Application Bridging for Federated Access Beyond Web (ABFAB) Architecture
draft-lear-abfab-arch-00.txt

Abstract

Over the last decade a substantial amount of work has occurred in the space of federated authentication and authorization. Most of this effort has focused on two common use cases: network and web-based access, with few common building blocks within the architecture. This memo describes an architecture that makes use of extensions to the commonly used mechanisms for both federated and non-federated authentication and authorization, including Radius/Diameter, GSS/GS2, and SAML, to primarily address non-web based authentication, in a that will scale to large numbers of federations.

Status of this Memo

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Table of Contents

1.  Introduction
    1.1.  Federation Description
2.  Terminology
3.  Architecture
4.  Privacy Considerations
5.  Security Considerations
6.  IANA Considerations
7.  Acknowledgments
8.  References
    8.1.  Normative References
    8.2.  Informative References
§  Authors' Addresses

1.  Introduction

XXX This document is a first draft. Comments and contributions are requested.

The Internet makes uses of numerous authentication methods to grant access to various resources. These mechanisms have been generalized and scaled over the last decade through mechanisms such as GS2, Security Assertion Markup Language (SAML) (Cantor, S., Kemp, J., Philpott, R., and E. Maler, “Assertions and Protocol for the OASIS Security Assertion Markup Language (SAML) V2.0,” March 2005.) [OASIS.saml‑core‑2.0‑os], Radius, and Diameter. So-called "federated" access has evolved over the last decade between web servers through such standards as SAML, OpenID, and OAUTH, allowing entire domains of individuals to be authorized for resources. The key scaling points that have been addressed are the following:

• An Internet service need not copy manually authentication information from a domain to allow for authentication and authorization.
• Individual users are able to make use of a single credential to authenticate to such services.

As the number of such federated services has proliferated, however, the role of the individual has become ambiguous in certain circumstances. For example, a school might provide online access to grades to a parent who is also a teacher. She must clearly distinguish her role upon access. After all, she is probably not allowed to edit her own child's grades.

Similarly, as the number of federations proliferates, it becomes increasingly difficult to discover which identity provider a user is associated with. This is true for both the web and non-web case, but particularly acute for the latter ans many non-web authentication systems are not semantically rich enough on their own to allow for such ambiguities. For instance, in the case of an email provider, the use of SMTP and IMAP protocols does not on its own provide for a way to select a federation. However, the building blocks do exist to add this functionality.

1.1.  Federation Description

The typical setup for a three party protocol involves the following entities:

• the End Host,
• the Identity Provider, and
• the Relying Party.

These entities are illustrated graphically in Figure 1 (Three Party Authentication Framework).

                       -----
                     /-     -\
                   //         \\
                   /           \
                  |             |
 ,----------\    |               |   ,---------\
 | Identity |    |               |   | Relying |
 | Provider +----+   Federation  +---+ Party   |
 `----------'    |               |   '---------'
          <      |               |        >
           \      |             |        /
            \      \           /        /
             \     \\         //       /
             \       \-     -/        /
              \        -----         /
               \                    /
                \  +------------+  /
                 \ |            | /
                  v|  End Host  |v
                   |            |
                   +------------+

Figure 1 (Three Party Authentication Framework) also shows the logical entity 'Federation'. In a federation, policy is agreed upon by some form of administrative management, and then instantiated through an operational framework that the members use, and where compliance is measured in some fashion. Some deployments may be required to deploy message routing intermediaries, such as application layer relays or proxies, to offer the required technical functionality while in other deployments those are missing.

Often a real world entity is associated with the end host and responsible for interacting with the identity provider, even if it is only as weak as completing a web form and confirming the verification email. The outcome of this initial registration step is that credentials are made available to the identity provider and to the end host. It is important to highlight that in some scenarios there might indeed be a human behind the device denoted as end host and in other cases there is no human involved in the actual protocol execution.

To support the more generic deployment case, we assume that the identity provider and the relying party belong to different administrative domains. The nature of federation dictates that there is some form of relationship between the identity provider and the relying party. This is particularly important when the relying party wants to use information obtained from the identity provider for authorization decisions and when the identity provider does not want to release information to every relying party (or only under certain conditions). While it is possible to have a bilateral agreement between every identity provider and every relying party; on an Internet scale this setup requires the introduction of a federation concept, as the management of such pair-wise relationships would otherwise prove burdensome. While many of the non-technical aspects of such a federation, such as business practices and operational arrangements, are outside the scope of the IETF they still impact the architecture setup on how to ensure the dynamic establishment of trust.

Our key design goals are as follows:

• Each party of a transaction will be authenticated, and the principal will be authorized for access to a specific resource .
• Means of authentication is decoupled so as to allow for multiple authentication methods.
• Hence, the architecture requires no sharing of long term private keys.
• The system will scale to large numbers of identity providers, relying parties, and users.
• The system will be designed primarily for non-Web-based authentication.
• The system will build upon existing standards, components, and operational practices.

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

Interestingly, for network access authentication the usage of the AAA framework with RADIUS [RFC2865] (Rigney, C., Willens, S., Rubens, A., and W. Simpson, “Remote Authentication Dial In User Service (RADIUS),” June 2000.) and Diameter [RFC3588] (Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, “Diameter Base Protocol,” September 2003.) was quite successful from a deployment point of view. To map the terminology used in Figure 1 (Three Party Authentication Framework) to the AAA framework the identity provider corresponds to the AAA server, the relying party corresponds to the AAA client, and the technical building blocks of a federation are AAA proxies, relays and redirect agents (particularly if they are operated by third parties, such as AAA brokers and clearing houses). The front-end, i.e. the end host to AAA client communication, is in case of network access authentication offered by link layer protocols that forward authentication protocol exchanges back-and-forth. An example of a large scale Radius-based federation is EDUROAM.

Is it possible to design a system that builds on top of successful protocols to offer non-Web-based protocols with a solid starting point for authentication and authorization in a distributed system?

2.  Terminology

This document uses identity management and privacy terminology from [I‑D.hansen‑privacy‑terminology] (Pfitzmann, A., Hansen, M., and H. Tschofenig, “Terminology for Talking about Privacy by Data Minimization: Anonymity, Unlinkability, Undetectability, Unobservability, Pseudonymity, and Identity Management,” August 2010.).

3.  Architecture

Section 1 (Introduction) already introduced the federated access architecture, with the illustration of the different actors that need to interact, but it did not expand on the specifics of providing support for non-Web based applications. This section details this aspect and motivates design decisions. The main theme of the work described in this document is focused on re-using existing building blocks that have been deployed already and to re-arrange them in a novel way.

A key design goal is the re-use an existing infrastructure, we build upon the AAA framework as utilized by RADIUS [RFC2138] (Rigney, C., Rigney, C., Rubens, A., Simpson, W., and S. Willens, “Remote Authentication Dial In User Service (RADIUS),” April 1997.) and Diameter [RFC3588] (Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, “Diameter Base Protocol,” September 2003.). Since this document does not aim to re-describe the AAA framework the interested reader is referred to [RFC2904] (Vollbrecht, J., Calhoun, P., Farrell, S., Gommans, L., Gross, G., de Bruijn, B., de Laat, C., Holdrege, M., and D. Spence, “AAA Authorization Framework,” August 2000.). Building on the AAA infrastructure, and RADIUS and Diameter as protocols, modifications to that infrastructure is to be avoided. Also, modifications to AAA servers should be kept at a minimum.

The astute reader will notice that RADIUS and Diameter have substantially similar characteristics. Why not pick one? A key difference is that today RADIUS is largely transported upon TCP, and its use is largely, though not exclusively, intra-domain. Diameter itself was designed to scale to broader uses. We leave as a deployment decision, which protocol will be appropriate.

Experience has taught us one key security and scalability requirement: it is important that the relying party not get in possession of the long-term secret of the entity being authenticated by the AAA server. Aside from a valuable secret being exposed, a synchronization problem can also often develop. Since there is no single authentication mechanism that will be used everywhere there is another associated requirement: The authentication framework must allow for the flexible integration of authentication mechanisms. For instance, some identity providers may require hardware tokens while others may use passwords. A service provider would want to support both sorts of federations, and others.

Fortunately, these requirements can be met by utilizing standardized and successfully deployed technology, namely by the Extensible Authentication Protocol (EAP) framework [RFC3748] (Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP),” June 2004.). Figure 2 (Architecture for Federated Access of non-Web based Applications) illustrates the integration graphically.

EAP is an end-to-end framework; it provides for two-way communication between a peer (i.e,service client or principal) through the authenticator (i.e., service provider) to the back-end (i.e., identity provider). Conveniently, this is precisely the communication path that is needed for federated identity. Although EAP support is already integrated in AAA systems (see [RFC3579] (Aboba, B. and P. Calhoun, “RADIUS (Remote Authentication Dial In User Service) Support For Extensible Authentication Protocol (EAP),” September 2003.) and [RFC4072] (Eronen, P., Hiller, T., and G. Zorn, “Diameter Extensible Authentication Protocol (EAP) Application,” August 2005.)) several challenges remain: one is to carry EAP payloads from the end host to the relying party, and the other is to determine where identity provider. We will look at these challenges in turn. [XXX but we don't get to discovery just yet.]

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

There are a number of ways for encapsulating EAP into an application protocol. For ease of integration with a wide range of non-Web based application protocols the usage of the GSS-API was chosen. Encapsulating EAP into the GSS-API also allows EAP to be used in SASL. A description of the technical specification can be found in [I‑D.ietf‑abfab‑gss‑eap] (Hartman, S. and J. Howlett, “A GSS-API Mechanism for the Extensible Authentication Protocol,” October 2010.). Other alternatives exist as well and may be considered later, such as "TLS using EAP Authentication" [I‑D.nir‑tls‑eap] (Nir, Y., Sheffer, Y., Tschofenig, H., and P. Gutmann, “TLS using EAP Authentication,” July 2010.).

                                 +--------------+
                                 |  AAA Server  |
                                 |  (Identity   |
                                 |  Provider)   |
                                 +-^----------^-+
                                   * EAP      | RADIUS/
                                   *          | Diameter
                                 --v----------v--
                              ///                \\\
                            //                      \\   ***
                           |        Federation        |  back-
                           |                          |  end
                            \\                      //   ***
                              \\\                ///
                                 --^----------^--
                                   * EAP      | RADIUS/
                 Application       *          | Diameter
+-------------+  Data            +-v----------v--+
|             |<---------------->|               |
| Client      |  EAP/EAP Method  | Server Side   |
| Application |<****************>| Application   |
| @ End Host  |  GSS-API         |(Relying Party)|
|             |<---------------->|               |
|             |  Application     |               |
|             |  Protocol        |               |
|             |<================>|               |
+-------------+                  +---------------+
               *** front-end ***

Legend:

 <****>: End-to-end exchange
 <---->: Hop-by-hop exchange
 <====>: Protocol through which GSS-API/GS2 exchanges are tunnelled

4.  Privacy Considerations

Sharing identity information may lead to privacy violations. A future verison of this document will provide a discussion of privacy considerations in a federated access environment.

5.  Security Considerations

This entire document is about security. A future version of the document will highlight some important security concepts.

6.  IANA Considerations

This document does not require actions by IANA.

7.  Acknowledgments

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

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

8.  References

8.1. Normative References

8.2. Informative References

Authors' Addresses