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Internet Engineering Task Force (IETF) M. Jones
Request for Comments: 7800 Microsoft
Category: Standards Track J. Bradley
ISSN: 2070-1721 Ping Identity
H. Tschofenig
ARM Limited
April 2016
Proof-of-Possession Key Semantics for JSON Web Tokens (JWTs)
Abstract
This specification describes how to declare in a JSON Web Token (JWT)
that the presenter of the JWT possesses a particular proof-of-
possession key and how the recipient can cryptographically confirm
proof of possession of the key by the presenter. Being able to prove
possession of a key is also sometimes described as the presenter
being a holder-of-key.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7800.
Copyright Notice
Copyright (c) 2016 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
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Jones, et al. Standards Track [Page 1]
RFC 7800 Proof-of-Possession Key for JWTs April 2016 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Notational Conventions . . . . . . . . . . . . . . . . . 5 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Representations for Proof-of-Possession Keys . . . . . . . . 5 3.1. Confirmation Claim . . . . . . . . . . . . . . . . . . . 6 3.2. Representation of an Asymmetric Proof-of-Possession Key . 7 3.3. Representation of an Encrypted Symmetric Proof-of- Possession Key . . . . . . . . . . . . . . . . . . . . . 7 3.4. Representation of a Key ID for a Proof-of-Possession Key 8 3.5. Representation of a URL for a Proof-of-Possession Key . . 9 3.6. Specifics Intentionally Not Specified . . . . . . . . . . 10 4. Security Considerations . . . . . . . . . . . . . . . . . . . 10 5. Privacy Considerations . . . . . . . . . . . . . . . . . . . 11 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 6.1. JSON Web Token Claims Registration . . . . . . . . . . . 12 6.1.1. Registry Contents . . . . . . . . . . . . . . . . . . 12 6.2. JWT Confirmation Methods Registry . . . . . . . . . . . . 12 6.2.1. Registration Template . . . . . . . . . . . . . . . . 12 6.2.2. Initial Registry Contents . . . . . . . . . . . . . . 13 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 7.1. Normative References . . . . . . . . . . . . . . . . . . 13 7.2. Informative References . . . . . . . . . . . . . . . . . 14 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 15 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15 1. Introduction This specification describes how a JSON Web Token [JWT] can declare that the presenter of the JWT possesses a particular proof-of- possession (PoP) key and how the recipient can cryptographically confirm proof of possession of the key by the presenter. Proof of possession of a key is also sometimes described as the presenter being a holder-of-key. The [OAUTH-POP-ARCH] specification describes key confirmation, among other confirmation mechanisms. This specification defines how to communicate confirmation key information in JWTs. Envision the following two use cases. The first use case employs a symmetric proof-of-possession key and the second use case employs an asymmetric proof-of-possession key. Jones, et al. Standards Track [Page 2]
RFC 7800 Proof-of-Possession Key for JWTs April 2016 +--------------+ | | +--------------+ | |--(3) Presentation of -->| | | | JWT w/ Encrypted | | | Presenter | PoP Key | | | | | | | |<-(4) Communication ---->| | | | Authenticated by | | +--------------+ PoP Key | | ^ ^ | | | | | | (1) Sym. (2) JWT w/ | Recipient | | PoP | Encrypted | | | Key | PoP Key | | v | | | +--------------+ | | | | | | | | | | | |<-(0) Key Exchange for ->| | | Issuer | Key Encryption Key | | | | | | | | | | | | +--------------+ +--------------+ Figure 1: Proof of Possession with a Symmetric Key In the case illustrated in Figure 1, (1) either the presenter generates a symmetric key and privately sends it to the issuer or the issuer generates a symmetric key and privately sends it to the presenter. The issuer generates a JWT with an encrypted copy of this symmetric key in the confirmation claim. This symmetric key is encrypted with a key known only to the issuer and the recipient, which was previously established in step (0). The entire JWT is integrity protected by the issuer. The JWT is then (2) sent to the presenter. Now, the presenter is in possession of the symmetric key as well as the JWT (which includes the confirmation claim). When the presenter (3) presents the JWT to the recipient, it also needs to demonstrate possession of the symmetric key; the presenter, for example, (4) 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 key in 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 (4). This symmetric key mechanism described above is conceptually similar to the use of Kerberos tickets. Jones, et al. Standards Track [Page 3]
RFC 7800 Proof-of-Possession Key for JWTs April 2016 Note that for simplicity, the diagram above and associated text describe the direct use of symmetric keys without the use of derived keys. A more secure practice is to derive the symmetric keys actually used from secrets exchanged, such as the key exchanged in step (0), using a Key Derivation Function (KDF) and use the derived keys, rather than directly using the secrets exchanged. +--------------+ | | +--------------+ | |--(3) Presentation of -->| | | | JWT w/ Public | | | Presenter | PoP Key | | | | | | | |<-(4) Communication ---->| | | | Authenticated by | | +--------------+ PoP Key | | | ^ | | | | | | (1) Public (2) JWT w/ | Recipient | | PoP | Public | | | Key | PoP Key | | v | | | +--------------+ | | | | | | | | | | | | | | | Issuer | | | | | | | | | | | | | +--------------+ +--------------+ Figure 2: Proof of Possession with an Asymmetric Key In the case illustrated in Figure 2, the presenter generates a public/private key pair and (1) sends the public key to the issuer, which creates a JWT that contains the public key (or an identifier for it) in the confirmation claim. The entire JWT is integrity protected using a digital signature to protect it against modifications. The JWT is then (2) sent to the presenter. When the presenter (3) presents the JWT to the recipient, it also needs to demonstrate possession of the private key. The presenter, for example, (4) uses the private key in a Transport Layer Security (TLS) exchange with the recipient or (4) 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) Jones, et al. Standards Track [Page 4]
RFC 7800 Proof-of-Possession Key for JWTs April 2016 and utilizing it with the private key in the TLS exchange or by checking the nonce signature. In both cases, the JWT may contain other claims that are needed by the 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], JSON Web Key [JWK], and JSON Web Encryption [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. Recipient Party that receives the JWT containing the proof-of-possession key information from the presenter. 3. Representations for Proof-of-Possession Keys By including a "cnf" (confirmation) claim in a JWT, the issuer of the JWT declares that the presenter possesses a particular key and that the recipient can cryptographically confirm that the presenter has possession of that key. The value of the "cnf" claim is a JSON object and the members of that object identify the proof-of- possession key. The presenter can be identified in one of several ways by the JWT depending upon the application requirements. If the JWT contains a "sub" (subject) claim [JWT], the presenter is normally the subject Jones, et al. Standards Track [Page 5]
RFC 7800 Proof-of-Possession Key for JWTs April 2016 identified by the JWT. (In some applications, the subject identifier will be relative to the issuer identified by the "iss" (issuer) claim [JWT].) If the JWT contains no "sub" claim, the presenter is normally the issuer identified by the JWT using the "iss" claim. The case in which the presenter is the subject of the JWT is analogous to Security Assertion Markup Language (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. Confirmation Claim The "cnf" claim is used in the JWT to contain members used to identify the proof-of-possession key. Other members of the "cnf" object may be defined 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). The set of confirmation members that a JWT must contain to be considered valid is context dependent and is outside the scope of this specification. Specific applications of JWTs will require implementations to understand and process some confirmation members in particular ways. However, in the absence of such requirements, all confirmation members that are not understood by implementations MUST be ignored. This specification establishes the IANA "JWT Confirmation Methods" registry for these members in Section 6.2 and registers the members defined by this specification. Other specifications can register other members used for confirmation, including other members for conveying proof-of-possession keys using different key representations. The "cnf" claim value MUST represent only a single proof-of- possession key; thus, at most one of the "jwk", "jwe", and "jku" (JWK Set URL) confirmation values defined below may be present. Note that if an application needs to represent multiple proof-of-possession keys in the same JWT, one way for it to achieve this is to use other claim names, in addition to "cnf", to hold the additional proof-of- Jones, et al. Standards Track [Page 6]
RFC 7800 Proof-of-Possession Key for JWTs April 2016 possession key information. These claims could use the same syntax and semantics as the "cnf" claim. Those claims would be defined by applications or other specifications and could be registered in the IANA "JSON Web Token Claims" registry [IANA.JWT.Claims]. 3.2. Representation of 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] 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, "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. The "jwk" member MAY also be used for a JWK representing a symmetric key, provided that the JWT is encrypted so that the key is not revealed to unintended parties. The means of encrypting a JWT is explained in [JWT]. If the JWT is not encrypted, the symmetric key MUST be encrypted as described below. 3.3. Representation of 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] 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 7 of the JSON Web Key [JWK] specification. Jones, et al. Standards Track [Page 7]
RFC 7800 Proof-of-Possession Key for JWTs April 2016 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 is 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": "RSA-OAEP", "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": "eyJhbGciOiJSU0EtT0FFUCIsImVuYyI6IkExMjhDQkMtSFMyNTYifQ. (remainder of JWE omitted for brevity)" } } 3.4. 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" claim in the JWT whose value is a JSON object with the JSON object containing a "kid" member identifying the key. Jones, et al. Standards Track [Page 8]
RFC 7800 Proof-of-Possession Key for JWTs April 2016 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, "cnf":{ "kid": "dfd1aa97-6d8d-4575-a0fe-34b96de2bfad" } } The content of the "kid" value is application specific. For instance, some applications may choose to use a JWK Thumbprint [JWK.Thumbprint] value as the "kid" value. 3.5. Representation of a URL for a Proof-of-Possession Key The proof-of-possession key can be passed by reference instead of being passed by value. This is done using the "jku" member. Its value is a URI [RFC3986] that refers to a resource for a set of JSON- encoded public keys represented as a JWK Set [JWK], one of which is the proof-of-possession key. If there are multiple keys in the referenced JWK Set document, a "kid" member MUST also be included with the referenced key's JWK also containing the same "kid" value. The protocol used to acquire the resource MUST provide integrity protection. An HTTP GET request to retrieve the JWK Set MUST use TLS [RFC5246] and the identity of the server MUST be validated, as per Section 6 of RFC 6125 [RFC6125]. The following example demonstrates such a declaration in the JWT Claims Set of a JWT: { "iss": "https://server.example.com", "sub": "17760704", "aud": "https://client.example.org", "exp": 1440804813, "cnf":{ "jku": "https://keys.example.net/pop-keys.json", "kid": "2015-08-28" } } Jones, et al. Standards Track [Page 9]
RFC 7800 Proof-of-Possession Key for JWTs April 2016 3.6. 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. 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 examples using the mechanisms defined in this specification, see [OAUTH-POP-ARCH]. 4. Security Considerations All of the security considerations that are discussed in [JWT] also apply here. In addition, proof of possession introduces its own unique security issues. Possessing a key is only valuable if it is kept secret. Appropriate means must be used to ensure that unintended parties do not learn private key or symmetric key values. Applications utilizing proof of possession should also utilize audience restriction, as described in Section 4.1.3 of [JWT], as it provides different protections. Proof of possession can be used by recipients to reject messages from unauthorized senders. Audience restriction can be used by recipients to reject messages intended for different recipients. A recipient might not understand the "cnf" claim. Applications that require the proof-of-possession keys communicated with it to be understood and processed must ensure that the parts of this specification that they use are implemented. Proof of possession via encrypted symmetric secrets is subject to replay attacks. This attack can, for example, be avoided when a signed nonce or challenge is used since the recipient can use a distinct nonce or challenge for each interaction. Replay can also be avoided if a sub-key is derived from a shared secret that is specific to the instance of the PoP demonstration. Jones, et al. Standards Track [Page 10]
RFC 7800 Proof-of-Possession Key for JWTs April 2016 As is the case with 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. 5. Privacy Considerations A proof-of-possession key can be used as a correlation handle if the same key is used with multiple parties. Thus, for privacy reasons, it is recommended that different proof-of-possession keys be used when interacting with different parties. 6. IANA Considerations The following registration procedure is used for all the registries established by this specification. Values are registered on a Specification Required [RFC5226] basis after a three-week review period on the jwt-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. Registration requests sent to the mailing list for review should use an appropriate subject (e.g., "Request to Register JWT Confirmation Method: example"). 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 include 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 evaluating the security properties of the item being registered and whether the registration makes sense. 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. Standards Track [Page 11]
RFC 7800 Proof-of-Possession Key for JWTs April 2016 Expert, that Expert should defer to the judgment of the other Experts. 6.1. JSON Web Token Claims Registration This specification registers the "cnf" claim in the IANA "JSON Web Token Claims" registry [IANA.JWT.Claims] established by [JWT]. 6.1.1. Registry Contents o Claim Name: "cnf" o Claim Description: Confirmation o Change Controller: IESG o Specification Document(s): Section 3.1 of [RFC7800] 6.2. 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. 6.2.1. 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 eight 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. Jones, et al. Standards Track [Page 12]
RFC 7800 Proof-of-Possession Key for JWTs April 2016 6.2.2. Initial Registry Contents o Confirmation Method Value: "jwk" o Confirmation Method Description: JSON Web Key Representing Public Key o Change Controller: IESG o Specification Document(s): Section 3.2 of [RFC7800] o Confirmation Method Value: "jwe" o Confirmation Method Description: Encrypted JSON Web Key o Change Controller: IESG o Specification Document(s): Section 3.3 of [RFC7800] o Confirmation Method Value: "kid" o Confirmation Method Description: Key Identifier o Change Controller: IESG o Specification Document(s): Section 3.4 of [RFC7800] o Confirmation Method Value: "jku" o Confirmation Method Description: JWK Set URL o Change Controller: IESG o Specification Document(s): Section 3.5 of [RFC7800] 7. References 7.1. Normative References [IANA.JWT.Claims] IANA, "JSON Web Token Claims", <http://www.iana.org/assignments/jwt>. [JWE] Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)", RFC 7516, DOI 10.17487/RFC7156, May 2015, <http://www.rfc-editor.org/info/rfc7516>. [JWK] Jones, M., "JSON Web Key (JWK)", RFC 7517, DOI 10.17487/RFC7157, May 2015, <http://www.rfc-editor.org/info/rfc7517>. [JWT] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token (JWT)", RFC 7519, DOI 10.17487/RFC7159, 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, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>. Jones, et al. Standards Track [Page 13]
RFC 7800 Proof-of-Possession Key for JWTs April 2016 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2003, <http://www.rfc-editor.org/info/rfc3629>. [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005, <http://www.rfc-editor.org/info/rfc3986>. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, DOI 10.17487/RFC5226, May 2008, <http://www.rfc-editor.org/info/rfc5226>. [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008, <http://www.rfc-editor.org/info/rfc5246>. [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 2011, <http://www.rfc-editor.org/info/rfc6125>. 7.2. Informative References [JWK.Thumbprint] Jones, M. and N. Sakimura, "JSON Web Key (JWK) Thumbprint", RFC 7638, DOI 10.17487/RFC7638, September 2015, <http://www.rfc-editor.org/info/rfc7638>. [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, <http://docs.oasis-open.org/security/saml/v2.0/>. [OAUTH-POP-ARCH] Hunt, P., Ed, Richer, J., Mills, W., Mishra, P., and H. Tschofenig, "OAuth 2.0 Proof-of-Possession (PoP) Security Architecture", Work in Progress, draft-ietf-oauth-pop- architecture-07, December 2015. Jones, et al. Standards Track [Page 14]
RFC 7800 Proof-of-Possession Key for JWTs April 2016 [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>. Acknowledgements The authors wish to thank Brian Campbell, Stephen Farrell, Barry Leiba, Kepeng Li, Chris Lonvick, James Manger, Kathleen Moriarty, Justin Richer, and Nat Sakimura for their reviews of the specification. 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. Standards Track [Page 15]