JOSE Working Group M. Jones
Internet-Draft Microsoft
Intended status: Standards Track N. Sakimura
Expires: August 23, 2015 NRI
February 19, 2015
JSON Web Key (JWK) Thumbprint
draft-ietf-jose-jwk-thumbprint-02
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.
Status of this Memo
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This Internet-Draft will expire on August 23, 2015.
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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 . . . . 7
3.4. JWK Thumbprints of Keys Not in JWK Format . . . . . . . . 7
4. Practical JSON and Unicode Considerations . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . . 10
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 10
Appendix B. Document History . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 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 value can be used for identifying or
selecting the key that is the subject of the thumbprint, for
instance, by using the base64url encoded JWK Thumbprint value as a
"kid" (key ID) value.
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
The thumbprint of a JSON Web Key (JWK) is computed as follows:
1. Construct a JSON object [RFC7159] 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
[UNICODE] code points of the member names. (This JSON object is
itself a legal JWK representation of the key.)
2. Hash the octets of the UTF-8 representation of this JSON object
with a cryptographic hash function H. For example, SHA-256 [SHS]
might be used as H.
The resulting value is the JWK Thumbprint with H of the JWK. The
details of this computation are further described in subsequent
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sections.
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:
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[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]
Using SHA-256 [SHS] as the hash function H, the JWK SHA-256
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 [JWS] of this JWK SHA-256 Thumbprint value
(which might, for instance, be used as a "kid" (key ID) value) 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 for the curves specified in Section 6.2.1.1
of [JWK], in lexicographic order, are:
o "crv"
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o "kty"
o "x"
o "y"
the REQUIRED members of an RSA public key, in lexicographic order,
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.
This specification defines the method of computing JWK Thumbprints of
private keys for interoperability reasons -- so that different
implementations computing JWK Thumbprints of private keys will
produce the same result.
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.
OPTIONAL members are not included so that the JWK Thumbprint refers
to a key -- not a key with an associated set of key attributes. This
has the benefit that while in different application contexts
different subsets of attributes about the key might or might not be
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included in the JWK, the JWK Thumbprint of the key remains the same
regardless of which optional attributes are present. Different kinds
of thumbprints could be defined by other specifications that might
include some or all additional JWK members, should use cases arise
where such different kinds of thumbprints would be useful. See
Section 9.1 of [JWK] for notes on some ways to cryptographically bind
attributes to a key.
3.3. Order and Representation of Members in Hash Input
The REQUIRED members in the input to the 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 JWKs 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 JWKs that use numbers that
are not integers are not defined by this specification. Also, as
noted in The I-JSON Message Format [I-D.ietf-json-i-json],
implementations cannot expect an integer whose absolute value is
greater than 9007199254740991 (i.e., that is outside the range
[-(2**53)+1, (2**53)-1]) to be treated as an exact value.
See Section 4 for a discussion of further practical considerations
pertaining to the representation of the hash input.
3.4. JWK Thumbprints of Keys Not in JWK Format
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
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possession of the necessary key material. These are sufficient to
create the hash input from the JWK representation of the key, as
described in Section 3.3.
4. 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 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. Note however, that JSON.stringify() cannot be counted on to
lexicographically sort the members of JSON objects, so while it may
be able to be used to emit some kinds of member values, different
code is likely to be needed to perform the sorting.
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."
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5. IANA Considerations
This specification makes no requests of IANA.
6. Security Considerations
The JSON Security Considerations and Unicode Comparison 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 4, 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.
A hash of a symmetric key has the potential to leak information about
the key value. Thus, the JWK Thumbprint of a symmetric key should be
typically be concealed from parties not in possession of the
symmetric key, unless in the application context, the cryptographic
hash used, such as SHA-256, is known to provide sufficient protection
against disclosure of the key value.
A JWK Thumbprint will only uniquely identify a particular key if a
single unambiguous JWK representation for that key is defined and
used when computing the JWK Thumbprint. (Such representations are
defined for all the key types defined in JSON Web Algorithms (JWA)
[JWA].) For example, if an RSA key were to use "e":"AAEAAQ"
(representing [0, 1, 0, 1]) rather than the specified correct
representation of "e":"AQAB" (representing [1, 0, 1]), a different
thumbprint value would be produced for what could be effectively the
same key, at least for implementations that are lax in validating the
JWK values that they accept. Thus, JWK Thumbprint values can only be
relied upon to be unique for a given key if the implementation also
validates that the correct representation of the key is used.
7. References
7.1. Normative References
[JWA] Jones, M., "JSON Web Algorithms (JWA)",
draft-ietf-jose-json-web-algorithms (work in progress),
January 2015.
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[JWE] Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
draft-ietf-jose-json-web-encryption (work in progress),
January 2015.
[JWK] Jones, M., "JSON Web Key (JWK)",
draft-ietf-jose-json-web-key (work in progress),
January 2015.
[JWS] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", draft-ietf-jose-json-web-signature (work
in progress), January 2015.
[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.
[SHS] National Institute of Standards and Technology, "Secure
Hash Standard (SHS)", FIPS PUB 180-4, March 2012.
[UNICODE] The Unicode Consortium, "The Unicode Standard", 1991-,
<http://www.unicode.org/versions/latest/>.
7.2. Informative References
[I-D.ietf-json-i-json]
Bray, T., "The I-JSON Message Format",
draft-ietf-json-i-json-06 (work in progress),
January 2015.
Appendix A. Acknowledgements
James Manger and John Bradley participated in discussions that led to
the creation of this specification. Jim Schaad also contributed to
this specification.
Appendix B. Document History
[[ to be removed by the RFC editor before publication as an RFC ]]
-02
o No longer register 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
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values.
o Added security considerations about the measures needed to ensure
that a unique JWK Thumbprint value is produced for a key.
o Added text saying that the base64url encoded JWK Thumbprint value
could be used as a "kid" (key ID) value.
o Broke a sentence up that used to be way too long.
-01
o Addressed issues pointed out by Jim Schaad, including defining the
JWK Thumbprint computation in a manner that allows different hash
functions to be used over time.
o Added Nat Sakimura as an editor.
-00
o Created draft-ietf-jose-jwk-thumbprint-00 from
draft-jones-jose-jwk-thumbprint-01 with no normative changes.
Authors' Addresses
Michael B. Jones
Microsoft
Email: mbj@microsoft.com
URI: http://self-issued.info/
Nat Sakimura
Nomura Research Institute
Email: n-sakimura@nri.co.jp
URI: http://nat.sakimura.org/
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