Internet DRAFT - draft-jones-jose-jwk-thumbprint
draft-jones-jose-jwk-thumbprint
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
Abstract
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.
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 January 24, 2015.
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. JSON Web Key (JWK) Thumbprint . . . . . . . . . . . . . . . . 3
3.1. Example JWK Thumbprint Computation . . . . . . . . . . . . 4
3.2. JWK Members Used in the Thumbprint Computation . . . . . . 5
3.2.1. JWK Thumbprint of a Private Key . . . . . . . . . . . 6
3.2.2. Why Not Include Optional Members? . . . . . . . . . . 6
3.3. Order and Representation of Members in Hash Input . . . . 6
3.4. JWK Thumbprints of Any Keys . . . . . . . . . . . . . . . 7
4. "jkt" Member Definitions . . . . . . . . . . . . . . . . . . . 7
4.1. "jkt" (JWK SHA-256 Thumbprint) JWS Header Parameter . . . 7
4.2. "jkt" (JWK SHA-256 Thumbprint) JWE Header Parameter . . . 7
4.3. "jkt" (JWK SHA-256 Thumbprint) JWK Parameter . . . . . . . 8
4.4. Possible Future Alternative Thumbprint Computations . . . 8
5. Practical JSON and Unicode Considerations . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6.1. JWS and JWE Header Parameter Registration . . . . . . . . 9
6.1.1. Registry Contents . . . . . . . . . . . . . . . . . . 9
6.2. JSON Web Key Parameters Registration . . . . . . . . . . . 9
6.2.1. Registry Contents . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. Normative References . . . . . . . . . . . . . . . . . . . . . 10
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 10
Appendix B. Document History . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
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.
1.1. Notational 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 Key
words for use in RFCs to Indicate Requirement Levels [RFC2119].
2. Terminology
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:
JWK Thumbprint
The digest value for a key that is the subject of this
specification.
3. JSON Web Key (JWK) Thumbprint
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.
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3.1. Example JWK Thumbprint Computation
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:
o "kty"
o "n"
o "e"
Therefore, these are the members used in the thumbprint computation.
Their lexicographic order (see more about this in Section 3.3) is:
o "e"
o "kty"
o "n"
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,
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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]
The base64url encoding of this JWK Thumbprint value (which would be
used in the "jkt" members registered below) is:
NzbLsXh8uDCcd-6MNwXF4W_7noWXFZAfHkxZsRGC9Xs
3.2. JWK Members Used in the Thumbprint Computation
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:
o "crv"
o "kty"
o "x"
o "y"
the REQUIRED members of an RSA public key, in lexicographic order,
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are:
o "e"
o "kty"
o "n"
and the REQUIRED members of a symmetric key, in lexicographic order,
are:
o "k"
o "kty"
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.
3.2.1. JWK Thumbprint of a Private Key
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.
3.2.2. Why Not Include Optional Members?
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.
3.3. Order and Representation of Members in Hash Input
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
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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.
3.4. JWK Thumbprints of Any Keys
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.
4. "jkt" Member Definitions
This section defines "jkt" (JWK SHA-256 Thumbprint) members used for
holding base64url encoded JWK Thumbprint values in JWK, JWS, and JWE
objects.
4.1. "jkt" (JWK SHA-256 Thumbprint) JWS Header Parameter
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.
4.2. "jkt" (JWK SHA-256 Thumbprint) JWE Header Parameter
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.
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4.3. "jkt" (JWK SHA-256 Thumbprint) JWK Parameter
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.
4.4. Possible Future Alternative Thumbprint Computations
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.
5. Practical JSON and Unicode Considerations
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
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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."
6. IANA Considerations
6.1. JWS and JWE Header Parameter Registration
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].
6.1.1. Registry Contents
o Header Parameter Name: "jkt"
o Header Parameter Description: JWS JWK Thumbprint
o Header Parameter Usage Location(s): JWS
o Change Controller: IETF
o Specification Document(s): Section 4.1 of [[ this document ]]
o Header Parameter Name: "jkt"
o Header Parameter Description: JWE JWK Thumbprint
o Header Parameter Usage Location(s): JWE
o Change Controller: IETF
o Specification Document(s): Section 4.2 of [[ this document ]]
6.2. JSON Web Key Parameters Registration
This specification registers the "jkt" JWK member defined in
Section 4.3 in the IANA JSON Web Key Parameters registry defined in
[JWK].
6.2.1. Registry Contents
o Parameter Name: "jkt"
o Parameter Description: JWK Thumbprint
o Used with "kty" Value(s): *
o Parameter Information Class: Public
o Change Controller: IESG
o Specification Document(s): Section 4.3 of [[ this document ]]
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7. Security Considerations
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.
8. Normative References
[JWA] Jones, M., "JSON Web Algorithms (JWA)",
draft-ietf-jose-json-web-algorithms (work in progress),
July 2014.
[JWE] Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
draft-ietf-jose-json-web-encryption (work in progress),
July 2014.
[JWK] Jones, M., "JSON Web Key (JWK)",
draft-ietf-jose-json-web-key (work in progress),
July 2014.
[JWS] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", draft-ietf-jose-json-web-signature (work
in progress), 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.
Appendix A. Acknowledgements
James Manger, Nat Sakimura, and John Bradley participated in
discussions that led to the creation of this specification.
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Appendix B. Document History
[[ to be removed by the RFC editor before publication as an RFC ]]
-01
o Based on input at IETF 90, revised the draft to say that the
result is undefined if characters requiring escaping are needed in
the hash input. If a canonical JSON representation standard is
ever adopted, this specification could be revised to use it,
resulting in unambiguous definitions for those (unlikely to ever
occur) values as well.
o Added instructions for representing integer numeric values in the
hash input.
-00
o Created draft-jones-jose-jwk-thumbprint.
Author's Address
Michael B. Jones
Microsoft
Email: mbj@microsoft.com
URI: http://self-issued.info/
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