OAuth Working Group M. Jones
Internet-Draft Microsoft
Intended status: Standards Track J. Bradley
Expires: January 7, 2016 Ping Identity
H. Tschofenig
ARM Limited
July 6, 2015
Proof-of-Possession Key Semantics for JSON Web Tokens (JWTs)
draft-ietf-oauth-proof-of-possession-03
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
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This Internet-Draft will expire on January 7, 2016.
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 . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Representations for Proof-of-Possession Keys . . . . . . . . . 4
3.1. Representation for an Asymmetric Proof-of-Possession
Key . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Representation for an Encrypted Symmetric
Proof-of-Possession Key . . . . . . . . . . . . . . . . . 5
3.3. Representation of a Key ID for a Proof-of-Possession
Key . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.4. Confirmation . . . . . . . . . . . . . . . . . . . . . . . 7
3.5. Specifics Intentionally Not Specified . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5.1. JSON Web Token Claims Registration . . . . . . . . . . . . 9
5.1.1. Registry Contents . . . . . . . . . . . . . . . . . . 10
5.2. JWT Confirmation Methods Registry . . . . . . . . . . . . 10
5.2.1. Registration Template . . . . . . . . . . . . . . . . 10
5.2.2. Initial Registry Contents . . . . . . . . . . . . . . 10
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.1. Normative References . . . . . . . . . . . . . . . . . . . 11
6.2. Informative References . . . . . . . . . . . . . . . . . . 11
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 12
Appendix B. Document History . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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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.
Envision the following two use cases. The first use case describes
the use of a symmetric proof-of-possession key and the second use
case uses an asymmetric proof-of-possession key.
An issuer 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 issuer and the recipient.
The entire JWT is then integrity protected by the issuer. The JWT is
then sent to the presenter. Since the presenter is unable to obtain
the encrypted symmetric key from the JWT itself, the issuer 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
present the JWT to the 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 then 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. This symmetric key
mechanism described above is conceptually similar to the use of
Kerberos tickets.
In the second case, consider a presenter that generates a public/
private key pair. It then sends the public key to an issuer, which
creates a JWT and places a public key (or an identifier for it)
inside the newly introduced confirmation claim. The entire JWT is
integrity protected using a digital signature to protect it against
modifications. The JWT is then sent to the presenter. When the
presenter needs to present the JWT to the recipient, it also needs to
demonstrate possession of the private key. The presenter, for
example, uses the private key in a TLS exchange with the recipient or
signs a nonce with the private key. The recipient is able to verify
that it is interacting with the genuine presenter by extracting the
public key from the confirmation claim of the JWT (after verifying
the digital signature of the JWT) and utilizing it with the private
key in the TLS exchange or checking the nonce signature.
In both cases the JWT may contain other claims that are needed by the
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application.
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 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:
Issuer
Party that creates the JWT and binds the proof-of-possession key
to it.
Presenter
Party that proves possession of a private key (for asymmetric key
cryptography) or secret key (for symmetric key cryptography) to a
recipient. The presenter may be the issuer or a party distinct
from the issuer.
Recipient
Party that receives the JWT containing the proof-of-possession key
information from the presenter.
3. Representations for Proof-of-Possession Keys
The issuer of a JWT declares that the presenter possesses a
particular key and that the recipient can cryptographically confirm
proof-of-possession of the key by the presenter by including a "cnf"
(confirmation) claim in the JWT whose value is a JSON object, with a
JSON object containing a "jwk" (JSON Web Key) member, a "jwe" (JSON
Web Encryption) member, or a "kid" (key ID) member identifying the
key.
The presenter can be identified in one of several ways by the JWT,
depending upon the application requirements. If the JWT contains a
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"sub" (subject) claim, the presenter is normally 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 normally 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. Some use cases may require that both be present.
Another means used by some applications to identify the presenter is
an explicit claim, such as the "azp" (authorized party) claim defined
by OpenID Connect [OpenID.Core]. Ultimately, the means of
identifying the presenter is application-specific, as is the means of
confirming possession of the key that is communicated.
3.1. Representation for an Asymmetric Proof-of-Possession Key
When the key held by the presenter is an asymmetric private key, 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": "https://server.example.com",
"aud": "https://client.example.org",
"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 members for a JWK of that key
type and MAY contain other JWK members, including the "kid" (key ID)
member.
3.2. Representation for an Encrypted Symmetric Proof-of-Possession Key
When the key held by the presenter is a symmetric key, the "jwe"
member is an encrypted JSON Web Key (JWK) [JWK] encrypted to a key
known to the recipient using the JWE Compact Serialization containing
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the symmetric key. The rules for encrypting a JWK are found in
Section 7 of the JSON Web Key [JWK] specification.
The following example illustrates a symmetric key that could
subsequently be encrypted for use in the "jwe" 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"
}
The following example JWT Claims Set of a JWT illustrates the use of
an encrypted symmetric key as the "jwe" member value:
{
"iss": "https://server.example.com",
"sub": "24400320",
"aud": "s6BhdRkqt3",
"nonce": "n-0S6_WzA2Mj",
"exp": 1311281970,
"iat": 1311280970,
"cnf":{
"jwe":
"eyJhbGciOiJSU0ExXzUiLCJlbmMiOiJBMTI4Q0JDLUhTMjU2In0.
(remainder of JWE omitted for brevity)"
}
}
3.3. Representation of a Key ID for a Proof-of-Possession Key
The proof-of-possession key can also be identified by the use of a
Key ID instead of communicating the actual key, provided the
recipient is able to obtain the identified key using the Key ID. In
this case, the issuer of a JWT declares that the presenter possesses
a particular key and that the recipient can cryptographically confirm
proof-of-possession of the key by the presenter by including a "cnf"
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(confirmation) claim in the JWT whose value is a JSON object, with
the JSON object containing a "kid" (key ID) member identifying the
key.
The following example demonstrates such a declaration in the JWT
Claims Set of a JWT:
{
"iss": "https://server.example.com",
"aud": "https://client.example.org",
"exp": "1361398824",
"nbf": "1360189224",
"cnf":{
"kid": "dfd1aa97-6d8d-4575-a0fe-34b96de2bfad"
}
}
3.4. Confirmation
The "cnf" (confirmation) claim is used in the JWT to contain the
"jwk", "jwe", or "kid" 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 establishes the IANA "JWT Confirmation Methods"
registry for these members in Section 5.2 and registers the "jwk",
"jwe", and "kid" members within the registry. Other specifications
can register other members used for confirmation, including members
for conveying other proof-of-possession keys, possibly using
different key representations.
3.5. Specifics Intentionally Not Specified
Proof-of-possession is typically demonstrated by having the presenter
sign a value determined by the recipient using the key possessed by
the presenter. 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.
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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.ietf-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 [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.ietf-oauth-pop-architecture].
5. IANA Considerations
The following registration procedure is used for all the registries
established by this specification.
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Values are registered on a Specification Required [RFC5226] basis
after a three-week review period on the oauth-pop-reg-review@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 Experts may approve registration once they are
satisfied that such a specification will be published. [[ Note to the
RFC Editor: The name of the mailing list should be determined in
consultation with the IESG and IANA. Suggested name:
oauth-pop-reg-review@ietf.org. ]]
Registration requests sent to the mailing list for review should use
an appropriate subject (e.g., "Request to register JWT Confirmation
Method: example").
Within the review period, the Designated Experts 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@ietf.org mailing list) for resolution.
Criteria that should be applied by the Designated Experts 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 Experts
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
Experts.
5.1. JSON Web Token Claims Registration
This specification registers the "cnf" claim in the IANA JSON Web
Token Claims registry defined in [JWT].
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5.1.1. Registry Contents
o Claim Name: "cnf"
o Claim Description: Confirmation
o Change Controller: IESG
o Specification Document(s): Section 3.4 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., "kid"). 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 Experts state that
there is a compelling reason to allow an exception.
Confirmation Method Description:
Brief description of the confirmation method (e.g., "Key
Identifier").
Change Controller:
For Standards Track RFCs, list the "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 or documents that specify the parameter,
preferably including URIs that can be used to retrieve copies of
the documents. 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 Representing Public
Key
o Change Controller: IESG
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o Specification Document(s): Section 3.1 of [[ this document ]]
o Confirmation Method Value: "jwe"
o Confirmation Method Description: Encrypted JSON Web Key
o Change Controller: IESG
o Specification Document(s): Section 3.2 of [[ this document ]]
o Confirmation Method Value: "kid"
o Confirmation Method Description: Key Identifier
o Change Controller: IESG
o Specification Document(s): Section 3.3 of [[ this document ]]
6. References
6.1. Normative References
[JWE] Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
RFC 7516, May 2015,
<http://www.rfc-editor.org/info/rfc7516>.
[JWK] Jones, M., "JSON Web Key (JWK)", RFC 7517, May 2015,
<http://www.rfc-editor.org/info/rfc7517>.
[JWT] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, May 2015,
<http://www.rfc-editor.org/info/rfc7519>.
[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.ietf-oauth-pop-architecture]
Hunt, P., Richer, J., Mills, W., Mishra, P., and H.
Tschofenig, "OAuth 2.0 Proof-of-Possession (PoP) Security
Architecture", draft-ietf-oauth-pop-architecture-01 (work
in progress), March 2015.
[OASIS.saml-core-2.0-os]
Cantor, S., Kemp, J., Philpott, R., and E. Maler,
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"Assertions and Protocol for the OASIS Security Assertion
Markup Language (SAML) V2.0", OASIS Standard saml-core-
2.0-os, March 2005.
[OpenID.Core]
Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
C. Mortimore, "OpenID Connect Core 1.0", November 2014,
<http://openid.net/specs/openid-connect-core-1_0.html>.
Appendix A. Acknowledgements
The authors wish to thank Brian Campbell, James Manger, Justin
Richer, and Nat Sakimura for their reviews of the specification.
Appendix B. Document History
[[ to be removed by the RFC Editor before publication as an RFC ]]
-03
o Separated the "jwk" and "jwe" confirmation members; the former
represents a public key as a JWK and the latter represents a
symmetric key as a JWE encrypted JWK.
o Changed the title to indicate that a proof-of-possession key is
being communicated.
o Updated language that formerly assumed that the issuer was an
OAuth 2.0 authorization server.
o Described ways that applications can choose to identify the
presenter, including use of the "iss", "sub", and "azp" claims.
o Harmonized the registry language with that used in JWT [RFC 7519].
o Addressed other issues identified during working group last call.
o Referenced the JWT and JOSE RFCs.
-02
o Defined the terms Issuer, Presenter, and Recipient and updated
their usage within the document.
o Added a description of a use case using an asymmetric proof-of-
possession key to the introduction.
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o Added the "kid" (key ID) confirmation method.
o These changes address the open issues identified in the previous
draft.
-01
o Updated references.
-00
o Created the initial working group draft from
draft-jones-oauth-proof-of-possession-02.
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
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