OAuth Working Group M. Jones
Internet-Draft Microsoft
Intended status: Standards Track J. Bradley
Expires: December 28, 2014 Ping Identity
H. Tschofenig
ARM Limited
June 26, 2014
Proof-Of-Possession Semantics for JSON Web Tokens (JWTs)
draft-jones-oauth-proof-of-possession-01.txt
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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 28, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
Jones, et al. Expires December 28, 2014 [Page 1]
Internet-Draft proof-of-possession for JWTs June 2014
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Proof-Of-Possession Representation . . . . . . . . . . . . . 3
3.1. Proof-of-Possession of an Asymmetric Key . . . . . . . . 3
3.2. Proof-of-Possession of a Symmetric Key . . . . . . . . . 4
3.3. Confirmation . . . . . . . . . . . . . . . . . . . . . . 5
3.4. Specifics Intentionally Not Specified . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5.1. JSON Web Token Claims Registration . . . . . . . . . . . 8
5.1.1. Registry Contents . . . . . . . . . . . . . . . . . . 8
5.2. JWT Confirmation Methods Registry . . . . . . . . . . . . 8
5.2.1. Registration Template . . . . . . . . . . . . . . . . 8
5.2.2. Initial Registry Contents . . . . . . . . . . . . . . 9
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Document History . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
This specification defines how to express a declaration in a JSON Web
Token (JWT) [I-D.ietf-oauth-json-web-token] 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.
Envision the following use case. An OAuth 2.0 authorization server
generates a JWT and places a symmetric key inside the newly
introduced confirmation claim. This symmetric key is encrypted with
a key known only to the authorization server and the recipient. The
JWT is then sent to the presenter. Since the presenter is unable to
obtain the encrypted symmetric key the authorization server conveys
that symmetric key separately to the presenter. Now, the presenter
is in possession of the symmetric key as well as the JWT (which
includes the confirmation claim element). When the presenter needs
to utilize the JWT to a recipient it also needs to demonstrate
possession of the symmetric key; the presenter, for example, uses the
symmetric key in a challenge / response protocol with the recipient.
Jones, et al. Expires December 28, 2014 [Page 2]
Internet-Draft proof-of-possession for JWTs June 2014
The recipient is able to verify that it indeed interacts with the
genuine presenter by decrypting the JWK contained inside the
confirmation claim of the JWT. By doing this the recipient obtains
the symmetric key, which it then uses to verify a cryptographically
protected messages exchanged with the presenter.
2. Terminology
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.
This specification uses terms defined in the JSON Web Token (JWT)
[I-D.ietf-oauth-json-web-token], JSON Web Key (JWK)
[I-D.ietf-jose-json-web-key], and JSON Web Encryption (JWE)
[I-D.ietf-jose-json-web-encryption] specifications.
3. 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
use cases both MUST be present. In cases where the presenter shall
be anonymous only the "iss" (issuer) claim may be present identifying
the party that issued the JWT.
3.1. Proof-of-Possession of an Asymmetric Key
When the key held by the issuer is an asymmetric key pair, the value
of the "jwk" member is a JSON Web Key (JWK)
[I-D.ietf-jose-json-web-key] representing the public key. The
Jones, et al. Expires December 28, 2014 [Page 3]
Internet-Draft proof-of-possession for JWTs June 2014
example shown in Figure 1 demonstrates such a declaration in the JWT
Claims Set of a JWT:
{
"iss":"xas.example.com",
"aud":"http://auth.example.com",
"exp":"1361398824",
"nbf":"1360189224",
"cnf":{
"jwk":{
"kid":"pk1"
"kty":"EC",
"use":"sig",
"crv":"P-256",
"x":"18wHLeIgW9wVN6VD1Txgpqy2LszYkMf6J8njVAibvhM",
"y":"-V4dS4UaLMgP_4fY4j8ir7cl1TXlFdAgcx55o7TkcSA"
}
}
}
Figure 1: JWT with a Confirmation Claim containing a Public Key
The JWK MUST contain the required elements of a JWK (as needed for
the particular type) and MAY contain other JWK elements. It is
highly recommended to include the "kid" (key ID) element in the JWK
as well since it allows the presenter and the recipient to later use
this key id to refer to the exchanged key.
3.2. 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)
[I-D.ietf-jose-json-web-key] encrypted with a key known to the
recipient. The rules for encrypting a JWK are found in Section 6 of
the JSON Web Key [I-D.ietf-jose-json-web-key] specification. Note
that the JWE compact serialization is used.
The example shown in Figure 2 illustrates a symmetric key that is
subsequently encrypted for use in the "jwk" member:
{
"kid":"sessionkey-1"
"kty":"oct",
"alg":"HS256",
"k":"ZoRSOrFzN_FzUA5XKMYoVHyzff5oRJxl-IXRtztJ6uE"
}
Figure 2: JWK with a Symmetric Key
Jones, et al. Expires December 28, 2014 [Page 4]
Internet-Draft proof-of-possession for JWTs June 2014
The UTF-8 [RFC3629] encoding of the JWK shown in Figure 2 would be
used as the JWE Plaintext.
An example JWE header is shown in Figure 3. It provides the
necessary information for encrypting the JWK.
{
"kid":"longterm-12345"
"alg":"RSA1_5",
"enc":"A128CBC-HS256",
"cty":"jwk+json",
}
Figure 3: Header of the JWE with Information about the Encryption of
the Symmetric Key
The example in Figure 4 illustrates a JWT with the 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)"
}
}
Figure 4: JWT with a Confirmation Claim containing the Encrypted
Symmetric Key
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.
3.3. Confirmation
The "cnf" (confirmation) claim is used in the JWT to contain the
"jwk" element because a proof-of-possession key may not be the only
means of confirming the authenticity of the token. This is analogous
Jones, et al. Expires December 28, 2014 [Page 5]
Internet-Draft proof-of-possession for JWTs June 2014
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 5.2 and registers the "jwk" member within the registry.
3.4. 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.
For a protocol that applies the mechanisms described in this document
to the OAuth 2.0 context please take a look at
[I-D.hunt-oauth-pop-architecture].
4. Security Considerations
All of the normal security issues, especially in relationship to
comparing URIs and dealing with unrecognized values, that are
discussed in JWT [I-D.ietf-oauth-json-web-token] also apply here.
Similarly to other information included in a JWT it is necessary to
apply data origin authentication and integrity protection (via a
keyed message digest or a digital signature). Data origin
authentication ensures that the recipient of the JWT learns about the
entity that created the JWT since this will be important for any
policy decisions. Integrity protection prevents an adversary from
changing any elements conveyed within the JWT payload. Special care
has to be applied when carrying symmetric keys inside the JWT since
those do not only require integrity protection but also
confidentiality protection.
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 symmetric key or the private key (in case of an
asymmetric crypto-system).
Jones, et al. Expires December 28, 2014 [Page 6]
Internet-Draft proof-of-possession for JWTs June 2014
A recipient may not understand the newly introduced "cnf" claim and
may consequently treat it as a bearer token. While this is a
legitimate concern it is outside the scope of this specification
since demonstration the possession of the key associated with the
"cnf" claim is not covered by this specification. For more details
please consult [I-D.hunt-oauth-pop-architecture].
5. 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
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
Jones, et al. Expires December 28, 2014 [Page 7]
Internet-Draft proof-of-possession for JWTs June 2014
Expert, that Expert should defer to the judgement of the other
Expert(s).
5.1. JSON Web Token Claims Registration
This specification registers the "cnf" claim in the IANA JSON Web
Token Claims registry defined in [I-D.ietf-oauth-json-web-token].
5.1.1. Registry Contents
o Claim Name: "cnf"
o Claim Description: Confirmation
o Change Controller: IESG
o Specification Document(s): Section 3.3 of this document
5.2. 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
defines it.
5.2.1. 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.
Jones, et al. Expires December 28, 2014 [Page 8]
Internet-Draft proof-of-possession for JWTs June 2014
5.2.2. 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 3 of [[ this document ]]
6. Acknowledgements
We would like to thanks James Manger for his review feedback.
7. References
7.1. Normative References
[I-D.ietf-jose-json-web-encryption]
Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
draft-ietf-jose-json-web-encryption-29 (work in progress),
June 2014.
[I-D.ietf-jose-json-web-key]
Jones, M., "JSON Web Key (JWK)", draft-ietf-jose-json-web-
key-29 (work in progress), June 2014.
[I-D.ietf-oauth-json-web-token]
Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", draft-ietf-oauth-json-web-token-23 (work in
progress), June 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.
7.2. Informative References
[I-D.hunt-oauth-pop-architecture]
Hunt, P., Richer, J., Mills, W., Mishra, P., and H.
Tschofenig, "OAuth 2.0 Proof-of-Possession (PoP) Security
Architecture", draft-hunt-oauth-pop-architecture-01 (work
in progress), April 2014.
Jones, et al. Expires December 28, 2014 [Page 9]
Internet-Draft proof-of-possession for JWTs June 2014
[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. Document History
[[ to be removed by the RFC Editor before publication as an RFC ]]
-01
o Updated affiliation.
o Various editorial changes.
o Updates to the security consideration section based on review
feedback by James Manager.
o Included the kid element in the examples (as requested by James
Manger).
o Expanded the introduction section.
o Moved the terminology/RFC2119 boilerplate text from the
introduction to a separate terminology section.
-00
o Wrote the first draft.
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
ARM Limited
Austria
Email: Hannes.Tschofenig@gmx.net
URI: http://www.tschofenig.priv.at
Jones, et al. Expires December 28, 2014 [Page 10]