| ABFAB | J. Howlett |
| Internet-Draft | JANET(UK) |
| Intended status: Informational | S. Hartman |
| Expires: January 08, 2013 | Painless Security |
| H. Tschofenig | |
| Nokia Siemens Networks | |
| E. Lear | |
| Cisco Systems GmbH | |
| J. Schaad | |
| Soaring Hawk Consulting | |
| July 09, 2012 |
Application Bridging for Federated Access Beyond Web (ABFAB) Architecture
draft-ietf-abfab-arch-03.txt
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.
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.
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This Internet-Draft will expire on January 08, 2013.
Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.
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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 Simple Authentication and Security Layer (SASL) with the Generic Security Server Application Program Interface (GSS-API) (known as the GS2 family) [RFC5801], Security Assertion Markup Language (SAML) [OASIS.saml-core-2.0-os], RADIUS [RFC2865], and Diameter [RFC3588].
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.
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'.
Federated access management has evolved over the last decade through such standards as SAML [OASIS.saml-core-2.0-os], OpenID, OAuth [RFC5849], [I-D.ietf-oauth-v2] and WS-Trust [WS-TRUST]. The benefits of federated access management include:
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.
This document uses identity management and privacy terminology from [I-D.iab-privacy-terminology]. In particular, this document uses the terms identity provider, relying party, (data) subject, identifier, pseudonymity, unlinkability, and anonymity.
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.
This document uses the term Network Access Identifier (NAI), as defined in [RFC4282].
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.
| Protocol | Subject | Relying Party | Identity Provider |
|---|---|---|---|
| ABFAB | Subject | Relying Party (RP) | Identity Provider (IdP) |
| Principal | |||
| Client | |||
| SAML | Subject | Service Provider | Issuer |
| GSS-API | Initiator | Acceptor | |
| EAP | EAP client | EAP server | |
| EAP peer | |||
| SASL | |||
| AAA | AAA Client | AAA server | |
| RADIUS | client | NAS | RADIUS server |
Note that in some cases a cell has been left empty, in these cases there is no direct name that represents this concept.
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.
In the previous section we introduced the following actors:
These entities and their relationships are illustrated graphically in Figure 1.
,----------\ ,---------\
| Identity | Federation | Relying |
| Provider + <-------------------> + Party |
`----------' '---------'
<
\
\ Authentication
\
\
\
\
\ +---------+
\ | | O
v| Client | \|/ Principal
| | |
+---------+ / \
A federation agreement typically encompasses operational specifications and legal rules:
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).
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.
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.
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.
Figure 1 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.
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).
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.
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.
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.
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.
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.
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.
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.
The steps taken in an ABFAB federated authentication/authorization exchange are as follows:
An example communication flow is given below:
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
Our key design goals are as follows:
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 Figure 1.
Interestingly, for network access authentication the usage of the AAA framework with RADIUS [RFC2865] and Diameter [RFC3588] was quite successful from a deployment point of view. To map the terminology used in Figure 1 to the AAA framework the IdP corresponds to the AAA server, the RP corresponds to the AAA client, and the technical building blocks of a federation are AAA proxies, relays and redirect agents (particularly if they are operated by third parties, such as AAA brokers and clearing houses). The front-end, i.e. the end host to AAA client communication, is in case of network access authentication offered by link layer protocols that forward authentication protocol exchanges back-and-forth. An example of a large scale RADIUS-based federation is EDUROAM.
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,
Expand here
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.
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.
There are a number of ways for encapsulating EAP into an application protocol. For ease of integration with a wide range of non-Web based application protocols the usage of the GSS-API was chosen. Encapsulating EAP into the GSS-API also allows EAP to be used in SASL. A description of the technical specification can be found in [I-D.ietf-abfab-gss-eap]. Other alternatives exist as well and may be considered later, such as "TLS using EAP Authentication" [I-D.nir-tls-eap].
The architecture consists of several building blocks, which is shown graphically in Figure 3. In the following sections, we discuss the data flow between each of the entities, the protocols used for that data flow and some of the trade-offs made in choosing the protocols.
+--------------+
| 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
Communications between the Relying Part and the Identity Provider is done by the federation substrate. This communication channel is responsible for:
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.
The IETF has defined a federation framework already with the Authentication, Authorization and Accounting (AAA) framework [RFC2903], [RFC2904]. Two implementations of the AAA framework exist in RADIUS [RFC2138] and Diameter [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.
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.
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.
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.
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 [RFC4282]. The NAI is represented and consumed by the GSS-API layer as GSS_C_NT_USER_NAME as specified in [RFC2743]. The GSS-API EAP mechanism includes the NAI in the EAP Response/Identity message.
While we are using the AAA protocols to communicate with the IdP, the RP may have multiple federation substrates to select from. The RP has a number of criteria that it will use in selecting which of the different federations to use:
The 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.
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.
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.
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:
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.
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.
Security Assertions Markup Language (SAML) [OASIS.saml-core-2.0-os] was designed in order to carry an extensible set of attributes about a subject. Since SAML is extensible in the attribute space, ABFAB has no immediate needs to update the core SAML specifications for our work. It will be necessary to update IdPs that need to return SAML assertions to 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 [I-D.ietf-abfab-aaa-saml].
There are two issues that need to be highlighted:
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.
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.
Looking at the communications between the client and the IdP, the following items need to be dealt with:
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.
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.
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.
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.
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.
Fortunately, these requirements can be met by utilizing standardized and successfully deployed technology, namely by the Extensible Authentication Protocol (EAP) framework [RFC3748]. Figure 3 illustrates the integration graphically.
EAP is an end-to-end framework; it provides for two-way communication between a peer (i.e,service client or principal) through the authenticator (i.e., service provider) to the back-end (i.e., identity provider). Conveniently, this is precisely the communication path that is needed for federated identity. Although EAP support is already integrated in AAA systems (see [RFC3579] and [RFC4072]) several challenges remain:
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 [I-D.hartman-emu-mutual-crypto-bind].
The recommended way to deal with channel binding can be found in RFC XXXX [I-D.ietf-emu-chbind].
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:
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.
Many applications such as SSH [RFC4462], NFS [RFC2203], DNS [RFC3645] and several non-IETF applications support the Generic Security Services Application Programming Interface [RFC2743]. Many applications such as IMAP, SMTP, XMPP and LDAP support the Simple Authentication and Security Layer (SASL) [RFC4422] framework. These two approaches work together nicely: by creating a GSS-API mechanism, SASL integration is also addressed. In effect, using a GSS-API mechanism with SASL simply requires placing some headers on the front of the mechanism and constraining certain GSS-API options.
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.
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.
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.
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 [RFC2743] provides some details on what requirements exist.
In addition we highlight the following:
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.
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.
RFC 2743, unfortunately, does not explicitly talk about what mutual authentication means. Within this document we therefore define it as:
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.
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.
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.
[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.
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.
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.
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 [I-D.ietf-abfab-gss-eap].
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.
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 [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.
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.
The GSS-EAP mechanism needs to support host-based service names in order to work with existing IETF protocols.
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.
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.
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.
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.)
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 [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.
Figure 3 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".
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.
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.
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.
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.
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.
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.
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:
Discuss the implications of needing EAP channel binding.
Discuss deployment implications of our proxy requirements.
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:
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
This document does not require actions by IANA.
We would like to thank Mayutan Arumaithurai and Klaas Wierenga for their feedback. Additionally, we would like to thank Eve Maler, Nicolas Williams, Bob Morgan, Scott Cantor, Jim Fenton, and Luke Howard for their feedback on the federation terminology question.
Furthermore, we would like to thank Klaas Wierenga for his review of the pre-00 draft version.