    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
       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
       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.

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    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.

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         +--------------+
         |              |                         +--------------+
         |              |--(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.

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       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)

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       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

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       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-

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       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.

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       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.

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       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"
           }
         }

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    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.

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       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

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       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.

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    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>.

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    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.

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       [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

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