OAuth                                                       P. Hunt, Ed.
Internet-Draft                                        Oracle Corporation
Intended status: Informational                                 J. Richer
Expires: March 28, April 21, 2016
                                                                W. Mills

                                                               P. Mishra
                                                      Oracle Corporation
                                                           H. Tschofenig
                                                             ARM Limited
                                                      September 25,
                                                        October 19, 2015

       OAuth 2.0 Proof-of-Possession (PoP) Security Architecture
                draft-ietf-oauth-pop-architecture-03.txt
                draft-ietf-oauth-pop-architecture-04.txt

Abstract

   The OAuth 2.0 bearer token specification, as defined in RFC 6750,
   allows any party in possession of a bearer token (a "bearer") to get
   access to the associated resources (without demonstrating possession
   of a cryptographic key).  To prevent misuse, bearer tokens must to be
   protected from disclosure in transit and at rest.

   Some scenarios demand additional security protection whereby a client
   needs to demonstrate possession of cryptographic keying material when
   accessing a protected resource.  This document motivates the
   development of the OAuth 2.0 proof-of-possession security mechanism.

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 March 28, April 21, 2016.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Preventing Access Token Re-Use by the Resource Server . .   4   3
     3.2.  TLS Channel Binding Support . . . . . . . . . . . . . . .   4
     3.3.  Access to a Non-TLS Protected Resource  . . . . . . . . .   4
     3.4.  Offering Application Layer End-to-End Security  . . . . .   5
   4.  Security and Privacy Threats  . . . . . . . . . . . . . . . .   5
   5.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  Threat Mitigation . . . . . . . . . . . . . . . . . . . . . .  10
     6.1.   6
     5.1.  Confidentiality Protection  . . . . . . . . . . . . . . .  11
     6.2.   7
     5.2.  Sender Constraint . . . . . . . . . . . . . . . . . . . .  11
     6.3.   7
     5.3.  Key Confirmation  . . . . . . . . . . . . . . . . . . . .  12
     6.4.   8
     5.4.  Summary . . . . . . . . . . . . . . . . . . . . . . . . .  13
   7.   9
   6.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  Client and Authorization Server Interaction . . . . . . .  14
       7.1.1.  Symmetric Keys  . . . . . . . . . . . . . . . . . . .  14
       7.1.2.  Asymmetric Keys . . . . . . . . . . . . . . . . . . .  16
     7.2.  Client and Resource Server Interaction  . . . . . .  10
   7.  Requirements  . . .  17
     7.3.  Resource and Authorization Server Interaction (Token
           Introspection) . . . . . . . . . . . . . . . . . . . . .  18  15
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  19  18
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  19
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     11.2.  Informative References . . . . . . . . . . . . . . . . .  20  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   The

   At the time of writing the OAuth 2.0 protocol family ([RFC6749],
   [RFC6750], and [RFC6819]) offer a single token type known as the "bearer" token standardized security
   mechanism to access protected resources. resources, namely the bearer token.
   RFC 6750 [RFC6750] specifies the bearer token mechanism and defines
   it as follows:

      "A security token with the property that any party in possession
      of the token (a "bearer") can use the token in any way that any
      other party in possession of it can.  Using a bearer token does
      not require a bearer to prove possession of cryptographic key
      material."

   The bearer token meets the security needs of a number of use cases
   the OAuth 2.0 protocol had originally been designed for.  There are,
   however, other scenarios that require stronger security properties
   and ask for active participation of the OAuth client in form of
   cryptographic computations when presenting an access token to a
   resource server.

   This document outlines additional use cases requiring stronger
   security protection in Section 3, identifies threats in Section 4,
   proposes different ways to mitigate those threats in Section 6, 5,
   outlines an architecture for a solution that builds on top of the
   existing OAuth 2.0 framework in Section 7, 6, and concludes with a
   requirements list in Section 5. 7.

2.  Terminology

   The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT',
   'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'MAY', and 'OPTIONAL' in this
   specification are to be interpreted as described in [RFC2119], with
   the important qualification that, unless otherwise stated, these
   terms apply to the design of the protocol, not its implementation or
   application.

3.  Use Cases

   The main use case that motivates improvement upon "bearer" better-than-bearer token security is
   the desire of resource servers to obtain additional assurance that
   the client is indeed authorized to present an access token.  The
   expectation is that the use of additional credentials (symmetric or
   asymmetric keying material) will encourage developers to take
   additional precautions when transferring and storing access token in
   combination with these credentials.

   Additional use cases listed below provide further requirements for
   the solution development.  Note that a single solution does not
   necessarily need to offer support for all use cases.

3.1.  Preventing Access Token Re-Use by the Resource Server

   In

   Imagine a scenario where a resource server that receives a valid
   access token,
   the resource server then token re-uses it with other resource server.  The reason for
   re-use may be malicious or may well be legitimate.  In a legitimate case, the intent is to support
   use case consider chaining of computations whereby a resource server
   needs to consult other third party resource servers to complete a the
   requested operation.  In both cases it may be assumed that the scope
   of the access token is sufficiently large that it allows such a re-use. re-
   use.  For example, imagine a case where a company operates email
   services as well as picture sharing services and that company had
   decided to issue access tokens with a scope that allows access to
   both services.

   With this use case the desire is to prevent such access token re-use.
   This also implies that the legitimate use cases require additional
   enhancements for request chaining.

3.2.  TLS Channel Binding Support

   In this use case we consider the scenario where an OAuth 2.0 request
   to a protected resource is secured using TLS but the client and the
   resource server demand that the underlying TLS exchange is bound to
   additional application layer security to prevent cases where the TLS
   connection is terminated at a TLS intermediary, which splits the TLS
   connection into two separate connections.

   In this use case additional information should be is conveyed to the resource
   server to ensure that no entity entity has tampered with the TLS
   connection.

3.3.  Access to a Non-TLS Protected Resource

   This use case is for a web client that needs to access a resource
   that makes data available (such as videos) without offering integrity
   and confidentiality protection using TLS.  Still, the initial
   resource request using OAuth, which includes the access token, must
   be protected against various threats (e.g., token replay, token
   modification).

   While it is possible to utilize bearer tokens in this scenario with
   TLS protection when the request to the protected resource is made, as
   described in [RFC6750], there may be the desire to avoid using TLS
   between the client and the resource server at all.  In such a case
   the bearer token approach is not possible since it relies on TLS for
   ensuring integrity and confidentiality protection of the access token
   exchange since otherwise replay attacks are possible: First, an
   eavesdropper may steal an access token and present represent it at a
   different resource server.  Second, an eavesdropper may steal an
   access token and replay it against the same resource server at a
   later point in time.  In both cases, if the attack is successful, the
   adversary gets access to the resource owners data or may perform an
   operation selected by the adversary (e.g., sending a message).  Note
   that the adversary may obtain the access token (if the
   recommendations in [RFC6749] and [RFC6750] are not followed) using a
   number of ways, including eavesdropping the communication on the
   wireless link.

   Consequently, the important assumption in this use case is that a
   resource server does not have TLS support and the security solution
   should work in such a scenario.  Furthermore, it may not be necessary
   to provide authentication of the resource server towards the client.

3.4.  Offering Application Layer End-to-End Security

   In Web deployments resource servers are often placed behind load
   balancers, which are deployed by the same organization that operates
   the resource servers.  These load balancers may terminate the TLS
   connection setup and HTTP traffic is transmitted without TLS
   protection in the clear from
   the load balancer to the resource server.  With application layer
   security in addition to the underlying TLS security it is possible to
   allow application servers to perform cryptographic verification on an
   end-to-end basis.

   The key aspect in this use case is therefore to offer end-to-end
   security in the presence of load balancers via application layer
   security.  Enterprise networks also deploy proxies that inspect
   traffic and thereby break TLS.

4.  Security and Privacy Threats

   The following list presents several common threats against protocols
   utilizing some form of token. tokens.  This list of threats is based on NIST
   Special Publication 800-63 [NIST800-63].  We exclude a discussion of
   threats related to any form of identity proofing and authentication
   of the resource owner to the authorization server since these
   procedures are not part of the OAuth 2.0 protocol specification
   itself.

   Token manufacture/modification:

      An attacker may generate a bogus tokens or modify the token
      content (such as authentication or attribute statements) of an
      existing token, causing resource server to grant inappropriate
      access to the client.  For example, an attacker may modify the
      token to extend the validity period.  A client may modify the
      token to have access to information that they should not be able
      to view.

   Token disclosure:

      Tokens may contain personal data, such as real name, age or
      birthday, payment information, etc.

   Token redirect:

      An attacker uses the token generated for consumption by the
      resource server to obtain access to another resource server.

   Token reuse:

      An attacker attempts to use a token that has already been used
      once with a resource server.  The attacker may be an eavesdropper
      who observes the communication exchange or, worse, one of the
      communication end points.  A client may, for example, leak access
      tokens because it cannot keep secrets confidential.  A client may
      also reuse re-use access tokens for some other resource servers.
      Finally, a resource server may use a token it had obtained from a
      client and use it with another resource server that the client
      interacts with.  A resource server, offering relatively
      unimportant application services, may attempt to use an access
      token obtained from a client to access a high-value service, such
      as a payment service, on behalf of the client using the same
      access token.

   Token repudiation:

      Token repudiation refers to a property whereby a resource server
      is given an assurance that the authorization server cannot deny to
      have created a token for the client.

5.  Requirements

   RFC 4962 [RFC4962] gives useful guidelines for designers  Threat Mitigation

   A large range of
   authentication and key management protocols.  While RFC 4962 was
   written with threats can be mitigated by protecting the AAA framework used for network access authentication
   in mind content
   of the offered suggestions are useful token, for example using a digital signature or a keyed
   message digest.  Alternatively, the design content of other
   key management systems as well.  The following requirements list
   applies OAuth 2.0 terminology to the requirements outlined in RFC
   4962.

   These requirements include

   Cryptographic Algorithm Independent:

      The key management protocol MUST be cryptographic algorithm
      independent.

   Strong, fresh session keys:

      Session keys MUST token could be strong and fresh.  Each session deserves an
      independent session key, i.e., one that is generated specifically
      for
   passed by reference rather than by value (requiring a separate
   message exchange to resolve the intended use.  In context of OAuth this means reference to the token content).  To
   simplify the subsequent description we assume that keying
      material the token itself
   is created in such a way that can only be used digitally signed by the
      combination of a client instance, protected resource, authorization server and therefore cannot
   be modified.

   To deal with token redirect it is important for the authorization scope.

   Limit Key Scope:

      Following
   server to include the principle identifier of least privilege, parties MUST NOT have
      access to keying material that is the intended recipient - the
   resource server.  A resource server must not needed be allowed to perform their
      role.  Any protocol accept
   access tokens that is used to establish session keys MUST
      specify the scope are not meant for session keys, clearly identifying the
      parties its consumption.

   To provide protection against token disclosure two approaches are
   possible, namely (a) not to whom include sensitive information inside the session key is available.

   Replay Detection Mechanism:

      The key management protocol exchanges MUST be replay protected.
      Replay protection allows a protocol message recipient
   token or (b) to discard
      any message that was recorded during a previous legitimate
      dialogue ensure confidentiality protection.  The latter
   approach requires at least the communication interaction between the
   client and presented the authorization server as well as though it belonged to the current
      dialogue.

   Authenticate All Parties:

      Each party in the key management protocol MUST be authenticated to
      the other parties with whom they communicate.  Authentication
      mechanisms MUST maintain interaction
   between the confidentiality of any secret values
      used in client and the authentication process.  Secrets MUST NOT be sent resource server to
      another party without experience
   confidentiality protection.

   Authorization:

      Client and resource server authorization MUST  As an example, TLS with a ciphersuite
   that offers confidentiality protection has to be performed.  These
      entities MUST demonstrate possession of applied (which is
   currently true for all ciphersuites, except for one).  Encrypting the appropriate keying
      material, without disclosing it.  Authorization
   token content itself is REQUIRED
      whenever another alternative.  In our scenario the
   authorization server would, for example, encrypt the token content
   with a client interacts symmetric key shared with an authorization the resource server.
      Authorization checking prevents an elevation of privilege attack.

   Keying Material

   To deal with token reuse more choices are available.

5.1.  Confidentiality and Integrity:

      While preserving algorithm independence, Protection

   In this approach confidentiality and
      integrity of all keying material MUST be maintained.

   Confirm Cryptographic Algorithm Selection:

      The selection protection of the "best" cryptographic algorithms SHOULD be
      securely confirmed.  The mechanism SHOULD detect attempted roll-
      back attacks.

   Uniquely Named Keys:

      Key management proposals require a robust key naming scheme,
      particularly where key caching exchange is supported.  The key name
      provides a way to refer
   provided on the communication interfaces between the client and the
   resource server, and between the client and the authorization server.
   No eavesdropper on the wire is able to observe the token exchange.
   Consequently, a key in replay by a protocol so that it third party is clear
      to all parties which key is being referenced.  Objects not possible.  An
   authorization server wants to ensure that cannot
      be named cannot be managed.  All keys MUST be uniquely named, it only hands out tokens to
   clients it has authenticated first and
      the key name MUST NOT directly or indirectly disclose the keying
      material.

   Prevent the Domino Effect:

      Compromise who are authorized.  For this
   purpose, authentication of a single client MUST NOT compromise keying material
      held by any other the client within to the system, including session keys
      and long-term keys.  Likewise, compromise of a single resource authorization server MUST NOT compromise keying material held by any other
      Resource Server within the system.  In the context of
   will be a key
      hierarchy, this means that the compromise requirement to ensure adequate protection against a range
   of one node attacks.  This is, however, true for the description in
   Section 5.2 and Section 5.3 as well.  Furthermore, the key
      hierarchy must client has to
   make sure it does not disclose distribute (or leak) the information necessary access token to
      compromise
   entities other branches in the key hierarchy.  Obviously, than the
      compromise of intended the root of resource server.  For that
   purpose the key hierarchy client will compromise all of have to authenticate the keys; however, a compromise in one branch MUST NOT result in resource server
   before transmitting the compromise of other branches.  There are many implications access token.

5.2.  Sender Constraint

   Instead of
      this requirement; however, two implications deserve highlighting.
      First, providing confidentiality protection the scope authorization
   server could also put the identifier of the keying material must be defined and
      understood by all parties that communicate client into the protected
   token with the following semantic: 'This token is only valid when
   presented by a party that holds
      that keying material.  Second, a party that holds keying material
      in a key hierarchy must not share that keying material with
      parties that are associated client with other branches in the key
      hierarchy.

   Bind Key to its Context:

      Keying material MUST be bound to the appropriate context.  The
      context includes the following.

      *  The manner in which following identifier.'  When the keying material
   access token is expected then presented to be used.

      *  The other parties the resource server how does it
   know that are expected it was provided by the client?  It has to have access authenticate the
   client!  There are many choices for authenticating the client to the
         keying material.

      *
   resource server, for example by using client certificates in TLS
   [RFC5246], or pre-shared secrets within TLS [RFC4279].  The expected lifetime choice of
   the keying material.  Lifetime of preferred authentication mechanism and credential type may depend
   on a
         child key SHOULD NOT be greater than the lifetime number of its parent
         in the key hierarchy.

      Any party with legitimate access to keying material can determine
      its context.  In addition, the protocol MUST ensure that all
      parties with legitimate access to keying material have factors, including

   o  security properties

   o  available infrastructure
   o  library support

   o  credential cost (financial)

   o  performance

   o  integration into the same
      context existing IT infrastructure

   o  operational overhead for the keying material.  This requires that the parties
      are properly identified configuration and authenticated, so that all distribution of the
      parties that have access
      credentials

   This long list hints to the keying material can be determined.
      The context will include the client and the resource server
      identities in more than challenge of selecting at least one form.

   Authorization Restriction:

      If
   mandatory-to-implement client authorization is restricted, then authentication mechanism.

5.3.  Key Confirmation

   A variation of the client SHOULD be
      made aware mechanism of sender authentication, described in
   Section 5.2, is to replace authentication with the restriction.

   Client Identity Confidentiality:

      A client has identity confidentiality when any party other than proof-of-
   possession of a specific (session) key, i.e., key confirmation.  In
   this model the resource server and would not authenticate the authorization server cannot
      sufficiently identify client
   itself but would rather verify whether the client within knows the anonymity set.  In
      comparison to anonymity and pseudonymity, identity confidentiality
      is concerned with eavesdroppers and intermediaries.  A session
   key
      management protocol SHOULD provide associated with a specific access token.  Examples of this property.

   Resource Owner Identity Confidentiality:

      Resource servers SHOULD
   approach can be prevented found with the OAuth 1.0 MAC token [RFC5849], and
   Kerberos [RFC4120] when utilizing the AP_REQ/AP_REP exchange (see
   also [I-D.hardjono-oauth-kerberos] for a comparison between Kerberos
   and OAuth).

   To illustrate key confirmation the first examples borrow from knowing
   Kerberos and use symmetric key cryptography.  Assume that the real or
      pseudonymous identity of
   authorization server shares a long-term secret with the resource owner, since
   server, called K(Authorization Server-Resource Server).  This secret
   would be established between them out-of-band.  When the client
   requests an access token the authorization server is creates a fresh and
   unique session key Ks and places it into the only entity involved in verifying token encrypted with the
      resource owner's identity.

   Collusion:

      Resource servers that collude can be prevented from using
      information related to
   long term key K(Authorization Server-Resource Server).  Additionally,
   the resource owner authorization server attaches Ks to track the individual.
      That is, two different resource servers can be prevented from
      determining that the same resource owner has authenticated response message to both
      of them.  Authorization servers MUST bind different keying
      material the
   client (in addition to access tokens used for resource servers from different
      origins (or similar concepts in the app world).

   AS-to-RS Relationship Anonymity:

      For solutions using asymmetric key cryptography access token itself) over a
   confidentiality protected channel.  When the client MAY
      conceal information about sends a request
   to the resource server it wants has to interact
      with.  The authorization server MAY reject such an attempt since
      it may not be able use Ks to enforce access control decisions.

   Channel Binding:

      A solution MUST enable support compute a keyed message
   digest for channel bindings. the request (in whatever form or whatever layer).  The concept
      of channel binding, as defined in [RFC5056], allows applications
   resource server, when receiving the message, retrieves the access
   token, verifies it and extracts K(Authorization Server-Resource
   Server) to establish that obtain Ks.  This key Ks is then used to verify the two end-points keyed
   message digest of a secure channel at one
      network layer are the same as at a higher layer by binding
      authentication at request message.

   Note that in this example one could imagine that the higher layer mechanism to
   protect the channel at the lower
      layer.

   There are performance concerns with the use of asymmetric
   cryptography.  Although token itself is based on a symmetric key cryptography offers better
   performance asymmetric cryptography offers additional security
   properties.  A solution MUST therefore offer the capability to
   support both symmetric as well as asymmetric keys.

   There are threats that relate based mechanism
   to the experience of the software
   developer as well as operational practices.  Verifying the servers
   identity in TLS is discussed at length in [RFC6125].

   A number of the threats listed in Section 4 demand protection of the
   access token content and a standardized solution, in avoid any form of a JSON-
   based format, public key infrastructure but this aspect is available with not
   further elaborated in the JWT [RFC7519].

6.  Threat Mitigation scenario.

   A large range of threats similar mechanism can also be mitigated by protecting the content
   of the token, for example designed using a digital signature or a keyed
   message digest.  Alternatively, the content of the token could be
   passed by reference rather than by value (requiring a separate
   message exchange to resolve the reference to the token content).  To
   simplify the subsequent description we assume that asymmetric
   cryptography.  When the client requests an access token itself
   is digitally signed by the
   authorization server creates an ephemeral public / privacy key pair
   (PK/SK) and therefore cannot
   be modified.

   To deal with token redirect it is important for the authorization
   server to include places the identifier of public key PK into the intended recipient - protected token.  When
   the
   resource server.  A resource authorization server must not be allowed to accept returns the access tokens that are not meant for its consumption.

   To provide protection against token disclosure two approaches are
   possible, namely (a) not to include sensitive information inside the
   token or (b) to ensure confidentiality protection.  The latter
   approach requires at least client it
   also provides the communication interaction between PK/SK key pair over a confidentiality protected
   channel.  When the client and sends a request to the authorization resource server as well as it
   has to use the interaction
   between privacy key SK to sign the client request.  The resource
   server, when receiving the message, retrieves the access token,
   verifies it and extracts the resource server public key PK.  It uses this ephemeral
   public key to experience
   confidentiality protection. verify the attached signature.

5.4.  Summary

   As an example, TLS with a ciphersuite
   that offers confidentiality protection has to high level message, there are various ways how the threats can
   be applied (which mitigated and while the details of each solution is
   currently true for somewhat
   different they all ciphersuites, except for one).  Encrypting ultimately accomplish the
   token content itself goal.

   The three approaches are:

   Confidentiality Protection:

      The weak point with this approach, which is another alternative.  In our scenario briefly described in
      Section 5.1, is that the
   authorization server would, for example, encrypt client has to be careful to whom it
      discloses the token content
   with a symmetric key shared access token.  What can be done with the resource server.

   To deal with token reuse more choices are available.

6.1.  Confidentiality Protection

   In this approach confidentiality protection of the exchange is
   provided
      entirely depends on what rights the communication interfaces between token entitles the client presenter
      and what constraints it contains.  A token could encode the
   resource server, and between
      identifier of the client and the authorization server.
   No eavesdropper on but there are scenarios where the wire client
      is able not authenticated to observe the token exchange.
   Consequently, a replay by a third party is not possible.  An
   authorization resource server wants to ensure that it only hands out tokens to
   clients it has authenticated first and who are authorized.  For this
   purpose, authentication or where the
      identifier of the client to the authorization server
   will be rather represents an application class
      rather than a requirement to ensure adequate protection against single application instance.  As such, it is
      possible that certain deployments choose a range
   of attacks.  This is, however, true for the description in
   Section 6.2 and Section 6.3 as well.  Furthermore, the client has rather liberal approach
      to
   make sure it does not distribute (or leak) security and that everyone who is in possession of the access
      token is granted access to
   entities other than the intended the resource server.  For that
   purpose the client will have data.

   Sender Constraint:

      The weak point with this approach, which is briefly described in
      Section 5.2, is to authenticate setup the authentication infrastructure such
      that clients can be authenticated towards resource server
   before transmitting the access token.

6.2.  Sender Constraint

   Instead of providing confidentiality protection servers.
      Additionally, the authorization server could also put must encode the identifier
      of the client into the protected
   token with in the following semantic: 'This token is only valid when
   presented for later verification by a client with the following identifier.'  When resource
      server.  Depending on the chosen layer for providing client-side
      authentication there may be additional challenges due Web server
      load balancing, lack of API access token is then presented to identity information, etc.

   Key Confirmation:

      The weak point with this approach, see Section 5.3, is the resource server how does it
   know that
      increased complexity: a complete key distribution protocol has to
      be defined.

   In all cases above it was provided by the client?  It has to authenticate the
   client!  There are many choices for authenticating be ensured that the client is able to
   keep the
   resource server, for example by using client certificates in TLS
   [RFC5246], or pre-shared secrets within TLS [RFC4279].  The choice of
   the preferred authentication mechanism and credential type may depend
   on a number of factors, including

   o credentials secret.

6.  Architecture

   The proof-of-possession security properties

   o  available infrastructure

   o  library support
   o  credential cost (financial)

   o  performance

   o  integration into concept assumes that the existing IT infrastructure

   o  operational overhead for configuration and distribution of
      credentials

   This long list hints
   authorization server acts as a trusted third party that binds keys to
   access tokens.  These keys are then used by the challenge of selecting at least one
   mandatory-to-implement client authentication mechanism.

6.3.  Key Confirmation

   A variation of the mechanism of sender authentication, described in
   Section 6.2, is to replace authentication with demonstrate
   the proof-of- possession of a specific (session) key, i.e., key confirmation.  In
   this model the secret to the resource server would not authenticate when accessing
   the client
   itself but would rather resource.  The resource server, when receiving an access token,
   needs to verify whether that the key used by the client knows matches the one
   included in the session
   key associated with a specific access token.  Examples of this
   approach can be found with the OAuth 1.0 MAC token [RFC5849], and
   Kerberos [RFC4120] when utilizing the AP_REQ/AP_REP exchange (see
   also [I-D.hardjono-oauth-kerberos] for a comparison

   There are slight differences between Kerberos
   and OAuth).

   To illustrate key confirmation, the first example is borrowed from
   Kerberos and use of symmetric key cryptography.  Assume that keys and
   asymmetric keys when they are bound to the
   authorization server shares a long-term secret with access token and the resource
   server, called K(Authorization Server-Resource Server).  This secret
   would be established
   subsequent interaction between them out-of-band.  When the client
   requests an access token and the authorization
   server creates a fresh and
   unique session key Ks and places it into the token encrypted with when demonstrating possession of these keys.  Figure 1 shows
   the
   long term symmetric key K(Authorization Server-Resource Server).  Additionally,
   the authorization server attaches Ks to the response message to the
   client (in addition to procedure and Figure 2 illustrates how asymmetric
   keys are used.  While symmetric cryptography provides better
   performance properties the access token itself) over a
   confidentiality protected channel.  When use of asymmetric cryptography allows the
   client sends a request to keep the resource server private key locally and never expose it has to use Ks to compute any
   other party.

   With the JSON Web Token (JWT) [RFC7519] a keyed message
   digest standardized format for the request (in whatever form or whatever layer).  The
   resource server, when receiving the message, retrieves the
   access
   token, verifies it and extracts K(Authorization Server-Resource
   Server) to obtain Ks.  This key Ks tokens is then used to verify the keyed
   message digest of the request message.

   Note that in this example one could imagine that the mechanism available.  The necessary elements to
   protect the token itself is based on a bind symmetric key based mechanism
   or asymmetric keys to avoid any form of public key infrastructure but this aspect is not
   further elaborated a JWT are described in the scenario.

   A similar mechanism can also be designed using asymmetric
   cryptography.  When
   [I-D.ietf-oauth-proof-of-possession].

   Note: The negotiation of cryptographic algorithms between the client requests an access token the
   authorization server creates an ephemeral public / privacy key pair
   (PK/SK)
   and places the public key PK into the protected token.  When the authorization server returns is not shown in the access token examples below and
   assumed to the client it
   also provides the PK/SK key pair over be present in a confidentiality protected
   channel.  When protocol solution to meet the client sends a request requirements
   for crypto-agility.

                        +---------------+
                       ^|               |
                     // | Authorization |
                    /   | Server        |
                  //    |               |
                 /      |               |
          (I)  //      /+---------------+
   Access     /      //
   Token     /      /
   Request //     //  (II) Access Token
   +Params /      /        +Symmetric Key
        //     //
       /      v
     +-----------+                       +------------+
     |           |                       |            |
     |           |                       | Resource   |
     | Client    |                       | Server     |
     |           |                       |            |
     |           |                       |            |
     +-----------+                       +------------+

   Figure 1: Interaction between the Client and the Authorization Server
                             (Symmetric Keys).

   In order to request an access token the resource client interacts with the
   authorization server as part of the a normal grant exchange, as shown
   in Figure 1.  However, it
   has needs to include additional information
   elements for use with the privacy key SK to sign PoP security mechanism, as depicted in
   message (I).  In message (II) the request.  The resource
   server, when receiving authorization server then returns
   the message, retrieves requested access token.  In addition to the access token,
   verifies it and extracts token itself,
   the public key PK.  It uses this ephemeral
   public symmetric key is communicated to verify the attached signature.

6.4.  Summary

   As client.  This symmetric key
   is a high level message, there are various ways unique and fresh session key with sufficient entropy for the threats can be
   mitigated.  While
   given lifetime.  Furthermore, information within the details of each solution are somewhat
   different, they all accomplish access token
   ties it to this specific symmetric key.

   Note: For this security mechanism to work the goal of mitigating client as well as the threats.

   The three approaches are:

   Confidentiality Protection:

      The weak point with this approach, which is briefly described in
      Section 6.1, is that the client has to be careful
   resource server need to whom it
      discloses the have access token.  What can be done with the token
      entirely depends on what rights to the token entitles session key.  While the presenter
      and what constraints it contains.  A token could encode
   key transport mechanism from the
      identifier of authorization server to the client but
   has been explained in the previous paragraph there are scenarios where three ways for
   communicating this session key from the client
      is not authenticated authorization server to the
   resource server or where the
      identifier of server, namely

      Embedding the client rather represents an application class
      rather than a single application instance.  As such, it is
      possible that certain deployments choose a rather liberal approach
      to security and that everyone who is in possession of symmetric key inside the access token is granted access to itself.  This
      requires that the data.

   Sender Constraint:

      The weak point with this approach, which is briefly described in
      Section 6.2, symmetric key is to setup the authentication infrastructure such
      that clients can be authenticated towards confidentiality protected.

      The resource servers.
      Additionally, server queries the authorization server must encode the identifier
      of for the client in
      symmetric key.  This is an approach envisioned by the token for later verification by
      introspection endpoint [I-D.ietf-oauth-introspection].

      The authorization server and the resource
      server.  Depending on the chosen layer for providing client-side
      authentication there may be additional challenges due to Web server load balancing, lack of API both have access
      to identity information,
      etc.

   Key Confirmation:

      The weak point with this approach, see Section 6.3, is the
      increased complexity: a complete key distribution protocol has to
      be defined.

   In all cases above it has to be ensured that same back-end database.  Smaller, tightly coupled systems
      might prefer such a deployment strategy.

                        +---------------+
                       ^|               |
   Access Token Req. // | Authorization |
   +Parameters      /   | Server        |
   +[Fingerprint] //    |               |
                 /      |               |
       (I)     //      /+---------------+
              /      //
             /      /     (II)
           //     //  Access Token
           /      /   +[ephemeral
        //     //      asymmetric key pair]
       /      v
     +-----------+                       +------------+
     |           |                       |            |
     |           |                       | Resource   |
     | Client    |                       | Server     |
     |           |                       |            |
     |           |                       |            |
     +-----------+                       +------------+

   Figure 2: Interaction between the client is able to
   keep Client and the credentials secret.

7.  Architecture Authorization Server
                            (Asymmetric Keys).

   The proof-of-possession security concept assumes that the
   authorization server acts as a trusted third party that binds keys to
   access tokens.  These use of asymmetric keys are then used by is slightly different since the client to demonstrate
   the possession of the secret to or
   the resource server when accessing could be involved in the resource.  The resource server, when receiving an access token,
   needs to verify that generation of the ephemeral key used by
   pair.  This exchange is shown in Figure 1.  If the client matches generates
   the one
   included in key pair it either includes a fingerprint of the access token.

   There are slight differences between public key or
   the use of symmetric keys and
   asymmetric keys when they are bound public key in the request to the authorization server.  The
   authorization server would include this fingerprint or public key in
   the confirmation claim inside the access token and thereby bind the
   asymmetric key pair to the token.  If the
   subsequent interaction between the client and did not provide a
   fingerprint or a public key in the request then the authorization
   server when demonstrating possession of these keys.  Figure 1 shows
   the symmetric key procedure and Figure 2 illustrates how is asked to create an ephemeral asymmetric
   keys are used.  While symmetric cryptography provides better
   performance properties key pair, binds the use
   fingerprint of asymmetric cryptography allows the
   client public key to keep the private access token, and returns the
   asymmetric key locally pair (public and never expose it private key) to any
   other party.

   With the JSON Web Token (JWT) [RFC7519] a standardized format for
   access tokens client.  Note
   that there is available.  The necessary elements to bind symmetric
   or asymmetric keys to a JWT are described in
   [I-D.ietf-oauth-proof-of-possession].

   Note: strong preference for generating the private/public
   key pair locally at the client rather than at the server.

   The negotiation of cryptographic algorithms specification describing the interaction between the client and
   the authorization server is not server, as shown in the examples below Figure 1 and
   assumed to in Figure 2, can
   be present found in a protocol solution [I-D.ietf-oauth-pop-key-distribution].

   Once the client has obtained the necessary access token and keying
   material it can start to meet interact with the requirements
   for crypto-agility.

7.1.  Client resource server.  To
   demonstrate possession of the key bound to the access token it needs
   to apply this key to the request by computing a keyed message digest
   (i.e., a symmetric key-based cryptographic primitive) or a digital
   signature (i.e., an asymmetric cryptographic computation).  When the
   resource server receives the request it verifies it and Authorization Server Interaction

7.1.1.  Symmetric Keys
                        +---------------+
                       ^|               |
                     // | Authorization |
                    /   | Server        |
                  // decides
   whether access to the protected resource can be granted.  This
   exchange is shown in Figure 3.

                      +---------------+
                      |               |
                 /
                      | Authorization |
          (I)  //      /+---------------+
   Access     /      //
   Token     /      /
                      | Server        |
                      |               |
                      |               |
                      +---------------+

                    Request //     //  (II) Access Token
   +Params /      /        +Symmetric Key
        //     //
       /      v
   +-----------+  + Signature/MAC (a)  +------------+
   |           |---------------------->|            |
   |           |
     |           |  [+Access Token]      | Resource   |
   | Client    |                       | Server     |
   |           |    Response (b)       |            |
   |           |                       |           |<----------------------|            |
   +-----------+  [+ Signature/MAC]    +------------+

        ^                                    ^
        |                                    |
        |                                    |
    Symmetric Key                       Symmetric Key
       or                                   or
    Asymmetric Key Pair                Public Key (Client)
       +                                     +
     Parameters                          Parameters

                    Figure 1: Interaction between the 3: Client and demonstrates PoP.

   The specification describing the Authorization Server
                             (Symmetric Keys).

   In order ability to sign the HTTP request an access token
   from the client interacts with to the
   authorization resource server as part of the a normal grant exchange, as shown can be found in Figure 1.  However, it needs
   [I-D.ietf-oauth-signed-http-request].

   So far the examples talked about access tokens that are passed by
   value and allow the resource server to include additional information
   elements for use with make authorization decisions
   immediately after verifying the PoP security mechanism, as depicted in
   message (I). request from the client.  In message (II) some
   deployments a real-time interaction between the authorization server then returns
   and the requested resource server is envisioned that lowers the need to pass
   self-contained access token. tokens around.  In addition to that case the access token itself,
   the symmetric key is communicated
   merely serves as a handle or a reference to state stored at the client.  This symmetric key
   is
   authorization server.  As a unique and fresh session key with sufficient entropy for the
   given lifetime.  Furthermore, information within the access token
   ties it to this specific symmetric key.

   Note: For this security mechanism to work the client as well as consequence, the resource server need to have access to the session key.  While the
   key transport mechanism from the cannot
   autonomously make an authorization server to the decision when receiving a request
   from a client but has been explained in the previous paragraph there are three ways for
   communicating this session key from the authorization server to the
   resource server, namely

      Embedding the symmetric key inside the access token itself.  This
      requires that the symmetric key is confidentiality protected.

      The resource server queries consult the authorization server for the
      symmetric key. server.  This is an approach envisioned by can,
   for example, be done using the token introspection endpoint [I-D.ietf-oauth-introspection].

      The authorization server (see
   [I-D.ietf-oauth-introspection]).  Figure 4 shows the protocol
   interaction graphically.  Despite the additional token exchange
   previous descriptions about associating symmetric and asymmetric keys
   to the resource server both have access token are still applicable to the same back-end database.  Smaller, tightly coupled systems
      might prefer such a deployment strategy.

7.1.2.  Asymmetric Keys this scenario.

                      +---------------+
        Access       ^|               |
   Access
        Token Req. // | Authorization |
   +Parameters |^
          (I)     /   | Server        |
   +[Fingerprint] \  (IV) Token
                //    |               |  \ Introspection Req.
               /      |               |
       (I)   \     +Access
             //      /+---------------+    \     Token
            /      // (II)             \    \\
           /      /     (II)   Access            \     \
         //     //  Access    Token              \ (V) \
         /      /   +[ephemeral                         \Resp.\
      //     //      asymmetric key pair]                            \     \
     /      v                               V     \
   +-----------+                       +------------+
     |           | Request +Signature/MAC+------------+
   |           |  (III)  +Access Token |            |
   |           |---------------------->| Resource   |
   | Client    |   (VI) Success or     | Server     |
   |           |        Failure        |            |
   |           |                       |           |<----------------------|            |
   +-----------+                       +------------+

          Figure 2: Interaction between the Client 4: Token Introspection and the Authorization Server
                            (Asymmetric Keys).

   The use Access Token Handles.

7.  Requirements

   RFC 4962 [RFC4962] gives useful guidelines for designers of asymmetric keys is slightly different since the client or
   the server could be involved in the generation of the ephemeral
   authentication and key
   pair.  This exchange is shown management protocols.  While RFC 4962 was
   written with the AAA framework used for network access authentication
   in Figure 1.  If mind the client generates offered suggestions are useful for the key pair it either includes a fingerprint design of the public key or
   the public other
   key in the request management systems as well.  The following requirements list
   applies OAuth 2.0 terminology to the authorization server.  The
   authorization server would requirements outlined in RFC
   4962.

   These requirements include this fingerprint or public

   Cryptographic Algorithm Independent:

      The key in management protocol MUST be cryptographic algorithm
      independent.

   Strong, fresh session keys:

      Session keys MUST be strong and fresh.  Each session deserves an
      independent session key, i.e., one that is generated specifically
      for the confirmation claim inside intended use.  In context of OAuth this means that keying
      material is created in such a way that can only be used by the access token
      combination of a client instance, protected resource, and thereby bind
      authorization scope.

   Limit Key Scope:

      Following the
   asymmetric key pair principle of least privilege, parties MUST NOT have
      access to the token.  If the client did keying material that is not provide a
   fingerprint or a public key in the request then the authorization
   server needed to perform their
      role.  Any protocol that is asked used to create an ephemeral asymmetric key pair, binds establish session keys MUST
      specify the
   fingerprint of scope for session keys, clearly identifying the public key
      parties to whom the access token, and returns the
   asymmetric session key pair (public and private key) to the client.  Note
   that there is available.

   Replay Detection Mechanism:

      The key management protocol exchanges MUST be replay protected.
      Replay protection allows a strong preference for generating protocol message recipient to discard
      any message that was recorded during a previous legitimate
      dialogue and presented as though it belonged to the private/public
   key pair locally at current
      dialogue.

   Authenticate All Parties:

      Each party in the client rather than at key management protocol MUST be authenticated to
      the server.

7.2.  Client and Resource Server Interaction

   The specification describing other parties with whom they communicate.  Authentication
      mechanisms MUST maintain the interaction between confidentiality of any secret values
      used in the client authentication process.  Secrets MUST NOT be sent to
      another party without confidentiality protection.

   Authorization:

      Client and
   the resource server authorization server, as shown in Figure 1 and in Figure 2, can MUST be found in [I-D.ietf-oauth-pop-key-distribution].

   Once performed.  These
      entities MUST demonstrate possession of the appropriate keying
      material, without disclosing it.  Authorization is REQUIRED
      whenever a client has obtained the necessary access token interacts with an authorization server.  The
      authorization checking prevents an elevation of privilege attack,
      and it ensures that an unauthorized authorized is detected.

   Keying Material Confidentiality and Integrity:

      While preserving algorithm independence, confidentiality and
      integrity of all keying material it can start to interact with the resource server.  To
   demonstrate possession MUST be maintained.

   Confirm Cryptographic Algorithm Selection:

      The selection of the "best" cryptographic algorithms SHOULD be
      securely confirmed.  The mechanism SHOULD detect attempted roll-
      back attacks.

   Uniquely Named Keys:

      Key management proposals require a robust key bound naming scheme,
      particularly where key caching is supported.  The key name
      provides a way to the access token it needs refer to apply this a key to the request by computing a keyed message digest
   (i.e., a symmetric key-based cryptographic primitive) or in a digital
   signature (i.e., an asymmetric cryptographic computation).  When the
   resource server receives the request it verifies protocol so that it and decides
   whether access is clear
      to the protected resource can be granted.  This
   exchange all parties which key is shown in Figure 3.

                      +---------------+
                      |               |
                      | Authorization |
                      | Server        |
                      |               |
                      |               |
                      +---------------+

                    Request
   +-----------+  + Signature/MAC (a)  +------------+
   |           |---------------------->|            |
   |           |  [+Access Token]      | Resource   |
   | Client    |                       | Server     |
   |           |    Response (b)       |            |
   |           |<----------------------|            |
   +-----------+  [+ Signature/MAC]    +------------+

        ^                                    ^
        |                                    |
        |                                    |
    Symmetric Key                       Symmetric Key
       or                                   or
    Asymmetric Key Pair                Public Key (Client)
       +                                     +
     Parameters                          Parameters

                    Figure 3: Client Demonstrates PoP.

   The specification describing being referenced.  Objects that cannot
      be named cannot be managed.  All keys MUST be uniquely named, and
      the ability to sign key name MUST NOT directly or indirectly disclose the HTTP request
   from keying
      material.

   Prevent the Domino Effect:

      Compromise of a single client to MUST NOT compromise keying material
      held by any other client within the system, including session keys
      and long-term keys.  Likewise, compromise of a single resource
      server can be found in
   [I-D.ietf-oauth-signed-http-request].

7.3. MUST NOT compromise keying material held by any other
      Resource and Authorization Server Interaction (Token
      Introspection)

   So far within the examples talked about access tokens that are passed by
   value and allow system.  In the resource server to make authorization decisions
   immediately after verifying context of a key
      hierarchy, this means that the request from compromise of one node in the client.  In some
   deployments a real-time interaction between key
      hierarchy must not disclose the authorization server
   and information necessary to
      compromise other branches in the resource server is envisioned that lowers key hierarchy.  Obviously, the need to pass
   self-contained access tokens around.  In that case
      compromise of the access token
   merely serves as a handle or a reference to state stored at root of the
   authorization server.  As key hierarchy will compromise all of
      the keys; however, a consequence, compromise in one branch MUST NOT result in
      the resource server cannot
   autonomously make an authorization decision when receiving compromise of other branches.  There are many implications of
      this requirement; however, two implications deserve highlighting.
      First, the scope of the keying material must be defined and
      understood by all parties that communicate with a request
   from party that holds
      that keying material.  Second, a client but has to consult party that holds keying material
      in a key hierarchy must not share that keying material with
      parties that are associated with other branches in the authorization server.  This can,
   for example, key
      hierarchy.

   Bind Key to its Context:

      Keying material MUST be done using bound to the token introspection endpoint (see
   [I-D.ietf-oauth-introspection]).  Figure 4 shows appropriate context.  The
      context includes the protocol
   interaction graphically.  Despite following.

      *  The manner in which the additional token exchange
   previous descriptions about associating symmetric and asymmetric keys keying material is expected to the access token be used.

      *  The other parties that are still applicable expected to this scenario.

                      +---------------+
        Access       ^|               |
        Token Req. // | Authorization |^
          (I)     /   | Server        | \  (IV) Token
                //    |               |  \ Introspection Req.
               /      |               |   \     +Access
             //      /+---------------+    \     Token
            /      // (II)             \    \\
           /      /   Access            \     \
         //     //    Token              \ (V) \
         /      /                         \Resp.\
      //     //                            \     \
     /      v                               V     \
   +-----------+ Request +Signature/MAC+------------+
   |           |  (III)  +Access Token |            |
   |           |---------------------->| Resource   |
   | have access to the
         keying material.

      *  The expected lifetime of the keying material.  Lifetime of a
         child key SHOULD NOT be greater than the lifetime of its parent
         in the key hierarchy.

      Any party with legitimate access to keying material can determine
      its context.  In addition, the protocol MUST ensure that all
      parties with legitimate access to keying material have the same
      context for the keying material.  This requires that the parties
      are properly identified and authenticated, so that all of the
      parties that have access to the keying material can be determined.
      The context will include the client and the resource server
      identities in more than one form.

   Authorization Restriction:

      If client authorization is restricted, then the client SHOULD be
      made aware of the restriction.

   Client    |   (VI) Success Identity Confidentiality:

      A client has identity confidentiality when any party other than
      the resource server and the authorization server cannot
      sufficiently identify the client within the anonymity set.  In
      comparison to anonymity and pseudonymity, identity confidentiality
      is concerned with eavesdroppers and intermediaries.  A key
      management protocol SHOULD provide this property.

   Resource Owner Identity Confidentiality:

      Resource servers SHOULD be prevented from knowing the real or     | Server     |
   |           |        Failure        |            |
   |           |<----------------------|            |
   +-----------+                       +------------+

          Figure 4: Token Introspection
      pseudonymous identity of the resource owner, since the
      authorization server is the only entity involved in verifying the
      resource owner's identity.

   Collusion:

      Resource servers that collude can be prevented from using
      information related to the resource owner to track the individual.
      That is, two different resource servers can be prevented from
      determining that the same resource owner has authenticated to both
      of them.  Authorization servers MUST bind different keying
      material to access tokens used for resource servers from different
      origins (or similar concepts in the app world).

   AS-to-RS Relationship Anonymity:

      For solutions using asymmetric key cryptography the client MAY
      conceal information about the resource server it wants to interact
      with.  The authorization server MAY reject such an attempt since
      it may not be able to enforce access control decisions.

   Channel Binding:

      A solution MUST enable support for channel bindings.  The concept
      of channel binding, as defined in [RFC5056], allows applications
      to establish that the two end-points of a secure channel at one
      network layer are the same as at a higher layer by binding
      authentication at the higher layer to the channel at the lower
      layer.

   There are performance concerns with the use of asymmetric
   cryptography.  Although symmetric key cryptography offers better
   performance asymmetric cryptography offers additional security
   properties.  A solution MUST therefore offer the capability to
   support both symmetric as well as asymmetric keys.

   There are threats that relate to the experience of the software
   developer as well as operational practices.  Verifying the servers
   identity in TLS is discussed at length in [RFC6125].

   A number of the threats listed in Section 4 demand protection of the
   access token content and Access Token Handles. a standardized solution, in form of a JSON-
   based format, is available with the JWT [RFC7519].

8.  Security Considerations

   The purpose of this document is to provide use cases, requirements,
   and motivation for developing an OAuth security solution extending
   Bearer Tokens.  As such, this document is only about security.

9.  IANA Considerations

   This document does not require actions by IANA.

10.  Acknowledgments

   This document is the result of conference calls late 2012/early 2013
   and in design team conference calls February 2013 of the IETF OAuth
   working group.  The following persons (in addition to the OAuth WG
   chairs, Hannes Tschofenig, and Derek Atkins) provided their input
   during these calls: Bill Mills, Justin Richer, Phil Hunt, Prateek
   Mishra, Mike Jones, George Fletcher, Leif Johansson, Lucy Lynch, John
   Bradley, Tony Nadalin, Klaas Wierenga, Thomas Hardjono, Brian
   Campbell

   In the appendix of this document we reuse re-use content from [RFC4962] and
   the authors would like thank Russ Housely and Bernard Aboba for their
   work on RFC 4962.

   We would like to thank Reddy Tirumaleswar for his review.

11.  References

11.1.  Normative References

   [I-D.ietf-oauth-introspection]
              Richer, J., "OAuth 2.0 Token Introspection", draft-ietf-
              oauth-introspection-11 (work in progress), July 2015.

   [I-D.ietf-oauth-pop-key-distribution]
              Bradley, J., Hunt, P., Jones, M., and H. Tschofenig,
              "OAuth 2.0 Proof-of-Possession: Authorization Server to
              Client Key Distribution", draft-ietf-oauth-pop-key-
              distribution-01 (work in progress), March 2015.

   [I-D.ietf-oauth-proof-of-possession]
              Jones, M., Bradley, J., and H. Tschofenig, "Proof-of-
              Possession Key Semantics for JSON Web Tokens (JWTs)",
              draft-ietf-oauth-proof-of-possession-04 (work in
              progress), August 2015.

   [I-D.ietf-oauth-signed-http-request]
              Richer, J., Bradley, J., and H. Tschofenig, "A Method for
              Signing an HTTP Requests for OAuth", draft-ietf-oauth-
              signed-http-request-01 (work in progress), March 2015.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, 10.17487/
              RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, 10.17487/
              RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <http://www.rfc-editor.org/info/rfc6749>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <http://www.rfc-editor.org/info/rfc7519>.

11.2.  Informative References

   [I-D.hardjono-oauth-kerberos]
              Hardjono, T., "OAuth 2.0 support for the Kerberos V5
              Authentication Protocol", draft-hardjono-oauth-kerberos-01
              (work in progress), December 2010.

   [I-D.ietf-oauth-introspection]
              Richer, J., "OAuth 2.0 Token Introspection", draft-ietf-
              oauth-introspection-11 (work in progress), July 2015.

   [I-D.ietf-oauth-pop-key-distribution]
              Bradley, J., Hunt, P., Jones, M., and H. Tschofenig,
              "OAuth 2.0 Proof-of-Possession: Authorization Server to
              Client Key Distribution", draft-ietf-oauth-pop-key-
              distribution-01 (work in progress), March 2015.

   [I-D.ietf-oauth-proof-of-possession]
              Jones, M., Bradley, J., and H. Tschofenig, "Proof-of-
              Possession Key Semantics for JSON Web Tokens (JWTs)",
              draft-ietf-oauth-proof-of-possession-04 (work in
              progress), August 2015.

   [I-D.ietf-oauth-signed-http-request]
              Richer, J., Bradley, J., and H. Tschofenig, "A Method for
              Signing an HTTP Requests for OAuth", draft-ietf-oauth-
              signed-http-request-01 (work in progress), March 2015.

   [NIST800-63]
              Burr, W., Dodson, D., Perlner, R., Polk, T., Gupta, S.,
              and E. Nabbus, "NIST Special Publication 800-63-1,
              INFORMATION SECURITY", December 2008.

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              DOI 10.17487/RFC4120, July 2005,
              <http://www.rfc-editor.org/info/rfc4120>.

   [RFC4279]  Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key
              Ciphersuites for Transport Layer Security (TLS)", RFC
              4279, DOI 10.17487/RFC4279, December 2005,
              <http://www.rfc-editor.org/info/rfc4279>.

   [RFC4962]  Housley, R. and B. Aboba, "Guidance for Authentication,
              Authorization, and Accounting (AAA) Key Management", BCP
              132, RFC 4962, DOI 10.17487/RFC4962, July 2007,
              <http://www.rfc-editor.org/info/rfc4962>.

   [RFC5056]  Williams, N., "On the Use of Channel Bindings to Secure
              Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007,
              <http://www.rfc-editor.org/info/rfc5056>.

   [RFC5849]  Hammer-Lahav, E., Ed., "The OAuth 1.0 Protocol", RFC 5849,
              DOI 10.17487/RFC5849, April 2010,
              <http://www.rfc-editor.org/info/rfc5849>.

   [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>.

   [RFC6750]  Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", RFC 6750, DOI 10.17487/RFC6750, 10.17487/
              RFC6750, October 2012,
              <http://www.rfc-editor.org/info/rfc6750>.

   [RFC6819]  Lodderstedt, T., Ed., McGloin, M., and P. Hunt, "OAuth 2.0
              Threat Model and Security Considerations", RFC 6819, DOI
              10.17487/RFC6819, January 2013,
              <http://www.rfc-editor.org/info/rfc6819>.

Authors' Addresses

   Phil Hunt (editor)
   Oracle Corporation

   Email: phil.hunt@yahoo.com

   Justin Richer

   Email: ietf@justin.richer.org

   William Mills

   Email: wmills@yahoo-inc.com

   Prateek Mishra
   Oracle Corporation

   Email: prateek.mishra@oracle.com
   Hannes Tschofenig
   ARM Limited
   Hall in Tirol  6060
   Austria

   Email: Hannes.Tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at