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rfc:rfc7831

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.

Howlett, et al. Informational [Page 1] RFC 7831 ABFAB Architecture May 2016

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.

Howlett, et al. Informational [Page 2] RFC 7831 ABFAB Architecture May 2016

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

Howlett, et al. Informational [Page 3] RFC 7831 ABFAB Architecture May 2016

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.

Howlett, et al. Informational [Page 4] RFC 7831 ABFAB Architecture May 2016

 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.

Howlett, et al. Informational [Page 5] RFC 7831 ABFAB Architecture May 2016

 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

Howlett, et al. Informational [Page 6] RFC 7831 ABFAB Architecture May 2016

 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.

Howlett, et al. Informational [Page 7] RFC 7831 ABFAB Architecture May 2016

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

Howlett, et al. Informational [Page 8] RFC 7831 ABFAB Architecture May 2016

 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.

Howlett, et al. Informational [Page 9] RFC 7831 ABFAB Architecture May 2016

 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).

Howlett, et al. Informational [Page 10] RFC 7831 ABFAB Architecture May 2016

 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.

Howlett, et al. Informational [Page 11] RFC 7831 ABFAB Architecture May 2016

 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

Howlett, et al. Informational [Page 12] RFC 7831 ABFAB Architecture May 2016

      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.

Howlett, et al. Informational [Page 13] RFC 7831 ABFAB Architecture May 2016

      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:
  1. —-: Between client and RP

=====: Between RP and IdP

  1. - -: 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.

Howlett, et al. Informational [Page 14] RFC 7831 ABFAB Architecture May 2016

 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.

2. Architecture

 We have already introduced the federated access architecture, with
 the illustration of the different actors that need to interact.  This
 section expands on the specifics of providing support for
 non-web-based applications and provides motivations for design
 decisions.  The main theme of the work described in this document is
 focused on reusing existing building blocks that have been deployed
 already and to rearrange them in a novel way.
 Although this architecture assumes updates to the RP, the client, and
 the IdP, those changes are kept at a minimum.  A mechanism that can
 demonstrate deployment benefits (based on ease of updates to 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 to encapsulate EAP into an application
 protocol.  For ease of integration with a wide range of non-web-based
 application protocols, GSS-API was chosen.  The technical
 specification of GSS-EAP can be found in [RFC7055].
 The architecture consists of several building blocks, as shown
 graphically in Figure 3.  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.

Howlett, et al. Informational [Page 15] RFC 7831 ABFAB Architecture May 2016

                                  +--------------+
                                  |   Identity   |
                                  |   Provider   |
                                  |    (IdP)     |
                                  +-^----------^-+
                                    * EAP      o RADIUS
                                    *          o
                                  --v----------v--
                               ///                \\\
                             //                      \\
                            |        Federation        |
                            |        Substrate         |
                             \\                      //
                               \\\                ///
                                  --^----------^--
                                    * EAP      o RADIUS
                                    *          o
 +-------------+                  +-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 3: ABFAB Protocol Instantiation

2.1. Relying Party to Identity Provider

 Communication between the RP and the IdP 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.
 o  Conveying authentication packets from the client to the IdP
    and back.

Howlett, et al. Informational [Page 16] RFC 7831 ABFAB Architecture May 2016

 o  Providing the means of establishing a trust relationship between
    the RP and the client.
 o  Providing a means for the RP to obtain attributes about the client
    from the IdP.
 The ABFAB working group has chosen the AAA framework for the messages
 transported between the RP and IdP.  The AAA framework supports the
 requirements stated above, as follows:
 o  The AAA backbone supplies the trust relationship between the RP
    and the IdP.
 o  The agreements governing a specific AAA backbone contain the rules
    governing the relationships within the AAA federation.
 o  A method exists for carrying EAP packets within RADIUS [RFC3579]
    and Diameter [RFC4072].
 o  The use of EAP channel binding [RFC6677] along with the core ABFAB
    protocol provide the pieces necessary to establish the identities
    of the RP and the client, while EAP provides the cryptographic
    methods for the RP and the client to validate that they are
    talking to each other.
 o  A method exists for carrying SAML packets within RADIUS [RFC7833];
    this method allows the RP to query attributes about the client
    from the IdP.
 Protocols that support the same framework but do different routing
 are expected to be defined and used in the future.  One such effort,
 called the Trust Router, is to set up a framework that creates a
 trusted point-to-point channel on the fly
 (https://www.ietf.org/proceedings/86/slides/slides-86-rtgarea-0.pdf).

2.1.1. AAA, RADIUS, and Diameter

 The usage of the AAA framework with RADIUS [RFC2865] and Diameter
 [RFC6733] for network access authentication has been 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
 clearinghouses).  In the case of network access authentication, the
 front end, i.e., the communication path between the end host and the
 AAA client, is offered by link-layer protocols that forward

Howlett, et al. Informational [Page 17] RFC 7831 ABFAB Architecture May 2016

 authentication protocol exchanges back and forth.  An example of a
 large-scale RADIUS-based federation is eduroam
 (https://www.eduroam.org).
 By using the AAA framework, ABFAB can be built on the federation
 agreements that already exist; the agreements can then merely be
 expanded to cover the ABFAB architecture.  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 reused.
 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; RADIUS is also used by
 operators of fixed networks.  Diameter, on the other hand, is
 deployed by operators of mobile networks.  Another key difference is
 that today RADIUS is largely transported over UDP.  The decision
 regarding which protocol will be appropriate to deploy is left to
 implementers.  The protocol defines all the necessary new AAA
 attributes as RADIUS attributes.  A future document could define the
 same AAA attributes for a Diameter environment.  We also note that
 there exist proxies that 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
 IdP can establish technical trust that messages are being sent by the
 appropriate RP.  Any given interaction will be associated with one
 federation at the policy level.  The legal or business relationship
 defines what statements the IdP is trusted to make and how these
 statements are interpreted by the RP.  The AAA framework also permits
 the RP or elements between the RP and IdP to make statements about
 the RP.
 The AAA framework provides transport for attributes.  Statements made
 about the client by the IdP, statements made about the RP, 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 endpoints 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

Howlett, et al. Informational [Page 18] RFC 7831 ABFAB Architecture May 2016

 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 [RFC7542].  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.
 At the time of this writing, no profiles for the use of Diameter have
 been created.

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.  The federation selected must
 o  be able to communicate with the IdP.
 o  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 criterion used during discovery is going to
 be the name of the IdP to be contacted.  The second selection
 criterion used during 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 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 client to the RP and what policies govern the RP's use of this
 information.  While rules determination is ultimately a business
 function, it has a 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.

Howlett, et al. Informational [Page 19] RFC 7831 ABFAB Architecture May 2016

 At the time of this writing, no protocol mechanism has been specified
 to allow a AAA client to determine whether a AAA proxy will indeed be
 able to route AAA requests to a specific IdP.  The AAA routing is
 impacted by business rules and technical policies that may be quite
 complex; at the present time, the route selection is based on manual
 configuration.

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 identities of both parties to the exchange.  The
 trust mechanism must ensure that the entity acting as the 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 is a AAA client and can send the
    request directly to a AAA proxy.  The hop-by-hop integrity
    protection of the AAA fabric provides technical trust.  An RP can
    submit a request directly to the correct 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 it routes a
    message to the appropriate federation.  The routing of messages
    across the fabric, plus attributes added to requests and
    responses, together provide 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 name of the IdP in the process.  The routing can be
    statically configured.  Alternatively, a routing protocol could be
    developed to exchange reachability information about a given IdP
    and to apply policy across the AAA fabric.  Such a routing
    protocol could flood naming constraints to the appropriate points
    in the fabric.

Howlett, et al. Informational [Page 20] RFC 7831 ABFAB Architecture May 2016

 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 that 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
 traffic to a AAA proxy within an organization.  That proxy could then
 use any of the above 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. AAA Security

 For the AAA framework, there are two different places where security
 needs to be examined.  The first is the security that is in place for
 the links in the AAA backbone being used.  The second are the nodes
 that form the AAA backbone.

Howlett, et al. Informational [Page 21] RFC 7831 ABFAB Architecture May 2016

 The default link security for RADIUS is showing its age, as it uses
 MD5 and a shared secret to both obfuscate passwords and provide
 integrity on the RADIUS messages.  While some EAP methods include the
 ability to protect the client authentication credentials, the MSK
 returned from the IdP to the RP is protected only by RADIUS security.
 In many environments, this is considered to be insufficient,
 especially as not all attributes are obfuscated and can thus leak
 information to a passive eavesdropper.  The use of RADIUS with
 Transport Layer Security (TLS) [RFC6614] and/or Datagram Transport
 Layer Security (DTLS) [RFC7360] addresses these attacks.  The same
 level of security is included in the base Diameter specifications.

2.1.5. SAML Assertions

 For the traditional use of AAA frameworks, i.e., granting access to a
 network, an affirmative response from the IdP is sufficient.  In the
 ABFAB world, the RP may need to get significantly more 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.
 The Security Assertion 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 RPs 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 [RFC7833].  As
 SAML statements will frequently be large, RADIUS servers and clients
 that deal with SAML statements will need to implement [RFC7499].
 There are several issues that need to be highlighted:
 o  The security of SAML Assertions.
 o  Namespaces and mapping of SAML attributes.
 o  Subject naming of entities.
 o  Making multiple queries about the subject(s).
 o  Level of assurance for authentication.
 SAML Assertions have an optional signature that can be used to
 protect and provide the 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

Howlett, et al. Informational [Page 22] RFC 7831 ABFAB Architecture May 2016

 operation but also the binding of the originator's 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 in a SAML Assertion are identified by a name string.  The
 name string is either assigned by the SAML issuer context or scoped
 by a namespace (for example, a URI or object identifier (OID)).  This
 means that the same attribute can have different name strings used to
 identify it.  In many cases, but not all, the federation agreements
 will determine what attributes and names can be used in a SAML
 statement.  This means that the RP needs to map from the SAML issuer
 or federation name, type, and semantic to the name, type, and
 semantics that the policies of the RP are written in.  In other
 cases, the Federation Substrate, in the form of proxies, will modify
 the SAML Assertions in transit to do the necessary name, type, and
 value mappings as the assertion crosses boundaries in the federation.
 If the proxies are modifying the SAML Assertion, then they will
 remove any signatures on the SAML Assertion, as changing the content
 of the SAML Assertion would invalidate the signature.  In this case,
 the technical trust is the required mechanism for validating the
 integrity of the assertion.  (The proxy could re-sign the SAML
 Assertion, but the same issues of establishing trust in the proxy
 would still exist.)  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.
 [RFC7833] has defined a new SAML name format that corresponds to the
 NAI name form defined by [RFC7542].  This allows for easy name
 matching in many cases, as the name form in the SAML statement and
 the name form used in RADIUS or Diameter will be the same.  In
 addition to the NAI name form, [RFC7833] also defines a pair of
 implicit name forms corresponding to the client and the client's
 machine.  These implicit name forms are based on the Identity-Type
 enumeration defined in the Tunnel Extensible Authentication Protocol
 (TEAP) specification [RFC7170].  If the name form returned in a SAML
 statement is not based on the NAI, then it is a requirement on the
 EAP server that it validate that the subject of the SAML Assertion,
 if any, is equivalent to the subject identified by the NAI used in
 the RADIUS or Diameter session.
 RADIUS has the ability to deal with multiple SAML queries for those
 EAP servers that follow [RFC5080].  In this case, a State attribute
 will always be returned with the Access-Accept.  The EAP client can
 then send a new Access-Request with the State attribute and the new

Howlett, et al. Informational [Page 23] RFC 7831 ABFAB Architecture May 2016

 SAML request.  Multiple SAML queries can then be done by making a new
 Access-Request, using the State attribute returned in the last
 Access-Accept to link together the different RADIUS sessions.
 Some RPs need to ensure that specific criteria are met during the
 authentication process.  This need is met by using levels of
 assurance.  A level of assurance is communicated to the RP from the
 EAP server by using a SAML Authentication Request, using the
 Authentication Profile described in [RFC7833].  When crossing
 boundaries between different federations, (1) the policy specified
 will need to be shared between the two federations, (2) the policy
 will need to be mapped by the proxy server on the boundary, or
 (3) the proxy server on the boundary will need to supply information
 to the EAP server so that the EAP server can do the required mapping.
 If this mapping is not done, then the EAP server will not be able to
 enforce the desired level of assurance, as it will not understand the
 policy requirements.

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 [RFC6677] that allows
 the IdP to check the identity of the RP provided by the AAA framework
 against the identity provided by the client.

2.2.1. Extensible Authentication Protocol (EAP)

 Traditional web federation does not describe how a client interacts
 with an IdP 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 recognize which entity is going to do the authentication,
 and thus which credentials to use and where in the authentication
 form the credentials are to be entered.  It is much easier for humans
 to correctly deal with these problems.  The use of browsers for
 authentication restricts the deployment of more secure forms of
 authentication beyond plaintext usernames and passwords known by the
 server.  In a number of cases, the authentication interface may be

Howlett, et al. Informational [Page 24] RFC 7831 ABFAB Architecture May 2016

 presented before the client has adequately validated that they are
 talking to the intended server.  By giving control of the
 authentication interface to a potential attacker, the security of the
 system may be reduced, and opportunities for phishing may be
 introduced.
 As a result, it is desirable to choose some standardized approach for
 communication between the client's end host and the IdP.  There are a
 number of requirements this approach must meet, as noted below.
 Experience has taught us one key security and scalability
 requirement: it is important that the RP 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, another associated requirement is that 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 also for other methods from IdPs
 not yet seen.
 These requirements can be met by utilizing standardized and
 successfully deployed technology, namely the EAP framework [RFC3748].
 Figure 3 illustrates the integration graphically.
 EAP is an end-to-end framework; it provides for two-way communication
 between a peer (i.e., client or Individual) through the EAP
 authenticator (i.e., RP) to the back end (i.e., IdP).  This is
 precisely -- and conveniently -- 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 RP.
 o  Another is to verify statements the RP has made to the client,
    confirm that these statements are consistent with statements made
    to the IdP, and confirm that all of the above are consistent with
    the federation and any federation-specific policy or
    configuration.
 o  Another challenge is choosing which IdP to use for which service.

Howlett, et al. Informational [Page 25] RFC 7831 ABFAB Architecture May 2016

 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 [RFC7170], either alone (if client 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
 that is also called "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.  See [RFC5056] for a
 discussion of the differences between these two facilities.
 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 [RFC6677].
 Since the time that [RFC6677] was published, a number of possible
 attacks were found.  Methods to address these attacks have been
 outlined in [RFC7029].

2.3. Client to Relying Party

 The final set of interactions between the 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.
 o  Dealing with client re-authentication where desired.

Howlett, et al. Informational [Page 26] RFC 7831 ABFAB Architecture May 2016

2.3.1. GSS-API

 One of the remaining layers is responsible for integration of
 federated authentication with the application.  Applications have
 adopted a number of approaches for providing security, so multiple
 strategies for integration of federated authentication with
 applications may be needed.  To this end, we start with a strategy
 that provides integration with a large number of application
 protocols.
 Many applications, such as Secure Shell (SSH) [RFC4462], NFS
 [RFC7530], DNS [RFC3645], and several non-IETF applications, support
 GSS-API [RFC2743].  Many applications, such as IMAP, SMTP, the
 Extensible Messaging and Presence Protocol (XMPP), and the
 Lightweight Directory Access Protocol (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 before the
 mechanism's messages and constraining certain GSS-API options.
 GSS-API is specified in terms of an abstract set of operations that
 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 perspective of authentication for non-web
 applications, GSS-API should be thought of as a protocol as well as
 an API.  When looked at as a protocol, it consists of abstract
 operations such as the initial context exchange, which includes two
 sub-operations (GSS_Init_sec_context and GSS_Accept_sec_context)
 [RFC2743].  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 of GSS-API does have significant value as well; since
 the abstract operations are well defined, the 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 the authentication mechanism defined in
 this document, 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

Howlett, et al. Informational [Page 27] RFC 7831 ABFAB Architecture May 2016

 often be limited to supporting a new naming and authorization model.
 For example, this work focuses on privacy; an application that
 assumes that it will always obtain an identifier for the client 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 that 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) encapsulates EAP between the end host and the service.  The AAA
 framework encapsulates EAP between the RP and the IdP.  The GSS-API
 mechanism includes rules about how initiators 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 RP 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 confidentiality 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.

Howlett, et al. Informational [Page 28] RFC 7831 ABFAB Architecture May 2016

2.3.3. Re-authentication

 There are circumstances where the RP will want to have the client
 re-authenticate itself.  These include very long sessions, where the
 original authentication is time limited or cases where in order to
 complete an operation a different authentication is required.
 GSS-EAP does not have any mechanism for the server to initiate a
 re-authentication, as all authentication operations start from the
 client.  If a protocol using GSS-EAP needs to support
 re-authentication that is initiated by the server, then a request
 from the server to the client for the re-authentication to start
 needs to be placed in the protocol.
 Clients can reuse the existing secure connection established by
 GSS-API, and run the new authentication in that connection, by
 calling GSS_Init_sec_context.  At this point, a full
 re-authentication will be done.

3. Application Security Services

 One of the key goals is to integrate federated authentication with
 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.
 In the context of ABFAB in particular, the naming of this service is
 confusing.  We still say that mutual authentication is provided when
 the identity of an acceptor is strongly authenticated to an anonymous
 initiator.

Howlett, et al. Informational [Page 29] RFC 7831 ABFAB Architecture May 2016

 Unfortunately, [RFC2743] does not explicitly talk about what mutual
 authentication means.  Within this document, we therefore define
 mutual authentication as follows:
 o  If a target name is configured for the initiator, then the
    initiator trusts that the supplied target name describes the
    acceptor.  This implies that (1) appropriate cryptographic
    exchanges took place for the initiator to make such a trust
    decision and (2) after evaluating the results of these exchanges,
    the initiator's policy trusts that the target name is accurate.
 o  If no target name is configured for 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 that 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, clients would like to assume that if some
 party asks for their credentials as part of authentication,
 successfully gaining access to the resource means that they are
 talking to the expected party.  Without mutual authentication, the
 server 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 capable of providing the necessary
 service can be used, and appropriate steps need to be taken to
 provide mutual authentication.  While a broader set of EAP methods
 could be supported by not requiring mutual authentication, it was
 decided that the client needs to always have the ability to request
 it.  In some cases, the IdP and the RP will not support mutual
 authentication; however, the client will always be able to detect
 this and make an appropriate security decision.
 The AAA infrastructure may hide the initiator's identity from the
 GSS-API acceptor, providing anonymity between the initiator and the
 acceptor.  At this time, whether the identity is disclosed is
 determined by EAP server policy rather than by an indication from the
 initiator.  Also, initiators are unlikely to be able to determine
 whether anonymous communication will be provided.  For this reason,
 initiators are unlikely to set the anonymous return flag from
 GSS_Init_sec_context (Section 2.2.1 of [RFC2743]).

Howlett, et al. Informational [Page 30] RFC 7831 ABFAB Architecture May 2016

3.2. GSS-API Channel Binding

 [RFC5056] defines a concept of channel binding that is used to
 prevent man-in-the-middle attacks.  This type of channel binding
 works by taking a cryptographic value from the transport security
 layer and checks to see that both sides of the GSS-API conversation
 know this value.  Transport Layer Security (TLS) [RFC5246] is the
 most common transport security layer used for this purpose.
 It needs to be stressed that channel binding as described in
 [RFC5056] (also called "GSS-API channel binding" when GSS-API is
 involved) is not the same thing as EAP channel binding.  GSS-API
 channel binding is used for detecting man-in-the-middle attacks.  EAP
 channel binding is used for mutual authentication and acceptor naming
 checks.  See [RFC7055] for details.  A more detailed description of
 the differences between the facilities can be found in [RFC5056].
 The use of TLS can provide both encryption and integrity on the
 channel.  It is common to provide SASL and GSS-API with these other
 security services.
 One of the benefits that the use of TLS provides is that a client has
 the ability to validate the name of the server.  However, this
 validation is predicated on a couple of things.  The TLS session
 needs to be using certificates and not be an anonymous session.  The
 client and the TLS server need to share a common trust point for the
 certificate used in validating the server.  TLS provides its own
 server authentication.  However, there are a variety of situations
 where, for policy or usability reasons, this authentication is not
 checked.  When the TLS authentication 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 endpoints 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 that this is the same on both sides, consistent with the
 GSS-API specification.

Howlett, et al. Informational [Page 31] RFC 7831 ABFAB Architecture May 2016

3.3. Host-Based Service Names

 IETF security mechanisms typically take a host name and perhaps a
 service, entered by a user, and make some trust decision about
 whether the remote party in the interaction is the intended party.
 This decision can be made via the use of certificates, preconfigured
 key information, or a previous leap of trust.  GSS-API has defined a
 relatively flexible naming convention; however, most of the IETF
 applications that use GSS-API (including SSH, NFS, IMAP, LDAP, and
 XMPP) have chosen to use a more restricted naming convention based on
 the host name.  The GSS-EAP mechanism needs to support host-based
 service names in order to work with existing IETF protocols.
 The use of host-based service names leads to a challenging trust
 delegation problem.  Who is allowed to decide whether a particular
 host name maps to a specific entity?  Possible solutions to this
 problem have been looked at.
 o  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 host name to a public key.
 o  A number of GSS-API mechanisms, such as Kerberos [RFC1964], have
    split the problem into two parts.  [RFC1964] introduced a new
    concept called a realm; the realm is responsible for host mapping
    within itself.  The mechanism then decides what realm is
    responsible for a given name.  This is the approach adopted by
    ABFAB.
 GSS-EAP defines a host naming convention that takes into account the
 host name, the realm, the service, and the service parameters.  An
 example of a GSS-API service name is "xmpp/foo@example.com".  This
 identifies the XMPP service on the host foo in the realm example.com.
 Any of the components, except for the service name, may be omitted
 from a name.  When omitted, a local default would be used for that
 component of the name.
 While there is no requirement that realm names map to Fully Qualified
 Domain Names (FQDNs) within DNS, in practice this is normally true.
 Doing so allows the realm portion of service names and the portion of
 NAIs to be the same.  It also allows for the use of DNS in locating
 the host of a service while establishing the transport channel
 between the client and the RP.
 It is the responsibility of the application to determine the server
 that it is going to communicate with; GSS-API has the ability to help
 confirm that the server is the desired server but not to determine
 the name of the server to use.  It is also the responsibility of the

Howlett, et al. Informational [Page 32] RFC 7831 ABFAB Architecture May 2016

 application to determine how much of the information identifying the
 service needs to be validated by the ABFAB system.  The information
 that needs to be validated is used to construct the service name
 passed into the GSS-EAP mechanism.  What information is to be
 validated will depend on (1) what information was provided by the
 client and (2) what information is considered significant.  If the
 client only cares about getting a specific service, then it does not
 need to validate the host and realm that provides the service.
 Applications may retrieve information about providers of services
 from DNS.  Service Records (SRVs) [RFC2782] and Naming Authority
 Pointer (NAPTR) [RFC3401] records are used to help find a host that
 provides a service; however, the necessity of having DNSSEC on the
 queries depends on how the information is going to be used.  If the
 host name returned is not going to be validated by EAP channel
 binding because only the service is being validated, then DNSSEC
 [RFC4033] is not required.  However, if the host name is going to be
 validated by EAP channel binding, then DNSSEC needs to be used to
 ensure that the correct host name is validated.  In general, if the
 information that is returned from the DNS query is to be validated,
 then it needs to be obtained in a secure manner.
 Another issue that needs to be addressed for host-based service names
 is that they do not work ideally when different instances of a
 service are running on different ports.  If the services are
 equivalent, then it does not matter.  However, if there are
 substantial differences in the quality of the service, that
 information needs to be part of the validation process.  If one has
 just a host name and not a port in the information being validated,
 then this is not going to be a successful strategy.

3.4. Additional GSS-API Services

 GSS-API provides per-message security services that can provide
 confidentiality and/or 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 security services will limit the set of EAP methods that
 can be used to those that generate a Master Session Key (MSK).  Any
 EAP method that produces an MSK is able to support per-message
 security services as described in [RFC2743].
 GSS-API provides a pseudorandom function.  This function generates a
 pseudorandom sequence using the shared session key as the seed for
 the bytes generated.  This provides an algorithm that both the
 initiator and acceptor can run in order to arrive at the same key
 value.  The use of this feature allows an application to generate
 keys or other shared secrets for use in other places in the protocol.

Howlett, et al. Informational [Page 33] RFC 7831 ABFAB Architecture May 2016

 In this regard, it is similar in concept to the mechanism (formerly
 known as "TLS Extractors") described in [RFC5705].  While no current
 IETF protocols require this feature, non-IETF protocols are expected
 to take advantage of it in the near future.  Additionally, a number
 of protocols have found the mechanism described in [RFC5705] to be
 useful in this regard, so it is highly probable that IETF protocols
 may also start using this feature.

4. Privacy Considerations

 As an architecture designed to enable federated authentication and
 allow for the secure transmission of identity information between
 entities, ABFAB obviously requires careful consideration regarding
 privacy and the potential for privacy violations.
 This section examines the privacy-related information presented in
 this document, summarizing 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 [RFC6973].
 Note that the ABFAB architecture uses at its core several existing
 technologies and protocols; detailed privacy discussion regarding
 these topics is not examined.  This section instead focuses on
 privacy considerations specifically related to the overall
 architecture and usage of ABFAB.
    +--------+       +---------------+       +--------------+
    | Client | <---> |      RP       | <---> | AAA Client   |
    +--------+       +---------------+       +--------------+
                                                   ^
                                                   |
                                                   v
                     +---------------+       +----------------+
                     | SAML Server   |       | AAA Proxy      |
                     +---------------+       | (or Proxies)   |
                              ^              +----------------+
                              |                       ^
                              |                       |
                              v                       v
    +------------+       +---------------+       +--------------+
    | EAP Server | <---> |   IdP         | <---> | AAA Server   |
    +------------+       +---------------+       +--------------+
                   Figure 4: Entities and Data Flow

Howlett, et al. Informational [Page 34] RFC 7831 ABFAB Architecture May 2016

4.1. Entities and Their Roles

 Categorizing the ABFAB entities shown in Figure 4 according to the
 taxonomy of terms from [RFC6973] is somewhat complicated, as the
 roles of each entity will change during the various phases of ABFAB
 communications.  The three main phases of relevance are the
 client-to-RP communication phase, the client-to-IdP (via the
 Federation Substrate) communication phase, and the IdP-to-RP (via the
 Federation Substrate) communication phase.
 In the client-to-RP communication phase, we have:
 Initiator:  Client.
 Observers:  Client, RP.
 Recipient:  RP.
 In the client-to-IdP (via the Federation Substrate) communication
 phase, we have:
 Initiator:  Client.
 Observers:  Client, RP, AAA Client, AAA Proxy (or Proxies), AAA
    Server, IdP.
 Recipient:  IdP
 In the IdP-to-RP (via the Federation Substrate) communication phase,
 we have:
 Initiator:  RP.
 Observers:  IdP, AAA Server, AAA Proxy (or Proxies), AAA Client, RP.
 Recipient:  IdP
 Eavesdroppers and attackers can reside on any or all communication
 links between the entities shown in Figure 4.

Howlett, et al. Informational [Page 35] RFC 7831 ABFAB Architecture May 2016

 The various entities in the system might also collude or be coerced
 into colluding.  Some of the significant collusions to look at are as
 follows:
 o  If two RPs are colluding, they have the information available to
    both nodes.  This can be analyzed as if a single RP were offering
    multiple services.
 o  If an RP and a AAA proxy are colluding, then the trust of the
    system is broken, as the RP would be able to lie about its own
    identity to the IdP.  There is no known way to deal with this
    situation.
 o  If multiple AAA proxies are colluding, they can be treated as a
    single node for analysis.
 The Federation Substrate consists of all of the AAA entities.  In
 some cases, the AAA proxies may not exist, as the AAA client can talk
 directly to the AAA server.  Specifications such as the Trust Router
 Protocol (https://www.ietf.org/proceedings/86/slides/
 slides-86-rtgarea-0.pdf) and RADIUS dynamic discovery [RFC7585] can
 be used to shorten the path between the AAA client and the AAA server
 (and thus stop these AAA proxies from being observers); however, even
 in these circumstances, there may be AAA proxies in the path.
 In Figure 4, the IdP has been divided into multiple logical pieces;
 in actual implementations, these pieces will frequently be tightly
 coupled.  The links between these pieces provide the greatest
 opportunity for attackers and eavesdroppers to acquire information;
 however, as they are all under the control of a single entity, they
 are also the easiest to have tightly secured.

4.2. Privacy Aspects of ABFAB Communication Flows

 In the ABFAB architecture, there are a few different types of data
 and identifiers in use.  The best way to understand them, and their
 potential privacy impacts, is to look at each phase of communication
 in ABFAB.

4.2.1. Client to RP

 The flow of data between the client and the RP is divided into two
 parts.  The first part consists of all of the data exchanged as part
 of the ABFAB authentication process.  The second part consists of all
 of the data exchanged after the authentication process has been
 finished.

Howlett, et al. Informational [Page 36] RFC 7831 ABFAB Architecture May 2016

 During the initial communication phase, the client sends an NAI (see
 [RFC7542]) to the RP.  Many EAP methods (but not all) allow the
 client to disclose an NAI to the RP in a form that includes only a
 realm component during this communication phase.  This is the minimum
 amount of identity information necessary for ABFAB to work -- it
 indicates an IdP that the principal has a relationship with.  EAP
 methods that do not allow this will necessarily also reveal an
 identifier for the principal in the IdP realm (e.g., a username).
 The data shared during the initial communication phase may be
 protected by a channel protocol such as TLS.  This will prevent the
 leakage of information to passive eavesdroppers; however, an active
 attacker may still be able to set itself up as a man-in-the-middle.
 The client may not be able to validate the certificates (if any)
 provided by the service, deferring the check of the identity of the
 RP until the completion of the ABFAB authentication protocol (using
 EAP channel binding rather than certificates).
 The data exchanged after the authentication process can have privacy
 and authentication using the GSS-API services.  If the overall
 application protocol allows for the process of re-authentication,
 then the same privacy implications as those discussed in previous
 paragraphs apply.

4.2.2. Client to IdP (via Federation Substrate)

 This phase includes a secure TLS tunnel set up between the client and
 the IdP via the RP and Federation Substrate.  The process is
 initiated by the RP using the realm information given to it by the
 client.  Once set up, the tunnel is used to send credentials to the
 IdP to authenticate.
 Various operational information is transported between the RP and the
 IdP over the AAA infrastructure -- for example, using RADIUS headers.
 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 on this
 information.  Some of this information may form identifiers or
 explicit identity information:
 o  The RP knows the IP address of the client.  It is possible that
    the RP could choose to expose this IP address by including it in a
    RADIUS header (e.g., using the Calling-Station-Id).  This is a
    privacy consideration to take into account for the application
    protocol.
 o  The EAP MSK is transported between the IdP and the RP over the AAA
    infrastructure -- for example, through RADIUS headers.  This is a
    particularly important privacy consideration, as any AAA proxy

Howlett, et al. Informational [Page 37] RFC 7831 ABFAB Architecture May 2016

    that has access to the EAP MSK is able to decrypt and eavesdrop on
    any traffic encrypted using that EAP MSK (i.e., all communications
    between the client and RP).  This problem can be mitigated if the
    application protocol sets up a secure tunnel between the client
    and the RP and performs a cryptographic binding between the tunnel
    and EAP MSK.
 o  Related to the bullet point above, the AAA server has access to
    the material necessary to derive the session key; thus, the AAA
    server can observe any traffic encrypted between the client and
    RP.  This "feature" was chosen as a simplification and to make
    performance faster; if it was decided that this trade-off was not
    desirable for privacy and security reasons, then extensions to
    ABFAB that make use of techniques such as Diffie-Hellman key
    exchange would mitigate this.
 The choice of EAP method used has other potential privacy
 implications.  For example, if the EAP method in use does not
 support mutual authentication, then there are no guarantees that the
 IdP is who it claims to be, and thus the full NAI, including a
 username and a realm, might be sent to any entity masquerading as a
 particular IdP.
 Note that ABFAB has not specified any AAA accounting requirements.
 Implementations that use the accounting portion of AAA should
 consider privacy appropriately when designing this aspect.

4.2.3. IdP to RP (via Federation Substrate)

 In this phase, the IdP communicates with the RP, informing it as to
 the success or failure of authentication of the user and, optionally,
 the sending of identity information about the principal.
 As in the previous flow (client to IdP), various operation
 information is transported between the IdP and RP over the AAA
 infrastructure, and the same privacy considerations apply.  However,
 in this flow, explicit identity information about the authenticated
 principal can be sent from the IdP to the RP.  This information can
 be sent through RADIUS headers, or using SAML [RFC7833].  This can
 include protocol-specific identifiers, such as SAML NameIDs, as well
 as arbitrary attribute information about the principal.  What
 information will be released is controlled by policy on the IdP.  As
 before, when sending this information through RADIUS headers, all AAA
 entities on the path between the RP and IdP have the ability to
 eavesdrop, unless additional security measures are taken (such as the
 use of TLS for RADIUS [RFC6614]).  However, when sending this

Howlett, et al. Informational [Page 38] RFC 7831 ABFAB Architecture May 2016

 information using SAML as specified in [RFC7833], confidentiality of
 the information should be guaranteed, as [RFC7833] requires the use
 of TLS for RADIUS.

4.3. Relationship between User and Entities

 o  Between user and IdP - The IdP is an entity the user will have a
    direct relationship with, created when the organization 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 RP - The RP 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 IdP.  This may well include the option
    that the user stays anonymous with respect to the RP (though,
    obviously, never anonymous to the IdP).  If attempting to preserve
    privacy via data minimization (Section 1), then the only attribute
    information about Individuals exposed to the RP should be
    attribute information that 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 IdP and/or RP
    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 the
    possibility of access to this information.

4.4. Accounting Information

 Alongside the core authentication and authorization that occur 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.

4.5. Collection and Retention of Data and Identifiers

 In cases where RPs are not required to identify a particular
 Individual when an Individual wishes to make use of their service,
 the ABFAB architecture enables anonymous or pseudonymous access.
 Thus, data and identifiers other than pseudonyms and unlinkable
 attribute information need not be stored and retained.

Howlett, et al. Informational [Page 39] RFC 7831 ABFAB Architecture May 2016

 However, in cases where RPs 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.  The de-provisioning of such accounts and
 information is out of scope for ABFAB, but for privacy protection, it
 is obvious that any identifiers collected should be deleted when they
 are no longer needed.

4.6. 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 the control of information related
 to them that is transmitted from the IdP to the RP for authorization
 purposes; rather, this is under the control of policy on the IdP.
 Due to the nature of the AAA communication flows, with the current
 ABFAB architecture there is no place for a process of gaining user
 consent for the information to be released from the IdP to the RP.

5. Security Considerations

 This document describes the architecture for Application Bridging for
 Federated Access Beyond web (ABFAB), and security is therefore the
 main focus.  Many of the items that are security considerations have
 already been discussed in Section 4 ("Privacy Considerations").
 Readers should be sure to read that section as well.
 There are many places in this document where TLS is used.  While in
 some places (e.g., client to RP) anonymous connections can be used,
 it is very important that TLS connections within the AAA
 infrastructure and between the client and the IdP be fully
 authenticated and, if using certificates, that revocation be checked
 as well.  When using anonymous connections between the client and the
 RP, all messages and data exchanged between those two entities will
 be visible to an active attacker.  In situations where the client is
 not yet on the network, the status_request extension [RFC6066] can be
 used to obtain revocation-checking data inside of the TLS protocol.
 Clients also need to get the trust anchor for the IdP configured
 correctly in order to prevent attacks; this is a difficult problem in
 general and is going to be even more difficult for kiosk
 environments.
 Selection of the EAP methods to be permitted by clients and IdPs is
 important.  The use of a tunneling method such as TEAP [RFC7170]
 allows other EAP methods to be used while hiding the contents of

Howlett, et al. Informational [Page 40] RFC 7831 ABFAB Architecture May 2016

 those EAP exchanges from the RP and the AAA framework.  When
 considering inner EAP methods, the considerations outlined in
 [RFC7029] about binding the inner and outer EAP methods need to be
 taken into account.  Finally, one wants to have the ability to
 support channel binding in those cases where the client needs to
 validate that it is talking to the correct RP.
 In those places where SAML statements are used, RPs will generally be
 unable to validate signatures on the SAML statement, either because
 the signature has been stripped off by the IdP or because the RP is
 unable to validate the binding between the signer, the key used to
 sign, and the realm represented by the IdP.  For these reasons, it is
 required that IdPs do the necessary trust checking on the SAML
 statements and that RPs can trust the AAA infrastructure to keep the
 SAML statements valid.
 When a pseudonym is generated as a unique long-term identifier for a
 client 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
 reverse-engineered, then the service provider can consult an oracle
 to determine if a given unique long-term identifier is associated
 with a different known identifier.

6. References

6.1. Normative References

 [RFC2743]  Linn, J., "Generic Security Service Application Program
            Interface Version 2, Update 1", RFC 2743,
            DOI 10.17487/RFC2743, January 2000,
            <http://www.rfc-editor.org/info/rfc2743>.
 [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
            "Remote Authentication Dial In User Service (RADIUS)",
            RFC 2865, DOI 10.17487/RFC2865, June 2000,
            <http://www.rfc-editor.org/info/rfc2865>.
 [RFC3579]  Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
            Dial In User Service) Support For Extensible
            Authentication Protocol (EAP)", RFC 3579,
            DOI 10.17487/RFC3579, September 2003,
            <http://www.rfc-editor.org/info/rfc3579>.
 [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
            Levkowetz, Ed., "Extensible Authentication Protocol
            (EAP)", RFC 3748, DOI 10.17487/RFC3748, June 2004,
            <http://www.rfc-editor.org/info/rfc3748>.

Howlett, et al. Informational [Page 41] RFC 7831 ABFAB Architecture May 2016

 [RFC4072]  Eronen, P., Ed., Hiller, T., and G. Zorn, "Diameter
            Extensible Authentication Protocol (EAP) Application",
            RFC 4072, DOI 10.17487/RFC4072, August 2005,
            <http://www.rfc-editor.org/info/rfc4072>.
 [RFC6677]  Hartman, S., Ed., Clancy, T., and K. Hoeper, "Channel-
            Binding Support for Extensible Authentication Protocol
            (EAP) Methods", RFC 6677, DOI 10.17487/RFC6677, July 2012,
            <http://www.rfc-editor.org/info/rfc6677>.
 [RFC7055]  Hartman, S., Ed., and J. Howlett, "A GSS-API Mechanism for
            the Extensible Authentication Protocol", RFC 7055,
            DOI 10.17487/RFC7055, December 2013,
            <http://www.rfc-editor.org/info/rfc7055>.
 [RFC7542]  DeKok, A., "The Network Access Identifier", RFC 7542,
            DOI 10.17487/RFC7542, May 2015,
            <http://www.rfc-editor.org/info/rfc7542>.
 [RFC7833]  Howlett, J., Hartman, S., and A. Perez-Mendez, Ed., "A
            RADIUS Attribute, Binding, Profiles, Name Identifier
            Format, and Confirmation Methods for the Security
            Assertion Markup Language (SAML)", RFC 7833,
            DOI 10.17487/RFC7833, May 2016,
            <http://www.rfc-editor.org/info/rfc7833>.

6.2. Informative References

 [NIST-SP.800-63-2]
            Burr, W., Dodson, D., Newton, E., Perlner, R., Polk, W.,
            Gupta, S., and E. Nabbus, "Electronic Authentication
            Guideline", NIST Special Publication 800-63-2,
            August 2013, <http://dx.doi.org/10.6028/NIST.SP.800-63-2>.
 [OASIS.saml-core-2.0-os]
            Cantor, S., Kemp, J., Philpott, R., and E. Maler,
            "Assertions and Protocols for the OASIS Security
            Assertion Markup Language (SAML) V2.0", OASIS
            Standard saml-core-2.0-os, March 2005,
            <http://docs.oasis-open.org/security/saml/v2.0/
            saml-core-2.0-os.pdf>.
 [RFC1964]  Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
            RFC 1964, DOI 10.17487/RFC1964, June 1996,
            <http://www.rfc-editor.org/info/rfc1964>.

Howlett, et al. Informational [Page 42] RFC 7831 ABFAB Architecture May 2016

 [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
            specifying the location of services (DNS SRV)", RFC 2782,
            DOI 10.17487/RFC2782, February 2000,
            <http://www.rfc-editor.org/info/rfc2782>.
 [RFC3401]  Mealling, M., "Dynamic Delegation Discovery System (DDDS)
            Part One: The Comprehensive DDDS", RFC 3401,
            DOI 10.17487/RFC3401, October 2002,
            <http://www.rfc-editor.org/info/rfc3401>.
 [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, DOI 10.17487/RFC3645, October 2003,
            <http://www.rfc-editor.org/info/rfc3645>.
 [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
            Rose, "DNS Security Introduction and Requirements",
            RFC 4033, DOI 10.17487/RFC4033, March 2005,
            <http://www.rfc-editor.org/info/rfc4033>.
 [RFC4422]  Melnikov, A., Ed., and K. Zeilenga, Ed., "Simple
            Authentication and Security Layer (SASL)", RFC 4422,
            DOI 10.17487/RFC4422, June 2006,
            <http://www.rfc-editor.org/info/rfc4422>.
 [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, DOI 10.17487/RFC4462,
            May 2006, <http://www.rfc-editor.org/info/rfc4462>.
 [RFC5056]  Williams, N., "On the Use of Channel Bindings to Secure
            Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007,
            <http://www.rfc-editor.org/info/rfc5056>.
 [RFC5080]  Nelson, D. and A. DeKok, "Common Remote Authentication
            Dial In User Service (RADIUS) Implementation Issues and
            Suggested Fixes", RFC 5080, DOI 10.17487/RFC5080,
            December 2007, <http://www.rfc-editor.org/info/rfc5080>.
 [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.2", RFC 5246,
            DOI 10.17487/RFC5246, August 2008,
            <http://www.rfc-editor.org/info/rfc5246>.

Howlett, et al. Informational [Page 43] RFC 7831 ABFAB Architecture May 2016

 [RFC5705]  Rescorla, E., "Keying Material Exporters for Transport
            Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
            March 2010, <http://www.rfc-editor.org/info/rfc5705>.
 [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, DOI 10.17487/RFC5801,
            July 2010, <http://www.rfc-editor.org/info/rfc5801>.
 [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
            Extensions: Extension Definitions", RFC 6066,
            DOI 10.17487/RFC6066, January 2011,
            <http://www.rfc-editor.org/info/rfc6066>.
 [RFC6614]  Winter, S., McCauley, M., Venaas, S., and K. Wierenga,
            "Transport Layer Security (TLS) Encryption for RADIUS",
            RFC 6614, DOI 10.17487/RFC6614, May 2012,
            <http://www.rfc-editor.org/info/rfc6614>.
 [RFC6733]  Fajardo, V., Ed., Arkko, J., Loughney, J., and G. Zorn,
            Ed., "Diameter Base Protocol", RFC 6733,
            DOI 10.17487/RFC6733, October 2012,
            <http://www.rfc-editor.org/info/rfc6733>.
 [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
            RFC 6749, DOI 10.17487/RFC6749, October 2012,
            <http://www.rfc-editor.org/info/rfc6749>.
 [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
            Morris, J., Hansen, M., and R. Smith, "Privacy
            Considerations for Internet Protocols", RFC 6973,
            DOI 10.17487/RFC6973, July 2013,
            <http://www.rfc-editor.org/info/rfc6973>.
 [RFC7029]  Hartman, S., Wasserman, M., and D. Zhang, "Extensible
            Authentication Protocol (EAP) Mutual Cryptographic
            Binding", RFC 7029, DOI 10.17487/RFC7029, October 2013,
            <http://www.rfc-editor.org/info/rfc7029>.
 [RFC7170]  Zhou, H., Cam-Winget, N., Salowey, J., and S. Hanna,
            "Tunnel Extensible Authentication Protocol (TEAP)
            Version 1", RFC 7170, DOI 10.17487/RFC7170, May 2014,
            <http://www.rfc-editor.org/info/rfc7170>.

Howlett, et al. Informational [Page 44] RFC 7831 ABFAB Architecture May 2016

 [RFC7360]  DeKok, A., "Datagram Transport Layer Security (DTLS) as a
            Transport Layer for RADIUS", RFC 7360,
            DOI 10.17487/RFC7360, September 2014,
            <http://www.rfc-editor.org/info/rfc7360>.
 [RFC7499]  Perez-Mendez, A., Ed., Marin-Lopez, R., Pereniguez-Garcia,
            F., Lopez-Millan, G., Lopez, D., and A. DeKok, "Support of
            Fragmentation of RADIUS Packets", RFC 7499,
            DOI 10.17487/RFC7499, April 2015,
            <http://www.rfc-editor.org/info/rfc7499>.
 [RFC7530]  Haynes, T., Ed., and D. Noveck, Ed., "Network File System
            (NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530,
            March 2015, <http://www.rfc-editor.org/info/rfc7530>.
 [RFC7585]  Winter, S. and M. McCauley, "Dynamic Peer Discovery for
            RADIUS/TLS and RADIUS/DTLS Based on the Network Access
            Identifier (NAI)", RFC 7585, DOI 10.17487/RFC7585,
            October 2015, <http://www.rfc-editor.org/info/rfc7585>.
 [WS-TRUST] Lawrence, K., Kaler, C., Nadalin, A., Goodner, M., Gudgin,
            M., Turner, D., Barbir, A., and H. Granqvist,
            "WS-Trust 1.4", OASIS Standard ws-trust-2012-04,
            April 2012, <http://docs.oasis-open.org/ws-sx/ws-trust/
            v1.4/ws-trust.html>.

Howlett, et al. Informational [Page 45] RFC 7831 ABFAB Architecture May 2016

Acknowledgments

 We would like to thank Mayutan Arumaithurai, Klaas Wierenga, and Rhys
 Smith 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 first draft version of this document.  We also thank Eliot Lear
 for his work on early draft versions of this document.

Authors' Addresses

 Josh Howlett
 Jisc
 Lumen House, Library Avenue, Harwell
 Oxford  OX11 0SG
 United Kingdom
 Phone: +44 1235 822363
 Email: Josh.Howlett@ja.net
 Sam Hartman
 Painless Security
 Email: hartmans-ietf@mit.edu
 Hannes Tschofenig
 ARM Ltd.
 110 Fulbourn Road
 Cambridge  CB1 9NJ
 United Kingdom
 Email: Hannes.tschofenig@gmx.net
 URI:   http://www.tschofenig.priv.at
 Jim Schaad
 August Cellars
 Email: ietf@augustcellars.com

Howlett, et al. Informational [Page 46]

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