Network Working Group M.B. Jones
Internet-Draft Microsoft
Intended status: Standards Track December 13, 2011
Expires: June 13, 2012

JSON Web Key (JWK)
draft-jones-json-web-key-03

Abstract

A JSON Web Key (JWK) is a JSON data structure that represents a set of public keys.

Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

Status of this Memo

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This Internet-Draft will expire on June 13, 2012.

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Table of Contents

1. Introduction

A JSON Web Key (JWK) is a JSON data structure that represents a set of public keys as a JSON object [RFC4627]. The JWK format is used to represent bare keys; representing certificate chains is an explicit non-goal of this specification. JSON Web Keys are referenced in JSON Web Signature (JWS) [JWS] using the jku (JSON Key URL) header parameter and in JSON Web Encryption (JWE) [JWE] using the jku (JSON Key URL) and epk (Ephemeral Public Key) header parameters.

2. Terminology

JSON Web Key (JWK)
A JSON data structure that represents a set of public keys. A JWK consists of a single JWK Container Object that contains an array of JWK Key Objects.
JWK Container Object
A JSON object that contains an array of JWK Key Objects as a member.
JWK Key Object
A JSON object that represents a single public key.
Base64url Encoding
For the purposes of this specification, this term always refers to the URL- and filename-safe Base64 encoding described in RFC 4648 [RFC4648], Section 5, with the (non URL-safe) '=' padding characters omitted, as permitted by Section 3.2. (See Appendix C of [JWS] for notes on implementing base64url encoding without padding.)

3. JSON Web Key (JWK) Overview

It is sometimes useful to be able to reference public key representations, for instance, in order to verify the signature on content signed with the corresponding private key. The JSON Web Key (JWK) data structure provides a convenient JSON representation for sets of public keys utilizing either the Elliptic Curve or RSA families of algorithms.

3.1. Example JWK

The following example JWK contains two public keys: one using an Elliptic Curve algorithm and a second one using an RSA algorithm. The first specifies that the key is to be used for encryption. Both provide a Key ID for matching purposes. In both cases, integers are represented using the base64url encoding of their big endian representations. (Long lines are broken are for display purposes only.)

{"jwk":
  [
    {"alg":"EC",
     "crv":"P-256",
     "x":"MKBCTNIcKUSDii11ySs3526iDZ8AiTo7Tu6KPAqv7D4",
     "y":"4Etl6SRW2YiLUrN5vfvVHuhp7x8PxltmWWlbbM4IFyM",
     "use":"enc",
     "kid":"1"},
 
    {"alg":"RSA",
     "mod": "0vx7agoebGcQSuuPiLJXZptN9nndrQmbXEps2aiAFbWhM78LhWx
4cbbfAAtVT86zwu1RK7aPFFxuhDR1L6tSoc_BJECPebWKRXjBZCiFV4n3oknjhMs
tn64tZ_2W-5JsGY4Hc5n9yBXArwl93lqt7_RN5w6Cf0h4QyQ5v-65YGjQR0_FDW2
QvzqY368QQMicAtaSqzs8KJZgnYb9c7d0zgdAZHzu6qMQvRL5hajrn1n91CbOpbI
SD08qNLyrdkt-bFTWhAI4vMQFh6WeZu0fM4lFd2NcRwr3XPksINHaQ-G_xBniIqb
w0Ls1jF44-csFCur-kEgU8awapJzKnqDKgw",
     "exp":"AQAB",
     "kid":"2011-04-29"}
  ]
}

4. JWK Format

A JWK consists of a JWK Container Object, which is a JSON object that contains an array of JWK Key Objects as a member. This section specifies the format of these objects.

4.1. JWK Container Object Format

A JWK Container Object is a JSON object containing a specific member. This member is:

JWK Container Object Member
Member Name JSON Value Type Container Object Member Semantics
jwk array The jwk member value contains an array of JWK Key Objects. This member is REQUIRED.

Additional members MAY be present in the JWK Container Object. If present, they MUST be understood by implementations using that JWK.

4.2. JWK Key Object Format

A JWK Key Object is a JSON object containing specific members. Those members that are common to all key types are as follows:

JWK Key Object Members
Member Name JSON Value Type Key Object Member Semantics
alg string The alg member identifies the cryptographic algorithm family used with the key. Values defined by this specification are EC and RSA. Specific additional members are required to represent the key, depending upon the alg value. The alg value is case sensitive. This member is REQUIRED.
use string The use member identifies the intended use of the key. Values defined by this specification are sig (signature) and enc (encryption). Other values MAY be used. The use value is case sensitive. This member is OPTIONAL.
kid string The kid (Key ID) member can be used to match a specific key. This can be used, for instance, to choose among a set of keys within the JWK during key rollover. The kid value MAY correspond to a JWS kid value. The interpretation of the kid value is unspecified. This member is OPTIONAL.

Additional members MAY be present in the JWK Key Object. If present, they MUST be understood by implementations using that key.

4.2.1. JWK Key Object Members for Elliptic Curve Keys

JWKs can represent Elliptic Curve [FIPS.186-3] keys. In this case, the alg member value MUST be EC. Furthermore, these additional members MUST be present:

Members for Elliptic Curve Keys
Member Name JSON Value Type Key Object Member Semantics
crv string The crv member identifies the cryptographic curve used with the key. Values defined by this specification are P-256, P-384 and P-521. Additional crv values MAY be used, provided they are understood by implementations using that Elliptic Curve key. The crv value is case sensitive.
x string The x member contains the x coordinate for the elliptic curve point. It is represented as the base64url encoding of the coordinate's big endian representation.
y string The y member contains the y coordinate for the elliptic curve point. It is represented as the base64url encoding of the coordinate's big endian representation.

4.2.2. JWK Key Object Members for RSA Keys

JWKs can represent RSA [RFC3447] keys. In this case, the alg member value MUST be RSA. Furthermore, these additional members MUST be present:

Members for RSA Keys
Member Name JSON Value Type Key Object Member Semantics
mod string The mod member contains the modulus value for the RSA public key. It is represented as the base64url encoding of the value's big endian representation.
exp string The exp member contains the exponent value for the RSA public key. It is represented as the base64url encoding of the value's big endian representation.

5. Base64url encoding as used by JWKs

JWKs make use of the base64url encoding as defined in RFC 4648 [RFC4648]. As allowed by Section 3.2 of the RFC, this specification mandates that base64url encoding when used with JWKs MUST NOT use padding. Notes on implementing base64url encoding can be found in the JWS [JWS] specification.

6. IANA Considerations

No IANA actions are required by this specification.

7. Security Considerations

TBD

8. Open Issues and Things To Be Done (TBD)

The following items remain to be done in this draft:

9. References

9.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447, February 2003.
[RFC4627] Crockford, D., "The application/json Media Type for JavaScript Object Notation (JSON)", RFC 4627, July 2006.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, October 2006.
[FIPS.186-3] National Institute of Standards and Technology, "Digital Signature Standard (DSS)", FIPS PUB 186-3, June 2009.

9.2. Informative References

[MagicSignatures] Panzer (editor), J., Laurie, B. and D. Balfanz, "Magic Signatures", August 2010.
[JWS] Jones, M.B., Balfanz, D., Bradley, J., Goland, Y.Y., Panzer, J., Sakimura, N. and P. Tarjan, "JSON Web Signature (JWS)", December 2011.
[JWE] Jones, M.B., Rescorla, E. and J. Hildebrand, "JSON Web Encryption (JWE)", December 2011.

Appendix A. Acknowledgements

A JSON representation for RSA public keys was previously introduced in Magic Signatures [MagicSignatures].

Appendix B. Document History

-03

-02

-01

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Author's Address

Michael B. Jones Microsoft EMail: mbj@microsoft.com URI: http://self-issued.info/