COSE Key Thumbprint and Thumbprint URI

Introduction This specification defines a method for computing a hash value (a.k.a. digest) over a COSE Key structure . 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. A future version of this document will describe the concept of a COSE Key Thumbprint URI.

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 when, and only when, they appear in all capitals, as shown here.

COSE Key Thumbprint The thumbprint of a COSE Key MUST be computed as follows:

Construct a COSE_Key structure (see Section 7 of ) 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.
Apply the deterministic encoding described in Section 4.2.1 of to the representation constructed in step (1).
Hash the bytes produced in step (2) with a cryptographic hash function H. For example, SHA-256 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.

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.

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 .

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 . Note: 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.

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)

HSS-LMS The required elements for HSS-LMS keys are:

"kty" (label: 1, data type: int, value: 5)
"pub" (label: -1, data type: bstr)

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.

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.

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 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.

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 , than to applications that use digests of complete certificate values, as the "x5t" (X.509 certificate SHA-1 thumbprint) 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.

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). The example above corresponds to the following CBOR encoding (with link breaks added for display purposes only): 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:

"kty"
"crv"
"x"
"y"

The required order based on Section 4.2.1 of 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. In CBOR encoding the result is (with line-breaks added for display purposes only): Using SHA-256, the resulting thumbprint is:

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. 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. Even more insidious is that an attacker may supply a key that is a transformation of a legal 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. Thus, while thumbprint values are valuable for identifying legitimate keys, comparing thumbprint values is not a reliable means of excluding (blacklisting) the use of particular keys (or transformations thereof).

IANA Considerations There are no actions for IANA.

Acknowledgements We would like to thank the authors of 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.