JSON Web Key (JWK) Thumbprint
draft-ietf-jose-jwk-thumbprint-03

JOSE Working Group                                              M. Jones
Internet-Draft                                                 Microsoft
Intended status: Standards Track                             N. Sakimura
Expires: August 30, 2015                                             NRI
                                                       February 26, 2015

                     JSON Web Key (JWK) Thumbprint
                   draft-ietf-jose-jwk-thumbprint-03

Abstract

   This specification defines a means of computing a thumbprint value
   (a.k.a. digest) of a key represented as a JSON Web Key (JWK).

Status of this Memo

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   This Internet-Draft will expire on August 30, 2015.

Copyright Notice

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   document authors.  All rights reserved.

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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.  Relationship to Digests of X.509 Values  . . . . . . . . . . . 10
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 11
   Appendix B.  Document History  . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12

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1.  Introduction

   This specification defines a means of computing a thumbprint value
   (a.k.a. digest) of a key represented as a JSON Web Key (JWK).  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], 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
   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]

   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"
   o  "kty"
   o  "x"

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   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
   included in the JWK, the JWK Thumbprint of the key remains the same
   regardless of which optional attributes are present.  Different kinds

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   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 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
   possession of the necessary key material.  These are sufficient to
   create the hash input from the JWK representation of the key, as

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   described in Section 3.3.

4.  Practical JSON and Unicode Considerations

   Implementations will almost certainly use functionality provided by
   the platform's JSON support 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.

   However, if new JWK members are defined that use non-ASCII member
   names, their definitions should specify the exact Unicode code point
   sequences used to represent them, particularly in cases in which
   Unicode normalization could result in the transformation of one set
   of code points into another under any circumstances.

   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.

   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).

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7.  Relationship to Digests of X.509 Values

   JWK Thumbprint values are computed on the members required to
   represent a key, rather than all members of a JWK 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) [JWS] value defined for X.509
   certificate objects does.  While logically equivalent to a digest of
   the SPKI representation of the key, a JWK Thumbprint is computed over
   a JSON representation of that key, rather than over an ASN.1
   representation of it.

8.  References

8.1.  Normative References

   [JWA]      Jones, M., "JSON Web Algorithms (JWA)",
              draft-ietf-jose-json-web-algorithms (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/>.

8.2.  Informative References

   [I-D.ietf-json-i-json]
              Bray, T., "The I-JSON Message Format",
              draft-ietf-json-i-json-06 (work in progress),

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              January 2015.

   [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, May 2008.

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 ]]

   -03

   o  Addressed review comments by James Manger and Jim Schaad,
      including adding a section on the relationship to digests of X.509
      values.

   -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
      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.

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   -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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