| JOSE Working Group | M. Jones |
| Internet-Draft | Microsoft |
| Intended status: Standards Track | July 23, 2014 |
| Expires: January 24, 2015 |
JSON Web Key (JWK) Thumbprint
draft-jones-jose-jwk-thumbprint-01
This specification defines a means of computing a thumbprint value (a.k.a. digest) of JSON Web Key (JWK) objects analogous to the x5t (X.509 Certificate SHA-1 Thumbprint) value defined for X.509 certificate objects. This specification also registers the new JSON Web Signature (JWS) and JSON Web Encryption (JWE) Header Parameters and the new JSON Web Key (JWK) member name jkt (JWK SHA-256 Thumbprint) for holding these values.
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This specification defines a means of computing a thumbprint value (a.k.a. digest) of JSON Web Key (JWK) [JWK] objects analogous to the x5t (X.509 Certificate SHA-1 Thumbprint) value defined for X.509 certificate objects. This specification also registers the new JSON Web Signature (JWS) [JWS] and JSON Web Encryption (JWE) [JWE] Header Parameters and the new JSON Web Key (JWK) [JWK] member name jkt (JWK SHA-256 Thumbprint) for holding these values.
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 Key words for use in RFCs to Indicate Requirement Levels [RFC2119].
This specification uses the same terminology as the JSON Web Key (JWK) [JWK], JSON Web Signature (JWS) [JWS], JSON Web Encryption (JWE) [JWE], and JSON Web Algorithms (JWA) [JWA] specifications.
This term is defined by this specification:
This specification defines the thumbprint of a JSON Web Key (JWK) value as being a function of the REQUIRED members of the key's JWK representation. Specifically, this function is the SHA-256 hash of the octets of the UTF-8 representation of a JSON object [RFC7159] constructed containing only the REQUIRED members of a JWK representing the key and with no white space or line breaks before or after any syntactic elements and with the REQUIRED members ordered lexicographically by the Unicode code points of the member names. This JSON object is itself a legal JWK representation of the key value. The details of this computation are further described in subsequent sections.
This section demonstrates the JWK Thumbprint computation for the JWK below (with long lines broken for display purposes only):
{
"kty": "RSA",
"n": "0vx7agoebGcQSuuPiLJXZptN9nndrQmbXEps2aiAFbWhM78LhWx4cbbfAAt
VT86zwu1RK7aPFFxuhDR1L6tSoc_BJECPebWKRXjBZCiFV4n3oknjhMstn6
4tZ_2W-5JsGY4Hc5n9yBXArwl93lqt7_RN5w6Cf0h4QyQ5v-65YGjQR0_FD
W2QvzqY368QQMicAtaSqzs8KJZgnYb9c7d0zgdAZHzu6qMQvRL5hajrn1n9
1CbOpbISD08qNLyrdkt-bFTWhAI4vMQFh6WeZu0fM4lFd2NcRwr3XPksINH
aQ-G_xBniIqbw0Ls1jF44-csFCur-kEgU8awapJzKnqDKgw",
"e": "AQAB",
"alg": "RS256",
"kid": "2011-04-29"
}
As defined in JSON Web Key (JWK) [JWK] and JSON Web Algorithms (JWA) [JWA], the REQUIRED members of an RSA public key are:
Therefore, these are the members used in the thumbprint computation.
Their lexicographic order (see more about this in Section 3.3) is:
Therefore the JSON object constructed as an intermediate step in the computation is as follows (with long lines broken for display purposes only):
{"e":"AQAB","kty":"RSA","n":"0vx7agoebGcQSuuPiLJXZptN9nndrQmbXEps2
aiAFbWhM78LhWx4cbbfAAtVT86zwu1RK7aPFFxuhDR1L6tSoc_BJECPebWKRXjBZCi
FV4n3oknjhMstn64tZ_2W-5JsGY4Hc5n9yBXArwl93lqt7_RN5w6Cf0h4QyQ5v-65Y
GjQR0_FDW2QvzqY368QQMicAtaSqzs8KJZgnYb9c7d0zgdAZHzu6qMQvRL5hajrn1n
91CbOpbISD08qNLyrdkt-bFTWhAI4vMQFh6WeZu0fM4lFd2NcRwr3XPksINHaQ-G_x
BniIqbw0Ls1jF44-csFCur-kEgU8awapJzKnqDKgw"}
The octets of the UTF-8 representation of this JSON object are:
[123, 34, 101, 34, 58, 34, 65, 81, 65, 66, 34, 44, 34, 107, 116, 121, 34, 58, 34, 82, 83, 65, 34, 44, 34, 110, 34, 58, 34, 48, 118, 120, 55, 97, 103, 111, 101, 98, 71, 99, 81, 83, 117, 117, 80, 105, 76, 74, 88, 90, 112, 116, 78, 57, 110, 110, 100, 114, 81, 109, 98, 88, 69, 112, 115, 50, 97, 105, 65, 70, 98, 87, 104, 77, 55, 56, 76, 104, 87, 120, 52, 99, 98, 98, 102, 65, 65, 116, 86, 84, 56, 54, 122, 119, 117, 49, 82, 75, 55, 97, 80, 70, 70, 120, 117, 104, 68, 82, 49, 76, 54, 116, 83, 111, 99, 95, 66, 74, 69, 67, 80, 101, 98, 87, 75, 82, 88, 106, 66, 90, 67, 105, 70, 86, 52, 110, 51, 111, 107, 110, 106, 104, 77, 115, 116, 110, 54, 52, 116, 90, 95, 50, 87, 45, 53, 74, 115, 71, 89, 52, 72, 99, 53, 110, 57, 121, 66, 88, 65, 114, 119, 108, 57, 51, 108, 113, 116, 55, 95, 82, 78, 53, 119, 54, 67, 102, 48, 104, 52, 81, 121, 81, 53, 118, 45, 54, 53, 89, 71, 106, 81, 82, 48, 95, 70, 68, 87, 50, 81, 118, 122, 113, 89, 51, 54, 56, 81, 81, 77, 105, 99, 65, 116, 97, 83, 113, 122, 115, 56, 75, 74, 90, 103, 110, 89, 98, 57, 99, 55, 100, 48, 122, 103, 100, 65, 90, 72, 122, 117, 54, 113, 77, 81, 118, 82, 76, 53, 104, 97, 106, 114, 110, 49, 110, 57, 49, 67, 98, 79, 112, 98, 73, 83, 68, 48, 56, 113, 78, 76, 121, 114, 100, 107, 116, 45, 98, 70, 84, 87, 104, 65, 73, 52, 118, 77, 81, 70, 104, 54, 87, 101, 90, 117, 48, 102, 77, 52, 108, 70, 100, 50, 78, 99, 82, 119, 114, 51, 88, 80, 107, 115, 73, 78, 72, 97, 81, 45, 71, 95, 120, 66, 110, 105, 73, 113, 98, 119, 48, 76, 115, 49, 106, 70, 52, 52, 45, 99, 115, 70, 67, 117, 114, 45, 107, 69, 103, 85, 56, 97, 119, 97, 112, 74, 122, 75, 110, 113, 68, 75, 103, 119, 34, 125]
The JWK Thumbprint value is the SHA-256 hash of these octets, specifically:
[55, 54, 203, 177, 120, 124, 184, 48, 156, 119, 238, 140, 55, 5, 197, 225, 111, 251, 158, 133, 151, 21, 144, 31, 30, 76, 89, 177, 17, 130, 245, 123]
NzbLsXh8uDCcd-6MNwXF4W_7noWXFZAfHkxZsRGC9Xs
The base64url encoding of this JWK Thumbprint value (which would be used in the jkt members registered below) is:
Only the REQUIRED members of a key's representation are used when computing its JWK Thumbprint value. As defined in JSON Web Key (JWK) [JWK] and JSON Web Algorithms (JWA) [JWA], the REQUIRED members of an elliptic curve public key, in lexicographic order, are:
the REQUIRED members of an RSA public key, in lexicographic order, are:
and the REQUIRED members of a symmetric key, in lexicographic order, are:
As other key type values are defined, the specifications defining them should be similarly consulted to determine which members, in addition to kty, are REQUIRED.
The JWK Thumbprint of a private key is computed as the JWK Thumbprint of the corresponding public key. This has the intentional benefit that the same JWK Thumbprint value can be computed both by parties using either the public or private key. The JWK Thumbprint can then be used to refer to both keys of the key pair. Application context can be used to determine whether the public or the private key is the one being referred to by the JWK Thumbprint.
OPTIONAL members of JWKs are intentionally not included in the JWK Thumbprint computation so that their absence or presence in the JWK doesn't alter the resulting value. The JWK Thumbprint value is a digest of the key value itself -- not of additional data that may also accompany the key.
The REQUIRED members in the input to the SHA-256 hash function are ordered lexicographically by the Unicode code points of the member names.
Characters in member names and member values MUST be represented without being escaped. This means that thumbprints of JWK values that require such characters are not defined by this specification. (This is not expected to limit the applicability of this specification, in practice, as the REQUIRED members of JWK representations are not expected to use any of these characters.) The characters specified as requiring escaping by Section 7 of [RFC7159] are quotation mark, reverse solidus (a.k.a. backslash), and the control characters U+0000 through U+001F.
If the JWK key type uses members whose values are themselves JSON objects (as of the time of this writing, none are defined that do), the members of those objects must likewise be lexicographically ordered.
If the JWK key type uses members whose values are JSON numbers (as of the time of this writing, none are defined that do), if the numbers are integers, they MUST be represented as a JSON number as defined in Section 6 of [RFC7159] without including a fraction part or exponent part. For instance, the value 1.024e3 MUST be represented as 1024. This means that thumbprints of JWK values that use numbers that are not integers are not defined by this specification.
See Section 5 for a discussion of further practical considerations pertaining to the representation of the hash input.
Note that a key need not be in JWK format to create a JWK Thumbprint of it. The only prerequisites are that the JWK representation of the key be defined and the party creating the JWK Thumbprint is in possession of the necessary key material. These are sufficient to create the hash input, as described in Section 3.3.
This section defines jkt (JWK SHA-256 Thumbprint) members used for holding base64url encoded JWK Thumbprint values in JWK, JWS, and JWE objects.
The jkt (JWK SHA-256 Thumbprint) JWS Header Parameter is a base64url encoded JWK Thumbprint (a.k.a. digest) of the public key that corresponds to the key used to digitally sign the JWS. Use of this JWS Header Parameter is OPTIONAL.
This parameter has the same meaning, syntax, and processing rules as the jkt JWS Header Parameter defined in Section 4.1, except that the JWK Thumbprint references the public key to which the JWE was encrypted; this can be used to determine the private key needed to decrypt the JWE.
The jkt (JWK SHA-256 Thumbprint) JWK parameter is a base64url encoded JWK Thumbprint (a.k.a. digest) of the JWK. If present, the JWK Thumbprint value represented MUST have been computed from the other members of the JWK as described in Section 3. Use of this member is OPTIONAL.
If, in the future, JWK Thumbprints need to be computed using hash functions other than SHA-256, it is suggested that additional related JWK, JWS, and JWE parameters be defined for that purpose. For example, it is suggested that a new jkt#S3-256 (X.509 Certificate Thumbprint using SHA-3-256) JWK parameter could be defined by registering it in the IANA JSON Web Key Parameters registry and the IANA JSON Web Signature and Encryption Header Parameters registry.
Implementations will almost certainly use functionality provided by the platform's JSON support, such as the JavaScript JSON.parse() JSON.stringify() functions, when parsing the JWK and emitting the JSON object used as the SHA-256 hash input. As a practical consideration, future JWK member names should be avoided for which different platforms or libraries might emit different representations. As of the time of this writing, currently all defined JWK member names use only printable ASCII characters, which should not exhibit this problem.
In particular, while the operation of lexicographically ordering member names by their Unicode code points is well defined, different platform sort functions may produce different results for non-ASCII characters, in ways that may not be obvious to developers. If writers of future specifications defining new JWK Key Type values choose to restrict themselves to ASCII member names (which are for machine and not human consumption anyway), some future interoperability problems might be avoided.
Use of escaped characters in the input JWK representation should be avoided.
While there is a natural representation to use for numeric values that are integers, this specification doesn't attempt to define a standard representation for numbers that are not integers or that contain an exponent component. This is not expected to be a problem in practice, as the REQUIRED members of JWK representations are not expected to use numbers that are not integers.
Use of number representations containing fraction or exponent parts in the input JWK representation should be avoided.
All of these practical considerations are really an instance of Jon Postel's principle: "Be liberal in what you accept, and conservative in what you send."
This specification registers the jkt Header Parameters defined in Sections 4.1 and 4.2 in the IANA JSON Web Signature and Encryption Header Parameters registry defined in [JWS].
This specification registers the jkt JWK member defined in Section 4.3 in the IANA JSON Web Key Parameters registry defined in [JWK].
The JSON Security Considerations and Unicode security considerations described in Sections 10.2 and 10.3 of JSON Web Signature (JWS) [JWS] also apply to this specification.
Also, as described in Section 5, some implementations may produce incorrect results if esoteric or escaped characters are used in the member names. The security implications of this appear to be limited for JWK Thumbprints of public keys, since while it may result in implementations failing to identify the intended key, it should not leak information, since the information in a public key is already public in nature, by definition.
| [JWA] | Jones, M., "JSON Web Algorithms (JWA)", Internet-Draft draft-ietf-jose-json-web-algorithms, July 2014. |
| [JWE] | Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)", Internet-Draft draft-ietf-jose-json-web-encryption, July 2014. |
| [JWK] | Jones, M., "JSON Web Key (JWK)", Internet-Draft draft-ietf-jose-json-web-key, July 2014. |
| [JWS] | Jones, M., Bradley, J. and N. Sakimura, "JSON Web Signature (JWS)", Internet-Draft draft-ietf-jose-json-web-signature, July 2014. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [RFC7159] | Bray, T., "The JavaScript Object Notation (JSON) Data Interchange Format", RFC 7159, March 2014. |
James Manger, Nat Sakimura, and John Bradley participated in discussions that led to the creation of this specification.
[[ to be removed by the RFC editor before publication as an RFC ]]
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