OAuth                                                       P. Hunt, Ed.
Internet-Draft                                        Oracle Corporation
Intended status: Informational                                 J. Richer
Expires: December 28, 2014                         The MITRE Corporation
                                                                W. Mills
                                                             Yahoo! Inc.
                                                               P. Mishra
                                                      Oracle Corporation
                                                           H. Tschofenig
                                                             ARM Limited
                                                           June 26, 2014

       OAuth 2.0 Proof-of-Possession (PoP) Security Architecture


   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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 28, 2014.

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Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Preventing Access Token Re-Use by the Resource Server . .   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.  Threat Mitigation . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Confidentiality Protection  . . . . . . . . . . . . . . .   7
     5.2.  Sender Constraint . . . . . . . . . . . . . . . . . . . .   7
     5.3.  Key Confirmation  . . . . . . . . . . . . . . . . . . . .   8
     5.4.  Summary . . . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .  15
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  19
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     11.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

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

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      "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

   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 5,
   outlines an architecture for a solution that builds on top of the
   existing OAuth 2.0 framework in Section 6, and concludes with a
   requirements list in Section 7.

2.  Terminology

   The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT',
   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

3.  Use Cases

   The main use case that motivates 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

   Imagine a scenario where a resource server that receives a valid
   access token re-uses it with other resource server.  The reason for
   re-use may be malicious or may well be legitimate.  In a legitimate

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   use case consider chaining of computations whereby a resource server
   needs to consult other third party resource servers to complete 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.  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 is conveyed to the resource
   server to ensure that no entity entity has tampered with the TLS

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

   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 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

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   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 in the clear from
   the load balancer to the resource server.  With application layer
   security independent of 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

4.  Security and Privacy Threats

   The following list presents several common threats against protocols
   utilizing some form of 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

   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.

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   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 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.  Threat Mitigation

   A large range of threats can be mitigated by protecting the content
   of the token, for example 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 the token itself
   is digitally signed by the authorization server and therefore cannot
   be modified.

   To deal with token redirect it is important for the authorization
   server to include the identifier of the intended recipient - the
   resource server.  A resource server must not be allowed to accept
   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 the communication interaction between the
   client and the authorization server as well as the interaction

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   between the client and the resource server to experience
   confidentiality protection.  As an example, TLS with a ciphersuite
   that offers confidentiality protection has to be applied (which is
   currently true for all ciphersuites, except for one).  Encrypting the
   token content itself is another alternative.  In our scenario the
   authorization server would, for example, encrypt the token content
   with a symmetric key shared with the resource server.

   To deal with token reuse more choices are available.

5.1.  Confidentiality Protection

   In this approach confidentiality protection of the exchange is
   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 replay by a third party is not possible.  An
   authorization server wants to ensure that it only hands out tokens to
   clients it has authenticated first and who are authorized.  For this
   purpose, authentication of the client to the authorization server
   will be a requirement to ensure adequate protection against a range
   of attacks.  This is, however, true for the description in
   Section 5.2 and Section 5.3 as well.  Furthermore, the client has to
   make sure it does not distribute (or leak) the access token to
   entities other than the intended the resource server.  For that
   purpose the client will have to authenticate the resource server
   before transmitting the access token.

5.2.  Sender Constraint

   Instead of providing confidentiality protection the authorization
   server could also put the identifier of the client into the protected
   token with the following semantic: 'This token is only valid when
   presented by a client with the following identifier.'  When the
   access token is then presented to the resource server how does it
   know that it was provided by the client?  It has to authenticate the
   client!  There are many choices for authenticating the client to 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  security properties

   o  available infrastructure

   o  library support

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   o  credential cost (financial)

   o  performance

   o  integration into the existing IT infrastructure

   o  operational overhead for configuration and distribution of

   This long list hints to the challenge of selecting at least one
   mandatory-to-implement client authentication mechanism.

5.3.  Key Confirmation

   A variation of the mechanism of sender authentication, described in
   Section 5.2, is to replace authentication with the proof-of-
   possession of a specific (session) key, i.e., key confirmation.  In
   this model the resource server would not authenticate the client
   itself but would rather verify whether the client knows 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 between Kerberos
   and OAuth).

   To illustrate key confirmation the first examples borrow from
   Kerberos and use symmetric key cryptography.  Assume that the
   authorization server shares a long-term secret with the resource
   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 creates a fresh and
   unique session key Ks and places it into the token encrypted with the
   long term key K(Authorization Server-Resource Server).  Additionally,
   the authorization server attaches Ks to the response message to the
   client (in addition to the access token itself) over a
   confidentiality protected channel.  When the client sends a request
   to the resource server it has to use Ks to compute a keyed message
   digest 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 is then used to verify the keyed
   message digest of the request message.

   Note that in this example one could imagine that the mechanism to
   protect the token itself is based on a symmetric key based mechanism
   to avoid any form of public key infrastructure but this aspect is not
   further elaborated in the scenario.

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   A similar mechanism can also be designed using asymmetric
   cryptography.  When 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 the access token to the client it
   also provides the PK/SK key pair over a confidentiality protected
   channel.  When the client sends a request to the resource server it
   has to use the privacy key SK to sign the request.  The resource
   server, when receiving the message, retrieves the access token,
   verifies it and extracts the public key PK.  It uses this ephemeral
   public key to verify the attached signature.

5.4.  Summary

   As a high level message, there are various ways how the threats can
   be mitigated and while the details of each solution is somewhat
   different they all ultimately accomplish the goal.

   The three approaches are:

   Confidentiality Protection:

      The weak point with this approach, which is briefly described in
      Section 5.1, is that the client has to be careful to whom it
      discloses the access token.  What can be done with the token
      entirely depends on what rights the token entitles the presenter
      and what constraints it contains.  A token could encode the
      identifier of the client but there are scenarios where the client
      is not authenticated to the resource server or where the
      identifier of 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 the access
      token is granted access to the data.

   Sender Constraint:

      The weak point with this approach, which is briefly described in
      Section 5.2, is to setup the authentication infrastructure such
      that clients can be authenticated towards resource servers.
      Additionally, the authorization server must encode the identifier
      of the client in the token for later verification by the 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 to identity information, etc.

   Key Confirmation:

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      The weak point with this approach, see Section 5.3, is the
      increased complexity: a complete key distribution protocol has to
      be defined.

   In all cases above it has to be ensured that the client is able to
   keep the credentials secret.

6.  Architecture

   The proof-of-possession security concept assumes that the
   authorization server acts as a trusted third party that binds keys to
   access tokens.  These keys are then used by the client to demonstrate
   the possession of the secret to the resource server when accessing
   the resource.  The resource server, when receiving an access token,
   needs to verify that the key used by the client matches the one
   included in the access token.

   There are slight differences between the use of symmetric keys and
   asymmetric keys when they are bound to the access token and the
   subsequent interaction between the client and the authorization
   server when demonstrating possession of these keys.  Figure 1 shows
   the symmetric key procedure and Figure 2 illustrates how asymmetric
   keys are used.

   With the JSON Web Token (JWT) a standardized format for access tokens
   is available.  The necessary elements to bind symmetric or asymmetric
   keys to the JWT are described in

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                       ^|               |
                     // | 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 client interacts with the
   authorization server as part of the a normal grant exchange, as shown
   in Figure 1.  However, it needs to include additional information
   elements for use with the PoP security mechanism, as depicted in
   message (I).  In message (II) the authorization server then returns
   the requested access token.  In addition to the access token itself,
   the symmetric key is communicated to the client.  This symmetric key
   is 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 the
   resource server need to have access to the session key.  While the
   key transport mechanism from the authorization server to the client
   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.

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      The resource server queries the authorization server for the
      symmetric key.  This is an approach envisioned by the token
      introspection endpoint [I-D.richer-oauth-introspection].

      The authorization server and the resource server both have access
      to the 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 and the Authorization Server
                            (Asymmetric Keys).

   The use of asymmetric keys is slightly different since the client or
   the server could be involved in the generation of the ephemeral key
   pair.  This exchange is shown in Figure 1.  If the client generates
   the key pair it either includes a fingerprint of the public key or
   the 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 client did not provide a
   fingerprint or a public key in the request then the authorization
   server is asked to create an ephemeral asymmetric key pair, binds the
   fingerprint of the public key to the access token, and returns the
   asymmetric key pair (public and private key) to the client.

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   The specification describing the interaction between the client and
   the authorization server, as shown in Figure 1 and in Figure 2, can
   be found in [I-D.bradley-oauth-pop-key-distribution].

   Once the client has obtained the necessary access token and keying
   material it can start to interact with the 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 primitive).  When the
   resource server receives the request it verifies it and decides
   whether access to the protected resource can be granted.  This
   exchange is shown in Figure 3.

                      |               |
                      | Authorization |
                      | Server        |
                      |               |
                      |               |

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

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

                    Figure 3: Client demonstrates PoP.

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   The specification describing the ability to sign the HTTP request
   from the client to the resource server can be found in

   So far the examples talked about access tokens that are passed by
   value and allow the resource server to make authorization decisions
   immediately after verifying the request from the client.  In some
   deployments a real-time interaction between the authorization server
   and the resource server is envisioned that lowers the need to pass
   self-contained access tokens around.  In that case the access token
   merely serves as a handle or a reference to state stored at the
   authorization server.  As a consequence, the resource server cannot
   autonomously make an authorization decision when receiving a request
   from a client but has to consult the authorization server.  This can,
   for example, be done using the token introspection endpoint (see
   [I-D.richer-oauth-introspection]).  Figure 4 shows the protocol
   interaction graphically.  Despite the additional token exchange
   previous descriptions about associating symmetric and asymmetric keys
   to the access token are still applicable to this scenario.

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

          Figure 4: Token Introspection and Access Token Handles.

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7.  Requirements

   RFC 4962 [RFC4962] gives useful guidelines for designers of
   authentication and key management protocols.  While RFC 4962 was
   written with the AAA framework used for network access authentication
   in mind the offered suggestions are useful for the design of other
   key management systems as well.  The following requirements list
   applies OAuth 2.0 terminology to the requirements outlined in RFC

   These requirements include

   Cryptographic Algorithm Independent:

      The key management protocol MUST be cryptographic algorithm

   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 intended use.  In context of OAuth this means that keying
      material is created in such a way that can only be used by the
      combination of a client instance, protected resource, and
      authorization scope.

   Limit Key Scope:

      Following the principle of least privilege, parties MUST NOT have
      access to keying material that is not needed to perform their
      role.  Any protocol that is used to establish session keys MUST
      specify the scope for session keys, clearly identifying the
      parties to whom the session key is available.

   Replay Detection Mechanism:

      The key management protocol exchanges MUST be replay protected.
      Replay protection allows a protocol message recipient to discard
      any message that was recorded during a previous legitimate
      dialogue and presented as though it belonged to the current

   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 the confidentiality of any secret values

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      used in the authentication process.  Secrets MUST NOT be sent to
      another party without confidentiality protection.


      Client and resource server authorization MUST be performed.  These
      entities MUST demonstrate possession of the appropriate keying
      material, without disclosing it.  Authorization is REQUIRED
      whenever a client 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 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 naming scheme,
      particularly where key caching is supported.  The key name
      provides a way to refer to a key in a protocol so that it is clear
      to all parties which key is being referenced.  Objects that cannot
      be named cannot be managed.  All keys MUST be uniquely named, and
      the key name MUST NOT directly or indirectly disclose the keying

   Prevent the Domino Effect:

      Compromise of a single client 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 MUST NOT compromise keying material held by any other
      Resource Server within the system.  In the context of a key
      hierarchy, this means that the compromise of one node in the key
      hierarchy must not disclose the information necessary to
      compromise other branches in the key hierarchy.  Obviously, the
      compromise of the root of the key hierarchy will compromise all of
      the keys; however, a compromise in one branch MUST NOT result in
      the 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

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      understood by all parties that communicate with 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 with other branches in the key

   Bind Key to its Context:

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

      *  The manner in which the keying material is expected to be used.

      *  The other parties that are expected to 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 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
      pseudonymous identity of the resource owner, since the

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      authorization server is the only entity involved in verifying the
      resource owner's identity.


      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.  This requires that each authorization server obtains
      different keying material as well as different access tokens with
      content that does not allow identification of the resource owner.

   AS-to-RS Relationship Anonymity:

      This MAC Token security does not provide AS-to-RS relationship
      anonymity since the client has to inform the resource server about
      the resource server it wants to talk to.  The authorization server
      needs to know how to encrypt the session key the client and the
      resource server will be using.

   As an additional requirement 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 a standardized solution, in form of a JSON-
   based format, is available with the JWT

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.

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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

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

11.  References

11.1.  Normative References

              Bradley, J., Hunt, P., Jones, M., and H. Tschofenig,
              "OAuth 2.0 Proof-of-Possession: Authorization Server to
              Client Key Distribution", draft-bradley-oauth-pop-key-
              distribution-00 (work in progress), April 2014.

              Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Message Syntax and Routing", draft-ietf-
              httpbis-p1-messaging-26 (work in progress), February 2014.

              Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
              draft-ietf-jose-json-web-encryption-29 (work in progress),
              June 2014.

              Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", draft-ietf-oauth-json-web-token-23 (work in
              progress), June 2014.

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              Jones, M., Bradley, J., and H. Tschofenig, "Proof-Of-
              Possession Semantics for JSON Web Tokens (JWTs)", draft-
              jones-oauth-proof-of-possession-00 (work in progress),
              April 2014.

              Richer, J., "OAuth Token Introspection", draft-richer-
              oauth-introspection-04 (work in progress), May 2013.

              Richer, J., Bradley, J., and H. Tschofenig, "A Method for
              Signing an HTTP Requests for OAuth", draft-richer-oauth-
              signed-http-request-01 (work in progress), April 2014.

              Tschofenig, H., "OAuth 2.0: Audience Information", draft-
              tschofenig-oauth-audience-00 (work in progress), February

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104, February

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
              Leach, P., Luotonen, A., and L. Stewart, "HTTP
              Authentication: Basic and Digest Access Authentication",
              RFC 2617, June 1999.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, January 2005.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

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   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              April 2011.

   [RFC6749]  Hardt, D., "The OAuth 2.0 Authorization Framework", RFC
              6749, October 2012.

              Raggett, D., Hors, A., and I. Jacobs, "HTML 4.01
              Specification", World Wide Web Consortium Recommendation
              REC-html401-19991224, December 1999,

11.2.  Informative References

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

              National Institute of Standards and Technology, "Secure
              Hash Standard (SHS). FIPS PUB 180-3, October 2008",
              October 2008.

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

   [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              July 2005.

   [RFC4279]  Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
              for Transport Layer Security (TLS)", RFC 4279, December

   [RFC4962]  Housley, R. and B. Aboba, "Guidance for Authentication,
              Authorization, and Accounting (AAA) Key Management", BCP
              132, RFC 4962, July 2007.

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   [RFC5056]  Williams, N., "On the Use of Channel Bindings to Secure
              Channels", RFC 5056, November 2007.

   [RFC5849]  Hammer-Lahav, E., "The OAuth 1.0 Protocol", RFC 5849,
              April 2010.

   [RFC5929]  Altman, J., Williams, N., and L. Zhu, "Channel Bindings
              for TLS", RFC 5929, July 2010.

   [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, March 2011.

   [RFC6750]  Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", RFC 6750, October 2012.

   [RFC6819]  Lodderstedt, T., McGloin, M., and P. Hunt, "OAuth 2.0
              Threat Model and Security Considerations", RFC 6819,
              January 2013.

Authors' Addresses

   Phil Hunt (editor)
   Oracle Corporation

   Email: phil.hunt@yahoo.com

   Justin Richer
   The MITRE Corporation

   Email: jricher@mitre.org

   William Mills
   Yahoo! Inc.

   Email: wmills@yahoo-inc.com

   Prateek Mishra
   Oracle Corporation

   Email: prateek.mishra@oracle.com

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   Hannes Tschofenig
   ARM Limited

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

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