OAuth Working Group M. Jones
Internet-Draft Microsoft
Intended status: Standards Track J. Bradley
Expires: October 3, 2014 Ping Identity
H. Tschofenig
April 1, 2014
Proof-Of-Possession Semantics for JSON Web Tokens (JWTs)
draft-jones-oauth-proof-of-possession-00
Abstract
This specification defines how to express a declaration in a JSON Web
Token (JWT) that the presenter of the JWT possesses a particular key
and that the recipient can cryptographically confirm proof-of-
possession of the key by the presenter. This property is also
sometimes described as the presenter being a holder-of-key.
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 October 3, 2014.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Proof-Of-Possession Representation . . . . . . . . . . . . . . 3
2.1. Proof-of-Possession of a Private Key . . . . . . . . . . . 4
2.2. Proof-of-Possession of a Symmetric Key . . . . . . . . . . 4
2.3. Confirmation . . . . . . . . . . . . . . . . . . . . . . . 5
2.4. Specifics Intentionally Not Specified . . . . . . . . . . . 6
3. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
4.1. JSON Web Token Claims Registration . . . . . . . . . . . . 7
4.1.1. Registry Contents . . . . . . . . . . . . . . . . . . . 7
4.2. JWT Confirmation Methods Registry . . . . . . . . . . . . . 7
4.2.1. Registration Template . . . . . . . . . . . . . . . . . 8
4.2.2. Initial Registry Contents . . . . . . . . . . . . . . . 8
5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Normative References . . . . . . . . . . . . . . . . . . . 8
5.2. Informative References . . . . . . . . . . . . . . . . . . 9
Appendix A. Open Issues . . . . . . . . . . . . . . . . . . . . . 9
Appendix B. Document History . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
This specification defines how to express a declaration in a JSON Web
Token (JWT) [JWT] that the presenter of the JWT possesses a
particular key and that the recipient can cryptographically confirm
proof-of-possession of the key by the presenter. This property is
also sometimes described as the presenter being a holder-of-key.
1.1. Notational Conventions
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].
Unless otherwise noted, all the protocol parameter names and values
are case sensitive.
1.2. Terminology
This specification uses terms defined in the JSON Web Token (JWT)
[JWT], JSON Web Key (JWK) [JWK], and JSON Web Encryption (JWE) [JWE]
specifications.
These terms are defined for use by this specification:
Presenter
Party that possesses the key identified by the JWT.
2. Proof-Of-Possession Representation
The presenter of a JWT declares that it possesses a particular key
and that the recipient can cryptographically confirm proof-of-
possession of the key by the issuer by including a "cnf"
(confirmation) claim in the JWT whose value is a JSON object, with
the JSON object containing a "jwk" (JSON Web Key) member identifying
the key.
The presenter can be identified in one of two ways by the JWT,
depending upon the application requirements. If the JWT contains a
"sub" (subject) claim, the presenter is the subject identified by the
JWT. (In some applications, the subject identifier will be relative
to the issuer identified by the "iss" (issuer) claim.) If the JWT
contains no "sub" (subject) claim, the presenter is the issuer
identified by the JWT using the "iss" (issuer) claim. The case in
which the presenter is the subject of the JWT is analogous to SAML
2.0 [OASIS.saml-core-2.0-os] SubjectConfirmation usage. At least one
of the "sub" and "iss" claims MUST be present in the JWT, and in some
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use cases, both MUST be present.
2.1. Proof-of-Possession of a Private Key
When the key held by the issuer is a private key, the value of the
"jwk" member is a JSON Web Key (JWK) [JWK] representing the
corresponding public key. The following example demonstrates such a
declaration in the JWT Claims Set of a JWT:
{
"iss":"xas.xboxlive.com",
"aud":"http://auth.xboxlive.com",
"exp":"1361398824",
"nbf":"1360189224",
"cnf":{
"jwk":{
"kty":"EC",
"use":"sig",
"crv":"P-256",
"x":"18wHLeIgW9wVN6VD1Txgpqy2LszYkMf6J8njVAibvhM",
"y":"-V4dS4UaLMgP_4fY4j8ir7cl1TXlFdAgcx55o7TkcSA"
}
}
}
The JWK MUST contain the REQUIRED key elements for a JWK of that key
type and MAY contain other JWK elements, including the "kid" (key ID)
element.
2.2. Proof-of-Possession of a Symmetric Key
When the key held by the issuer is a symmetric key, the value of the
"jwk" member is an encrypted JSON Web Key (JWK) [JWK] encrypted to
the recipient using the JWE Compact Serialization containing the
symmetric key. The rules for encrypting a JWK are found in Section 6
of the JSON Web Key [JWK] specification.
The following is an example symmetric key that could be encrypted for
use in the "jwk" member:
{
"kty":"oct",
"alg":"HS256",
"k":"ZoRSOrFzN_FzUA5XKMYoVHyzff5oRJxl-IXRtztJ6uE"
}
The UTF-8 [RFC3629] encoding of this JWK would be used as the JWE
Plaintext.
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The following is an example JWE Header could be used when encrypting
this key:
{
"alg":"RSA1_5",
"enc":"A128CBC-HS256",
"cty":"jwk+json"
}
The following example JWT Claims Set of a JWT illustrates the use of
an encrypted symmetric key as the "jwk" claim value:
{
"iss": "https://server.example.com",
"sub": "24400320",
"aud": "s6BhdRkqt3",
"nonce": "n-0S6_WzA2Mj",
"exp": 1311281970,
"iat": 1311280970,
"cnf":{
"jwk":
"eyJhbGciOiJSU0ExXzUiLCJlbmMiOiJBMTI4Q0JDLUhTMjU2IiwiY3R5Ijoi
andrK2pzb24ifQ. ... (remainder of JWE omitted for brevity)"
}
}
Note that the case in which the "jwk" claim contains an unencoded JWK
value and the case in which it contains an encrypted JWK value can be
distinguished by the type of the member value. In the first case,
the value is a JSON object containing the JWK and in the second case,
the value is a string containing the JWE JSON Serialization of the
encrypted JWK representation.
2.3. Confirmation
The "cnf" (confirmation) claim is used in the JWT to contain the
"jwk" member because a proof-of-possession key may not be the only
means of confirming the authenticity of the token. This is analogous
to the SAML 2.0 [OASIS.saml-core-2.0-os] SubjectConfirmation element,
in which a number of different subject confirmation methods can be
included, including proof-of-possession key information. When a
recipient receives a "cnf" claim with a member that it does not
understand, it MUST ignore that member.
This specification defines a registry for these elements in
Section 4.2 and registers the "jwk" member within the registry.
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2.4. Specifics Intentionally Not Specified
Proof-of-possession is typically demonstrated by having the issuer
sign a value determined by the recipient using the key possessed by
the issuer. This value is sometimes called a "nonce" or a
"challenge".
The means of communicating the nonce and the nature of its contents
are intentionally not described in this specification, as different
protocols will communicate this information in different ways.
Likewise, the means of communicating the signed nonce is also not
specified, as this is also protocol-specific.
Note that another means of proving possession of the key when it is a
symmetric key is to encrypt the key to the recipient. The means of
obtaining a key for the recipient is likewise protocol-specific.
3. Security Considerations
All of the normal security issues, especially in relationship to
comparing URIs and dealing with unrecognized values, that are
discussed in JWT [JWT] also apply here.
In addition, proof-of-possession introduces its own unique security
issues. Possessing the key is only valuable if it is kept secret.
Appropriate means must be used to ensure that unintended parties do
not learn the secret or symmetric key value.
Proof-of-possession via encrypted symmetric secrets is subject to
replay attacks. This attack can be avoided when a signed nonce or
challenge is used, since the recipient can use a distinct nonce or
challenged for each interaction.
4. IANA Considerations
The following registration procedure is used for all the registries
established by this specification.
Values are registered with a Specification Required [RFC5226] after a
two-week review period on the [TBD]@ietf.org mailing list, on the
advice of one or more Designated Experts. However, to allow for the
allocation of values prior to publication, the Designated Expert(s)
may approve registration once they are satisfied that such a
specification will be published.
Registration requests must be sent to the [TBD]@ietf.org mailing list
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for review and comment, with an appropriate subject (e.g., "Request
for access token type: example"). [[ Note to the RFC Editor: The name
of the mailing list should be determined in consultation with the
IESG and IANA. Suggested name: jwt-reg-review. ]]
Within the review period, the Designated Expert(s) will either
approve or deny the registration request, communicating this decision
to the review list and IANA. Denials should include an explanation
and, if applicable, suggestions as to how to make the request
successful. Registration requests that are undetermined for a period
longer than 21 days can be brought to the IESG's attention (using the
iesg@iesg.org mailing list) for resolution.
Criteria that should be applied by the Designated Expert(s) includes
determining whether the proposed registration duplicates existing
functionality, determining whether it is likely to be of general
applicability or whether it is useful only for a single application,
and whether the registration makes sense.
IANA must only accept registry updates from the Designated Expert(s)
and should direct all requests for registration to the review mailing
list.
It is suggested that multiple Designated Experts be appointed who are
able to represent the perspectives of different applications using
this specification, in order to enable broadly-informed review of
registration decisions. In cases where a registration decision could
be perceived as creating a conflict of interest for a particular
Expert, that Expert should defer to the judgment of the other
Expert(s).
4.1. JSON Web Token Claims Registration
This specification registers the "cnf" claim in the IANA JSON Web
Token Claims registry defined in [JWT].
4.1.1. Registry Contents
o Claim Name: "cnf"
o Claim Description: Confirmation
o Change Controller: IESG
o Specification Document(s): Section 2.3 of this document
4.2. JWT Confirmation Methods Registry
This specification establishes the IANA JWT Confirmation Methods
registry for JWT "cnf" member values. The registry records the
confirmation method member and a reference to the specification that
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defines it.
4.2.1. Registration Template
Confirmation Method Value:
The name requested (e.g., "example"). Because a core goal of this
specification is for the resulting representations to be compact,
it is RECOMMENDED that the name be short -- not to exceed 8
characters without a compelling reason to do so. This name is
case-sensitive. Names may not match other registered names in a
case-insensitive manner unless the Designated Expert(s) state that
there is a compelling reason to allow an exception in this
particular case.
Confirmation Method Description:
Brief description of the confirmation method (e.g., "Example
description").
Change Controller:
For Standards Track RFCs, state "IESG". For others, give the name
of the responsible party. Other details (e.g., postal address,
email address, home page URI) may also be included.
Specification Document(s):
Reference to the document(s) that specify the parameter,
preferably including URI(s) that can be used to retrieve copies of
the document(s). An indication of the relevant sections may also
be included but is not required.
4.2.2. Initial Registry Contents
o Confirmation Method Value: "jwk"
o Confirmation Method Description: JSON Web Key or Encrypted JSON
Web Key
o Change Controller: IESG
o Specification Document(s): Section 2 of [[ this document ]]
5. References
5.1. Normative References
[JWE] Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
draft-ietf-jose-json-web-encryption (work in progress),
March 2014.
[JWK] Jones, M., "JSON Web Key (JWK)",
draft-ietf-jose-json-web-key (work in progress),
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March 2014.
[JWT] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", draft-ietf-oauth-json-web-token (work in
progress), March 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
5.2. Informative References
[OASIS.saml-core-2.0-os]
Cantor, S., Kemp, J., Philpott, R., and E. Maler,
"Assertions and Protocol for the OASIS Security Assertion
Markup Language (SAML) V2.0", OASIS Standard saml-core-
2.0-os, March 2005.
Appendix A. Open Issues
In some conversations, we have said that it is the issuer of the JWT
that possesses the key, and in some conversations, we have said that
it is the presenter of the JWT that possesses the key. Which
description should we use?
Appendix B. Document History
[[ to be removed by the RFC Editor before publication as an RFC ]]
-00
o Wrote the first draft.
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Authors' Addresses
Michael B. Jones
Microsoft
Email: mbj@microsoft.com
URI: http://self-issued.info/
John Bradley
Ping Identity
Email: ve7jtb@ve7jtb.com
URI: http://www.thread-safe.com/
Hannes Tschofenig
Email: Hannes.Tschofenig@gmx.net
URI: http://www.tschofenig.priv.at
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