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RFC 7831

Informational
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Application Bridging for Federated Access Beyond Web (ABFAB) Architecture

Part 1 of 3, p. 1 to 15
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Internet Engineering Task Force (IETF)                        J. Howlett
Request for Comments: 7831                                          Jisc
Category: Informational                                       S. Hartman
ISSN: 2070-1721                                        Painless Security
                                                           H. Tschofenig
                                                                ARM Ltd.
                                                               J. Schaad
                                                          August Cellars
                                                                May 2016


      Application Bridging for Federated Access Beyond Web (ABFAB)
                              Architecture

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 access and web-based access.
   However, the solutions to these use cases that have been proposed and
   deployed tend to have few 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), the Generic Security Service
   Application Program Interface (GSS-API), 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 document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7831.

Page 2 
Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

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Table of Contents

   1. Introduction ....................................................4
      1.1. Terminology ................................................5
           1.1.1. Channel Binding .....................................6
      1.2. An Overview of Federation ..................................8
      1.3. Challenges for Contemporary Federation ....................11
      1.4. An Overview of ABFAB-Based Federation .....................11
      1.5. Design Goals ..............................................14
   2. Architecture ...................................................15
      2.1. Relying Party to Identity Provider ........................16
           2.1.1. AAA, RADIUS, and Diameter ..........................17
           2.1.2. Discovery and Rules Determination ..................19
           2.1.3. Routing and Technical Trust ........................20
           2.1.4. AAA Security .......................................21
           2.1.5. SAML Assertions ....................................22
      2.2. Client to Identity Provider ...............................24
           2.2.1. Extensible Authentication Protocol (EAP) ...........24
           2.2.2. EAP Channel Binding ................................26
      2.3. Client to Relying Party ...................................26
           2.3.1. GSS-API ............................................27
           2.3.2. Protocol Transport .................................28
           2.3.3. Re-authentication ..................................29
   3. Application Security Services ..................................29
      3.1. Authentication ............................................29
      3.2. GSS-API Channel Binding ...................................31
      3.3. Host-Based Service Names ..................................32
      3.4. Additional GSS-API Services ...............................33
   4. Privacy Considerations .........................................34
      4.1. Entities and Their Roles ..................................35
      4.2. Privacy Aspects of ABFAB Communication Flows ..............36
           4.2.1. Client to RP .......................................36
           4.2.2. Client to IdP (via Federation Substrate) ...........37
           4.2.3. IdP to RP (via Federation Substrate) ...............38
      4.3. Relationship between User and Entities ....................39
      4.4. Accounting Information ....................................39
      4.5. Collection and Retention of Data and Identifiers ..........39
      4.6. User Participation ........................................40
   5. Security Considerations ........................................40
   6. References .....................................................41
      6.1. Normative References ......................................41
      6.2. Informative References ....................................42
   Acknowledgments ...................................................46
   Authors' Addresses ................................................46

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1.  Introduction

   Numerous security mechanisms have been deployed on the Internet to
   manage access to various resources.  These mechanisms have been
   generalized and scaled over the last decade through mechanisms such
   as the Simple Authentication and Security Layer (SASL) with the
   Generic Security Server Application Program Interface (GSS-API)
   (known as the GS2 family) [RFC5801]; the 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 [RFC6733].

   A Relying Party (RP) is the entity that manages access to some
   resource.  The entity 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 entities.
   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 entity 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
   (http://www.openid.net), OAuth [RFC6749], 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 organization that already has a long-term
      relationship with the client.  This is often attractive, as RPs
      frequently do not want these responsibilities.  The client also
      requires fewer credentials, which is also desirable.

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   Data minimization and user participation:

      Often, an RP does not need to know the identity of a client to
      reach an access management decision.  It is frequently only
      necessary for the RP to know specific attributes about the client
      -- for example, that the client is affiliated with a particular
      organization or has a certain role or entitlement.  Sometimes, the
      RP only needs to know a pseudonym of the client.

      Prior to the release of attributes to the RP 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, an RP needs, or would like, to know more about a client
      than an affiliation or a pseudonym.  For example, an RP may want
      the client'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 web (ABFAB) architecture.  This architecture addresses the
   problem of federated access management primarily for non-web-based
   services.  This architecture makes use of extensions to the commonly
   used security mechanisms for both federated and non-federated access
   management, including RADIUS, the Generic Security Service (GSS), the
   Extensible Authentication Protocol (EAP), and SAML.  The architecture
   should be extended to use Diameter in the future.  It does so in a
   manner that is designed to scale to large numbers of IdPs, RPs, and
   federations.

1.1.  Terminology

   This document uses identity management and privacy terminology from
   [RFC6973].  In particular, this document uses the terms
   "identity provider", "relying party", "identifier", "pseudonymity",
   "unlinkability", and "anonymity".

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

   This document uses the term "Network Access Identifier" (NAI) as
   defined in [RFC7542].  An NAI consists of a realm identifier, which
   is associated with a AAA server, and thus an IdP and a username, that
   are associated with a specific client of the IdP.

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   One of the problems some people have found with reading this document
   is that the terminology sometimes appears to be inconsistent.  This
   is because the various standards that we refer to use different terms
   for the same concept.  In general, this document uses either the
   ABFAB term or the term associated with the standard under discussion,
   as appropriate.  For reference, we include Table 1 below, which
   provides a mapping for these different terms.  (Note that items
   marked "N/A" (not applicable) indicate that there is no name that
   represents the entity.)

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

                           Table 1: Terminology

1.1.1.  Channel Binding

   This document uses the term "channel binding" in two different
   contexts; this term has a different meaning in each of these
   contexts.

   EAP channel binding is used to implement GSS-API naming semantics.
   EAP channel binding sends a set of attributes from the peer to the
   EAP server either as part of the EAP conversation or as part of a
   secure association protocol.  In addition, attributes are sent in the
   back-end protocol from the EAP authenticator to the EAP server.  The

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   EAP server confirms the consistency of these attributes and provides
   the confirmation back to the peer.  In this document, channel binding
   without qualification refers to EAP channel binding.

   GSS-API channel binding provides protection against man-in-the-middle
   attacks when GSS-API is used for authentication inside of some
   tunnel; it is similar to a facility called "cryptographic binding" in
   EAP.  The binding works by each side deriving a cryptographic value
   from the tunnel itself and then using that cryptographic value to
   prove to the other side that it knows the value.

   See [RFC5056] for a discussion of the differences between these two
   facilities.  These differences can be summarized as follows:

   o  GSS-API channel binding specifies that there is nobody between the
      client and the EAP authenticator.

   o  EAP channel binding allows the client to have knowledge of such
      EAP authenticator attributes as the EAP authenticator's name.

   Typically, when considering both EAP and GSS-API channel binding,
   people think of channel binding in combination with mutual
   authentication.  This is sufficiently common that, without additional
   qualification, channel binding should be assumed to imply mutual
   authentication.  In GSS-API, without mutual authentication, only the
   acceptor has authenticated the initiator.  Similarly, in EAP, only
   the EAP server has authenticated the peer.  Sometimes, one-way
   authentication is useful.  Consider, for example, a user who wishes
   to access a protected resource for a shared whiteboard in a
   conference room.  The whiteboard is the acceptor; it knows that the
   initiator is authorized to give it a presentation, and the user can
   validate that the whiteboard got the correct presentation by visual
   means.  (The presentation should not be confidential in this case.)
   If channel binding is used without mutual authentication, it is
   effectively a request to 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, note also that
   although 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.

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1.2.  An Overview of Federation

   In the previous section, we introduced the following entities:

   o  the client,

   o  the IdP, and

   o  the RP.

   The final entity that needs to be introduced is the Individual.  An
   Individual is a human being that is using the client.  In any given
   situation, an Individual may or may not exist.  Clients can act as
   front ends for Individuals, or clients may be independent entities
   that are set up and allowed to run autonomously.  An example of such
   an independent entity can be found in the Trust Router Protocol
   (https://www.ietf.org/proceedings/86/slides/slides-86-rtgarea-0.pdf),
   where the routers use ABFAB to authenticate to each other.

   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

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   The relationships between the entities in Figure 1 are as follows:

   Federation

      The IdP and the RPs are part of a federation.  The relationship
      may be direct (they have an explicit trust relationship) or
      transitive (the trust relationship is mediated by one or more
      entities).  The federation relationship is governed by a
      federation agreement.  Within a single federation, there may be
      multiple IdPs as well as multiple RPs.

   Authentication

      There is a direct relationship between the client and the IdP.
      This relationship provides the means by which they trust each
      other and can securely authenticate each other.

   A federation agreement typically encompasses operational
   specifications and legal rules:

   Operational Specifications:

      The goal of operational specifications is to provide enough
      definition that the system works and interoperability is possible.
      These include 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.

   Legal Rules:

      The legal rules take the legal framework into consideration and
      provide 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, and 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.

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   The operational specifications can demand the usage of a specific
   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 on
   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 between 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.
   Within the framework of ABFAB, it will generally be required that a
   mechanism exist 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.

   The nature of federation dictates that there exists some form of
   relationship between the IdP and the RP.  This is particularly
   important when the RP wants to use information obtained from the IdP
   for access management decisions and when the IdP does not want to
   release information to every RP (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 multilateral 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 the time of employment
   within an organization).  When dealing with Individuals, this process
   is called "identity proofing" [NIST-SP.800-63-2].  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 stipulate the appropriate use of credentials and so forth).

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   The nature and quality of the relationship between the client and the
   IdP are important contributors to the level of trust that an RP may
   assign to an assertion describing a client made by an IdP.  This is
   sometimes described as the level of assurance [NIST-SP.800-63-2].

   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 or the possibility that the RP and
   client 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 federated IdPs and RPs (services) proliferate, the role of an
   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 their role upon access.

   Similarly, as federations proliferate, it becomes increasingly
   difficult to discover which IdP(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, SMTP and IMAP do not
   have the ability for the server to request information from the
   client, beyond the client NAI, that the server would then use to
   decide between the multiple federations it is 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
   the application of access management within the federation.  This
   section provides a brief overview of ABFAB in the context of this
   model.

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   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 architecture (a
   graphical view of the steps can be found in Figure 2):

   1.   Client configuration: The client 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 client is configured to
        use the GSS-EAP GSS-API mechanism for authentication/
        authorization.

   3.   Client provides an NAI to RP: The client sets up a transport to
        the RP and begins GSS-EAP authentication.  In response, the RP
        sends an EAP request message (nested in GSS-EAP) 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 its
        full NAI.

   4.   Discovery of federated IdP: The RP uses preconfigured
        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 RP to IdP: Once the RP knows who the IdP is, it (or
        its agent) will send a RADIUS request to the IdP.  The RADIUS
        Access-Request encapsulates the EAP response.  At this stage,
        the RP will likely have no idea who the client is.  The RP sends
        its identity to the IdP in AAA attributes, and it may send a
        SAML request in a AAA attribute.  The AAA network checks to see
        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 used.  The IdP should not
        re-request the client's name in this message, but clients need
        to be able to handle it.  In this case, the IdP must accept a
        realm only in order to protect the client's name from the RP.
        The available and appropriate methods are discussed below
        (Section 2.2.1).

   7.   EAP 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 depend on the EAP method selected.  If
        the IdP is unable to authenticate the client, the IdP sends an

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        EAP failure message to the RP.  As part of the EAP method, the
        client sends an EAP channel-binding 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 against the data 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 client 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
        regarding 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 attributes, if any,
        will be released to the RP.  If the IdP gets a policy failure,
        it sends an EAP failure message to the RP and client.  (The RP
        will have done its policy checks during the discovery process.)

   10.  IdP provides the RP with the MSK: The IdP sends a success 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
        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 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 using the EAP MSK value.

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        Relying         Client         Identity
        Party                          Provider

        |              (1)             | Client configuration
        |               |              |
        |<-----(2)----->|              | Mechanism selection
        |               |              |
        |<-----(3)-----<|              | NAI transmitted to RP
        |               |              |
        |<=====(4)====================>| IdP 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)====================<| Results to RP
        |               |              |
      (11)              |              | RP processes results
        |               |              |
        |>----(12)----->|              | Results to client

        Legend:

          -----: Between client and RP
          =====: Between RP and IdP
          - - -: Between client and IdP (via RP)

                   Figure 2: ABFAB Authentication Steps

1.5.  Design Goals

   Our key design goals are as follows:

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

   o  The means of authentication is decoupled from the application
      protocol so as to allow for multiple authentication methods with
      minimal changes to the application.

   o  The architecture requires no sharing of long-term private keys
      between clients and RPs.

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   o  The system will scale to large numbers of IdPs, RPs, 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 difficult and fraught with the risk of cryptographic flaws.
   Achieving widespread 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 growing trend
   to layer 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.



(page 15 continued on part 2)

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