
}
In CBOR encoding, the result is (with line breaks added for display
purposes only):
A40102200121582065EDA5A12577C2BAE829437FE338701A10AAA375E1BB5B5DE
108DE439C08551D2258201E52ED75701163F7F9E40DDF9F341B3DC9BA860AF7E0
CA7CA7E9EECD0084D19C
Using SHA-256, the resulting thumbprint is:
496bd8afadf307e5b08c64b0421bf9dc01528a344a43bda88fadd1669da253ec
7. Security Considerations
A COSE Key Thumbprint will only uniquely identify a particular key if
a single unambiguous COSE Key representation for that key is defined
and used when computing the COSE Key Thumbprint.
A COSE Key Thumbprint will only uniquely identify a particular key if
a single unambiguous COSE Key representation for that key is defined
and used when computing the COSE Key Thumbprint. Key identifiers are
not included in the thumbprint calculation (similarly to other
optional parameters in the COSE_Key structure). If the inclusion of
specific optional parameters in the thumbprint calculation is
important for a particular application, this specification would not
be suitable.
While thumbprint values are useful for identifying legitimate keys,
comparing thumbprint values is not a reliable means of excluding the
use of particular keys (or transformations thereof). The reason is
because an attacker may supply a key that is a transformation of a
key in order for it to appear as a different key. For instance, if a
legitimate RSA key uses a modulus value N and an attacker supplies a
key with modulus 3*N, the modified key would still work about 1/3 of
the time, but it would appear to be a different key.
Producing thumbprints of symmetric keys needs to be done with care.
Developers MUST ensure that the symmetric key has sufficient entropy
to prevent attackers from precomputing tables of symmetric keys with
their corresponding hash values. This can be prevented if the
symmetric key is a randomly selected key of at least a 128-bit
length. Thumbprints MUST NOT be used with passwords or other low-
entropy secrets. If a developer is unable to determine whether all
symmetric keys used in an application have sufficient entropy, then
thumbprints of symmetric keys MUST NOT be used. In general, using
thumbprints of symmetric keys should only be used in special
applications. In most other deployment scenarios, it is more
appropriate to utilize a different naming scheme for key identifiers.
8. IANA Considerations
IANA has added the following entry to the "CWT Confirmation Methods"
registry [IANA-CWT] established by [RFC8747]:
Confirmation Method Name: ckt
Confirmation Method Description: COSE Key SHA-256 Thumbprint
JWT Confirmation Method Name: jkt
Confirmation Key: 5
Confirmation Value Type(s): binary string
Change Controller: IETF
Specification Document(s): RFC 9679
Furthermore, IANA has added a value to the "OAuth URI" registry
[IANA-OAuth] established by [RFC6755]:
URN: urn:ietf:params:oauth:ckt
Common Name: COSE Key Thumbprint URI
Change Controller: IETF
Specification Document(s): RFC 9679
9. References
9.1. Normative References
[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>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
[RFC6755] Campbell, B. and H. Tschofenig, "An IETF URN Sub-Namespace
for OAuth", RFC 6755, DOI 10.17487/RFC6755, October 2012,
<https://www.rfc-editor.org/info/rfc6755>.
[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>.
[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>.
[RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
May 2018, <https://www.rfc-editor.org/info/rfc8392>.
[RFC8747] Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
Tschofenig, "Proof-of-Possession Key Semantics for CBOR
Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March
2020, <https://www.rfc-editor.org/info/rfc8747>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>.
[RFC9052] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", STD 96, RFC 9052,
DOI 10.17487/RFC9052, August 2022,
<https://www.rfc-editor.org/info/rfc9052>.
[RFC9053] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053,
August 2022, <https://www.rfc-editor.org/info/rfc9053>.
9.2. Informative References
[IANA-CWT] IANA, "CWT Confirmation Methods",
<https://www.iana.org/assignments/cwt>.
[IANA-OAuth]
IANA, "OAuth URI",
<https://www.iana.org/assignments/oauth-parameters>.
[IANA.Hash.Algorithms]
IANA, "Named Information Hash Algorithm Registry",
<https://www.iana.org/assignments/named-information>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011,
<https://www.rfc-editor.org/info/rfc6234>.
[RFC7638] Jones, M. and N. Sakimura, "JSON Web Key (JWK)
Thumbprint", RFC 7638, DOI 10.17487/RFC7638, September
2015, <https://www.rfc-editor.org/info/rfc7638>.
[RFC9278] Jones, M. and K. Yasuda, "JWK Thumbprint URI", RFC 9278,
DOI 10.17487/RFC9278, August 2022,
<https://www.rfc-editor.org/info/rfc9278>.
[RFC9360] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Header Parameters for Carrying and Referencing X.509
Certificates", RFC 9360, DOI 10.17487/RFC9360, February
2023, <https://www.rfc-editor.org/info/rfc9360>.
Acknowledgements
We would like to thank the authors of [RFC7638] for their work on the
JWK Thumbprint specification. This document applies JWK Thumbprints
to COSE Key structures.
Additionally, we would like to thank Carsten Bormann, Ilari
Liusvaara, Laurence Lundblade, Daisuke Ajitomi, Michael Richardson,
Michael B. Jones, Mallory Knodel, Joel Jaeggli, Derrell Piper, Patrik
Fältström, Warren Kumari, Deb Cooley, and Brendan Moran for their
feedback.
Authors' Addresses
Kohei Isobe
SECOM CO., LTD.
Email: isobekohei@gmail.com
Hannes Tschofenig
University of Applied Sciences Bonn-Rhein-Sieg
Germany
Email: hannes.tschofenig@gmx.net
Orie Steele
Transmute
United States of America
Email: orie@transmute.industries