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ABFAB                                                         J. Howlett
Internet-Draft                                                 JANET(UK)
Intended status: Informational                                S. Hartman
Expires: April 25, 2013                                Painless Security
                                                           H. Tschofenig
                                                  Nokia Siemens Networks
                                                                 E. Lear
                                                      Cisco Systems GmbH
                                                               J. Schaad
                                                 Soaring Hawk Consulting
                                                        October 22, 2012


      Application Bridging for Federated Access Beyond Web (ABFAB)
                              Architecture
                      draft-ietf-abfab-arch-04.txt

Abstract

   Over the last decade a substantial amount of work has occurred in the
   space of federated access management.  Most of this effort has
   focused on two use-cases: network and web-based access.  However, the
   solutions to these use-cases that have been proposed and deployed
   tend to have few common building blocks in common.

   This memo describes an architecture that makes use of extensions to
   the commonly used security mechanisms for both federated and non-
   federated access management, including the Remote Authentication Dial
   In User Service (RADIUS) and the Diameter protocol, the Generic
   Security Service (GSS), the GS2 family, the Extensible Authentication
   Protocol (EAP) and the Security Assertion Markup Language (SAML).
   The architecture addresses the problem of federated access management
   to primarily non-web-based services, in a manner that will scale to
   large numbers of identity providers, relying parties, and
   federations.

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



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   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 April 25, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.
































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
     1.2.  An Overview of Federation  . . . . . . . . . . . . . . . .  6
     1.3.  Challenges for Contemporary Federation . . . . . . . . . .  9
     1.4.  An Overview of ABFAB-based Federation  . . . . . . . . . . 10
     1.5.  Design Goals . . . . . . . . . . . . . . . . . . . . . . . 13
   2.  Architecture . . . . . . . . . . . . . . . . . . . . . . . . . 14
     2.1.  Relying Party to Identity Provider . . . . . . . . . . . . 15
       2.1.1.  AAA, RADIUS and Diameter . . . . . . . . . . . . . . . 16
       2.1.2.  Discovery and Rules Determination  . . . . . . . . . . 17
       2.1.3.  Routing and Technical Trust  . . . . . . . . . . . . . 18
       2.1.4.  SAML Assertions  . . . . . . . . . . . . . . . . . . . 20
     2.2.  Client To Identity Provider  . . . . . . . . . . . . . . . 21
       2.2.1.  Extensible Authentication Protocol (EAP) . . . . . . . 21
       2.2.2.  EAP Channel Binding  . . . . . . . . . . . . . . . . . 23
     2.3.  Client to Relying Party  . . . . . . . . . . . . . . . . . 23
       2.3.1.  GSS-API  . . . . . . . . . . . . . . . . . . . . . . . 23
       2.3.2.  Protocol Transport . . . . . . . . . . . . . . . . . . 25
   3.  Application Security Services  . . . . . . . . . . . . . . . . 26
     3.1.  Authentication . . . . . . . . . . . . . . . . . . . . . . 26
     3.2.  GSS-API Channel Binding  . . . . . . . . . . . . . . . . . 27
     3.3.  Host-Based Service Names . . . . . . . . . . . . . . . . . 28
     3.4.  Per-Message Tokens . . . . . . . . . . . . . . . . . . . . 29
   4.  Future Work: Attribute Providers . . . . . . . . . . . . . . . 30
   5.  Privacy Considerations . . . . . . . . . . . . . . . . . . . . 31
     5.1.  Entities and their roles . . . . . . . . . . . . . . . . . 31
     5.2.  Relationship between user and entities . . . . . . . . . . 32
     5.3.  Data and Identifiers in use  . . . . . . . . . . . . . . . 32
       5.3.1.  NAI  . . . . . . . . . . . . . . . . . . . . . . . . . 32
       5.3.2.  Identity Information . . . . . . . . . . . . . . . . . 33
       5.3.3.  Accounting Information . . . . . . . . . . . . . . . . 33
       5.3.4.  Collection and retention of data and identifiers . . . 33
     5.4.  User Participation . . . . . . . . . . . . . . . . . . . . 34
   6.  Deployment Considerations  . . . . . . . . . . . . . . . . . . 35
     6.1.  EAP Channel Binding  . . . . . . . . . . . . . . . . . . . 35
     6.2.  AAA Proxy Behavior . . . . . . . . . . . . . . . . . . . . 35
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 36
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 38
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 39
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 40
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 40
     10.2. Informative References . . . . . . . . . . . . . . . . . . 40
   Editorial Comments . . . . . . . . . . . . . . . . . . . . . . . .
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 45





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1.  Introduction

   The Internet uses numerous security mechanisms to manage access to
   various resources.  These mechanisms have been generalized and scaled
   over the last decade through mechanisms such as Simple Authentication
   and Security Layer (SASL) with the Generic Security Server
   Application Program Interface (GSS-API) (known as the GS2 family)
   [RFC5801], Security Assertion Markup Language (SAML)
   [OASIS.saml-core-2.0-os], and the Authentication, Authorization, and
   Accounting (AAA) architecture as embodied in RADIUS [RFC2865] and
   Diameter [RFC3588].

   A Relying Party (RP) is the entity that manages access to some
   resource.  The actor that is requesting access to that resource is
   often described as the Client.  Many security mechanisms are
   manifested as an exchange of information between these actors.  The
   RP is therefore able to decide whether the Client is authorized, or
   not.

   Some security mechanisms allow the RP to delegate aspects of the
   access management decision to an actor called the Identity Provider
   (IdP).  This delegation requires technical signaling, trust and a
   common understanding of semantics between the RP and IdP.  These
   aspects are generally managed within a relationship known as a
   'federation'.  This style of access management is accordingly
   described as 'federated access management'.

   Federated access management has evolved over the last decade through
   specifications like SAML [OASIS.saml-core-2.0-os], OpenID [1], OAuth
   [RFC5849], [I-D.ietf-oauth-v2] and WS-Trust [WS-TRUST].  The benefits
   of federated access management include:

   Single or Simplified sign-on:

      An Internet service can delegate access management, and the
      associated responsibilities such as identity management and
      credentialing, to an organisation that already has a long-term
      relationship with the Subject.  This is often attractive for
      Relying Parties who frequently do not want these responsibilities.
      The Subject also requires fewer credentials, which is also
      desirable.

      Data Minimization and User Participation:

      Often a Relying Party does not need to know the identity of a
      Subject to reach an access management decision.  It is frequently
      only necessary for the Relying Party know specific attributes
      about the subject, for example, that the Subject is affiliated



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      with a particular organisation or has a certain role or
      entitlement.  Sometimes the RP only needs to know a pseudonym of
      the Subject.

      Prior to the release of attributes to the IdP from the IdP, the
      IdP will check configuration and policy to determine if the
      attributes are to be released.  There is currently no direct
      client participation in this decision.

   Provisioning

      Sometimes a Relying Party needs, or would like, to know more about
      a subject than an affiliation or a pseudonym.  For example, a
      Relying Party may want the Subject's email address or name.  Some
      federated access management technologies provide the ability for
      the IdP to supply this information, either on request by the RP or
      unsolicited.

   This memo describes the Application Bridging for Federated Access
   Beyond the Web (ABFAB) architecture.  This architecture makes use of
   extensions to the commonly used security mechanisms for both
   federated and non-federated access management, including the RADIUS
   and the Diameter protocols, the Generic Security Service (GSS), the
   GS2 family, the Extensible Authentication Protocol (EAP) and SAML.
   The architecture addresses the problem of federated access management
   primarily for non-web-based services.  It does so in a manner that
   will scale to large numbers of identity providers, relying parties,
   and federations.

1.1.  Terminology

   This document uses identity management and privacy terminology from
   [I-D.iab-privacy-considerations].  In particular, this document uses
   the terms identity provider, relying party, (data) subject,
   identifier, pseudonymity, unlinkability, and anonymity.

   In this architecture the IdP consists of the following components: an
   EAP server, a RADIUS or a Diameter server, and optionally a SAML
   Assertion service.

   This document uses the term Network Access Identifier (NAI), as
   defined in [RFC4282].  An NAI consists of a realm identifier, which
   is associated with an IdP and a username which is associated with a
   specific client of the IdP.

   One of the problems people will find with reading this document is
   that the terminology sometimes appears to be inconsistent.  This is
   due the fact that the terms used by the different standards we are



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   picking up don't use the same terms.  In general the document uses
   either a consistent term or the term associated with the standard
   under discussion as appropriate.  For reference we include this table
   which maps the different terms into a single table.

   +----------+-----------+--------------------+-----------------------+
   | Protocol | Subject   | Relying Party      | Identity Provider     |
   +----------+-----------+--------------------+-----------------------+
   | ABFAB    | Client    | Relying Party (RP) | Identity Provider     |
   |          |           |                    | (IdP)                 |
   |          |           |                    |                       |
   |          | Initiator | Acceptor           |                       |
   |          |           |                    |                       |
   | SAML     | Subject   | Service Provider   | Issuer                |
   |          |           |                    |                       |
   | GSS-API  | Initiator | Acceptor           |                       |
   |          |           |                    |                       |
   | EAP      | EAP peer  |                    | EAP server            |
   |          |           |                    |                       |
   | AAA      |           | AAA Client         | AAA server            |
   |          |           |                    |                       |
   | RADIUS   | user      | NAS                | RADIUS server         |
   |          |           |                    |                       |
   |          |           | RADIUS client      |                       |
   +----------+-----------+--------------------+-----------------------+

   Note that in some cases a cell has been left empty, in these cases
   there is no direct name that represents this concept.

   Note to reviewers - I have most likely missed some entries in the
   table.  Please provide me with both correct names from the protocol
   and missing names that are used in the text below.

1.2.  An Overview of Federation

   In the previous section we introduced the following actors:

   o  the Client,

   o  the Identity Provider, and

   o  the Relying Party.

   One additional actor in can be an Individual.  An individual is a
   human being that is using a client.  Individuals may or may not exist
   in any given deployment.  The client may be either a front end on an
   individual or an independent automated entity.




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   These entities and their relationships are illustrated graphically in
   Figure 1.



    ,----------\                        ,---------\
    | Identity |       Federation       | Relying |
    | Provider +  <-------------------> + Party   |
    `----------'                        '---------'
           <
            \
             \ Authentication
              \
               \
                \
                 \
                  \  +---------+
                   \ |         |  O
                    v| Client  | \|/ Individual
                     |         |  |
                     +---------+ / \

                Figure 1: Entities and their Relationships

   The relationships between the entities in Figure 1 are:

   Federation

      The Identity Provider and the Relying Parties are part of a
      Federation.  The relationship may be direct (they have an explicit
      trust relationship) or transitive (the trust releationship is
      mediated by one or more entities).  The federation relationship is
      governed by a federation agreement.  Within a single federation,
      there may be multiple Identity Providers as well as multiple
      Relying Parties.  A federation is governed by a federation
      agreement.

   Authentication

      There is a direct relationship between the Client and the Identity
      Provider by which the entities trust and can securely authenticate
      each other.

   A federation agreement typically encompasses operational
   specifications and legal rules:






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   Operational Specifications:

      These includes the technical specifications (e.g. protocols used
      to communicate between the three parties), process standards,
      policies, identity proofing, credential and authentication
      algorithm requirements, performance requirements, assessment and
      audit criteria, etc.  The goal of operational specifications is to
      provide enough definition that the system works and
      interoperability is possible.

   Legal Rules:

      The legal rules take the legal framework into consideration and
      provides contractual obligations for each entity.  The rules
      define the responsibilities of each party and provide further
      clarification of the operational specifications.  These legal
      rules regulate the operational specifications, make operational
      specifications legally binding to the participants, define and
      govern the rights and responsibilities of the participants.  The
      legal rules may, for example, describe liability for losses,
      termination rights, enforcement mechanisms, measures of damage,
      dispute resolution, warranties, etc.

   The Operational Specifications can demand the usage of a
   sophisticated technical infrastructure, including requirements on the
   message routing intermediaries, to offer the required technical
   functionality.  In other environments, the Operational Specifications
   require fewer technical components in order to meet the required
   technical functionality.

   The Legal Rules include many non-technical aspects of federation,
   such as business practices and legal arrangements, which are outside
   the scope of the IETF.  The Legal Rules can still have an impact the
   architectural setup or on how to ensure the dynamic establishment of
   trust.

   While a federation agreement is often discussed within the context of
   formal relationships, such as between an enterprise and an employee
   or a government and a citizen, a federation agreement does not have
   to require any particular level of formality.  For an IdP and a
   Client, it is sufficient for a relationship to be established by
   something as simple as using a web form and confirmation email.  For
   an IdP and an RP, it is sufficient for the IdP to publish contact
   information along with a public key and for the RP to use that data.
   With in the framework of ABFAB, it will generally be required that a
   mechanism exists for the IdP to be able to trust the identity of the
   RP, if this is not present then the IdP cannot provide the assurances
   to the client that the identity of the RP has been established.



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   The nature of federation dictates that there is some form of
   relationship between the identity provider and the relying party.
   This is particularly important when the relying party wants to use
   information obtained from the identity provider for access management
   decisions and when the identity provider does not want to release
   information to every relying party (or only under certain
   conditions).

   While it is possible to have a bilateral agreement between every IdP
   and every RP; on an Internet scale this setup requires the
   introduction of the multi-lateral federation concept, as the
   management of such pair-wise relationships would otherwise prove
   burdensome.

   The IdP will typically have a long-term relationship with the Client.
   This relationship typically involves the IdP positively identifying
   and credentialing the Client (for example, at time of employment
   within an organization).  The relationship will often be instantiated
   within an agreement between the IdP and the Client (for example,
   within an employment contract or terms of use that stipulates the
   appropriate use of credentials and so forth).

   The nature and quality of the relationship between the Subject and
   the IdP is an important contributor to the level of trust that an RP
   may attribute to an assertion describing a Client made by an IdP.
   This is sometimes described as the Level of Assurance.

   Federation does not require an a priori relationship or a long-term
   relationship between the RP and the Client; it is this property of
   federation that yields many of its benefits.  However, federation
   does not preclude the possibility of a pre-existing relationship
   between the RP and the Client, nor that they may use the introduction
   to create a new long-term relationship independent of the federation.

   Finally, it is important to reiterate that in some scenarios there
   might indeed be an Individual behind the Client and in other cases
   the Client may be autonomous.

1.3.  Challenges for Contemporary Federation

   As the number of federated services has proliferated, the role of the
   individual can become ambiguous in certain circumstances.  For
   example, a school might provide online access for a student's grades
   to their parents for review, and to the student's teacher for
   modification.  A teacher who is also a parent must clearly
   distinguish her role upon access.

   Similarly, as the number of federations proliferates, it becomes



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   increasingly difficult to discover which identity provider(s) a user
   is associated with.  This is true for both the web and non-web case,
   but is particularly acute for the latter as many non-web
   authentication systems are not semantically rich enough on their own
   to allow for such ambiguities.  For instance, in the case of an email
   provider, the use of SMTP and IMAP protocols do not have the ability
   for the server to get additional information, beyond the clients NAI,
   in order to provide additional input to decide between multiple
   federations it may be associated with.  However, the building blocks
   do exist to add this functionality.

1.4.  An Overview of ABFAB-based Federation

   The previous section described the general model of federation, and
   its the application of federated access management.  This section
   provides a brief overview of ABFAB in the context of this model.

   In this example, a client is attempting to connect to a server in
   order to either get access to some data or perform some type of
   transaction.  In order for the client to mutually authenticate with
   the server, the following steps are taken in an ABFAB federated
   architecture:

   1.   Client Configuration: The Client Application is configured with
        an NAI assigned by the IdP.  It is also configured with any
        keys, certificates, passwords or other secret and public
        information needed to run the EAP protocols between it and the
        IdP.

   2.   Authentication mechanism selection: The GSS-EAP GSS-API
        mechanism is selected for authentication/authorization.

   3.   Client provides an NAI to RP: The client application sets up a
        transport to the RP and begins the GSS-EAP authentication.  In
        response, the RP sends an EAP request message (nested in the
        GSS-EAP protocol) asking for the Client's name.  The Client
        sends an EAP response with an NAI name form that at a minimum,
        contains the realm portion of it's full NAI.

   4.   Discovery of federated IdP: The RP uses pre-configured
        information or a federation proxy to determine what IdP to use
        based on policy and the realm portion of the provided Client
        NAI.  This is discussed in detail below (Section 2.1.2).

   5.   Request from Relying Party to IdP: Once the RP knows who the IdP
        is, it (or its agent) will send a RADIUS/Diameter request to the
        IdP.  The RADIUS/Diameter access request encapsulates the EAP
        response.  At this stage, the RP will likely have no idea who



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        the client is.  The RP sends its identity to the IdP in AAA
        attributes, and it may send a SAML Attribute Requests in a AAA
        attribute.  The AAA network checks that the identity claimed by
        the RP is valid.

   6.   IdP begins EAP with the client: The IdP sends an EAP message to
        the client with an EAP method to be run.  The IdP may re-request
        the clients name in this message, but this is unexpected
        behavior.  The available and appropriate methods are discussed
        below in this memo (Section 2.2.1).

   7.   The EAP protocol is run: A bunch of EAP messages are passed
        between the client (EAP peer) and the IdP (EAP server), until
        the result of the authentication protocol is determined.  The
        number and content of those messages depends on the EAP method
        selected.  If the IdP is unable to authenticate the client, the
        IdP sends a EAP failure message to the RP.  As part of the EAP
        protocol, the client sends a channel bindings EAP message to the
        IdP (Section 2.2.2).  In the channel binding message the client
        identifies, among other things, the RP to which it is attempting
        to authenticate.  The IdP checks the channel binding data from
        the client with that provided by the RP via the AAA protocol.
        If the bindings do not match the IdP sends an EAP failure
        message to the RP.

   8.   Successful EAP Authentication: At this point, the IdP (EAP
        server) and client (EAP peer) have mutually authenticated each
        other.  As a result, the subject and the IdP hold two
        cryptographic keys: a Master Session Key (MSK), and an Extended
        MSK (EMSK).  At this point the client has a level of assurance
        about the identity of the RP based on the name checking the IdP
        has done using the RP naming information from the AAA framework
        and from the client (by the channel binding data).

   9.   Local IdP Policy Check: At this stage, the IdP checks local
        policy to determine whether the RP and client are authorized for
        a given transaction/service, and if so, what if any, attributes
        will be released to the RP.  If the IdP gets a policy failure,
        it sends an EAP failure message to the RP.[anchor4] (The RP will
        have done its policy checks during the discovery process.)

   10.  IdP provide the RP with the MSK: The IdP sends a positive result
        EAP to the RP, along with an optional set of AAA attributes
        associated with the client (usually as one or more SAML
        assertions).  In addition, the EAP MSK is returned to the RP.

   11.  RP Processes Results: When the RP receives the result from the
        IdP, it should have enough information to either grant or refuse



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        a resource access request.  It may have information that
        associates the client with specific authorization identities.
        If additional attributes are needed from the IdP the RP may make
        a new SAML Request to the IdP.  It will apply these results in
        an application-specific way.

   12.  RP returns results to client: Once the RP has a response it must
        inform the client application of the result.  If all has gone
        well, all are authenticated, and the application proceeds with
        appropriate authorization levels.  The client can now complete
        the authentication of the RP by the use of the EAP MSK value.

   An example communication flow is given below:


       Relying           Client        Identity
       Party            App          Provider

       |              (1)             | Client Configuration
       |               |              |
       |<-----(2)----->|              | Mechanism Selection
       |               |              |
       |<-----(3)-----<|              | NAI transmitted to RP
       |               |              |
       |<=====(4)====================>| Discovery
       |               |              |
       |>=====(5)====================>| Access request from RP to IdP
       |               |              |
       |               |< - - (6) - -<| EAP method to Client
       |               |              |
       |               |< - - (7) - ->| EAP Exchange to authenticate
       |               |              | Client
       |               |              |
       |               |           (8 & 9) Local Policy Check
       |               |              |
       |<====(10)====================<| IdP Assertion to RP
       |               |              |
       (11)              |              | RP processes results
       |               |              |
       |>----(12)----->|              | Results to client app.



       ----- = Between Client App and RP
       ===== = Between RP and IdP
       - - - = Between Client App and IdP





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1.5.  Design Goals

   Our key design goals are as follows:

   o  Each party of a transaction will be authenticated, although
      perhaps not identified, and the client will be authorized for
      access to a specific resource.

   o  Means of authentication is decoupled so as to allow for multiple
      authentication methods.

   o  Hence, the architecture requires no sharing of long term private
      keys between clients and servers.

   o  The system will scale to large numbers of identity providers,
      relying parties, and users.

   o  The system will be designed primarily for non-Web-based
      authentication.

   o  The system will build upon existing standards, components, and
      operational practices.

   Designing new three party authentication and authorization protocols
   is hard and fraught with risk of cryptographic flaws.  Achieving
   widespead deployment is even more difficult.  A lot of attention on
   federated access has been devoted to the Web. This document instead
   focuses on a non-Web-based environment and focuses on those protocols
   where HTTP is not used.  Despite the increased excitement for
   layering every protocol on top of HTTP there are still a number of
   protocols available that do not use HTTP-based transports.  Many of
   these protocols are lacking a native authentication and authorization
   framework of the style shown in Figure 1.


















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2.  Architecture

   We have already introduced the federated access architecture, with
   the illustration of the different actors that need to interact, but
   did not expand on the specifics of providing support for non-Web
   based applications.  This section details this aspect and motivates
   design decisions.  The main theme of the work described in this
   document is focused on re-using existing building blocks that have
   been deployed already and to re-arrange them in a novel way.

   Although this architecture assumes updates to the relying party, the
   client application, and the Identity Provider, those changes are kept
   at a minimum.  A mechanism that can demonstrate deployment benefits
   (based on ease of update of existing software, low implementation
   effort, etc.) is preferred and there may be a need to specify
   multiple mechanisms to support the range of different deployment
   scenarios.

   There are a number of ways for encapsulating EAP into an application
   protocol.  For ease of integration with a wide range of non-Web based
   application protocols the usage of the GSS-API was chosen.  A
   description of the technical specification can be found in
   [I-D.ietf-abfab-gss-eap].  Other alternatives exist as well and may
   be considered later, such as "TLS using EAP Authentication"
   [I-D.nir-tls-eap].[anchor7]

   The architecture consists of several building blocks, which is shown
   graphically in Figure 2.  In the following sections, we discuss the
   data flow between each of the entities, the protocols used for that
   data flow and some of the trade-offs made in choosing the protocols.





















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                                    +--------------+
                                    |   Identity   |
                                    |   Provider   |
                                    |    (IdP)     |
                                    +-^----------^-+
                                      * EAP      o RADIUS/
                                      *          o Diameter
                                    --v----------v--
                                 ///                \\\
                               //                      \\
                              |        Federation        |
                              |        Substrate         |
                               \\                      //
                                 \\\                ///
                                    --^----------^--
                                      * EAP      o RADIUS/
                                      *          o Diameter
   +-------------+                  +-v----------v--+
   |             |<---------------->|               |
   | Client      |  EAP/EAP Method  | Relying Party |
   | Application |<****************>|     (RP)      |
   |             |  GSS-API         |               |
   |             |<---------------->|               |
   |             |  Application     |               |
   |             |  Protocol        |               |
   |             |<================>|               |
   +-------------+                  +---------------+

   Legend:

    <****>: Client-to-IdP Exchange
    <---->: Client-to-RP Exchange
    <oooo>: RP-to-IdP Exchange
    <====>: Protocol through which GSS-API/GS2 exchanges are tunneled

                  Figure 2: ABFAB Protocol Instantiation

2.1.  Relying Party to Identity Provider

   Communications between the Relying Party and the Identity Provider is
   done by the federation substrate.  This communication channel is
   responsible for:

   o  Establishing the trust relationship between the RP and the IdP.

   o  Determining the rules governing the relationship.





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   o  Conveying EAP packets between the RP and IdP.

   The ABFAB working group has chosen the AAA framework for the messages
   transported between the RP and IdP.  This allows for the current AAA
   protocols to be used to establish the trust relationship between the
   RP and the IdP.  Future protocols that support the same framework but
   do different routing may be used in the future.  There is currently
   an effort to setup a framework that creates a trusted point-to-point
   channel on the fly.  The ABFAB protocol itself details the method of
   establishing the trust relationship between the RP and the client.

2.1.1.  AAA, RADIUS and Diameter

   Interestingly, for network access authentication the usage of the AAA
   framework with RADIUS [RFC2865] and Diameter [RFC3588] was quite
   successful from a deployment point of view.  To map the terminology
   used in Figure 1 to the AAA framework the IdP corresponds to the AAA
   server, the RP corresponds to the AAA client, and the technical
   building blocks of a federation are AAA proxies, relays and redirect
   agents (particularly if they are operated by third parties, such as
   AAA brokers and clearing houses).  The front-end, i.e. the end host
   to AAA client communication, is in case of network access
   authentication offered by link layer protocols that forward
   authentication protocol exchanges back-and-forth.  An example of a
   large scale RADIUS-based federation is EDUROAM [2].

   By using the AAA framework, ABFAB gets a lot of mileage as many of
   the federation agreements already exist and merely need to be
   expanded to cover the ABFAB additions.  The AAA framework has already
   addressed some of the problems outlined above.  For example,

   o  It already has a method for routing requests based on a domain.

   o  It already has an extensible architecture allowing for new
      attributes to be defined and transported.

   o  Pre-existing relationships can be re-used.

   The astute reader will notice that RADIUS and Diameter have
   substantially similar characteristics.  Why not pick one?  RADIUS and
   Diameter are deployed in different environments.  RADIUS can often be
   found in enterprise and university networks, and is also in use by
   fixed network operators.  Diameter, on the other hand, is deployed by
   mobile operators.  Another key difference is that today RADIUS is
   largely transported upon UDP.  We leave as a deployment decision,
   which protocol will be appropriate.  The protocol defines all the
   necessary new AAA attributes as RADIUS attributes.  A future document
   would defined the same AAA attributes for a Diameter environment.  We



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   also note that there exist proxies which convert from RADIUS to
   Diameter and back.  This makes it possible for both to be deployed in
   a single federation substrate.

   Through the integrity protection mechanisms in the AAA framework, the
   identity provider can establish technical trust that messages are
   being sent by the appropriate relying party.  Any given interaction
   will be associated with one federation at the policy level.  The
   legal or business relationship defines what statements the identity
   provider is trusted to make and how these statements are interpreted
   by the relying party.  The AAA framework also permits the relying
   party or elements between the relying party and identity provider to
   make statements about the relying party.

   The AAA framework provides transport for attributes.  Statements made
   about the subject by the identity provider, statements made about the
   relying party and other information are transported as attributes.

   One demand that the AAA substrate makes of the upper layers is that
   they must properly identify the end points of the communication.  It
   must be possible for the AAA client at the RP to determine where to
   send each RADIUS or Diameter message.  Without this requirement, it
   would be the RP's responsibility to determine the identity of the
   client on its own, without the assistance of an IdP.  This
   architecture makes use of the Network Access Identifier (NAI), where
   the IdP is indicated by the realm component [RFC4282].  The NAI is
   represented and consumed by the GSS-API layer as GSS_C_NT_USER_NAME
   as specified in [RFC2743].  The GSS-API EAP mechanism includes the
   NAI in the EAP Response/Identity message.

2.1.2.  Discovery and Rules Determination

   While we are using the AAA protocols to communicate with the IdP, the
   RP may have multiple federation substrates to select from.  The RP
   has a number of criteria that it will use in selecting which of the
   different federations to use:

   o  The federation selected must be able to communicate with the IdP.

   o  The federation selected must match the business rules and
      technical policies required for the RP security requirements.

   The RP needs to discover which federation will be used to contact the
   IdP.  The first selection criteria in discovery is going to be the
   name of the IdP to be contacted.  The second selection criteria in
   discovery is going to be the set of business rules and technical
   policies governing the relationship; this is called rules
   determination.  The RP also needs to establish technical trust in the



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   communications with the IdP.

   Rules determination covers a broad range of decisions about the
   exchange.  One of these is whether the given RP is permitted to talk
   to the IdP using a given federation at all, so rules determination
   encompasses the basic authorization decision.  Other factors are
   included, such as what policies govern release of information about
   the principal to the RP and what policies govern the RP's use of this
   information.  While rules determination is ultimately a business
   function, it has significant impact on the technical exchanges.  The
   protocols need to communicate the result of authorization.  When
   multiple sets of rules are possible, the protocol must disambiguate
   which set of rules are in play.  Some rules have technical
   enforcement mechanisms; for example in some federations
   intermediaries validate information that is being communicated within
   the federation.

   ABFAB has not formally defined any part of discovery at this point.
   The process of specifying and evaluating the business rules and
   technical policies is too complex to provide a simple framework.
   There is not currently a way to know if a AAA proxy is able to
   communicate with a specific IdP, although this may change with some
   of the routing protocols that are being considered.  At the present
   time, the discovery process is going to be a manual configuration
   process.

2.1.3.  Routing and Technical Trust

   Several approaches to having messages routed through the federation
   substrate are possible.  These routing methods can most easily be
   classified based on the mechanism for technical trust that is used.
   The choice of technical trust mechanism constrains how rules
   determination is implemented.  Regardless of what deployment strategy
   is chosen, it is important that the technical trust mechanism be able
   to validate the names of both parties to the exchange.  The trust
   mechanism must to ensure that the entity acting as IdP for a given
   NAI is permitted to be the IdP for that realm, and that any service
   name claimed by the RP is permitted to be claimed by that entity.
   Here are the categories of technical trust determination:

   AAA Proxy:
      The simplest model is that an RP supports a request directly to an
      AAA proxy.  The hop-by-hop integrity protection of the AAA fabric
      provides technical trust.  An RP can submit a request directly to
      a federation.  Alternatively, a federation disambiguation fabric
      can be used.  Such a fabric takes information about what
      federations the RP is part of and what federations the IdP is part
      of and routes a message to the appropriate federation.  The



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      routing of messages across the fabric plus attributes added to
      requests and responses provides rules determination.  For example,
      when a disambiguation fabric routes a message to a given
      federation, that federation's rules are chosen.  Name validation
      is enforced as messages travel across the fabric.  The entities
      near the RP confirm its identity and validate names it claims.
      The fabric routes the message towards the appropriate IdP,
      validating the IdP's name in the process.  The routing can be
      statically configured.  Alternatively a routing protocol could be
      developed to exchange reachability information about given IdPs
      and to apply policy across the AAA fabric.  Such a routing
      protocol could flood naming constraints to the appropriate points
      in the fabric.

   Trust Broker:
      Instead of routing messages through AAA proxies, some trust broker
      could establish keys between entities near the RP and entities
      near the IdP.  The advantage of this approach is efficiency of
      message handling.  Fewer entities are needed to be involved for
      each message.  Security may be improved by sending individual
      messages over fewer hops.  Rules determination involves decisions
      made by trust brokers about what keys to grant.  Also, associated
      with each credential is context about rules and about other
      aspects of technical trust including names that may be claimed.  A
      routing protocol similar to the one for AAA proxies is likely to
      be useful to trust brokers in flooding rules and naming
      constraints.

   Global Credential:
      A global credential such as a public key and certificate in a
      public key infrastructure can be used to establish technical
      trust.  A directory or distributed database such as the Domain
      Name System is used by the RP to discover the endpoint to contact
      for a given NAI.  Either the database or certificates can provide
      a place to store information about rules determination and naming
      constraints.  Provided that no intermediates are required (or
      appear to be required) and that the RP and IdP are sufficient to
      enforce and determine rules, rules determination is reasonably
      simple.  However applying certain rules is likely to be quite
      complex.  For example if multiple sets of rules are possible
      between an IdP and RP, confirming the correct set is used may be
      difficult.  This is particularly true if intermediates are
      involved in making the decision.  Also, to the extent that
      directory information needs to be trusted, rules determination may
      be more complex.

   Real-world deployments are likely to be mixtures of these basic
   approaches.  For example, it will be quite common for an RP to route



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   traffic to a AAA proxy within an organization.  That proxy could then
   use any of the three methods to get closer to the IdP.  It is also
   likely that rather than being directly reachable, the IdP may have a
   proxy on the edge of its organization.  Federations will likely
   provide a traditional AAA proxy interface even if they also provide
   another mechanism for increased efficiency or security.

2.1.4.  SAML Assertions

   For the traditional use of AAA frameworks, network access, the only
   requirement that was necessary to grant access was an affirmative
   response from the IdP.  In the ABFAB world, the RP may need to get
   additional information about the client before granting access.
   ABFAB therefore has a requirement that it can transport an arbitrary
   set of attributes about the client from the IdP to the RP.

   Security Assertions Markup Language (SAML) [OASIS.saml-core-2.0-os]
   was designed in order to carry an extensible set of attributes about
   a subject.  Since SAML is extensible in the attribute space, ABFAB
   has no immediate needs to update the core SAML specifications for our
   work.  It will be necessary to update IdPs that need to return SAML
   assertions to IdPs and for both the IdP and the RP to implement a new
   SAML profile designed to carry SAML assertions in AAA.  The new
   profile can be found in RFCXXXX [I-D.ietf-abfab-aaa-saml].

   There are two issues that need to be highlighted:

   o  The security of SAML assertions.

   o  Namespaces and mapping of SAML attributes.

   SAML assertions have an optional signature that can be used to
   protect and provide origination of the assertion.  These signatures
   are normally based on asymmetric key operations and require that the
   verifier be able to check not only the cryptographic operation, but
   also the binding of the originators name and the public key.  In a
   federated environment it will not always be possible for the RP to
   validate the binding, for this reason the technical trust established
   in the federation is used as an alternate method of validating the
   origination and integrity of the SAML Assertion.

   Attributes placed in SAML assertions can have different namespaces
   assigned to the same name.  In many, but not all, cases the
   federation agreements will determine what attributes can be used in a
   SAML statement.  This means that the RP needs to map from the
   federation names, types and semantics into the ones that the policies
   of the RP are written in.  In other cases the federation substrate
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   namespace, naming and semantic mappings as the assertion crosses the
   different boundaries in the federation.  If the proxies are modifying
   the SAML Assertion, then will obviously remove any signatures on the
   SAML assertions as they would no longer validate.  In this case the
   technical trust is the required mechanism for validating the
   integrity of the assertion.  Finally, the attributes may still be in
   the namespace of the originating IdP.  When this occurs the RP will
   need to get the required mapping operations from the federation
   agreements and do the appropriate mappings itself.

2.2.  Client To Identity Provider

   Looking at the communications between the client and the IdP, the
   following items need to be dealt with:

   o  The client and the IdP need to mutually authenticate each other.

   o  The client and the IdP need to mutually agree on the identity of
      the RP.

   ABFAB selected EAP for the purposes of mutual authentication and
   assisted in creating some new EAP channel binding documents for
   dealing with determining the identity of the RP.  A framework for the
   channel binding mechanism has been defined in RFC 6677 [RFC6677] that
   allows the IdP to check the identity of the RP provided by the AAA
   framework with that provided by the client.

2.2.1.  Extensible Authentication Protocol (EAP)

   Traditional web federation does not describe how a subject interacts
   with an identity provider for authentication.  As a result, this
   communication is not standardized.  There are several disadvantages
   to this approach.  Since the communication is not standardized, it is
   difficult for machines to correctly enter their credentials with
   different authentications, where Individuals can correctly identify
   the entyr mechanism on the fly.  The use of browsers for
   authentication restricts the deployment of more secure forms of
   authentication beyond plaintext username and password known by the
   server.  In a number of cases the authentication interface may be
   presented before the subject has adequately validated they are
   talking to the intended server.  By giving control of the
   authentication interface to a potential attacker, then the security
   of the system may be reduced and phishing opportunities introduced.

   As a result, it is desirable to choose some standardized approach for
   communication between the subject's end-host and the identity
   provider.  There are a number of requirements this approach must
   meet.



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   Experience has taught us one key security and scalability
   requirement: it is important that the relying party not get
   possession of the long-term secret of the client.  Aside from a
   valuable secret being exposed, a synchronization problem can develop
   when the client changes keys with the IdP.

   Since there is no single authentication mechanism that will be used
   everywhere there is another associated requirement: The
   authentication framework must allow for the flexible integration of
   authentication mechanisms.  For instance, some IdPs require hardware
   tokens while others use passwords.  A service provider wants to
   provide support for both authentication methods, and other methods
   from IdPs not yet seen.

   Fortunately, these requirements can be met by utilizing standardized
   and successfully deployed technology, namely by the Extensible
   Authentication Protocol (EAP) framework [RFC3748].  Figure 2
   illustrates the integration graphically.

   EAP is an end-to-end framework; it provides for two-way communication
   between a peer (i.e,service client or principal) through the
   authenticator (i.e., service provider) to the back-end (i.e.,
   identity provider).  Conveniently, this is precisely the
   communication path that is needed for federated identity.  Although
   EAP support is already integrated in AAA systems (see [RFC3579] and
   [RFC4072]) several challenges remain:

   o  The first is how to carry EAP payloads from the end host to the
      relying party.

   o  Another is to verify statements the relying party has made to the
      subject, confirm these statements are consistent with statements
      made to the identity provider and confirm all the above are
      consistent with the federation and any federation-specific policy
      or configuration.

   o  Another challenge is choosing which identity provider to use for
      which service.

   The EAP method used for ABFAB needs to meet the following
   requirements:

   o  It needs to provide mutual authentication of the client and IdP.

   o  It needs to support channel binding.

   As of this writing, the only EAP method that meets these criteria is
   TEAP [I-D.ietf-emu-eap-tunnel-method] either alone (if client



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   certificates are used) or with an inner EAP method that does mutual
   authentication.

2.2.2.  EAP Channel Binding

   EAP channel binding is easily confused with a facility in GSS-API
   also called channel binding.  GSS-API channel binding provides
   protection against man-in-the-middle attacks when GSS-API is used as
   authentication inside some tunnel; it is similar to a facility called
   cryptographic binding in EAP.  See [RFC5056] for a discussion of the
   differences between these two facilities and Section 6.1 for how GSS-
   API channel binding is handled in this mechanism.

   The client knows, in theory, the name of the RP that it attempted to
   connect to, however in the event that an attacker has intercepted the
   protocol, the client and the IdP need to be able to detect this
   situation.  A general overview of the problem along with a
   recommended way to deal with the channel binding issues can be found
   in RFC 6677 [RFC6677].

   Since that document was published, a number of possible attacks were
   found and methods to address these attacks have been outlined in
   [I-D.hartman-emu-mutual-crypto-bind].

2.3.  Client to Relying Party

   The final set of interactions between parties to consider are those
   between the client and the RP.  In some ways this is the most complex
   set since at least part of it is outside the scope of the ABFAB work.
   The interactions between these parties include:

   o  Running the protocol that implements the service that is provided
      by the RP and desired by the client.

   o  Authenticating the client to the RP and the RP to the client.

   o  Providing the necessary security services to the service protocol
      that it needs beyond authentication.

2.3.1.  GSS-API

   One of the remaining layers is responsible for integration of
   federated authentication into the application.  There are a number of
   approaches that applications have adopted for security.  So, there
   may need to be multiple strategies for integration of federated
   authentication into applications.  However, we have started with a
   strategy that provides integration to a large number of application
   protocols.



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   Many applications such as SSH [RFC4462], NFS [RFC2203], DNS [RFC3645]
   and several non-IETF applications support the Generic Security
   Services Application Programming Interface [RFC2743].  Many
   applications such as IMAP, SMTP, XMPP and LDAP support the Simple
   Authentication and Security Layer (SASL) [RFC4422] framework.  These
   two approaches work together nicely: by creating a GSS-API mechanism,
   SASL integration is also addressed.  In effect, using a GSS-API
   mechanism with SASL simply requires placing some headers on the front
   of the mechanism and constraining certain GSS-API options.

   GSS-API is specified in terms of an abstract set of operations which
   can be mapped into a programming language to form an API.  When
   people are first introduced to GSS-API, they focus on it as an API.
   However, from the prospective of authentication for non-web
   applications, GSS-API should be thought of as a protocol not an API.
   It consists of some abstract operations such as the initial context
   exchange, which includes two sub-operations (gss_init_sec_context and
   gss_accept_sec_context).  An application defines which abstract
   operations it is going to use and where messages produced by these
   operations fit into the application architecture.  A GSS-API
   mechanism will define what actual protocol messages result from that
   abstract message for a given abstract operation.  So, since this work
   is focusing on a particular GSS-API mechanism, we generally focus on
   protocol elements rather than the API view of GSS-API.

   The API view has significant value.  Since the abstract operations
   are well defined, the set of information that a mechanism gets from
   the application is well defined.  Also, the set of assumptions the
   application is permitted to make is generally well defined.  As a
   result, an application protocol that supports GSS-API or SASL is very
   likely to be usable with a new approach to authentication including
   this one with no required modifications.  In some cases, support for
   a new authentication mechanism has been added using plugin interfaces
   to applications without the application being modified at all.  Even
   when modifications are required, they can often be limited to
   supporting a new naming and authorization model.  For example, this
   work focuses on privacy; an application that assumes it will always
   obtain an identifier for the principal will need to be modified to
   support anonymity, unlinkability or pseudonymity.

   So, we use GSS-API and SASL because a number of the application
   protocols we wish to federate support these strategies for security
   integration.  What does this mean from a protocol standpoint and how
   does this relate to other layers?  This means we need to design a
   concrete GSS-API mechanism.  We have chosen to use a GSS-API
   mechanism that encapsulates EAP authentication.  So, GSS-API (and
   SASL) encapsulate EAP between the end-host and the service.  The AAA
   framework encapsulates EAP between the relying party and the identity



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   provider.  The GSS-API mechanism includes rules about how principals
   and services are named as well as per-message security and other
   facilities required by the applications we wish to support.

2.3.2.  Protocol Transport

   The transport of data between the client and the relying party is not
   provided by GSS-API.  GSS-API creates and consumes messages, but it
   does not provide the transport itself, instead the protocol using
   GSS-API needs to provide the transport.  In many cases HTTP or HTTPS
   is used for this transport, but other transports are perfectly
   acceptable.  The core GSS-API document [RFC2743] provides some
   details on what requirements exist.

   In addition we highlight the following:

   o  The transport does not need to provide either privacy or
      integrity.  After GSS-EAP has finished negotiation, GSS-API can be
      used to provide both services.  If the negotiation process itself
      needs protection from eavesdroppers then the transport would need
      to provide the necessary services.

   o  The transport needs to provide reliable transport of the messages.

   o  The transport needs to ensure that tokens are delivered in order
      during the negotiation process.

   o  GSS-API messages need to be delivered atomically.  If the
      transport breaks up a message it must also reassemble the message
      before delivery.





















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3.  Application Security Services

   One of the key goals is to integrate federated authentication into
   existing application protocols and where possible, existing
   implementations of these protocols.  Another goal is to perform this
   integration while meeting the best security practices of the
   technologies used to perform the integration.  This section describes
   security services and properties required by the EAP GSS-API
   mechanism in order to meet these goals.  This information could be
   viewed as specific to that mechanism.  However, other future
   application integration strategies are very likely to need similar
   services.  So, it is likely that these services will be expanded
   across application integration strategies if new application
   integration strategies are adopted.

3.1.  Authentication

   GSS-API provides an optional security service called mutual
   authentication.  This service means that in addition to the initiator
   providing (potentially anonymous or pseudonymous) identity to the
   acceptor, the acceptor confirms its identity to the initiator.
   Especially for the ABFAB context, this service is confusingly named.
   We still say that mutual authentication is provided when the identity
   of an acceptor is strongly authenticated to an anonymous initiator.

   RFC 2743, unfortunately, does not explicitly talk about what mutual
   authentication means.  Within this document we therefore define it
   as:

   o  If a target name is supplied to the initiator, then the initiator
      trusts that the supplied target name describes the acceptor.  This
      implies both that appropriate cryptographic exchanges took place
      for the initiator to make such a trust decision, and that after
      evaluating the results of these exchanges, the initiator's policy
      trusts that the target name is accurate.

   o  If no target name is supplied to the initiator, then the initiator
      trusts that the acceptor name, supplied by the acceptor, correctly
      names the entity it is communicating with.

   o  Both the initiator and acceptor have the same key material for
      per-message keys and both parties have confirmed they actually
      have the key material.  In EAP terms, there is a protected
      indication of success.

   Mutual authentication is an important defense against certain aspects
   of phishing.  Intuitively, users would like to assume that if some
   party asks for their credentials as part of authentication,



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   successfully gaining access to the resource means that they are
   talking to the expected party.  Without mutual authentication, the
   acceptor could "grant access" regardless of what credentials are
   supplied.  Mutual authentication better matches this user intuition.

   It is important, therefore, that the GSS-EAP mechanism implement
   mutual authentication.  That is, an initiator needs to be able to
   request mutual authentication.  When mutual authentication is
   requested, only EAP methods capabale of providing the necessary
   service can be used, and appropriate steps need to be taken to
   provide mutual authentication.  A broader set of EAP methods could be
   supported when a particular application does not request mutual
   authentication.  It is an open question whether the mechanism will
   permit this.

   The AAA infrastructure MAY hide the peer's identity from the GSS-API
   acceptor, providing anonymity between the peer and initiator.  At
   this time, whether the identity is disclosed is determined by EAP
   server policy rather than by an indication from the peer.  Also,
   peers are unlikely to be able to determine whether anonymous
   communication will be provided.  For this reason, peers are unlikely
   to set the anonymous return flag from GSS_Init_Sec_context.

3.2.  GSS-API Channel Binding

   [RFC5056] defines a concept of channel binding to prevent man-in-the-
   middle attacks.  It is common to provide SASL and GSS-API with
   another layer to provide transport security; Transport Layer Security
   (TLS) is the most common such layer.  TLS provides its own server
   authentication.  However there are a variety of situations where this
   authentication is not checked for policy or usability reasons.  Even
   when it is checked, if the trust infrastructure behind the TLS
   authentication is different from the trust infrastructure behind the
   GSS-API mutual authentication then confirming the end-points using
   both trust infrastructures is likely to enhance security.  If the
   endpoints of the GSS-API authentication are different than the
   endpoints of the lower layer, this is a strong indication of a
   problem such as a man-in-the-middle attack.  Channel binding provides
   a facility to determine whether these endpoints are the same.

   The GSS-EAP mechanism needs to support channel binding.  When an
   application provides channel binding data, the mechanism needs to
   confirm this is the same on both sides consistent with the GSS-API
   specification.

   Typically when considering channel binding, people think of channel
   binding in combination with mutual authentication.  This is
   sufficiently common that without additional qualification channel



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   binding should be assumed to imply mutual authentication.  Without
   mutual authentication, only one party knows that the endpoints are
   correct.  That's sometimes useful.  Consider for example a user who
   wishes to access a protected resource from a shared whiteboard in a
   conference room.  The whiteboard is the initiator; it does not need
   to actually authenticate that it is talking to the correct resource
   because the user will be able to recognize whether the displayed
   content is correct.  If channel binding were used without mutual
   authentication, it would in effect be a request to only disclose the
   resource in the context of a particular channel.  Such an
   authentication would be similar in concept to a holder-of-key SAML
   assertion.  However, also note that while it is not happening in the
   protocol, mutual authentication is happening in the overall system:
   the user is able to visually authenticate the content.  This is
   consistent with all uses of channel binding without protocol level
   mutual authentication found so far.

   RFC 5056 channel binding (also called GSS-API channel binding when
   GSS-API is involved) is not the same thing as EAP channel binding.
   EAP channel binding is also used in the ABFAB context in order to
   implement acceptor naming and mutual authentication.  Details are
   discussed in the mechanisms specification [I-D.ietf-abfab-gss-eap].

3.3.  Host-Based Service Names

   IETF security mechanisms typically take the name of a service entered
   by a user and make some trust decision about whether the remote party
   in an interaction is the intended party.  GSS-API has a relatively
   flexible naming architecture.  However most of the IETF applications
   that use GSS-API, including SSH, NFS, IMAP, LDAP and XMPP, have
   chosen to use host-based service names when they use GSS-API.  In
   this model, the initiator names an acceptor based on a service such
   as "imap" or "host" (for login services such as SSH) and a host name.

   Using host-based service names leads to a challenging trust
   delegation problem.  Who is allowed to decide whether a particular
   hostname maps to an entity.  The public-key infrastructure (PKI) used
   by the web has chosen to have a number of trust anchors (root
   certificate authorities) each of which can map any name to a public
   key.  A number of GSS-API mechanisms, such as Kerberos [RFC1964],
   split the problem into two parts.  A new concept called a realm is
   introduced.  Then the mechanism decides what realm is responsible for
   a given name.  That realm is responsible for deciding if the acceptor
   entity is allowed to claim the name.  ABFAB needs to adopt this
   approach.

   Host-based service names do not work ideally when different instances
   of a service are running on different ports.  Also, these do not work



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   ideally when SRV record or other insecure referrals are used.

   The GSS-EAP mechanism needs to support host-based service names in
   order to work with existing IETF protocols.

3.4.  Per-Message Tokens

   GSS-API provides per-message security services that can provide
   confidentiality and integrity.  Some IETF protocols such as NFS and
   SSH take advantage of these services.  As a result GSS-EAP needs to
   support these services.  As with mutual authentication, per-message
   services will limit the set of EAP methods that are available.  Any
   EAP method that produces a Master Session Key (MSK) is able to
   support per-message security services described in [X].

   GSS-API provides a pseudo-random function.  While the pseudo-random
   function does not involve sending data over the wire, it provides an
   algorithm that both the initiator and acceptor can run in order to
   arrive at the same key value.  This is useful for designs where a
   successful authentication is used to key some other function.  This
   is similar in concept to the TLS extractor.  No current IETF
   protocols require this.  However GSS-EAP supports this service
   because it is valuable for the future and easy to do given per-
   message services.  Non-IETF protocols are expected to take advantage
   of this in the near future.


























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4.  Future Work: Attribute Providers

   This architecture provides for a federated authentication and
   authorization framework between IdPs, RPs, principals, and subjects.
   It does not at this time provide for a means to retrieve attributes
   from 3rd parties.  However, it envisions such a possibility.  We note
   that in any extension to the model, an attribute provider must be
   authorized to release specific attributes to a specific RP for a
   specific principal.  In addition, we note that it is an open question
   beyond this architecture as to how the RP should know to trust a
   particular attribute provider.

   There are a number of possible technical means to provide attribute
   provider capabilities.  One possible approach is for the IdP to
   provide a signed attribute request to RP that it in turn will provide
   to the attribute authority.  Another approach would be for the IdP to
   provide a URI to the RP that contains a token of some form.  The form
   of communications between the IdP and attribute provider as well as
   other considerations are left for the future.  One thing we can say
   now is that the IdP would merely be asserting who the attribute
   authority is, and not the contents of what the attribute authority
   would return.  (Otherwise, the IdP might as well make the query to
   the attribute authority and then resign it.)




























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5.  Privacy Considerations

   ABFAB, as an architecture designed to enable federated authentication
   and allow for the secure transmission of identity information between
   entities, obviously requires careful consideration around privacy and
   the potential for privacy violations.

   This section examines the privacy related information presented in
   this document, summarising the entities that are involved in ABFAB
   communications and what exposure they have to identity information.
   In discussing these privacy considerations in this section, we use
   terminology and ideas from [I-D.iab-privacy-considerations].

   Note that the ABFAB architecture uses at its core several existing
   technologies and protocols; detailed privacy discussion around these
   is not examined.  This section instead focuses on privacy
   considerations specifically related to overall architecture and usage
   of ABFAB.

5.1.  Entities and their roles

   In an ABFAB environment, there are four distinct types of entities
   involved in communication paths.  Figure 2 shows the ABFAB
   architecture with these entity types.  We have:

   o  The client application: usually a piece of software running on a
      user's device.  This communicates with a service (the Relying
      Party) that the user wishes to interact with.

   o  The Identity Provider: The home AAA server for the user.

   o  The Relying Party: The service the user wishes to connect to.

   o  The federation substrate: A set of entities through which messages
      pass on their path between RP and AAA server.

   As described in detail earlier in this document, when a user wishes
   to access a Relying Party, a secure tunnel is set up between their
   client application and their Identity Provider (via the Relying Party
   and the federation substrate) through which credentials are
   exchanged.  An indication of success or failure, alongside a set of
   AAA attributes about a principal is then passed from the Identity
   Provider to the Relying Party (usually in the form of a SAML
   assertion).







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5.2.  Relationship between user and entities

   o  Between User and Identity Provider - the identity Provider is an
      entity the user will have a direct relationship with, created when
      the organisation that operates the entity provisioned and
      exchanged the user's credentials.  Privacy and data protection
      guarantees may form a part of this relationship.

   o  Between User and Relying Party - the Relying Party is an entity
      the user may or may not have a direct relationship with, depending
      on the service in question.  Some services may only be offered to
      those users where such a direct relationship exists (for
      particularly sensitive services, for example), while some may not
      require this and would instead be satisfied with basic federation
      trust guarantees between themselves and the Identity Provider).
      This may well include the option that the user stays anonymous
      with respect to the Relying Party (though obviously not to the
      Identity Provider).  If attempting to preserve privacy through the
      mitigation of data minimisation, then the only attribute
      information about individuals exposed to the Relying Party should
      be that which is strictly necessary for the operation of the
      service.

   o  Between User and Federation substrate - the user is highly likely
      to have no knowledge of, or relationship with, any entities
      involved with the federation substrate (not that the Identity
      Provider and/or Relying Party may, however).  Knowledge of
      attribute information about individuals for these entities is not
      necessary, and thus such information should be protected in such a
      way as to prevent access to this information from being possible.

5.3.  Data and Identifiers in use

   In the ABFAB architecture, there are a few different types of data
   and identifiers in use.

5.3.1.  NAI

   In order for the Relying Party to be able to route messages to enable
   an EAP transaction to occur between client application and the
   correct identity Provider, it is necessary for the client application
   to provide enough information to the Relying Party to enable the
   identification of the correct Identity Provider.  This takes the form
   of an Network Access Identifier (NAI) (as specified in [RFC4282]).
   Note that an NAI can have inner and outer forms in a AAA
   architecture.





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   o  The outer part of NAI is exposed to the Relying Party; this can
      simply contain realm information.  Doing so (i.e. not including
      user identification details such as a username) minimises the data
      given to the Relying Part to that which is purely necessary to
      support the necessary routing decision.

   o  The inner part of NAI is sent through the secure tunnel as
      established by the EAP protocol; this form of the NAI will contain
      credentials for the user suitable for authenticating them
      successfully (e.g. a username and password).  Since the entire
      purpose of the secure tunnel is to protect communications between
      client application (EAP client) and Identity Provider (EAP
      server), then it is considered secure from eavesdroppers or
      malicious intermediaries and no further privacy discussion is
      necessary.

5.3.2.  Identity Information

   As a part of the ABFAB process, after a successful authentication has
   occurred between client application and Identity Provider, an
   indication of this success is sent to the Relying Party.  Alongside
   this message, information about the user may be returned through AAA
   attributes, usually in form of a SAML assertion.  This information is
   arbitrary and may include either only attributes that prevent an
   individual from being identified by the Relying Party (thus enabling
   anonymous or pseudonymous access) or attributes that contain
   personally identifiable information.

   Depending on the method used, this information carried through AAA
   attributes may or may not be accessible to intermediaries involved in
   communications - e.g. in the case of RADIUS and unencrypted SAML,
   these headers are plain text and could be seen by any observer,
   whereas if using RADSEC or encrypted SAML, these headers are
   protected from observers.  Obviously, where the protection of the
   privacy of an individual is required then this information needs to
   be protected by some appropriate means.

5.3.3.  Accounting Information

   Alongside the core authentication and authorization that occurs in
   AAA communications, accounting information about resource consumption
   may be delivered as part of the accounting exchange during the
   lifetime of the granted application session.

5.3.4.  Collection and retention of data and identifiers

   In cases where Relying Parties do not require to identify a
   particular individual when an individual wishes to make use of their



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   service, the ABFAB architecture enable anonymous or pseudonymous
   access.  Thus data and identifiers other than pseudonyms and
   unlinkable attribute information need not be stored and retained.

   However, in cases where Relying Parties require the ability to
   identify a particular individual (e.g. so they can link this identity
   information to a particular account in their service, or where
   identity information is required for audit purposes), the service
   will need to collect and store such information, and to retain it for
   as long as they require.  Deprovisioning of such accounts and
   information is out of scope for ABFAB, but obviously for privacy
   protection any identifiers collected should be deleted when they are
   no longer needed.

5.4.  User Participation

   In the ABFAB architecture, by its very nature users are active
   participants in the sharing of their identifiers as they initiate the
   communications exchange every time they wish to access a server.
   They are, however, not involved in control of the set of information
   related to them that transmitted from Identity Provider to Relying
   Party for authorisation purposes.





























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6.  Deployment Considerations

6.1.  EAP Channel Binding

   Discuss the implications of needing EAP channel binding.

6.2.  AAA Proxy Behavior

   Discuss deployment implications of our proxy requirements.










































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7.  Security Considerations

   This document describes the architecture for Application Bridging for
   Federated Access Beyond Web (ABFAB) and security is therefore the
   main focus.  This section highlights the main communication channels
   and their security properties:

   Client-to-RP Channel:

      The channel binding material is provided by any certificates and
      the final message (i.e., a cryptographic token for the channel).
      Authentication may be provided by the RP to the client but a
      deployment without authentication at the TLS layer is possible as
      well.  In addition, there is a channel between the GSS requestor
      and the GSS acceptor, but the keying material is provided by a
      "third party" to both entities.  The client can derive keying
      material locally, but the RP gets the material from the IdP.  In
      the absence of a transport that provides encryption and/or
      integrity, the channel between the client and the RP has no
      ability to have any cryptographic protection until the EAP
      authentication has been completed and the MSK is transfered from
      the IdP to the RP.

   RP-to-IdP Channel:

      The security of this communication channel is mainly provided by
      the functionality offered via RADIUS and Diameter.  At the time of
      writing there are no end-to-end security mechanisms standardized
      and thereby the architecture has to rely on hop-by-hop security
      with trusted AAA entities or, as an alternative but possible
      deployment variant, direct communication between the AAA client to
      the AAA server.  Note that the authorization result the IdP
      provides to the RP in the form of a SAML assertion may, however,
      be protected such that the SAML related components are secured
      end-to-end.

      The MSK is transported from the IdP to the RP over this channel.
      As no end-to-end security is provided by AAA, all AAA entities on
      the path between the RP and IdP have the ability to eavesdrop if
      no additional security measures are taken.  One such measure is to
      use a transport between the client and the IdP that provides
      confidentiality.

   Client-to-IdP Channel:

      This communication interaction is accomplished with the help of
      EAP and EAP methods.  The offered security protection will depend
      on the EAP method that is chosen but a minimum requirement is to



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      offer mutual authentication, and key derivation.  The IdP is
      responsible during this process to determine that the RP that is
      communication to the client over the RP-to-IdP channel is the same
      one talking to the IdP.  This is accomplished via the EAP channel
      binding.

   Partial list of issues to be addressed in this section: Privacy,
   SAML, Trust Anchors, EAP Algorithm Selection, Diameter/RADIUS/AAA
   Issues, Naming of Entities, Protection of passwords, Channel Binding,
   End-point-connections (TLS), Proxy problems

   When a psuedonym is generated as a unique long term identifier for a
   Subject by an IdP, care MUST be taken in the algorithm that it cannot
   easily be reverse engineered by the service provider.  If it can be
   reversed then the service provider can consult an oracle to determine
   if a given unique long term identifier is associated with a different
   known identifier.


































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8.  IANA Considerations

   This document does not require actions by IANA.
















































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9.  Acknowledgments

   We would like to thank Mayutan Arumaithurai and Klaas Wierenga for
   their feedback.  Additionally, we would like to thank Eve Maler,
   Nicolas Williams, Bob Morgan, Scott Cantor, Jim Fenton, Paul Leach,
   and Luke Howard for their feedback on the federation terminology
   question.

   Furthermore, we would like to thank Klaas Wierenga for his review of
   the pre-00 draft version.









































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10.  References

10.1.  Normative References

   [RFC2743]  Linn, J., "Generic Security Service Application Program
              Interface Version 2, Update 1", RFC 2743, January 2000.

   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2865, June 2000.

   [RFC3588]  Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
              Arkko, "Diameter Base Protocol", RFC 3588, September 2003.

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, "Extensible Authentication Protocol (EAP)",
              RFC 3748, June 2004.

   [RFC3579]  Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
              Dial In User Service) Support For Extensible
              Authentication Protocol (EAP)", RFC 3579, September 2003.

   [RFC4072]  Eronen, P., Hiller, T., and G. Zorn, "Diameter Extensible
              Authentication Protocol (EAP) Application", RFC 4072,
              August 2005.

   [RFC4282]  Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
              Network Access Identifier", RFC 4282, December 2005.

   [I-D.ietf-abfab-gss-eap]
              Hartman, S. and J. Howlett, "A GSS-API Mechanism for the
              Extensible Authentication Protocol",
              draft-ietf-abfab-gss-eap-09 (work in progress),
              August 2012.

   [I-D.ietf-abfab-aaa-saml]
              Howlett, J. and S. Hartman, "A RADIUS Attribute, Binding
              and Profiles for SAML", draft-ietf-abfab-aaa-saml-04 (work
              in progress), October 2012.

   [RFC6677]  Hartman, S., Clancy, T., and K. Hoeper, "Channel-Binding
              Support for Extensible Authentication Protocol (EAP)
              Methods", RFC 6677, July 2012.

10.2.  Informative References

   [RFC2903]  de Laat, C., Gross, G., Gommans, L., Vollbrecht, J., and
              D. Spence, "Generic AAA Architecture", RFC 2903,



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              August 2000.

   [I-D.nir-tls-eap]
              Nir, Y., Sheffer, Y., Tschofenig, H., and P. Gutmann, "A
              Flexible Authentication Framework for the Transport Layer
              Security (TLS) Protocol using the Extensible
              Authentication Protocol (EAP)", draft-nir-tls-eap-13 (work
              in progress), December 2011.

   [I-D.ietf-oauth-v2]
              Hardt, D., "The OAuth 2.0 Authorization Framework",
              draft-ietf-oauth-v2-31 (work in progress), August 2012.

   [I-D.iab-privacy-considerations]
              Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols",
              draft-iab-privacy-considerations-03 (work in progress),
              July 2012.

   [RFC4017]  Stanley, D., Walker, J., and B. Aboba, "Extensible
              Authentication Protocol (EAP) Method Requirements for
              Wireless LANs", RFC 4017, March 2005.

   [RFC5106]  Tschofenig, H., Kroeselberg, D., Pashalidis, A., Ohba, Y.,
              and F. Bersani, "The Extensible Authentication Protocol-
              Internet Key Exchange Protocol version 2 (EAP-IKEv2)
              Method", RFC 5106, February 2008.

   [RFC1964]  Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
              RFC 1964, June 1996.

   [RFC2203]  Eisler, M., Chiu, A., and L. Ling, "RPCSEC_GSS Protocol
              Specification", RFC 2203, September 1997.

   [RFC3645]  Kwan, S., Garg, P., Gilroy, J., Esibov, L., Westhead, J.,
              and R. Hall, "Generic Security Service Algorithm for
              Secret Key Transaction Authentication for DNS (GSS-TSIG)",
              RFC 3645, October 2003.

   [RFC2138]  Rigney, C., Rigney, C., Rubens, A., Simpson, W., and S.
              Willens, "Remote Authentication Dial In User Service
              (RADIUS)", RFC 2138, April 1997.

   [RFC4462]  Hutzelman, J., Salowey, J., Galbraith, J., and V. Welch,
              "Generic Security Service Application Program Interface
              (GSS-API) Authentication and Key Exchange for the Secure
              Shell (SSH) Protocol", RFC 4462, May 2006.



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   [RFC4422]  Melnikov, A. and K. Zeilenga, "Simple Authentication and
              Security Layer (SASL)", RFC 4422, June 2006.

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

   [RFC5801]  Josefsson, S. and N. Williams, "Using Generic Security
              Service Application Program Interface (GSS-API) Mechanisms
              in Simple Authentication and Security Layer (SASL): The
              GS2 Mechanism Family", RFC 5801, July 2010.

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

   [OASIS.saml-core-2.0-os]
              Cantor, S., Kemp, J., Philpott, R., and E. Maler,
              "Assertions and Protocol for the OASIS Security Assertion
              Markup Language (SAML) V2.0", OASIS Standard saml-core-
              2.0-os, March 2005.

   [RFC2904]  Vollbrecht, J., Calhoun, P., Farrell, S., Gommans, L.,
              Gross, G., de Bruijn, B., de Laat, C., Holdrege, M., and
              D. Spence, "AAA Authorization Framework", RFC 2904,
              August 2000.

   [I-D.hartman-emu-mutual-crypto-bind]
              Hartman, S., Wasserman, M., and D. Zhang, "EAP Mutual
              Cryptographic Binding",
              draft-hartman-emu-mutual-crypto-bind-00 (work in
              progress), March 2012.

   [I-D.ietf-emu-eap-tunnel-method]
              Zhou, H., Cam-Winget, N., Salowey, J., and S. Hanna,
              "Tunnel EAP Method (TEAP) Version 1",
              draft-ietf-emu-eap-tunnel-method-04 (work in progress),
              October 2012.

   [WS-TRUST]
              Lawrence, K., Kaler, C., Nadalin, A., Goodner, M., Gudgin,
              M., Barbir, A., and H. Granqvist, "WS-Trust 1.4", OASIS
              Standard ws-trust-200902, February 2009, <http://
              docs.oasis-open.org/ws-sx/ws-trust/v1.4/ws-trust.html>.









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URIs

   [1]  <http://www.openid.net>

   [2]  <http://www.eduroam.org>














































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Editorial Comments

   [anchor4]  JLS: Should this be an EAP failure to the client as well?

   [anchor7]  JLS: I don't believe this is a true statement - check it
              with Josh and Sam.













































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Authors' Addresses

   Josh Howlett
   JANET(UK)
   Lumen House, Library Avenue, Harwell
   Oxford  OX11 0SG
   UK

   Phone: +44 1235 822363
   Email: Josh.Howlett@ja.net


   Sam Hartman
   Painless Security


   Phone:
   Email: hartmans-ietf@mit.edu


   Hannes Tschofenig
   Nokia Siemens Networks
   Linnoitustie 6
   Espoo  02600
   Finland

   Phone: +358 (50) 4871445
   Email: Hannes.Tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at


   Eliot Lear
   Cisco Systems GmbH
   Richtistrasse 7
   Wallisellen, ZH  CH-8304
   Switzerland

   Phone: +41 44 878 9200
   Email: lear@cisco.com


   Jim Schaad
   Soaring Hawk Consulting

   Email: ietf@augustcellars.com






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