Federated Authentication Beyond The Web: Problem Statement and Requirements
draft-tschofenig-moonshot-ps-01.txt

Abstract

It is quite common that application developers and system architects are in need for authentication and authorization support in a distributed environment. At least three parties need to cooperate, namely the end host, the identity provider, and the relying party. At the end of the exchange the identity provider asserts identity information or certain attributes to the relying party without exposing the user's long-term secret to the relying party.

Although the problem sounds challenging and interesting, it is not new. In fact, various IETF groups have produced specifications to solve this problem, such as Kerberos, RADIUS, and Diameter. Outside the IETF various Single-Sign-On solution for HTTP-based applications have been developed as well.

The reader might therefore wonder about the need for new work given the existence of readily available solutions. This document tries to answer this question in a compact fashion. Note that the description in this document focuses on the scope of the new work as part of the "Federated Authentication Beyond The Web" BOF being proposed rather than what could be theoretically done.

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Table of Contents

1.  Introduction
2.  Terminology
3.  Assumptions and Requirements
4.  Security Considerations
5.  IANA Considerations
6.  Acknowledgments
7.  References
    7.1.  Normative References
    7.2.  Informative References
§  Author's Address

1.  Introduction

The typical setup for a three party protocol involves the End-host, the identity-provider and relying party as illustrated in Figure 1 (Three Party Authentication Framework). It might be of surprise that there are actually four parties shown in Figure 1 (Three Party Authentication Framework); we will address the invisible party in the middle a little bit later.

With three party protocols there are a number of different protocol variants possible, as the available crypto-literature shows. We will not discuss the different options in this document. What is relevant is that a real world entity is behind the end host and responsible for establishing some form of contract 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 (or the user). 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.

We assume that the identity provider and the relying party belong to different administrative domains. Very often 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 does require some intermediary, the "stuff-in-the-middle". Please note that the lack of scalability is not caused by technical limitations but rather by business limitations since the agreements between identity providers and the relying parties are often business contracts that are financially motivated. The "stuff-in-the-middle" is a placeholder for technical interoperability as well as business practices and operational arrangements, many aspects are outside the scope of the IETF.

Agreed terminology for what is labeled as generically "stuff-in-the-middle" is unfortunately not available. Sometimes the term "identity federation", or "trust framework" are used. To make it worse, different terminology is used when looking at specific protocols.

                         -----
                       /-     -\
                     //         \\
                     /           \
        ----        |             |        ----
    ///-    -\\\   |               |   ///-    -\\\
   /            \  |   Stuff-in-   |  /            \
  |              |-+   the-Middle  +-|              |
  |   Identity   | |               | |  Relying     |
  |   Provider   | |               | |  Party       |
  |              |  |             |  |              |
   \            /    \           /    \            /
    \\\-    -///     \\         //     \\\-    -///
        ----           \-     -/           ----
            <            -----              >
             \                             /
              \                           /
               \                         /
               \                        /
                \                      /
                 \                    /
                  \  +------------+  /
                   \ |            | /
                    v|  End Host  |v
                     |            |
                     |            |
                     +------------+

Designing new three party authentication and authorization protocols is hard and cryptographic flaws common in designs. Achieving widespead deployment is even more difficult. The HTTP-based Web has enjoyed a lot of attention from the industry with respect to this problem and some amount of success can be noticed even though many of the business aspects with the "stuff-in-the-middle" still has to be sorted out. This document does not focus on an HTTP-based environment and instead 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 an 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 "stuff-in-the-middle" are AAA proxies and relays (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.

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

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

3.  Assumptions and Requirements

Some requirements restrict the solution space more than others. In this particular case the main requirement is to re-use an existing infrastructure, namely the AAA framework. Briefly stated: The solution MUST make use of the AAA infrastructure (RADIUS and Diameter). Ideally, modifications at AAA servers SHOULD be kept at a minimum. Modifications to the AAA infrastructure that affect operational aspects MUST NOT be made.

The next requirement concerns security: The relying party MUST NOT get in possession of the long-term secret of the entity that is authenticated towards the AAA server. 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.

Those who are familiar with the AAA framework might realize that the choices are limited. The standardized Extensible Authentication Protocol (EAP) framework [RFC3748] (Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP),” June 2004.) fits the above requirements and is widely deployed.

Assuming that this design decision is taken for granted the remaining work is with the integration of the AAA infrastructure into non-Web-based application protocols. Figure 2 (Front-End Integration) illustrates it graphically.

                                  +--------------+ business
                                  |AAA Server    | agreements
                                  |(Identity     | <......+
                                  |Provider)     |        .
                                  |              |        .
                                  +------------+-+        .
                                  --^----------|--   .    .
                             /////  |          |  \\\\\   .
                           //       |          |       \\ . ***
                          |         |  AAA     |         |. back-
                          |         | protocol |         |. end
                          |         |          |         |. ***
                           \\       |          |       // .
                             \\\\\  |          |  /////   .
                                  --|----------|--   .    .
                  Authentication    |          |          .
                  and Security      |          v          .
 +-------------+  Layer           +-+----------+--+       .
 |             |<---------------->|               |<-.....+
 | Application |                  | Server Side   |
 | @ End Host  |  Application     | Application   |
 |             |<================>|(Relying Party)|
 +-------------+  Application     +---------------+
                  Data

                  *** front-end ***

The front-end (end host to relying party) communication MUST be integrated into the authentication framework available to the back-end. As argued previously the end-to-end authentication framework is EAP. 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.)) the challenge remains to carry EAP payloads from the end host to the relying party using some mechanism.

For illustrative purposes, some examples of the front-end protocols suitable for carrying EAP are "TLS using EAP Authentication" [I‑D.nir‑tls‑eap] (Nir, Y., Sheffer, Y., Tschofenig, H., and P. Gutmann, “TLS using EAP Authentication,” July 2010.), and GSS-API Mechanism for the EAP [I‑D.howlett‑eap‑gss] (Hartman, S. and J. Howlett, “A GSS-API Mechanism for the Extensible Authentication Protocol,” March 2010.).

The changes to the end host and the changes to the relying party SHOULD be kept at a minimum. A mechanism that can demonstrate deployment benefits (based on ease of update of existing software, low implementation effort, etc.) MUST be preferred. There MAY be a need to specify multiple mechanisms to support the range of different deployment scenarios.

4.  Security Considerations

This entire document is about security.

5.  IANA Considerations

This document does not require actions by IANA.

6.  Acknowledgments

The author would like to thank Sam Hartman for a discussion about all aspects of the "Federated Authentication Beyond The Web" effort when he was visiting MIT in June 2010.

I would like to thank Mayutan Arumaithurai and Klaas Wierenga for their feedback.

7.  References

7.1. Normative References

7.2. Informative References

Author's Address