One document matched: draft-ietf-abfab-arch-03.xml
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<rfc category="info" docName="draft-ietf-abfab-arch-03.txt" ipr="trust200902">
<front>
<title abbrev="ABFAB Architecture">Application Bridging for Federated Access Beyond Web (ABFAB) Architecture</title>
<author initials="J." surname="Howlett" fullname="Josh Howlett">
<organization>JANET(UK)</organization>
<address>
<postal>
<street>Lumen House, Library Avenue, Harwell</street>
<city>Oxford</city>
<code>OX11 0SG</code>
<country>UK</country>
</postal>
<phone>+44 1235 822363</phone>
<email>Josh.Howlett@ja.net</email>
</address>
</author>
<author initials="S." surname="Hartman" fullname="Sam Hartman">
<organization>Painless Security</organization>
<address>
<postal>
<street> </street>
<city> </city>
<code> </code>
<country> </country>
</postal>
<phone> </phone>
<email>hartmans-ietf@mit.edu</email>
</address>
</author>
<author initials="H." surname="Tschofenig" fullname="Hannes Tschofenig">
<organization>Nokia Siemens Networks</organization>
<address>
<postal>
<street>Linnoitustie 6</street>
<city>Espoo</city>
<code>02600</code>
<country>Finland</country>
</postal>
<phone>+358 (50) 4871445</phone>
<email>Hannes.Tschofenig@gmx.net</email>
<uri>http://www.tschofenig.priv.at</uri>
</address>
</author>
<author fullname="Eliot Lear" initials="E." surname="Lear">
<organization>Cisco Systems GmbH</organization>
<address>
<postal>
<street>Richtistrasse 7</street>
<city>Wallisellen</city>
<code>CH-8304</code>
<region>ZH</region>
<country>Switzerland</country>
</postal>
<phone>+41 44 878 9200</phone>
<email>lear@cisco.com</email>
</address>
</author>
<author fullname="Jim Schaad" initials="J." surname="Schaad">
<organization>Soaring Hawk Consulting</organization>
<address>
<email>ietf@augustcellars.com</email>
</address>
</author>
<date/>
<area>Security</area>
<workgroup>ABFAB</workgroup>
<keyword>Internet-Draft</keyword>
<keyword>Federated Authentication</keyword>
<keyword>AAA</keyword>
<keyword>RADIUS</keyword>
<keyword>Diameter</keyword>
<keyword>GSS-API</keyword>
<keyword>EAP</keyword>
<keyword>SASL</keyword>
<keyword>SAML</keyword>
<abstract>
<t>
Over the last decade a substantial amount of work has occurred in
the space of federated access management. Most of
this effort has focused on two use-cases: network and
web-based access. However, the solutions to these use-cases that
have been proposed and deployed tend to have few common building blocks in common.
</t>
<t>
This memo describes an architecture that makes use of
extensions to the commonly used security mechanisms for both federated and
non-federated access management, including
the Remote Authentication Dial In User Service (RADIUS)
and the Diameter protocol, the Generic Security Service (GSS), the GS2 family, the Extensible Authentication Protocol (EAP) and the Security Assertion Markup Language (SAML). The architecture addresses
the problem of federated access management to primarily non-web-based
services, in a manner that will scale to large numbers of identity providers, relying parties, and
federations.
</t>
</abstract>
</front>
<middle>
<!-- ////////////////////////////////////////////////////////////////////////////////// -->
<section anchor="introduction" title="Introduction">
<t>
The Internet uses numerous security mechanisms to manage
access to various resources. These mechanisms have been generalized
and scaled over the last decade through mechanisms such as
<xref target="RFC5801">Simple Authentication and Security Layer (SASL) with the Generic Security Server Application Program Interface (GSS-API) (known as the GS2 family)</xref>, <xref target="OASIS.saml-core-2.0-os">Security Assertion Markup Language (SAML)</xref>,
RADIUS <xref target="RFC2865"/>, and Diameter <xref target="RFC3588"/>.
</t>
<t>
A Relying Party (RP) is the entity that manages access to some resource.
The actor that is requesting access to that resource is often described as the Subject.
Many security mechanisms are manifested as an exchange of information between these actors.
The RP is therefore able to decide whether the Subject is authorised, or not.
</t>
<t>
Some security mechanisms allow the RP to delegate aspects
of the access management decision to an actor called the
Identity Provider (IdP).
This delegation requires technical signaling, trust and a common understanding of
semantics between the RP and IdP. These aspects are generally managed within a
relationship known as a 'federation'. This style of access management is accordingly
described as 'federated access management'.
</t>
<t>
Federated access management has evolved over
the last decade through such standards as SAML <xref target="OASIS.saml-core-2.0-os"/>,
<eref target="http://www.openid.net">OpenID</eref>, OAuth <xref target="RFC5849"/>, <xref target="I-D.ietf-oauth-v2"/> and WS-Trust <xref target="WS-TRUST"/>. The benefits of federated access management include:
<list style="hanging">
<t hangText="Single or Simplified sign-on:">
<vspace blankLines="1"/> An Internet service can delegate access management,
and the associated responsibilities such as identity management and credentialing,
to an organisation that already has a long-term relationship with the Subject.
This is often attractive for Relying Parties who frequently do not want these responsibilities.
The Subject also requires fewer credentials, which is also desirable.
</t>
<t hangText="Privacy:"><vspace blankLines="1"/>
Often a Relying Party does not need to know the identity of a Subject to
reach an access management decision. It is frequently only necessary for the
Relying Party know specific attributes about the subject, for example, that the Subject is affiliated with
a particular organisation or has a certain role or entitlement. Sometimes the
RP does not need to know the identity of the Subject, but does require a unique identifier for the Subject (for example, so that it can recognise
the Subject subsequently); in this case, it is a common practise for the IdP to only
release a pseudonym that is specific to that particular Relying Party.
Federated access management therefore provides various strategies for protecting
the Subject's privacy. Other privacy aspects typically of concern are the policy for releasing
personal data about the Subject from the IdP to the RP, the purpose of the usage, the retention period
of the data, and many more.
</t>
<t hangText="Provisioning"><vspace blankLines="1"/>
Sometimes a Relying Party needs, or would like, to know more about
a subject than an affiliation or a pseudonym. For example, a Relying Party may want
the Subject's email address or name. Some federated access management technologies
provide the ability for the IdP to supply this information, either on request
by the RP or unsolicited.
</t>
</list>
</t>
<t>
This memo describes the Application Bridging for Federated Access Beyond the Web (ABFAB) architecture.
This architecture makes use of
extensions to the commonly used security mechanisms for both federated and
non-federated access management, including
the RADIUS and the Diameter protocol, the Generic Security Service (GSS), the GS2 family,
the Extensible Authentication Protocol (EAP) and 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.
</t>
<!-- ////////////////////////////////////////////////////////////////////////////////// -->
<section anchor="terminology" title="Terminology">
<t>This document uses identity management and privacy
terminology from
<xref target="I-D.iab-privacy-terminology"/>.
In particular, this document uses the terms identity provider, relying party, (data) subject, identifier,
pseudonymity, unlinkability, and anonymity.</t>
<t>In this architecture the IdP
consists of the following components: an EAP server, a RADIUS or a Diameter
server, and optionally a SAML Assertion service.
<!-- The reader is advised that for succinctness, in most cases the
term IdP is used, except where additional clarity seems
appropriate.-->
</t>
<t>This document uses the term Network Access Identifier (NAI), as defined in <xref target="RFC4282"/>.</t>
<t>One of the problems people will find with reading this document is that the terminology sometimes appears to be inconsistent. This is due the fact that the terms used by the different standards we are picking up don't use the same terms. In general the document uses either a consistent term or the term associated with the standard under discussion as appropriate. For reference we include this table which maps the different terms into a single table.</t>
<!-- Expand EAP -->
<!-- Expand ABFAB -->
<texttable>
<ttcol align='left'>Protocol</ttcol> <ttcol align='left'>Subject</ttcol> <ttcol align='left'>Relying Party</ttcol> <ttcol align='left'>Identity Provider</ttcol>
<c>ABFAB</c> <c>Subject</c> <c>Relying Party (RP)</c> <c>Identity Provider (IdP)</c>
<c></c> <c>Principal</c> <c></c> <c></c>
<c></c> <c>Client</c> <c></c> <c></c>
<c>SAML</c> <c>Subject</c> <c>Service Provider</c> <c>Issuer</c>
<c>GSS-API</c> <c>Initiator</c> <c>Acceptor</c> <c></c>
<c>EAP</c> <c>EAP client</c> <c></c> <c>EAP server</c>
<c></c> <c>EAP peer</c> <c></c> <c></c>
<c>SASL</c> <c></c> <c></c> <c></c>
<c>AAA</c> <c></c> <c>AAA Client</c> <c>AAA server</c>
<c>RADIUS</c> <c>client</c> <c>NAS</c> <c>RADIUS server</c>
</texttable>
<t>Note that in some cases a cell has been left empty, in these cases there is no direct name that represents this concept.</t>
<t>Note to reviewers - I have most likely missed some entries in the table. Please provide me with both correct names from the protocol and missing names that are used in the text below.</t>
</section>
<section title="An Overview of Federation">
<t>
In the previous section we introduced the following actors:
</t>
<t>
<list style="symbols">
<t>the Subject,</t>
<t>the Identity Provider, and </t>
<t>the Relying Party.</t>
</list>
</t>
<t>
These entities and their relationships are illustrated graphically in <xref
target="framework"/>.
</t>
<t>
<figure title="Entities and their Relationships" anchor="framework">
<artwork><![CDATA[
,----------\ ,---------\
| Identity | Federation | Relying |
| Provider + <-------------------> + Party |
`----------' '---------'
<
\
\ Authentication
\
\
\
\
\ +---------+
\ | | O
v| Client | \|/ Principal
| | |
+---------+ / \
]]></artwork>
</figure>
</t>
<!-- M00QUEST - talk about Subject, Federation and Authentication? as in give some type of definition before below-->
<t>
A federation agreement typically encompasses
operational specifications and legal rules:
</t>
<t>
<list style="hanging">
<t hangText="Operational Specifications:"> <vspace blankLines="1"/>
These includes the technical specifications (e.g. protocols used to communicate between the three parties),
process standards, policies, identity proofing, credential and authentication algorithm requirements, performance requirements, assessment and audit criteria, etc. The goal of operational specifications is to provide enough definition that the system works and interoperability is possible.</t>
<t hangText="Legal Rules:"> <vspace blankLines="1"/>
The legal rules takes the legal framework into consideration and provides contractual obligations for each entity, defines the responsibilities and provides further clarification of the operational specifications. These legal rules regulate the operational specifications, make operational specifications legally binding to the participants, define and govern the rights and responsibilities of the participants.
The legal rules may, for example, describe liability for losses, termination rights, enforcement mechanisms, measures of damage, dispute resolution, warranties, etc.</t>
</list>
</t>
<t>
The nature of federation dictates that there is some
form of relationship between the identity provider and the relying party.
This is particularly important when the relying party wants to use
information obtained from the identity provider for access management
decisions and when the identity provider does not want to release
information to every relying party (or only under certain conditions).
</t>
<t>
While it is possible to have a bilateral agreement between every
IdP and every RP; on an Internet scale this setup requires the introduction of the
multi-lateral federation concept, as the management of such pair-wise
relationships would otherwise prove burdensome.
</t>
<t>
While many of the non-technical aspects of federation, such as business practices and legal arrangements, are outside the
scope of the IETF, they still impact the architectural setup on how to ensure the dynamic establishment of trust.
</t>
<t>
Some deployments demand the deployment of sophisticated technical
infrastructure, including message routing intermediaries, to offer the
required technical functionality. In other deployments fewer technical components are needed.
</t>
<t>
<xref target="framework"/> also shows the relationship between
the IdP and the Subject. Often a real world entity is associated with the Subject; for example, a person
or some software.
</t>
<t>
The IdP will typically have a long-term relationship with the Subject. This relationship
would typically involve the IdP positively identifying and credentialing the Subject
(for example, at time of enrollment in the context of employment within an organisation).
The relationship will often be instantiated within an agreement between the IdP and the
Subject (for example, within an employment contract or terms of use that stipulates the
appropriate use of credentials and so forth).
</t>
<t>
While federation is often discussed within the context of formal relationships,
such as between an enterprise and an employee or a government and a citizen, federation does
not in require any particular level of formality.
For an IdP and a subject, it is entirely compatible with a relationship between the IdP and Subject
that is only Requiems completing a web form and confirming the verification email.
For an IdP and an RP, it is entirely compatible with the IdP publishing a usage point and the RP using that data.
</t>
<t>
However, the nature and quality of the relationship between the Subject and the IdP is
an important contributor to the level of trust that an RP may attribute to an
assertion describing a Subject made by an IdP. This is sometimes described
as the Level of Assurance.
</t>
<t>
Federation does not imposes requirement of an a priori relationship or a long-term relationship between the RP and the Subject.
This is a property of federation that yields many of its benefits.
However, federation does not preclude the possibility of a pre-existing relationship existing between the RP and the Subject, nor that they may use the introduction to create a new long-term relationship independent of the federation.
</t>
<t>
Finally, it is important to reiterate that in some
scenarios there might indeed be a human behind the device denoted as Client and in other
cases there is no human involved in the actual protocol execution.
</t>
</section>
<section title="Challenges to Contemporary Federation">
<!-- <t>
JH: Much more content needed!
</t>
--> <t>
As the number of federated services has proliferated,
the role of the individual can become ambiguous in certain
circumstances.
For example, a school might
provide online access for a student's grades to their parents
for review, and to the student's teacher for modification.
A teacher who is also a parent must clearly
distinguish her role upon access.
</t>
<t>
Similarly, as the number of federations proliferates, it becomes
increasingly difficult to discover which identity provider(s) a user is
associated with. This is true for both the web and non-web case,
but is particularly acute for the latter as many non-web
authentication systems are not semantically rich enough on their own
to allow for such ambiguities. For instance, in the case of an
email provider, the use of SMTP and IMAP protocols does not on its
own provide for a way to select a federation. However, the building
blocks do exist to add this functionality.
</t>
</section>
<section title="An Overview of ABFAB-based Federation">
<t>
The previous section described the general model of federation,
and its the application of federated access management. This section provides
a brief overview of ABFAB in the context of this model.
</t>
<t>
The steps taken in an ABFAB federated authentication/authorization
exchange are as follows:
</t>
<t>
<list style="numbers">
<t>Principal provides an NAI to Application:
The client application is
configured with an NAI.
The provided name contains, at a minimum, the realm of
an NAI. The realm represents the IdP to be discovered.</t>
<t>
Authentication mechanism selection:
The GSS-EAP SASL/GS2 mechanism is selected for authentication/authorization.
</t>
<t>
Client Application provides the NAI to RP:
The client application setups a transport to the RP and begins the GSS-EAP authentication.
The RP initiates the EAP protocol to the client application, and the client provides the NAI to the RP in the form of the EAP response.
</t>
<t>
Discovery of federated IdP:
The RP uses pre-configured information or a federation proxy to determine what IdP to use based on policy and the provided NAI.
This is discussed in detail below (<xref target="discovery"/>).
</t>
<t>
Request from Relying Party to IdP:
Once the RP knows who the IdP is, it (or its agent) will send a RADIUS/Diameter request to the IdP.
The RADIUS/Diameter access request encapsulates the EAP response.
At this stage, the RP will likely have no idea who the client is.
The RP claims its identity to the IdP in AAA attributes, and
it may contain a SAML Attribute Requests in a AAA attribute.
</t>
<t>
IdP informs the client of which EAP method to use:
The available and appropriate methods are discussed below in this memo.<cref source="JLS">Add section on discussion EAP methods and requirements there on</cref>
</t>
<t>
The EAP protocol is run:
A bunch of EAP messages are passed between the EAP peer and the EAP server, i.e., the client and the IdP in our identity management terminology, until the
result of the authentication protocol is determined. The number and content of those messages will
depend on the EAP method.
If the IdP is unable to authenticate the client, the process concludes here.
As part of the EAP protocol, the client will create a channel bindings message to the IdP identifying, among other things, the RP to which it is attempting to authenticate.
The IdP checks the channel binding data from the client with that provided by the RP via the AAA protocol. If the bindings do not match the IdP fails the EAP protocol.
</t>
<t>
Successful Authentication:
The IdP (its EAP server) and EAP peer / subject have mutually authenticated each other.
As a result of this step, the subject and the IdP hold two cryptographic
keys- a Master Session Key (MSK), and an Extended MSK (EMSK).
There is some confidence that the RP is the one the client is communicating with as the channel binding data validated.
This allows for a claim of authentication for the RP to the client.
</t>
<t>
Local IdP Policy Check:
At this stage, the IdP checks local policy to determine whether the RP and subject are authorized for a given transaction/service, and if so, what if any,
attributes will be released to the RP.
The RP will have done its policy checks during the discovery process.
</t>
<t>
Response from the IdP to the Relying Party:
Once the IdP has made a determination of whether and how to authenticate and authorize the client to the RP, it returns either a negative AAA result to the RP, or it returns a positive result to the RP, along with an optional set of AAA attributes associated with the client (usually as one or more SAML assertions).
In addition, an EAP MSK is returned to the RP.
</t>
<t>
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.
</t>
<t>
RP returns results to client:
Once the RP has a response it must inform the client application of the result.
If all has gone well, all are authenticated, and the application proceeds with appropriate authorization levels.
</t>
</list>
</t>
<t>
An example communication flow is given below:
</t>
<t>
<figure>
<artwork><![CDATA[
Relying Client Identity
Party App Provider
| (1) | Client App gets NAI (somehow)
| | |
|<-----(2)----->| | Mechanism Selection
| | |
|<-----(3)-----<| | NAI transmitted to RP
| | |
|<=====(4)====================>| Discovery
| | |
|>=====(5)====================>| Access request from RP to IdP
| | |
| |< - - (6) - -<| EAP method to Client
| | |
| |< - - (7) - ->| EAP Exchange to authenticate
| | | Client
| | |
| | (8 & 9) Local Policy Check
| | |
|<====(10)====================<| IdP Assertion to RP
| | |
(11) | | RP processes results
| | |
|>----(12)----->| | Results to client app.
----- = Between Client App and RP
===== = Between RP and IdP
- - - = Between Client App and IdP
]]>
</artwork>
</figure>
</t>
</section>
<section title="Design Goals">
<t>Our key design goals are as follows:</t>
<t>
<list style="symbols">
<t>Each party of a transaction will be authenticated, and the
client will be authorized for access to a specific resource.</t>
<t>Means of authentication is decoupled so as to allow for multiple
authentication methods.</t>
<t>Hence, the architecture requires no sharing of long term private
keys.</t>
<t>The system will scale to large numbers of identity providers,
relying parties, and users.</t>
<t>The system will be designed primarily for non-Web-based
authentication.</t>
<t>The system will build upon existing standards, components, and
operational practices.
</t>
</list>
</t>
<t>Designing new three party authentication and authorization
protocols is hard and fraught with risk of cryptographic
flaws. Achieving widespead deployment is even more
difficult. A lot of attention on federated access has been devoted to the Web. This document instead
focuses on a non-Web-based environment and focuses on those protocols where HTTP is not
used. Despite the increased excitement for layering every protocol on top of HTTP there are
still a number of protocols available that do not use HTTP-based transports. Many of these
protocols are lacking a native authentication and authorization framework of the style shown in
<xref target="framework"/>.</t>
</section>
<section title="Use of AAA">
<t>Interestingly, for network access authentication the usage of the AAA framework with RADIUS
<xref target="RFC2865"/> and Diameter <xref target="RFC3588"/> was quite successful from a
deployment point of view. To map the terminology used in <xref target="framework"/> to the
AAA framework the IdP corresponds to the AAA server, the RP
corresponds to the AAA client, and the technical building blocks of a federation are AAA proxies, relays
and redirect agents
(particularly if they are operated by third parties, such as AAA brokers and clearing
houses). The front-end, i.e. the end host to AAA client communication, is in case of network
access authentication offered by link layer protocols that forward authentication protocol
exchanges back-and-forth. An example of a large scale
RADIUS-based federation
is <eref target="http://www.eduroam.org">EDUROAM</eref>.</t>
<t>
By using the AAA framework, ABFAB gets a lot of mileage as many of the federation agreements already exist and merely need to be expanded to cover the ABFAB additions.
The AAA framework has already addressed some of the problems outlined above. For example,
<list style="symbols">
<t>It already needs a method of doing routing of requests based on a domain.</t>
<t>It already has an extensible architecture allowing for new attributes to be defined and transported.</t>
<t>Pre-existing relationships can be re-used.</t>
</list>
</t>
</section>
<section title="Use of GSS-API">
<t>Expand here</t>
</section>
<!-- ////////////////////////////////////////////////////////////////////////////////// -->
</section>
<section title="Architecture">
<t>
We have already introduced the federated access architecture, with the illustration of the different actors that need to interact, but did not expand on the specifics of providing support for non-Web based applications.
This section details this aspect and motivates design decisions.
The main theme of the work described in this document is focused on re-using existing building blocks that have been deployed already and to re-arrange them in a novel way.
</t>
<t>
Although this architecture assumes updates to the relying party, the client application and the Identity Provider, those changes are kept at a minimum.
A mechanism that can demonstrate deployment benefits (based on ease of update of existing software, low implementation effort, etc.) is preferred and there may be a need to specify multiple mechanisms to support the range of different deployment scenarios.
</t>
<t>
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 <xref target="I-D.ietf-abfab-gss-eap"/>.
Other alternatives exist as well and may be considered later, such as "TLS using EAP Authentication" <xref target="I-D.nir-tls-eap"/>.<cref source="JLS">I don't believe this is a true statement - check it with Josh and Sam.</cref>
</t>
<t>
The architecture consists of several building blocks, which is shown graphically in <xref target="abfab-arch"/>.
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.
</t>
<!--
<t>We have 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.</t>
<t>
Although this architecture assumes updates both to the relying party and the client 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.
</t>
<t>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 <xref target="I-D.ietf-abfab-gss-eap"/>. Other alternatives exist as well and may be considered later, such as "TLS using EAP Authentication" <xref target="I-D.nir-tls-eap"/>.<cref source="JLS">I don't believe this is a true statement - check it with Josh and Sam.</cref></t>
<t>The architecture consists of several building blocks, which is shown graphically in <xref target="abfab-arch"/>.
The subsections below explain relationship of the protocol components in more detail.
</t>
-->
<t><figure title="ABFAB Protocol Instantiation" anchor="abfab-arch">
<artwork><![CDATA[
+--------------+
| Identity |
| Provider |
| (IdP) |
+-^----------^-+
* EAP o RADIUS/
* o Diameter
--v----------v--
/// \\\
// \\
| Federation |
| Substrate |
\\ //
\\\ ///
--^----------^--
* EAP o RADIUS/
* o Diameter
+-------------+ +-v----------v--+
| |<---------------->| |
| Client | EAP/EAP Method | Relying Party |
| Application |<****************>| (RP) |
| | GSS-API | |
| |<---------------->| |
| | Application | |
| | Protocol | |
| |<================>| |
+-------------+ +---------------+
Legend:
<****>: Client-to-IdP Exchange
<---->: Client-to-RP Exchange
<oooo>: RP-to-IdP Exchange
<====>: Protocol through which GSS-API/GS2 exchanges are tunnelled
]]></artwork>
</figure>
</t>
<section title="Relying Party to Identity Provider">
<t>
Communications between the Relying Part and the Identity Provider is done by the federation substrate.
This communication channel is responsible for:
<list style="symbols">
<t>Establishing the trust relationship between the RP and the IdP.</t>
<t>Determining the Rules governing the relationship.</t>
<t>Conveying packets between the RP and IdP.</t>
<t>Providing the means of establishing a trust relationship between the RP and the client.</t>
</list>
</t>
<t>
The ABFAB working group has chosen the AAA framework for the messages transported between the RP and IdP.
This allows for the standard AAA framework to be used to establish the trust relationship between the RP and the IdP while allowing other newer routing mechanisms using the same message format to be used in the future.
The ABFAB protocol itself details the method of establishing the trust relationship between the RP and the client.
</t>
<section title="AAA, RADIUS and Diameter">
<t>
The IETF has defined a federation framework already with the Authentication, Authorization and Accounting (AAA) framework <xref target="RFC2903"/>, <xref target="RFC2904"/>.
Two implementations of the AAA framework exist in RADIUS <xref target="RFC2138"/> and Diameter <xref target="RFC3588"/> protocols.
The existence of these protocols allows us to re-use a pair of existing protocols that have been widely deployed and are reasonable well understood.
These protocols are nicely designed so that they can carry additional attributes with minimal changes to either the protocol or existing AAA servers.
</t>
<t>
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 UDP, 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.
The protocol defines all the necessary new AAA attributes as RADIUS attributes, this allows for the same structures and attributes to be used in both RADIUS and Diameter.
We also note that there exist proxies which convert from RADIUS to Diameter and back.
This makes it possible for both to be deployed in a single federation substrate.
</t>
<t>
Through the integrity protection mechanisms in the AAA framework, the identity provider can establish technical trust that messages are being sent by the appropriate relying party.
Any given interaction will be associated with one federation at the policy level.
The legal or business relationship defines what statements the identity provider is trusted to make and how these statements are interpreted by the relying party.
The AAA framework also permits the relying party or elements between the relying party and identity provider to make statements about the relying party.
</t>
<t>
The AAA framework provides transport for attributes.
Statements made about the subject by the identity provider, statements made about the relying party and other information are transported as attributes.
</t>
<t>
One demand that the AAA substrate makes of the upper layers is that they must properly identify the end points of the communication.
It must be possible for the AAA client at the RP to determine where to send each RADIUS or Diameter message.
Without this requirement, it would be the RP's responsibility to determine the identity of the client on its own, without the assistance of an IdP.
This architecture makes use of the Network Access Identifier (NAI), where the IdP is indicated by the realm component <xref target="RFC4282"/>.
The NAI is represented and consumed by the GSS-API layer as GSS_C_NT_USER_NAME as specified in <xref target="RFC2743"/>.
The GSS-API EAP mechanism includes the NAI in the EAP Response/Identity message.
</t>
</section>
<section title="Discovery and Rules Determination" anchor="discovery">
<t>
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:
<list style="symbols">
<t>The federation selected must be able to communicate with the IdP.</t>
<t>The federation selected must match the business rules and technical policies required for the RP security requirements.</t>
</list>
</t>
<t>
The RP needs to discover which federation will be used to contact the IdP.
The first selection criteria in discovery is going to be the name of the IdP to be contacted.
The second selection criteria in discovery is going to be the set of business rules and technical policies governing the relationship; this is called rules determination.
The RP also needs to establish technical trust in the communications with the IdP.
</t>
<t>
Rules determination covers a broad range of decisions about the exchange.
One of these is whether the given RP is permitted to talk to the IdP using a given federation at all, so rules determination encompasses the basic authorization decision.
Other factors are included, such as what policies govern release of information about the principal to the RP and what policies govern the RP's use of this information.
While rules determination is ultimately a business function, it has significant impact on the technical exchanges.
The protocols need to communicate the result of authorization.
When multiple sets of rules are possible, the protocol must disambiguate which set of rules are in play.
Some rules have technical enforcement mechanisms; for example in some federations intermediates validate information that is being communicated within the federation.
</t>
<t>
ABFAB has not formally defined any part of discovery at this point. The process of specifying and evaluating the business rules and technical policies is too complex to provide a simple framework. There is not currently a way to know if a AAA proxy is able to communicate with a specific IdP, although this may change with some of the routing protocols that are being considered. At the present time, the discovery process is going to be a manual configuration process.
</t>
</section>
<section title="Routing and Technical Trust">
<t>
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 constrain the names of both parties to the exchange.
The trust mechanism ought to ensure that the entity acting as IdP for a given NAI is permitted to be the IdP for that realm, and that any service name claimed by the RP is permitted to be claimed by that entity.
Here are the categories of technical trust determination:
<list style="hanging">
<t hangText="AAA Proxy:"><vspace/>
The simplest model is that an RP supports a request directly to an AAA proxy.
The hop-by-hop integrity protection of the AAA fabric provides technical trust.
An RP can submit a request directly to a federation.
Alternatively, a federation disambiguation fabric can be used.
Such a fabric takes information about what federations the RP is part of and what federations the IdP is part of and routes a message to the appropriate federation.
The routing of messages across the fabric plus attributes added to requests and responses provides rules determination.
For example, when a disambiguation fabric routes a message to a given federation, that federation's rules are chosen.
Naming is enforced as messages travel across the fabric.
The entities near the RP confirm its identity and validate names it claims.
The fabric routes the message towards the appropriate IdP, validating the IdP's name in the process.
The routing can be statically configured.
Alternatively a routing protocol could be developed to exchange reachability information about given IdPs and to apply policy across the AAA fabric.
Such a routing protocol could flood naming constraints to the appropriate points in the fabric.
</t>
<t hangText="Trust Broker:"><vspace/>
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.
</t>
<t hangText="Global Credential:"><vspace/>
A global credential such as a public key and certificate in a public key infrastructure can be used to establish technical trust.
A directory or distributed database such as the Domain Name System is used by the RP to discover the endpoint to contact for a given NAI.
Either the database or certificates can provide a place to store information about rules determination and naming constraints.
Provided that no intermediates are required (or appear to be required) and that the RP and IdP are sufficient to enforce and determine rules, rules determination is reasonably simple.
However applying certain rules is likely to be quite complex.
For example if multiple sets of rules are possible between an IdP and RP, confirming the correct set is used may be difficult.
This is particularly true if intermediates are involved in making the decision.
Also, to the extent that directory information needs to be trusted, rules determination may be more complex.
</t>
</list>
</t>
<t>
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 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.
</t>
</section>
<section title="SAML Assertions">
<t>For the traditional use of AAA frameworks, network access, the only requirement that was necessary to grant access was an affirmative response from the IdP. In the ABFAB world, the RP may need to get additional information about the client before granting access. ABFAB therefore has a requirement that it can transport an arbitrary set of attributes about the client from the IdP to the RP.</t>
<t>Security Assertions Markup Language (SAML) <xref target="OASIS.saml-core-2.0-os"/> was designed in order to carry an extensible set of attributes about a subject. Since SAML is extensible in the attribute space, ABFAB has no immediate needs to update the core SAML specifications for our work. It will be necessary to update IdPs that need to return SAML assertions to IdPs and for both the IdP and the RP to implement a new SAML profile designed to carry SAML assertions in AAA. The new profile can be found in RFCXXXX <xref target="I-D.ietf-abfab-aaa-saml"/>.</t>
<t>There are two issues that need to be highlighted:
<list style="symbols">
<t>The security of SAML Assertions.</t>
<t>Namespaces and mapping of SAML attributes.</t>
</list>
</t>
<t>SAML Assertions have an optional signature that can be used to protect and provide origination of the assertion. These signatures are normally based on asymmetric key operations and require that the verifier be able to check not only the cryptographic operation, but also the binding of the originators name and the public key. In a federated environment it will not always be possible for the RP to validate the binding, for this reason the technical trust established in the federation is used as an alternate method of validating the origination and integrity of the SAML Assertion.</t>
<t>Attributes placed in SAML Assertions can have different namespaces assigned to the same name. In many, but not all, cases a the federation agreements will determine what attributes can be used in a SAML statement. This means that the RP needs to map from the federation names, types and semantics into the onces that the policies of the RP are written in. In other cases the federation substrate may modify the SAML Assertions in transit to do the necessary namespace, naming and semantic mappings as the assertion crosses the different boundaries in the federation. If the proxies are modifying the SAML Assertion, then will obviously remove any signatures on the SAML Assertions as they would no longer validate. In this case the technical trust is the required mechanism for validating the integrity of the assertion. In the last case, 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.</t>
</section>
</section>
<section title="Client To Identity Provider">
<t>
Looking at the communications between the client and the IdP, the following items need to be dealt with:
<list style="symbols">
<t>The client and the IdP need to mutually authenticate each other.</t>
<t>The client and the IdP need to mutually agree on the identity of the RP.</t>
</list>
</t>
<t>
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.
ABFAB has defined and specified a new channel binding mechanism that operates as an EAP method for the purpose of agreeing on the identity of the RP.
</t>
<section title="Extensible Authentication Protocol (EAP)">
<t>
Traditional web federation does not describe how a subject interacts with an identity provider for authentication.
As a result, this communication is not standardized.
There are several disadvantages to this approach.
It is difficult to have subjects that are machines rather than humans that use some sort of programatic credential.
In addition, use of browsers for authentication restricts the deployment of more secure forms of authentication beyond plaintext username and password known by the server.
In a number of cases the authentication interface may be presented before the subject has adequately validated they are talking to the intended server.
By giving control of the authentication interface to a potential attacker, then the security of the system may be reduced and phishing opportunities introduced.
</t>
<t>
As a result, it is desirable to choose some standardized approach for communication between the subject's end-host and the identity provider.
There are a number of requirements this approach must meet.
</t>
<t>
Experience has taught us one key security and scalability requirement: it is important that the relying party not get possession of the long-term secret of the client.
Aside from a valuable secret being exposed, a synchronization problem can develop when the client changes keys with the IdP.
</t>
<t>
Since there is no single authentication mechanism that will be used everywhere there is another associated requirement:
The authentication framework must allow for the flexible integration of authentication mechanisms.
For instance, some IdPs require hardware tokens while others use passwords.
A service provider wants to provide support for both authentication methods, and other methods from IdPs not yet seen.
</t>
<t>
Fortunately, these requirements can be met by utilizing standardized and successfully deployed technology, namely by the Extensible Authentication Protocol (EAP) framework <xref target="RFC3748"/>.
<xref target="abfab-arch"/> illustrates the integration graphically.
</t>
<t>
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 <xref target="RFC3579"/> and <xref target="RFC4072"/>) several challenges remain:
<list style="symbols">
<t>The first is how to carry EAP payloads from the end host to the relying party.</t>
<t>Another is to verify statements the relying party has made to the subject, confirm these statements are consistent with statements made to the identity provider and confirm all the above are consistent with the federation and any federation-specific policy or configuration.</t>
<t>Another challenge is choosing which identity provider to use for which service.</t>
</list>
</t>
</section>
<section title="Channel Binding">
<t>
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 can be found in <xref target="I-D.hartman-emu-mutual-crypto-bind"/>.
</t>
<t>
The recommended way to deal with channel binding can be found in RFC XXXX <xref target="I-D.ietf-emu-chbind"/>.
</t>
</section>
</section>
<section title="Client to Relying Party">
<t>
The final set of interactions between parties to consider are those between the client and the RP. In some ways this is the most complex set since at least part of it is outside the scope of the ABFAB work. The interactions between these parties include:
<list style="symbols">
<t>Running the protocol that implements the web service that is provided by the RP and desired by the client.</t>
<t>Authenticating the client to the RP and the RP to the client.</t>
<t>Providing the necessary security services to the web service protocol that it needs beyond authentication.</t>
</list>
</t>
<section title="GSS-API">
<t>
One of the remaining layers is responsible for integration of federated authentication into the application.
There are a number of approaches that applications have adopted for security.
So, there may need to be multiple strategies for integration of federated authentication into applications.
However, we have started with a strategy that provides integration to a large number of application protocols.
</t>
<t>
Many applications such as SSH <xref target="RFC4462"/>, NFS <xref target="RFC2203"/>, DNS <xref target="RFC3645"/> and several non-IETF applications support the Generic Security Services Application Programming Interface <xref target="RFC2743"/>.
Many applications such as IMAP, SMTP, XMPP and LDAP support the Simple Authentication and Security Layer (SASL) <xref target="RFC4422"/> framework.
These two approaches work together nicely: by creating a GSS-API mechanism, SASL integration is also addressed.
In effect, using a GSS-API mechanism with SASL simply requires placing some headers on the front of the mechanism and constraining certain GSS-API options.
</t>
<t>
GSS-API is specified in terms of an abstract set of operations which can be mapped into a programming language to form an API.
When people are first introduced to GSS-API, they focus on it as an API.
However, from the prospective of authentication for non-web applications, GSS-API should be thought of as a protocol not an API.
It consists of some abstract operations such as the initial context exchange, which includes two sub-operations (gss_init_sec_context and gss_accept_sec_context).
An application defines which abstract operations it is going to use and where messages produced by these operations fit into the application architecture.
A GSS-API mechanism will define what actual protocol messages result from that abstract message for a given abstract operation.
So, since this work is focusing on a particular GSS-API mechanism, we generally focus on protocol elements rather than the API view of GSS-API.
</t>
<t>
The API view has significant value.
Since the abstract operations are well defined, the set of information that a mechanism gets from the application is well defined.
Also, the set of assumptions the application is permitted to make is generally well defined.
As a result, an application protocol that supports GSS-API or SASL is very likely to be usable with a new approach to authentication including this one with no required modifications.
In some cases, support for a new authentication mechanism has been added using plugin interfaces to applications without the application being modified at all.
Even when modifications are required, they can often be limited to supporting a new naming and authorization model.
For example, this work focuses on privacy; an application that assumes it will always obtain an identifier for the principal will need to be modified to support anonymity, unlinkability or pseudonymity.
</t>
<t>
So, we use GSS-API and SASL because a number of the application protocols we wish to federate support these strategies for security integration.
What does this mean from a protocol standpoint and how does this relate to other layers?
This means we need to design a concrete GSS-API mechanism.
We have chosen to use a GSS-API mechanism that encapsulates EAP authentication.
So, GSS-API (and SASL) encapsulate EAP between the end-host and the service.
The AAA framework encapsulates EAP between the relying party and the identity provider.
The GSS-API mechanism includes rules about how principals and services are named as well as per-message security and other facilities required by the applications we wish to support.
</t>
</section>
<section title="Protocol Transport">
<t>The transport of data between the client and the relying party is not provided by GSS-API. GSS-API creates and consumes messages, but it does not provide the transport itself, instead the protocol using GSS-API needs to provide the transport. In many cases HTTP or HTTPS is used for this transport, but other transports are perfectly acceptable. The core GSS-API document <xref target="RFC2743"/> provides some details on what requirements exist.</t>
<t>
In addition we highlight the following:
<list style="symbols">
<t>The transport does not need to provide either privacy or integrity. After GSS-EAP has finished negotiation, GSS-API can be used to provide both services. If the negotiation process itself needs protection from eavesdroppers then the transport would need to provide the necessary services.</t>
<t>The transport needs to provide reliable transport of the messages.</t>
<t>The transport needs to ensure that tokens are delivered in order during the negotiation process.</t>
<t>GSS-API messages need to be delivered atomically. If the transport breaks up a message it must also reassemble the message before delivery.</t>
</list>
</t>
</section>
</section>
<!-- <section title="Personalization Layer">
<t>The AAA framework provides a way to transport statements
from the identity provider to the relying party. However, we
also need to say more about the content of these
statements. In simple cases, attributes particular to the AAA
protocol can be defined. However in more complicated
situations it is strongly desirable to re-use an existing
protocol for asking questions and receiving information about
subjects. SAML is used for this. </t>
<t>SAML usage may be as simple as the identity provider
including a SAML Response message in the AAA
response. Alternatively the relying party may generate a SAML
request XXX to whom, how, and at what point? (see above XXX).</t>
</section>
-->
<!--
<section title="Relying Party to Identity Provider">
<t>The federation substrate is responsible for the communication between the relying party and the identity provider. This layer is responsible for the inter-domain communication and for the technical mechanisms necessary to establish inter-domain trust.</t>
<t>A key design goal is the re-use of an existing infrastructure,
we build upon the AAA framework as utilized by RADIUS
<xref target="RFC2138"/> and Diameter
<xref target="RFC3588"/>. Since this document does not aim to
re-describe the AAA framework the interested reader is referred
to <xref target="RFC2904"/>. 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.</t>
<t>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 UDP, 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.
</t>
<t>Through the integrity protection mechanisms in the AAA framework, the relying party can establish technical trust that messages are being sent by the appropriate relying party. Any given interaction will be associated with one federation at the policy level. The legal or business relationship defines what statements the identity provider is trusted to make and how these statements are interpreted by the relying party. The AAA framework also permits the relying party or elements between the relying party and identity provider to make statements about the relying party. </t>
<t>The AAA framework provides transport for attributes. Statements made about the subject by the identity provider, statements made about the relying party and other information is transported as attributes.</t>
<section title="Discovery, Rules Determination, and Technical Trust" anchor="Discovery">
<t>
One demand that the AAA substrate makes of the upper layers is that they must properly identify the end points of the communication.
It must be possible for the AAA client at the RP to determine where to send each RADIUS or Diameter message.
Without this requirement, it would be the RP's responsibility to determine the identity of the principal on its own, without the assistance of an IdP.
This architecture makes use of the Network Access Identifier (NAI), where the IdP is indicated in the realm component <xref target="RFC4282"/>.
The NAI is represented and consumed by the GSS-API layer as GSS_C_NT_USER_NAME as specified in <xref target="RFC2743"/>.
The GSS-API EAP mechanism includes the NAI in the EAP Response/Identity message.
</t>
<t>
The RP needs to discover which federation will be used to contact the IdP.
As part of this process, the RP determines 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.
</t>
<t>
Rules determination covers a broad range of decisions about the exchange.
One of these is whether the given RP is permitted to talk to the IdP using a given federation at all, so rules determination encompasses the basic authorization decision.
Other factors are included, such as what policies govern release of information about the principal to the RP and what policies govern the RP's use of this information.
While rules determination is ultimately a business function, it has significant impact on the technical exchanges.
The protocols need to communicate the result of authorization.
When multiple sets of rules are possible, the protocol must disambiguate which set of rules are in play.
Some rules have technical enforcement mechanisms; for example in some federations intermediates validate information that is being communicated within the federation.
</t>
<t>
Several deployment approaches are possible.
These 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 constrain the names of both parties to the exchange.
The trust mechanism ought to ensure that the entity acting as IdP for a given NAI is permitted to be the IdP for that realm, and that any service name claimed by the RP is permitted to be claimed by that entity.
Here are the categories of technical trust determination:
<list style="hanging">
<t hangText="AAA Proxy:">The simplest model is that an
RP supports a request directly to an AAA proxy. The
hop-by-hop integrity protection of the AAA fabric provides
technical trust. An RP can submit a request directly to a
federation. Alternatively, a federation disambiguation
fabric can be used. Such a fabric takes information about
what federations the RP is part of and what federations the
IdP is part of and routes a message to the appropriate
federation. The routing of messages across the fabric plus
attributes added to requests and responses provides rules
determination. For example, when a disambiguation fabric
routes a message to a given federation, that federation's
rules are chosen. Naming is enforced as messages travel
across the fabric. The entities near the RP confirm its
identity and validate names it claims. The fabric routes the
message towards the appropriate IdP, validating the IdP's
name in the process. The routing can be statically
configured. Alternatively a routing protocol could be
developed to exchange reachability information about given
IdPs and to apply policy across the AAA fabric. Such a
routing protocol could flood naming constraints to the
appropriate points in the fabric.</t>
<t hangText="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.</t>
<t hangText="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 needed for an RP to discover what endpoint to
contact for a given NAI. Certificates provide a place to
store information about rules determination and naming
constraints. Provided that no intermediates are required and
that the RP and IdP are sufficient to enforce and determine
rules, rules determination is reasonably simple. However
applying certain rules is likely to be quite complex. For
example if multiple sets of rules are possible between an
IdP and RP, confirming the correct set is used may be
difficult. This is particularly true if intermediates are
involved in making the decision. Also, to the extent that
directory information needs to be trusted, rules
determination may be more complex.</t>
</list>
</t>
<t>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 MAY use any of the three methods to
get closer to the IdP. It is also likely that rather than
being directly reachable, an IdP may have a proxy within its
organization. Federations MAY provide a traditional AAA
proxy interface even if they also provide another mechanism
for increased efficiency or security.</t>
</section>
<section title="Client To Identity Provider">
<t>
Traditional web federation does not describe how a subject interacts with an identity provider for authentication.
As a result, this communication is not standardized.
There are several disadvantages to this approach.
It is difficult to have subjects that are machines rather than humans that use some sort of programatic credential.
In addition, use of browsers for authentication restricts the deployment of more secure forms of authentication beyond plaintext username and password known by the server.
In a number of cases the authentication interface may be presented before the subject has adequately validated they are talking to the intended server.
By giving control of the authentication interface to a potential attacker, then the security of the system may be reduced and phishing opportunities introduced.
</t>
<t>
As a result, it is desirable to choose some standardized approach for communication between the subject's end-host and the identity provider.
There are a number of requirements this approach must meet.
</t>
<t>
Experience has taught us one key security and scalability requirement: it is important that the relying party not get possession of the long-term secret of the client.
Aside from a valuable secret being exposed, a synchronization problem can develop when the client changes keys with the IdP.
</t>
<t>
Since there is no single authentication mechanism that will be used everywhere there is another associated requirement:
The authentication framework must allow for the flexible integration of authentication mechanisms.
For instance, some IdPs require hardware tokens while others use passwords.
A service provider wants to provide support for both authentication methods, and other methods from IdPs not yet seen.
</t>
<t>
Fortunately, these requirements can be met by utilizing standardized and successfully deployed technology, namely by the Extensible Authentication Protocol (EAP) framework <xref target="RFC3748"/>.
<xref target="abfab-arch"/> illustrates the integration graphically.
</t>
<t> 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
<xref target="RFC3579"/> and <xref target="RFC4072"/>)
several challenges remain: one is to carry EAP payloads
from the end host to the relying party. Another is to
verify statements the relying party has made to the
subject, confirm these statements are consistent with
statements made to the identity provider and confirm all
the above are consistent with the federation and any
federation-specific policy or configuration. Another
challenge is choosing which identity provider to use for
which service.</t>
</section>
<section title="Client to Relying Party">
<t>One of the remaining layers is responsible for integration
of federated authentication into the application. There are a
number of approaches that applications have adopted for
security. So, there may need to be multiple strategies for
integration of federated authentication into
applications. However, we have started with a strategy that
provides integration to a large number of application
protocols.</t>
<t>Many applications such as SSH <xref target="RFC4462"/>, NFS
<xref target="RFC2203"/>, DNS <xref target="RFC3645"/> and
several non-IETF applications support the Generic Security
Services Application Programming Interface <xref
target="RFC2743"/>. Many applications such as IMAP, SMTP, XMPP
and LDAP support the Simple Authentication and Security Layer
(SASL) <xref target="RFC4422"/> framework. These two
approaches work together nicely: by creating a GSS-API
mechanism, SASL integration is also addressed. In effect,
using a GSS-API mechanism with
SASL simply requires placing some headers on the front of the
mechanism and constraining certain GSS-API options.</t>
<t>GSS-API is specified in terms of an abstract set of
operations which can be mapped into a programming language to
form an API. When people are first introduced to GSS-API, they
focus on it as an API. However, from the prospective of
authentication for non-web applications, GSS-API should be
thought of as a protocol not an API. It consists of some
abstract operations such as the initial context exchange,
which includes two sub-operations (gss_init_sec_context and
gss_accept_sec_context). An application defines which abstract
operations it is going to use and where messages produced by
these operations fit into the application architecture. A
GSS-API mechanism will define what actual protocol messages
result from that abstract message for a given abstract
operation. So, since this work is focusing on a particular
GSS-API mechanism, we generally focus on protocol elements
rather than the API view of GSS-API.</t>
<t>The API view has significant value. Since the abstract
operations are well defined, the set of information that a
mechanism gets from the application is well defined. Also, the
set of assumptions the application is permitted to make is
generally well defined. As a result, an application protocol
that supports GSS-API or SASL is very likely to be usable with
a new approach to authentication including this one with no
required modifications. In some cases, support for a new
authentication mechanism has been added using plugin
interfaces to applications without the application being
modified at all. Even when modifications are required, they
can often be limited to supporting a new naming and
authorization model. For example, this work focuses on
privacy; an application that assumes it will always obtain an
identifier for the principal will need to be modified to
support anonymity, unlinkability or pseudonymity.</t>
<t>So, we use GSS-API and SASL because a number of the
application protocols we wish to federate support these
strategies for security integration. What does this mean from
a protocol standpoint and how does this relate to other
layers? This means we need to design a concrete GSS-API
mechanism. We have chosen to use a GSS-API mechanism that
encapsulates EAP authentication. So, GSS-API (and SASL)
encapsulate EAP between the end-host and the service. The AAA
framework encapsulates EAP between the relying party and the
identity provider. The GSS-API mechanism includes rules about
how principals and services are named as well as per-message
security and other facilities required by the applications we
wish to support.</t>
</section>
-->
<!-- <section title="Personalization Layer">
<t>The AAA framework provides a way to transport statements
from the identity provider to the relying party. However, we
also need to say more about the content of these
statements. In simple cases, attributes particular to the AAA
protocol can be defined. However in more complicated
situations it is strongly desirable to re-use an existing
protocol for asking questions and receiving information about
subjects. SAML is used for this. </t>
<t>SAML usage may be as simple as the identity provider
including a SAML Response message in the AAA
response. Alternatively the relying party may generate a SAML
request XXX to whom, how, and at what point? (see above XXX).</t>
</section>
-->
</section>
<!-- ////////////////////////////////////////////////////////////////////////////////// -->
<section title="Application Security Services">
<t>One of the key goals is to integrate federated authentication
into existing application protocols and where possible, existing
implementations of these protocols. Another goal is to perform
this integration while meeting the best security practices of
the technologies used to perform the integration. This section
describes security services and properties required by the EAP
GSS-API mechanism in order to meet these goals. This information
could be viewed as specific to that mechanism. However, other
future application integration strategies are very likely to
need similar services. So, it is likely that these services will
be expanded across application integration strategies if new
application integration strategies are adopted.</t>
<section title="Authentication">
<t>
GSS-API provides an optional security service called mutual authentication.
This service means that in addition to the initiator providing (potentially anonymous or pseudonymous) identity to the acceptor, the acceptor confirms its identity to the initiator.
Especially for the ABFAB context, this service is confusingly named.
We still say that mutual authentication is provided when the identity of an acceptor is strongly authenticated to an anonymous initiator.
</t>
<t>
RFC 2743, unfortunately, does not explicitly talk about what mutual authentication means.
Within this document we therefore define it as:
<list style="symbols">
<t>
If a target name is supplied by the initiator, then the initiator trusts that the supplied target name describes the acceptor.
This implies both that appropriate cryptographic <!----> exchanges took place for the initiator to make such a trust decision, and that after evaluating the results of these exchanges, the initiator's policy trusts that the target name is accurate.
</t>
<t>
If no target name is supplied by the initiator, then the initiator trusts that its idea of the acceptor name correctly names the entity it is communicating with.
</t>
<t>
Both the initiator and acceptor have the same key material for per-message keys and both parties have confirmed they actually have the key material.
In EAP terms, there is a protected indication of success.
</t>
</list>
</t>
<t>
Mutual authentication is an important defense against certain aspects of phishing.
Intuitively, users would like to assume that if some party asks for their credentials as part of authentication, successfully gaining access to the resource means that they are talking to the expected party.
Without mutual authentication, the acceptor could "grant access" regardless of what credentials are supplied.
Mutual authentication better matches this user intuition.
</t>
<t>
It is important, therefore, that the GSS-EAP mechanism implement mutual authentication.
That is, an initiator needs to be able to request mutual authentication.
When mutual authentication is requested, only EAP methods capabale of providing the necessary service can be used, and appropriate steps need to be taken to provide mutual authentication.
A broader set of EAP methods could be supported when a particular application does not request mutual authentication.
It is an open question whether the mechanism will permit this.
</t>
<t>
<!-- May want to move this paragraph to a different section -->
The AAA infrastructure MAY hide the peer's identity from the GSS-API acceptor, providing anonymity between the peer and initiator.
At this time, whether the identity is disclosed is determined by EAP server policy rather than by an indication from the peer.
Also, peers are unlikely to be able to determine whether anonymous communication will be provided.
For this reason, peers are unlikely to set the anonymous return flag from GSS_Init_Sec_context.
</t>
</section>
<section title="GSS-API Channel Binding">
<t><xref target="RFC5056"/> defines a concept of channel
binding to prevent man-in-the-middle attacks. It is common to
provide SASL and GSS-API with another layer to provide
transport security; Transport Layer Security (TLS) is the most
common such layer. TLS provides its own server
authentication. However there are a variety of situations
where this authentication is not checked for policy or
usability reasons. Even when it is checked, if the trust
infrastructure behind the TLS authentication is different from
the trust infrastructure behind the GSS-API mutual
authentication then confirming the end-points using both trust infrastructures is likely to enhance security. If the endpoints of the GSS-API authentication
are different than the endpoints of the lower layer, this is a
strong indication of a problem such as a man-in-the-middle
attack. Channel binding provides a facility to determine
whether these endpoints are the same.</t>
<t>The GSS-EAP mechanism needs to support channel
binding. When an application provides channel binding data,
the mechanism needs to confirm this is the same on both sides
consistent with the GSS-API specification. XXXThere is an open
question here as to the details; today RFC 5554 governs. We
could use that and the current draft assumes we will. However
in Beijing we became aware of some changes to these details
that would make life much better for GSS authentication of
HTTP. We should resolve this with kitten and replace this note
with a reference to the spec we're actually following.</t>
<t>Typically when considering 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. Without mutual authentication, only one party
knows that the endpoints are correct. That's sometimes
useful. Consider for example a user who wishes to access a
protected resource from a shared whiteboard in a conference
room. The whiteboard is the initiator; it does not need to
actually authenticate that it is talking to the correct
resource because the user will be able to recognize whether
the displayed content is correct. If channel binding were used
without mutual authentication, it would in effect be a request
to only disclose the resource in the context of a particular
channel. Such an authentication would be similar in concept to
a holder-of-key SAML assertion. However, also note that while
it is not happening in the protocol, mutual authentication is
happening in the overall system: the user is able to visually
authenticate the content. This is consistent with all uses of
channel binding without protocol level mutual authentication
found so far.</t>
<t>RFC 5056 channel binding (also called GSS-API channel
binding when GSS-API is involved) is not the same thing as EAP
channel binding. EAP channel binding is also used in the ABFAB
context in order to implement acceptor naming and mutual
authentication. Details are discussed in the mechanisms
specification <xref target="I-D.ietf-abfab-gss-eap"/>.</t>
</section>
<section title="Host-Based Service Names">
<t>IETF security mechanisms typically take the name of a
service entered by a user and make some trust decision about
whether the remote party in an interaction is the intended
party. GSS-API has a relatively flexible naming
architecture. However most of the IETF applications that use
GSS-API, including SSH, NFS, IMAP, LDAP and XMPP, have chosen
to use host-based service names when they use GSS-API. In this
model, the initiator names an acceptor based on a service such
as "imap" or "host" (for login services such as SSH) and a
host name.</t>
<t>Using host-based service names leads to a challenging trust
delegation problem. Who is allowed to decide whether a
particular hostname maps to an entity. The public-key
infrastructure (PKI) used by the web has chosen to have a
number of trust anchors (root certificate authorities) each of
wich can map any name to a public key. A number of GSS-API
mechanisms suchs as Kerberos <xref target="RFC1964"/> split
the problem into two parts. A new concept called a realm is
introduced. Then the mechanism decides what realm is
responsible for a given name. That realm is responsible for
deciding if the acceptor entity is allowed to claim the
name. ABFAB needs to adopt this approach.</t>
<t>Host-based service names do not work ideally when different
instances of a service are running on different ports. Also,
these do not work ideally when SRV record or other insecure
referrals are used.</t>
<t>The GSS-EAP mechanism needs to support host-based service
names in order to work with existing IETF protocols.</t>
</section>
<section title="Per-Message Tokens">
<t>GSS-API provides per-message security services that can
provide confidentiality and integrity. Some IETF protocols
such as NFS and SSH take advantage of these services. As a
result GSS-EAP needs to support these services. As with mutual
authentication, per-message services will limit the set of EAP
methods that are available. Any method that produces a Master
Session Key (MSK) should be able to support per-message
security services.</t>
<t>GSS-API provides a pseudo-random function. While the
pseudo-random function does not involve sending data over the
wire, it provides an algorithm that both the initiator and
acceptor can run in order to arrive at the same key
value. This is useful for designs where a successful
authentication is used to key some other function. This is
similar in concept to the TLS extractor. No current IETF
protocols require this. However GSS-EAP supports this service
because it is valuable for the future and easy to do given
per-message services. Non-IETF protocols are expected to take
advantage of this in the near future.</t>
</section>
</section>
<section anchor="attribute-providers" title="Future Work:
Attribute Providers">
<t>
This architecture provides for a federated authentication and
authorization framework between IdPs, RPs, principals, and
subjects. It does not at this time provide for a means to
retrieve attributes from 3rd parties. However, it envisions
such a possibility. We note that in any extension to the
model, an attribute provider must be authorized to release
specific attributes to a specific RP for a specific
principal. In addition, we note that it is an open question
beyond this architecture as to how the RP should know to trust
a particular attribute provider.
</t>
<t>There are a number of possible technical means to provide
attribute provider capabilities. One possible approach is for the
IdP to provide a signed attribute request to RP that it in turn
will provide to the attribute authority. Another approach would be
for the IdP to provide a URI to the RP that contains a token of
some form. The form of communications between the IdP and attribute
provider as well as other considerations are left for the future.
One thing we can say now is that the IdP would merely be asserting
who the attribute authority is, and not the contents of what the
attribute authority would return. (Otherwise, the IdP might as well
make the query to the attribute authority and then resign it.)
</t>
</section>
<section anchor="privacy-cons" title="Privacy Considerations">
<t>Sharing identity information raises privacy violations and as described throughout this document an existing architecture is re-used for a different usage environment. As such, a discussion about the privacy properties has to take these pre-conditions into consideration. We use the approach suggested in <xref target="I-D.iab-privacy-considerations"/> to shed light into what data is collected and used by which entity, what the relationship between these entities and the end user is, what data about the user is likely needed to be collected, and what the identification level of the data is.</t>
<section title="What Entities collect and use Data?">
<t><xref target="abfab-arch"/> shows the architecture with the involved entities. Message exchanges are exchanged between the client application, via the relying part to the AAA server. There will likely be intermediaries between the relying party and the AAA server, labeled generically as "federation".
</t>
<t>In order for the relying party to route messages to the AAA server it is necessary for the client application to provide enough information to enable the identification of the AAA server's domain. While often the username is attached to the domain (in the form of a Network Access Identity (NAI) this is not necessary for the actual protocol operation. The EAP server component within the AAA server needs to authenticate the user and therefore needs to execute the respective authentication protocol. Once the authentication exchange is complete authorization information needs to be conveyed to the relying party to grant the user the necessary application rights. Information about resource consumption may be delivered as part of the accounting exchange during the lifetime of the granted application session.</t>
<t>The authentication exchange may reveal an identifier that can be linked to a user. Additionally, a sequence of authentication protocol exchanges may be linked together. Depending on the chosen authentication protocol information at varying degrees may be revealed to all parties along the communication path. This authorization information exchange may disclose identity information about the user. While accounting information is created by the relying party it is likely to needed by intermediaries in the federation for financial settlement purposes and will be stored for billing, fraud detection, statistical purposes, and for service improvement by the AAA server operator.</t>
</section>
<section title="Relationship between User's and other Entities">
<t>The AAA server is a first-party site the user typically has a relationship with. This relationship will be created at the time when the security credentials are exchange and provisioned. The relying party and potential intermediares in the federation are typically operate under the contract of the first-party site. The user typically does not know about the intermediaries in the federation nor does he have any relationship with them. The protocol interaction triggered by the client application happens with the relying party at the time of application access. The relying party does not have a direct contractual relationship with the user but depending on the application the interaction may expose the brand of the application running by the relying party to the end user via some user interface.</t>
</section>
<section title="What Data about the User is likely Needed to be Collected?">
<t>The data that is likely going to be collected as part of a protocol exchange with application access at the relying party is accounting information and authorization information. This information is likely to be kept beyond the terminated application usage for trouble shooting, statistical purposes, etc. There is also like to be additional data collected to to improve application service usage by the relying party that is not conveyed to the AAA server as part of the accounting stream.
</t>
</section>
<section title="What is the Identification Level of the Data?">
<t>With regard to identification there are several protocol layers that need to be considered separately. First, there is the EAP method exchange, which as an authentication and key exchange protocol has properties related to identification and protocol linkage. Second, there is identification at the EAP layer for routing of messages. Then, in the exchange between the client application and the relying party the identification depends on the underlying application layer protocol the EAP/GSS-API exchange is tunneled over. Finally, there is the backend exchange via the AAA infrastructure, which involves a range of RADIUS and Diameter extensions and yet to be defined extensions, such as encoding authorization information inside SAML assertions.</t>
<t>Since this document does not attempt to define any of these exchanges but rather re-uses existing mechanisms the level of identification heavily depends on the selected mechanisms. The following two examples aim to illustrate the amount of existing work with respect to decrease exposure of personal data.
</t>
<t><list style="numbers">
<t>When designing EAP methods a number of different requirements may need to get considered; some of them are conflicting. RFC 4017 <xref target="RFC4017"/>, for example, tried to list requirements for EAP methods utilized for network access over Wireless LANs. It also recommends the end-user identity hiding requirement, which is privacy-relevant. Some EAP methods, such as EAP-IKEv2 <xref target="RFC5106"/>, also fulfill this requirement.</t>
<t>EAP, as the layer encapsulating EAP method specific information, needs identity information to allow routing requests towards the user's home AAA server. This information is carried within the Network Access Identifier (NAI) and the username part of the NAI (as supported by RFC 4282 <xref target="RFC4282"/>) can be obfuscated.</t>
</list>
</t>
</section>
<section title="Privacy Challenges">
<t>While a lot of standarization work was done to avoid leakage of identity information to intermediaries (such as eavesdroppers on the communication path between the client application and the relying party) in the area of authentication and key exchange protocols. However, from current deployments the weak aspects with respect to security are:
<list style="numbers">
<t>Today business contracts are used to create federations between identity providers and relying parties. These contracts are not only financial agreements but they also define the rules about what information is exchanged between the AAA server and the relying party and the potential involvement of AAA proxies and brokers as intermediaries. While these contracts are openly available for university federations they are not public in case of commercial deployments. Quite naturally, there is a lack of transparency for external parties.</t>
<t>In today's deployments the ability for users to determine the amount of information exchanged with other parties over time, as well as the possibility to control the amount of information exposed via an explict consent is limited. This is partially due the nature of application capabilities at the time of network access authentication. However, with the envisioned extension of the usage, as described in this document, it is desirable to offer these capabilities.</t>
</list>
</t>
</section>
</section>
<!-- ////////////////////////////////////////////////////////////////////////////////// -->
<section title="Deployment Considerations">
<section title="EAP Channel Binding">
<t>Discuss the implications of needing EAP channel
binding.</t>
</section>
<section title="AAA Proxy Behavior">
<t>Discuss deployment implications of our proxy requirements.</t>
</section>
</section>
<!-- ////////////////////////////////////////////////////////////////////////////////// -->
<section anchor="sec-cons" title="Security Considerations">
<t>This document describes the architecture for Application Bridging for Federated Access Beyond Web (ABFAB) and security is therefore the main focus. This section highlights the main communication channels and their security properties:
<list style="hanging">
<t hangText="Client-to-RP Channel:">
<vspace blankLines="1"/>
The channel binding material is provided by any certificates and the final message (i.e., a cryptographic token for the channel).
Authentication may be provided by the RP to the client but a deployment without authentication at the TLS layer is possible as well.
In addition, there is a channel between the GSS requestor and the GSS acceptor, but the keying material is provided by a "third party" to both entities.
The client can derive keying material locally, but the RP gets the material from the IdP.
In the absence of a transport that provides encryption and/or integrity, the channel between the client and the RP has no ability to have any cryptographic protection until the EAP authentication has been completed and the MSK is transfered from the IdP to the RP.
</t>
<t hangText="RP-to-IdP Channel:">
<vspace blankLines="1"/>
The security of this communication channel is mainly provided by the functionality offered via RADIUS and Diameter.
At the time of writing there are no end-to-end security mechanisms standardized and thereby the architecture has to rely on hop-by-hop security with trusted AAA entities or, as an alternative but possible deployment variant, direct communication between the AAA client to the AAA server.
Note that the authorization result the IdP provides to the RP in the form of a SAML assertion may, however, be protected such that the SAML related components are secured end-to-end.
<vspace blankLines="1"/>
The MSK is transported from the IdP to the RP over this channel. As no end-to-end security is provided by AAA, all AAA entities on the path between the RP and IdP have the ability to eavesdrop if no additional security measures are taken. One such measure is to use a transport between the client and the IdP that provides confidentiality.
</t>
<t hangText="Client-to-IdP Channel:"><vspace blankLines="1"/>This communication interaction is accomplished with the help of EAP and EAP methods. The offered security protection will depend on the EAP method that is chosen but a minimum requirement fis to offer mutual authentication, and key derivation. The IdP is responsible during this process to determine that the RP that is communication to the client over the RP-to-IdP channel is the same one talking to the IdP. This is accomplished via the EAP channel binding.</t>
</list>
</t>
<t>Partial list of issues to be addressed in this section: Privacy, SAML, Trust Anchors, EAP Algorithm Selection, Diameter/RADIUS/AAA Issues, Naming of Entities, Protection of passwords, Channel Binding, End-point-connections (TLS), Proxy problems</t>
</section>
<!-- ////////////////////////////////////////////////////////////////////////////////// -->
<section anchor="iana" title="IANA Considerations">
<t>This document does not require actions by IANA.</t>
</section>
<!-- ////////////////////////////////////////////////////////////////////////////////// -->
<section title="Acknowledgments">
<!-- <t>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.</t>-->
<t>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.</t>
<t>Furthermore, we would like to thank Klaas Wierenga for his review of the pre-00 draft version.</t>
</section>
<!-- ////////////////////////////////////////////////////////////////////////////////// -->
</middle>
<!-- ////////////////////////////////////////////////////////////////////////////////// -->
<back>
<references title="Normative References">
&RFC2743;
&RFC2865; &RFC3588;
&RFC3748; &RFC3579; &RFC4072; &RFC4282;
&I-D.iab-privacy-terminology;
&I-D.ietf-abfab-gss-eap;
&I-D.ietf-abfab-aaa-saml;
&I-D.ietf-emu-chbind;
</references>
<references title="Informative References">
&RFC2903;
&I-D.nir-tls-eap;
&I-D.ietf-oauth-v2;
&I-D.iab-privacy-considerations;
&RFC4017;
&RFC5106;
&RFC1964;
&RFC2203;
&RFC3645;
&RFC2138;
&RFC4462;
&RFC4422;
&RFC5056;
&RFC5801;
&RFC5849;
&SAML20;
&RFC2904;
&I-D.hartman-emu-mutual-crypto-bind;
<reference anchor="WS-TRUST" target="http://docs.oasis-open.org/ws-sx/ws-trust/v1.4/ws-trust.html">
<front>
<title>WS-Trust 1.4</title>
<author initials="K" surname="Lawrence"/>
<author initials="C" surname="Kaler"/>
<author initials="A" surname="Nadalin"/>
<author initials="M" surname="Goodner"/>
<author initials="M" surname="Gudgin"/>
<author initials="A" surname="Barbir"/>
<author initials="H" surname="Granqvist"/>
<date month="February" day='2' year='2009'/>
</front>
<seriesInfo name="OASIS Standard" value="ws-trust-200902"/>
<format type="HTML" target="http://docs.oasis-open.org/ws-sx/ws-trust/v1.4/ws-trust.html">
</format>
</reference>
</references>
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-22 21:57:34 |