Internet DRAFT - draft-isobe-cose-key-thumbprint
draft-isobe-cose-key-thumbprint
COSE K. Isobe
Internet-Draft SECOM CO., LTD.
Intended status: Standards Track H. Tschofenig
Expires: 17 December 2023 15 June 2023
COSE Key Thumbprint
draft-isobe-cose-key-thumbprint-02
Abstract
This specification defines a method for computing a hash value over a
COSE Key. It defines which fields in a COSE Key structure are used in
the hash computation, the method of creating a canonical form of the
fields, and how to hash the byte sequence. The resulting hash value
can be used for identifying or selecting a key that is the subject of
the thumbprint.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 17 December 2023.
Copyright Notice
Copyright (c) 2023 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
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Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. COSE Key Thumbprint . . . . . . . . . . . . . . . . . . . . . 3
4. Required COSE Key Elements . . . . . . . . . . . . . . . . . 3
4.1. Octet Key Pair (OKP) . . . . . . . . . . . . . . . . . . 3
4.2. Elliptic Curve Keys w/ x- and y-coordinate pair . . . . . 4
4.3. RSA Public Keys . . . . . . . . . . . . . . . . . . . . . 4
4.4. HSS-LMS . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.5. Others . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.6. Why Not Include Optional COSE Key Elements? . . . . . . . 5
4.7. Selection of Hash Function . . . . . . . . . . . . . . . 5
4.8. Relationship to Digests of X.509 Values . . . . . . . . . 6
5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
This specification defines a method for computing a hash value
(a.k.a. digest) over a COSE Key structure [RFC9052]. It defines
which fields in a COSE Key structure are used in the hash
computation, the method of creating a canonical form for those
fields, and how to hash the byte sequence. The resulting hash value
can be used for identifying or selecting the key that is the subject
of the thumbprint, for instance, by using the COSE Key Thumbprint
value as a "kid" (key ID) value.
This specification only defines how thumbprints of public keys are
created, not private keys or symmetric keys.
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.
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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 elements representing the key. This
specification describes what those required elements are and
what, if necessary, what the unique encoding is.
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 resulting value is the COSE Key Thumbprint with H of the COSE
Key. The details of this computation are further described in
subsequent sections.
4. Required COSE Key Elements
Only the required elements of a key's representation are used when
computing its COSE Key Thumbprint value. This section summarizes the
required elements.
The "kty" (label: 1) element MUST be present for all key types and
the integer value found in the IANA COSE Key Types registry MUST be
used. The tstr data type is not used with the kty element.
Many COSE Key elements depend on the chosen key type. The subsection
below list the required elements for commonly used key types.
4.1. Octet Key Pair (OKP)
The required elements for elliptic curve public keys that use the 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)
Details can be found in Section 7.1 of [RFC9053].
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4.2. Elliptic Curve Keys w/ x- and y-coordinate pair
The required elements for elliptic curve public keys that use the 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)
Details can be found in Section 7.1 of [RFC9053].
Note: [RFC9052] offers both compressed as well as uncompressed point
representations. For interoperability, implementations following
this specification MUST use the uncompressed point representation.
Hence, the y-coordinate is expressed as a bstr. An implementation
that uses the compressed point representation MUST compute the
uncompressed representation for the purpose of the thumbprint
calculation.
4.3. RSA Public Keys
The required elements 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. HSS-LMS
The required elements for HSS-LMS keys are:
* "kty" (label: 1, data type: int, value: 5)
* "pub" (label: -1, data type: bstr)
4.5. Others
As other key type values are defined, the specifications defining
them should be similarly consulted to determine which elements, in
addition to the "kty" element, are required.
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4.6. Why Not Include Optional COSE Key Elements?
Optional elements 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 value is a digest of the elements required to represent
the key as a COSE Key -- not of additional data that may also
accompany the key.
Optional elements are not included so that the COSE Key Thumbprint
refers to a key -- not a key with an associated set of key
attributes. Different application contexts might or might not
include different subsets of optional attributes about the key in the
COSE Key structure. If these were included in the calculation of the
COSE Key Thumbprint, the values would be different for those COSE
Keys, even though the keys are the same. The benefit of including
only the required elements is that the COSE Key Thumbprint of any
COSE Key representing the key remains the same, regardless of any
other attributes that are present.
Different kinds of thumbprints could be defined by other
specifications that might include some or all additional COSE Key
elements, if use cases arise where such different kinds of
thumbprints would be useful.
4.7. Selection of Hash Function
A specific hash function must be chosen by an application to compute
the hash value of the hash input. For example, SHA-256 [RFC6234]
might be used as the hash function by the application. While SHA-256
is a good default choice at the time of writing, the hash function of
choice can be expected to change over time as the cryptographic
landscape evolves.
Note that in many cases, only the party that creates a key will need
to know the hash function used. A typical usage is for the producer
of the key to use the thumbprint value as a "kid" (key ID) value. In
this case, the consumer of the "kid" treats it as an opaque value
that it uses to select the key.
However, in some cases, multiple parties will be reproducing the COSE
Key Thumbprint calculation and comparing the results. In these
cases, the parties will need to know which hash function was used and
use the same one.
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4.8. Relationship to Digests of X.509 Values
COSE Key Thumbprint values are computed on the COSE Key element
required to represent a key, rather than all members of a COSE Key
that the key is represented in. 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. Example
This section demonstrates the COSE Key Thumbprint computation for the
following example COSE Key containing an ECC public key.
For better readability, the example is first presented in JSON (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 /
2:'meriadoc.brandybuck@buckland.example'
}
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 elements from the example above are used in the COSE
Key Thumbprint since the required elements of an elliptic curve
public key are:
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* "kty"
* "crv"
* "x"
* "y"
The required order based on Section 4.2.1 of [RFC8949] is:
* "y" (label: -3, data type: bstr)
* "x" (label: -2, data type: bstr)
* "crv" (label: -1, data type: int)
* "kty" (label: 1, data type: int)
The resulting COSE Key structure, in CBOR diagnostic format with
line-breaks added for better readability, with the minimum elements
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
6. 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.
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If two asymmetric keys are used by different parties with different
key identifiers then the COSE Key Thumbprints will still be equal
since the key identifier itself is not included in the thumbprint
calculation (similarly to other optional elements in the COSE_Key
structure). When the inclusion of certain optinal elements in the
thumbprint calcuation is important for a given application, this
specification is not the appropriate choice.
To promote interoperability among implementations, the SHA-256 hash
algorithm is mandatory to implement.
While thumbprint values are valuable 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
that an attacker may supply a key that is a transformation of a key
in order to have it appear to be 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 would appear to be a different key.
7. IANA Considerations
There are no actions for IANA.
8. Acknowledgements
We would like to thank the authors of [RFC7638] for their work on the
JSON Web Key (JWK) Thumbprint specification. This document applies
JWK Thumbprints to COSE Key structures.
Additionally, we would like to thank Carsten Bormann, Orie Steele,
Ilari Liusvaara, Laurence Lundblade, Daisuke Ajitomi, and Michael
Richardson for their feedback.
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/rfc/rfc2119>.
[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/rfc/rfc8174>.
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[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/rfc/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/rfc/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/rfc/rfc9053>.
9.2. Informative References
[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/rfc/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/rfc/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/rfc/rfc7638>.
[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/rfc/rfc9360>.
Authors' Addresses
Kohei Isobe
SECOM CO., LTD.
Email: isobekohei@gmail.com
Hannes Tschofenig
Email: hannes.tschofenig@gmx.net
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