Internet DRAFT - draft-jones-oauth-proof-of-possession
draft-jones-oauth-proof-of-possession
OAuth Working Group M. Jones
Internet-Draft Microsoft
Intended status: Standards Track J. Bradley
Expires: January 5, 2015 Ping Identity
H. Tschofenig
ARM Limited
July 4, 2014
Proof-Of-Possession Semantics for JSON Web Tokens (JWTs)
draft-jones-oauth-proof-of-possession-02
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
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 5, 2015.
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
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include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Proof-Of-Possession Representation . . . . . . . . . . . . . . 4
3.1. Proof-of-Possession of an Asymmetric Key . . . . . . . . . 4
3.2. Proof-of-Possession of a Symmetric Key . . . . . . . . . . 5
3.3. Confirmation . . . . . . . . . . . . . . . . . . . . . . . 6
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 . . . . . . . . . . . . . . . . 9
5.2.2. Initial Registry Contents . . . . . . . . . . . . . . 9
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. Normative References . . . . . . . . . . . . . . . . . . . 9
6.2. Informative References . . . . . . . . . . . . . . . . . . 10
Appendix A. Open Issues . . . . . . . . . . . . . . . . . . . . . 10
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 10
Appendix C. Document History . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
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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.
[[ Editorial Note: This paragraph needs to be updated to provide more
context and possibly also to describe the use of asymmetric keys
instead. It's not clear that the symmetric case is as useful or
valuable, and it is certainly more complicated.]] Envision the
following use case: An OAuth 2.0 authorization server generates a JWT
and places an encrypted 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 member). When the presenter needs to utilize
the JWT to at 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. The recipient
is able to verify that it is interacting 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 cryptographically protected messages exchanged
with the presenter.
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.
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 by this specification:
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Presenter
Party that possesses the key identified by the JWT.
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.
3.1. Proof-of-Possession of an Asymmetric Key
When the key held by the issuer is an asymmetric private key, the
value of the "jwk" member is a JSON Web Key (JWK) [JWK] representing
the corresponding asymmetric public key. The following example
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":{
"kty":"EC",
"use":"sig",
"crv":"P-256",
"x":"18wHLeIgW9wVN6VD1Txgpqy2LszYkMf6J8njVAibvhM",
"y":"-V4dS4UaLMgP_4fY4j8ir7cl1TXlFdAgcx55o7TkcSA"
}
}
}
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The JWK MUST contain the required key members for a JWK of that key
type and MAY contain other JWK members, including the "kid" (key ID)
member.
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) [JWK] encrypted to a
key known 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 example illustrates a symmetric key that could
subsequently 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 when encrypting the key.
The following example is a JWE Header that 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)"
}
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}
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" 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 members 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.
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.
For an example specification that uses the mechanisms defined in this
document, see [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
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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 private key 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.
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 not only require integrity protection, but also confidentiality
protection.
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. ]]
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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).
5.1. JSON Web Token Claims Registration
This specification registers the "cnf" claim in the IANA JSON Web
Token Claims registry defined in [JWT].
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.
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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.
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. References
6.1. Normative References
[JWE] Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
draft-ietf-jose-json-web-encryption (work in progress),
July 2014.
[JWK] Jones, M., "JSON Web Key (JWK)",
draft-ietf-jose-json-web-key (work in progress),
July 2014.
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[JWT] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", draft-ietf-oauth-json-web-token (work in
progress), July 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.
6.2. 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-02 (work
in progress), 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. 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. Acknowledgements
The authors wish to thank James Manger for his review of the
specification.
Appendix C. Document History
[[ to be removed by the RFC Editor before publication as an RFC ]]
-02
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o Used the same section structuring conventions as the JWT
specification.
o Reverted some changes introduced in -01 without adequate prior
review.
o Applied some editorial corrections.
-01
o Updated affiliation.
o Various editorial changes.
o Updates to the security considerations 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/
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Hannes Tschofenig
ARM Limited
Austria
Email: Hannes.Tschofenig@gmx.net
URI: http://www.tschofenig.priv.at
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