| ABFAB | J. Howlett |
| Internet-Draft | JANET(UK) |
| Intended status: Informational | S. Hartman |
| Expires: January 22, 2015 | Painless Security |
| H. Tschofenig | |
| ARM Ltd. | |
| E. Lear | |
| Cisco Systems GmbH | |
| J. Schaad | |
| Soaring Hawk Consulting | |
| July 21, 2014 |
Application Bridging for Federated Access Beyond Web (ABFAB) Architecture
draft-ietf-abfab-arch-13.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 access and web-based access. However, the solutions to these use cases that have been proposed and deployed tend to have few building blocks in common.
This memo describes an architecture that makes use of extensions to the commonly used security mechanisms for both federated and non-federated access management, including the Remote Authentication Dial In User Service (RADIUS) the Generic Security Service Application Program Interface (GSS-API), the Extensible Authentication Protocol (EAP) and the Security Assertion Markup Language (SAML). The architecture addresses the problem of federated access management to primarily non-web-based services, in a manner that will scale to large numbers of identity providers, relying parties, and federations.
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This Internet-Draft will expire on January 22, 2015.
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Numerous security mechanisms have been deployed on the Internet to manage access to various resources. These mechanisms have been generalized and scaled over the last decade through mechanisms such as 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], and the Authentication, Authorization, and Accounting (AAA) architecture as embodied in RADIUS [RFC2865] and Diameter [RFC6733].
A Relying Party (RP) is the entity that manages access to some resource. The entity that is requesting access to that resource is often described as the Client. Many security mechanisms are manifested as an exchange of information between these entities. The RP is therefore able to decide whether the Client is authorized, or not.
Some security mechanisms allow the RP to delegate aspects of the access management decision to an entity called the Identity Provider (IdP). This delegation requires technical signaling, trust and a common understanding of semantics between the RP and IdP. These aspects are generally managed within a relationship known as a 'federation'. This style of access management is accordingly described as 'federated access management'.
Federated access management has evolved over the last decade through specifications like SAML [OASIS.saml-core-2.0-os], OpenID, OAuth [RFC6749] 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. The architecture addresses the problem of federated access management primarily for non-web-based services. This architecture makes use of extensions to the commonly used security mechanisms for both federated and non-federated access management, including RADIUS, the Generic Security Service (GSS), the Extensible Authentication Protocol (EAP) and SAML. The architecture should be extended to use Diameter in the future. It does so in a manner that designed to scale to large numbers of identity providers, relying parties, and federations.
This document uses identity management and privacy terminology from [RFC6973]. In particular, this document uses the terms identity provider, relying party, identifier, pseudonymity, unlinkability, and anonymity.
In this architecture the IdP consists of the following components: an EAP server, a RADIUS server, and optionally a SAML Assertion service.
This document uses the term Network Access Identifier (NAI), as defined in [I-D.ietf-radext-nai]. An NAI consists of a realm identifier, which is associated with an AAA server and thus an IdP and a username which is associated with a specific client of the IdP.
One of the problems some people have found 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 referencing are not consistent with each other. In general the document uses either the ABFAB 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 | Client | Relying Party | Identity Provider |
|---|---|---|---|
| ABFAB | Client | Relying Party (RP) | Identity Provider (IdP) |
| Initiator | Acceptor | ||
| Server | |||
| SAML | Subject | Service Provider | Issuer |
| GSS-API | Initiator | Acceptor | |
| EAP | EAP peer | EAP Authenticator | EAP server |
| AAA | AAA Client | AAA server | |
| RADIUS | user | NAS | RADIUS server |
| RADIUS client |
Table 1. Terminology
Note that in some cases a cell has been left empty; in these cases there is no name that represents the entity.
This document uses the term channel binding in two different contexts. The term channel binding has a different meaning in each of these contexts.
EAP channel binding is used to implement GSS-API naming semantics. EAP channel binding sends a set of attributes from the peer to the EAP server either as part of the EAP conversation or as part of a secure association protocol. In addition, attributes are sent in the backend protocol from the EAP authenticator to the EAP server. The EAP server confirms the consistency of these attributes and provides the confirmation back to the peer. In this document, channel binding without qualification refers to EAP channel binding.
GSS-API channel binding provides protection against man-in-the-middle attacks when GSS-API is used for authentication inside of some tunnel; it is similar to a facility called cryptographic binding in EAP. The binding works by each side deriving a cryptographic value from the tunnel itself and then using that cryptographic value to prove to the other side that it knows the value.
See [RFC5056] for a discussion of the differences between these two facilities. However, the difference can be summarized as GSS-API channel binding says that there is nobody between the client and the EAP authenticator while EAP channel binding allows the client to have knowledge about attributes of the EAP authenticator (such as its name).
Typically when considering both EAP and GSS-API channel binding, people think of channel binding in combination with mutual authentication. This is sufficiently common that without additional qualification channel binding should be assumed to imply mutual authentication. In GSS-API, without mutual authentication only the acceptor has authenticated the initiator. Similarly in EAP, only the EAP server has authenticated the peer. That's sometimes useful. Consider for example a user who wishes to access a protected resource for a shared whiteboard in a conference room. The whiteboard is the acceptor; it knows that the initiator is authorized to give it a presentation and the user can validate the whitebord got the correct presentation by visual means. (The presention should not be confidential in this case.) If channel binding is used without mutual authentication, it is effectively a request to disclose the resource in the context of a particular channel. Such an authentication would be similar in concept to a holder-of-key SAML assertion. However, 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.
In the previous section we introduced the following entities: https://community.ja.net/system/files/288/Trust-Router-Overview-IETF86.pptx where the routers use ABFAB to authenticate to each other.
The final entity that needs to be introduced is the Individual. An Individual is a human being that is using the Client. In any given situation, an Individual may or may not exist. Clients can act either as front ends for Individuals or they may be independent entities that are setup and allowed to run autonomously. An example of such an independent entity can be found in the trust routing protocol
These entities and their relationships are illustrated graphically in Figure 1.
,----------\ ,---------\
| Identity | Federation | Relying |
| Provider + <-------------------> + Party |
`----------' '---------'
<
\
\ Authentication
\
\
\
\
\ +---------+
\ | | O
v| Client | \|/ Individual
| | |
+---------+ / \
Figure 1: Entities and their Relationships
The relationships between the entities in Figure 1 are:
A federation agreement typically encompasses operational specifications and legal rules:
The Operational Specifications can demand the usage of a specific technical infrastructure, including requirements on the message routing intermediaries, to offer the required technical functionality. In other environments, the Operational Specifications require fewer technical components in order to meet the required technical functionality.
The Legal Rules include many non-technical aspects of federation, such as business practices and legal arrangements, which are outside the scope of the IETF. The Legal Rules can still have an impact on the architectural setup or on how to ensure the dynamic establishment of trust.
While a federation agreement is often discussed within the context of formal relationships, such as between an enterprise and an employee or a government and a citizen, a federation agreement does not have to require any particular level of formality. For an IdP and a Client, it is sufficient for a relationship to be established by something as simple as using a web form and confirmation email. For an IdP and an RP, it is sufficient for the IdP to publish contact information along with a public key and for the RP to use that data. Within the framework of ABFAB, it will generally be required that a mechanism exists for the IdP to be able to trust the identity of the RP, if this is not present then the IdP cannot provide the assurances to the client that the identity of the RP has been established.
The nature of federation dictates that there 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.
The IdP will typically have a long-term relationship with the Client. This relationship typically involves the IdP positively identifying and credentialing the Client (for example, at time of employment within an organization). When dealing with individuals, this process is called identity proofing [NIST-SP.800-63]. The relationship will often be instantiated within an agreement between the IdP and the Client (for example, within an employment contract or terms of use that stipulates the appropriate use of credentials and so forth).
The nature and quality of the relationship between the Client and the IdP is an important contributor to the level of trust that an RP may assign to an assertion describing a Client made by an IdP. This is sometimes described as the Level of Assurance [NIST-SP.800-63].
Federation does not require an a priori relationship or a long-term relationship between the RP and the Client; it is this property of federation that yields many of the federation benefits. However, federation does not preclude the possibility of a pre-existing relationship between the RP and the Client, 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 an Individual behind the Client and in other cases the Client may be autonomous.
As the number of federated IdPs and RPs (services) proliferats, 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 SMTP and IMAP protocols do not have the ability for the server to request information from the client, beyond the clients NAI, that the server would then use to decide between the multiple federations it is associated with. However, the building blocks do exist to add this functionality.
The previous section described the general model of federation, and the application of access management within the federation. This section provides a brief overview of ABFAB in the context of this model.
In this example, a client is attempting to connect to a server in order to either get access to some data or perform some type of transaction. In order for the client to mutually authenticate with the server, the following steps are taken in an ABFAB architecture (a graphical view of the steps can be found in figure Figure 2):
Relying Client Identity
Party App Provider
| (1) | Client Configuration
| | |
|<-----(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 (via RP)
Figure 2: ABFAB Authentication Steps
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 widespread deployment is even more difficult. A lot of attention on federated access has been devoted to the Web. This document instead focuses on a non-Web-based environment and focuses on those protocols where HTTP is not used. Despite the growing trend to layer every protocol on top of HTTP there are still a number of protocols available that do not use HTTP-based transports. Many of these protocols are lacking a native authentication and authorization framework of the style shown in Figure 1.
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 IdP, 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 to encapsulate EAP into an application protocol. For ease of integration with a wide range of non-Web based application protocols, GSS-API was chosen. The technical specification of GSS-EAP can be found in [RFC7055].
The architecture consists of several building blocks, 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
--v----------v--
/// \\\
// \\
| Federation |
| Substrate |
\\ //
\\\ ///
--^----------^--
* EAP o RADIUS
* o
+-------------+ +-v----------v--+
| | | |
| Client | EAP/EAP Method | Relying Party |
| Application |<****************>| (RP) |
| | GSS-API | |
| |<---------------->| |
| | Application | |
| | Protocol | |
| |<================>| |
+-------------+ +---------------+
Legend:
<****>: Client-to-IdP Exchange
<---->: Client-to-RP Exchange
<oooo>: RP-to-IdP Exchange
<====>: Protocol through which GSS-API/GS2 exchanges are tunneled
Figure 3: ABFAB Protocol Instantiation
Communications between the Relying Party 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. The AAA framework supports the requirements stated above as follows:
Protocols that support the same framework, but do different routing are expected to be defined and used the future. One such effort call the Trust Router is to setup a framework that creates a trusted point-to-point channel on the fly https://commmunity.ja.net/system/files/288/Trust-Router-Overview-IETF86.pptx.
The usage of the AAA framework with RADIUS [RFC2865] and Diameter [RFC6733] for network access authentication has been successful from a deployment point of view. To map to 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 can be built on the federation agreements already exist, the agreements can then merely be expanded to cover the ABFAB. The AAA framework has already addressed some of the problems outlined above. For example,
The astute reader will notice that RADIUS and Diameter have substantially similar characteristics. Why not pick one? RADIUS and Diameter are deployed in different environments. RADIUS can often be found in enterprise and university networks, and is also in use by fixed network operators. Diameter, on the other hand, is deployed by mobile operators. Another key difference is that today RADIUS is largely transported upon UDP. We leave as a deployment decision, which protocol will be appropriate. The protocol defines all the necessary new AAA attributes as RADIUS attributes. A future document could define the same AAA attributes for a Diameter environment. We also note that there exist proxies 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 client 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 [I-D.ietf-radext-nai]. 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.
As of the time this document was published, no profiles for the use of Diameter have been created.
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 criterion used during discovery is going to be the name of the IdP to be contacted. The second selection criteria used during discovery is going to be the set of business rules and technical policies governing the relationship; this is called rules determination. The RP also needs to establish technical trust in the communications with the IdP.
Rules determination covers a broad range of decisions about the exchange. One of these is whether the given RP is permitted to talk to the IdP using a given federation at all, so rules determination encompasses the basic authorization decision. Other factors are included, such as what policies govern release of information about the client to the RP and what policies govern the RP's use of this information. While rules determination is ultimately a business function, it has significant impact on the technical exchanges. The protocols need to communicate the result of authorization. When multiple sets of rules are possible, the protocol must disambiguate which set of rules are in play. Some rules have technical enforcement mechanisms; for example in some federations intermediaries validate information that is being communicated within the federation.
At the time of writing no protocol mechanism has been specified to allow a AAA client to determine whether a AAA proxy will indeed be able to route AAA requests to a specific IdP. The AAA routing is impacted by business rules and technical policies that may be quite complex and at the present time, the route selection is based on manual configuration.
Several approaches to having messages routed through the federation substrate are possible. These routing methods can most easily be classified based on the mechanism for technical trust that is used. The choice of technical trust mechanism constrains how rules determination is implemented. Regardless of what deployment strategy is chosen, it is important that the technical trust mechanism be able to validate the identities of both parties to the exchange. The trust mechanism must ensure that the entity acting as 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 AAA framework there are two different places where security needs to be examined. The first is the security that is in place for the links in the AAA backbone being used. The second are the nodes that form the AAA backbone.
The default link security for RADIUS is showing its age as it uses MD5 and a shared secret to both obfuscate passwords and to provide integrity on the RADIUS messages. While some EAP methods include the ability to protect the client authentication credentials, the MSK returned from the IdP to the RP is protected only by the RADIUS security. In many environments this is considered to be insufficient, especially as not all attributes are obfuscated and can thus leak information to a passive eavesdropper. The use of RADIUS with TLS [RFC6614] and/or DTLS [I-D.ietf-radext-dtls] addresses these attacks. The same level of security is included in the base Diameter specifications.
For the traditional use of AAA frameworks, network access, an affirmative response from the IdP can be sufficient to grant access. In the ABFAB world, the RP may need to get significantly more additional information about the client before granting access. ABFAB therefore has a requirement that it can transport an arbitrary set of attributes about the client from the IdP to the RP.
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 RPs and for both the IdP and the RP to implement a new SAML profile designed to carry SAML assertions in AAA. The new profile can be found in RFCXXXX [I-D.ietf-abfab-aaa-saml]. As SAML statements will frequently be large, RADIUS servers and clients that deal with SAML statements will need to implement RFC XXXX [I-D.ietf-radext-radius-fragmentation]
There are several 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 in a SAML assertion are identified by a name string. The name string is either assigned by the SAML issuer context or is scoped by a namespace (for example a URI or object identifier (OID)). This means that the same attribute can have different name strings used to identify it. In many, but not all, cases the federation agreements will determine what attributes and names can be used in a SAML statement. This means that the RP needs to map from the SAML issuer or federation name, type and semantic into the name, type and semantics that the policies of the RP are written in. In other cases the federation substrate, in the form of proxies, will modify the SAML assertions in transit to do the necessary name, type and value mappings as the assertion crosses boundaries in the federation. If the proxies are modifying the SAML Assertion, then they will remove any signatures on the SAML as changing the content of the SAML statement would invalidate the signature. In this case the technical trust is the required mechanism for validating the integrity of the assertion. (The proxy could re-sign the SAML assertion, but the same issues of establishing trust in the proxy would still exist.) Finally, the attributes may still be in the namespace of the originating IdP. When this occurs the RP will need to get the required mapping operations from the federation agreements and do the appropriate mappings itself.
The RADIUS SAML RFC [I-D.ietf-abfab-aaa-saml] has defined a new SAML name format that corresponds to the NAI name form defined by RFC XXXX [I-D.ietf-radext-nai]. This allows for easy name matching in many cases as the name form in the SAML statement and the name form used in RADIUS or Diameter will be the same. In addition to the NAI name form, the document also defines a pair of implicit name forms corresponding to the Client and the Client's machine. These implicit name forms are based on the Identity-Type enumeration defined in TEAP [I-D.ietf-emu-eap-tunnel-method]. If the name form returned in a SAML statement is not based on the NAI, then it is a requirement on the EAP server that it validate that the subject of the SAML assertion, if any, is equivalent to the subject identified by the NAI used in the RADIUS or Diameter session.
RADIUS has the ability to deal with multiple SAML queries for those EAP Servers which follow RFC 5080 [RFC5080]. In this case a State attribute will always be returned with the Access-Accept. The EAP client can then send a new Access-Request with the State attribute and the new SAML Request Multiple SAML queries can then be done by making a new Access-Request using the State attribute returned in the last Access-Accept to link together the different RADIUS sessions.
Some RPs need to ensure that specific criteria are met during the authentication process. This need is met by using Levels of Assurance. The way a Level of Assurance is communicated to the RP from the EAP server is by the use of a SAML Authentication Request using the Authentication Profile from RFC XXX [I-D.ietf-abfab-aaa-saml] When crossing boundaries between different federations, either the policy specified will need to be shared between the two federations, the policy will need to be mapped by the proxy server on the boundary or the proxy server on the boundary will need to supply information the EAP server so that it can do the required mapping. If this mapping is not done, then the EAP server will not be able to enforce the desired Level of Assurance as it will not understand the policy requirements.
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. A framework for the channel binding mechanism has been defined in RFC 6677 [RFC6677] that allows the IdP to check the identity of the RP provided by the AAA framework with that provided by the client.
Traditional web federation does not describe how a client interacts with an identity provider for authentication. As a result, this communication is not standardized. There are several disadvantages to this approach. Since the communication is not standardized, it is difficult for machines to recognize which entity is going to do the authentication and thus which credentials to use and where in the authentication form that the credentials are to be entered. Humans have a much easier time to correctly deal with these problems. The 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 client has adequately validated they are talking to the intended server. By giving control of the authentication interface to a potential attacker, the security of the system may be reduced and phishing opportunities introduced.
As a result, it is desirable to choose some standardized approach for communication between the client'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.
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. client or individual) through the EAP authenticator (i.e., relying party) 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 EAP method used for ABFAB needs to meet the following requirements: [I-D.ietf-emu-eap-tunnel-method] either alone (if client certificates are used) or with an inner EAP method that does mutual authentication.
As of this writing, the only EAP method that meets these criteria is TEAP
EAP channel binding is easily confused with a facility in GSS-API also called channel binding. GSS-API channel binding provides protection against man-in-the-middle attacks when GSS-API is used as authentication inside some tunnel; it is similar to a facility called cryptographic binding in EAP. See [RFC5056] for a discussion of the differences between these two facilities.
The client knows, in theory, the name of the RP that it attempted to connect to, however in the event that an attacker has intercepted the protocol, the client and the IdP need to be able to detect this situation. A general overview of the problem along with a recommended way to deal with the channel binding issues can be found in RFC 6677 [RFC6677].
Since that document was published, a number of possible attacks were found and methods to address these attacks have been outlined in [RFC7029].
The final set of interactions between the parties to consider are those between the client and the RP. In some ways this is the most complex set since at least part of it is outside the scope of the ABFAB work. The interactions between these parties include:
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 as well as an API. When looked at as a protocol, it consists of abstract operations such as the initial context exchange, which includes two sub-operations (gss_init_sec_context and gss_accept_sec_context). An application defines which abstract operations it is going to use and where messages produced by these operations fit into the application architecture. A GSS-API mechanism will define what actual protocol messages result from that abstract message for a given abstract operation. So, since this work is focusing on a particular GSS-API mechanism, we generally focus on protocol elements rather than the API view of GSS-API.
The API view of GSS-API does have significant value as well, since the abstract operations are well defined, the 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 client will need to be modified to support anonymity, unlinkability or pseudonymity.
So, we use GSS-API and SASL because a number of the application protocols we wish to federate support these strategies for security integration. What does this mean from a protocol standpoint and how does this relate to other layers? This means we need to design a concrete GSS-API mechanism. We have chosen to use a GSS-API mechanism that encapsulates EAP authentication. So, GSS-API (and SASL) encapsulates EAP between the end-host and the service. The AAA framework encapsulates EAP between the relying party and the identity provider. The GSS-API mechanism includes rules about how initiators and services are named as well as per-message security and other facilities required by the applications we wish to support.
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:
There are circumstances where the RP will want to have the client reauthenticate itself. These include very long sessions, where the original authentication is time limited or cases where in order to complete an operation a different authentication is required. GSS-EAP does not have any mechanism for the server to initiate a reauthentication as all authentication operations start from the client. If a protocol using GSS-EAP needs to support reauthentication that is initiated by the server, then a request from the server to the client for the reauthentiction to start needs to be placed in the protocol.
Clients can re-use the existing secure connection established by GSS-API to run the new authentication in by calling GSS_Init_sec_context. At this point a full reauthentication will be done.
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 mutual authentication as:
Mutual authentication is an important defense against certain aspects of phishing. Intuitively, clients would like to assume that if some party asks for their credentials as part of authentication, successfully gaining access to the resource means that they are talking to the expected party. Without mutual authentication, the server could "grant access" regardless of what credentials are supplied. Mutual authentication better matches this user intuition.
It is important, therefore, that the GSS-EAP mechanism implement mutual authentication. That is, an initiator needs to be able to request mutual authentication. When mutual authentication is requested, only EAP methods capable of providing the necessary service can be used, and appropriate steps need to be taken to provide mutual authentication. While a broader set of EAP methods could be supported by not requiring mutual authentication, it was decided that the client needs to always have the ability to request it. In some cases the IdP and the RP will not support mutual authentication, however the client will always be able to detect this and make an appropriate security decision.
The AAA infrastructure may hide the initiator's identity from the GSS-API acceptor, providing anonymity between the initiator and the acceptor. At this time, whether the identity is disclosed is determined by EAP server policy rather than by an indication from the initiator. Also, initiators are unlikely to be able to determine whether anonymous communication will be provided. For this reason, initiators are unlikely to set the anonymous return flag from GSS_Init_Sec_context (Section 4.2.1 in [RFC4178].
[RFC5056] defines a concept of channel binding which is used prevent man-in-the-middle attacks. The channel binding works by taking a cryptographic value from the transport security and checks that both sides of the GSS-API conversation know this value. Transport Layer Security (TLS) [RFC5246] is the most common transport security layer used for this purpose.
It needs to be stressed that RFC 5056 channel binding (also called GSS-API channel binding when GSS-API is involved) is not the same thing as EAP channel binding. GSS-API channel binding is used for detecting Man-In-The-Middle attacks. EAP channel binding is used for mutual authentication and acceptor naming checks. Details are discussed in the mechanisms specification [RFC7055]. A fuller description of the differences between the facilities can be found in RFC 5056 [RFC5056].
The use of TLS can provide both encryption and integrity on the channel. It is common to provide SASL and GSS-API with these other security services.
One of the benefits that the use of TLS provides, is that client has the ability to validate the name of the server. However this validation is predicated on a couple of things. The TLS sessions needs to be using certificates and not be an anonymous session. The client and the TLS server need to share a common trust point for the certificate used in validating the server. TLS provides its own server authentication. However there are a variety of situations where this authentication is not checked for policy or usability reasons. When the TLS authentication is checked, if the trust infrastructure behind the TLS authentication is different from the trust infrastructure behind the GSS-API mutual authentication then confirming the 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.
IETF security mechanisms typically take a host name and perhaps a service, entered by a user, and make some trust decision about whether the remote party in the interaction is the intended party. This decision can be made by the use of certificates, pre-configured key information or a previous leap of trust. GSS-API has defined a relatively flexible name convention, however most of the IETF applications that use GSS-API (including SSH, NFS, IMAP, LDAP and XMPP) have chosen to use a more restricted naming convention based on the host name. The GSS-EAP mechanism needs to support host-based service names in order to work with existing IETF protocols.
The use of host-based service names leads to a challenging trust delegation problem. Who is allowed to decide whether a particular host name maps to a specific entity? Possible solutions to this problem have been looked at.
GSS-EAP defines a host naming convention that takes into account the host name, the realm, the service and the service parameters. An example of GSS-API service name is "xmpp/foo@example.com". This identifies the XMPP service on the host foo in the realm example.com. Any of the components, except for the service name may be omitted from a name. When omitted, then a local default would be used for that component of the name.
While there is no requirement that realm names map to Fully Qualified Domain Names (FQDN) within DNS, in practice this is normally true. Doing so allows for the realm portion of service names and the portion of NAIs to be the same. It also allows for the use of DNS in locating the host of a service while establishing the transport channel between the client and the relying party.
It is the responsibility of the application to determine the server that it is going to communicate with; GSS-API has the ability to help confirm that the server is the desired server but not to determine the name of the server to use. It is also the responsibility of the application to determine how much of the information identifying the service needs to be validated by the ABFAB system. The information that needs to be validated is used to build up the service name passed into the GSS-EAP mechanism. What information is to be validated will depend on both what information was provided by the client, and what information is considered significant. If the client only cares about getting a specific service, then the host and realm that provides the service does not need to be validated.
Applications may retrieve information about providers of services from DNS. Service Records (SRV) [RFC2782] and Naming Authority Pointer (NAPTR) [RFC3401] records are used to help find a host that provides a service; however the necessity of having DNSSEC on the queries depends on how the information is going to be used. If the host name returned is not going to be validated by EAP channel binding, because only the service is being validated, then DNSSEC [RFC4033] is not required. However, if the host name is going to be validated by EAP channel binding then DNSSEC needs to be use to ensure that the correct host name is validated. In general, if the information that is returned from the DNS query is to be validated, then it needs to be obtained in a secure manner.
Another issue that needs to be addressed for host-based service names is that they do not work ideally when different instances of a service are running on different ports. If the services are equivalent, then it does not matter. However if there are substantial differences in the quality of the service that information needs to be part of the validation process. If one has just a host name and not a port in the information being validated, then this is not going to be a successful strategy.
GSS-API provides per-message security services that can provide confidentiality and/or integrity. Some IETF protocols such as NFS and SSH take advantage of these services. As a result GSS-EAP needs to support these services. As with mutual authentication, per-message security services will limit the set of EAP methods that can be used to those that generate a Master Session Key (MSK). Any EAP method that produces an MSK is able to support per-message security services described in [RFC2743].
GSS-API provides a pseudo-random function. This function generates a pseudo-random sequence using the shared session key as the seed for the bytes generated. This provides an algorithm that both the initiator and acceptor can run in order to arrive at the same key value. The use of this feature allows for an application to generate keys or other shared secrets for use in other places in the protocol. In this regards, it is similar in concept to the TLS extractor (RFC 5705 [RFC5705].). While no current IETF protocols require this, non-IETF protocols are expected to take advantage of this in the near future. Additionally, a number of protocols have found the TLS extractor to be useful in this regards so it is highly probable that IETF protocols may also start using this feature.
ABFAB, as an architecture designed to enable federated authentication and allow for the secure transmission of identity information between entities, obviously requires careful consideration around privacy and the potential for privacy violations.
This section examines the privacy related information presented in this document, summarizing the entities that are involved in ABFAB communications and what exposure they have to identity information. In discussing these privacy considerations in this section, we use terminology and ideas from [RFC6973].
Note that the ABFAB architecture uses at its core several existing technologies and protocols; detailed privacy discussion around these is not examined. This section instead focuses on privacy considerations specifically related to overall architecture and usage of ABFAB.
+--------+ +---------------+ +--------------+
| Client | <---> | RP | <---> | AAA Client |
+--------+ +---------------+ +--------------+
^
|
v
+---------------+ +--------------+
| SAML Server | | AAA Proxy(s) |
+---------------+ +--------------+
^ ^
| |
v v
+------------+ +---------------+ +--------------+
| EAP Server | <---> | IdP | <---> | AAA Server |
+------------+ +---------------+ +--------------+
Figure 4: Entities and Data Flow
Categorizing the ABFAB entities shown in the Figure 4 according to the taxonomy of terms from [RFC6973] the entities shown in Figure 4 is somewhat complicated as during the various phases of ABFAB communications the roles of each entity changes. The three main phases of relevance are the Client to RP communication phase, the Client to IdP (via the Federation Substrate) phase, and the IdP to RP (via the Federation Substrate) phase.
In the Client to RP communication phase, we have:
In the Client to IdP (via the Federation Substrate) communication phase, we have:
In the IdP to Relying party (via the Federation Substrate) communication phase, we have:
Eavesdroppers and Attackers can reside on any or all communication links between entities in Figure 4.
The various entities in the system might also collude or be coerced into colluding. Some of the significant collusions to look at are:
The Federation Substrate consists of all of the AAA entities. In some cases the AAA Proxies entities may not exist as the AAA Client can talk directly to the AAA Server. Specifications such as the Trust Router Protocol https://community.ja.net/system/files/288/Trust-Router-Overview-IETF86.pptx and RADIUS dynamic discovery [I-D.ietf-radext-dynamic-discovery] can be used to shorten the path between the AAA client and the AAA server (and thus stop these AAA Proxies from being Observers); however even in these circumstances there may be AAA Proxies in the path.
In Figure 4 the IdP has been divided into multiple logical pieces, in actual implementations these pieces will frequently be tightly coupled. The links between these pieces provide the greatest opportunity for attackers and eavesdroppers to acquire information, however, as they are all under the control of a single entity they are also the easiest to have tightly secured.
In the ABFAB architecture, there are a few different types of data and identifiers in use. The best way to understand them, and the potential privacy impacts of them, is to look at each phase of communication in ABFAB.
The flow of data between the client and the RP is divided into two parts. The first part consists of all of the data exchanged as part of the ABFAB authentication process. The second part consists of all of the data exchanged after the authentication process has been finished.
During the initial communications phase, the client sends an NAI (see [I-D.ietf-radext-nai]) to the RP. Many EAP methods (but not all) allow for the client to disclose an NAI to RP the in a form that includes only a realm component during this communications phase. This is the minimum amount of identity information necessary for ABFAB to work - it indicates an IdP that the principal has a relationship with. EAP methods that do not allow this will necessarily also reveal an identifier for the principal in the IdP realm (e.g. a username).
The data shared during the initial communication phase may be protected by a channel protocol such as TLS. This will prevent the leak of information to passive eavesdroppers, however an active attacker may still be able to setup as a man-in-the-middle. The client may not be able to validate the certificates (if any) provided by the service, deferring the check of the identity of the RP until the completion of the ABFAB authentication protocol (i.e., using EAP channel binding).
The data exchanged after the authentication process can have privacy and authentication using the GSS-API services. If the overall application protocol allows for the process of re-authentication, then the same privacy implications as discussed in previous paragraphs apply.
This phase sees a secure TLS tunnel initiated between the Client and the IdP via the RP and federation substrate. The process is initiated by the RP using the realm information given to it by the client. Once set up, the tunnel is used to send credentials to IdP to authenticate.
Various operational information is transported between RP and IdP, over the AAA infrastructure, for example using RADIUS headers. As no end-to-end security is provided by AAA, all AAA entities on the path between the RP and IdP have the ability to eavesdrop on this information unless additional security measures are taken (such as the use of TLS for RADIUS [I-D.ietf-radext-dtls]). Some of this information may form identifiers or explicit identity information:
The choice of EAP method used has other potential privacy implications. For example, if the EAP method in use does not support trust anchors to enable mutual authentication, then there are no guarantees that the IdP is who it claims to be, and thus the full NAI including a username and a realm might be sent to any entity masquerading as a particular IdP.
Note that ABFAB has not specified any AAA accounting requirements. Implementations that use the accounting portion of AAA should consider privacy appropriately when designing this aspect.
In this phase, the IdP communicates with the RP informing it as to the success or failure of authentication of the user, and optionally, the sending of identity information about the principal.
As in the previous flow (Client to IdP), various operation information is transported between IdP and RP over the AAA infrastructure, and the same privacy considerations apply. However, in this flow, explicit identity information about the authenticated principal can be sent from the IdP to the RP. This information can be sent through RADIUS headers, or using SAML [I-D.ietf-abfab-aaa-saml]. This can include protocol specific identifiers, such as SAML NameIDs, as well as arbitrary attribute information about the principal. What information will be released is controlled by policy on the Identity Provider. As before, when sending this through RADIUS headers, all AAA entities on the path between the RP and IdP have the ability to eavesdrop unless additional security measures are taken (such as the use of TLS for RADIUS [I-D.ietf-radext-dtls]). When sending this using SAML, as specified in [I-D.ietf-abfab-aaa-saml], confidentiality of the information should however be guaranteed as [I-D.ietf-abfab-aaa-saml] requires the use of TLS for RADIUS.
Alongside the core authentication and authorization that occurs in AAA communications, accounting information about resource consumption may be delivered as part of the accounting exchange during the lifetime of the granted application session.
In cases where Relying Parties are not required to identify a particular individual when an individual wishes to make use of their service, the ABFAB architecture enables anonymous or pseudonymous access. Thus data and identifiers other than pseudonyms and unlinkable attribute information need not be stored and retained.
However, in cases where Relying Parties require the ability to identify a particular individual (e.g. so they can link this identity information to a particular account in their service, or where identity information is required for audit purposes), the service will need to collect and store such information, and to retain it for as long as they require. Deprovisioning of such accounts and information is out of scope for ABFAB, but obviously for privacy protection any identifiers collected should be deleted when they are no longer needed.
In the ABFAB architecture, by its very nature users are active participants in the sharing of their identifiers as they initiate the communications exchange every time they wish to access a server. They are, however, not involved in control of the set of information related to them that transmitted from the IdP to RP for authorization purposes; rather, this is under the control of policy on the IdP. Due to the nature of the AAA communication flows, with the current ABFAB architecture there is no place for a process of gaining user consent for the information to be released from IdP to RP.
This document describes the architecture for Application Bridging for Federated Access Beyond Web (ABFAB) and security is therefore the main focus. Many of the items that are security considerations have already been discussed in the Privacy Considerations section. Readers should be sure to read that section as well.
There are many places in this document where TLS is used. While in some places (i.e. client to RP) anonymous connections can be used, it is very important that TLS connections within the AAA infrastructure and between the client and the IdP be fully authenticated and, if using certificates, that revocation be checked as well. When using anonymous connections between the client and the RP, all messages and data exchanged between those two entities will be visible to an active attacker. In situations where the client is not yet on the net, the status_request extension [RFC6066] can be used to obtain revocation checking data inside of the TLS protocol. Clients also need to get the Trust Anchor for the IdP configured correctly in order to prevent attacks, this is a hard problem in general and is going to be even harder for kiosk environments.
Selection of the EAP methods to be permitted by clients and IdPs is important. The use of a tunneling method such as TEAP [I-D.ietf-emu-eap-tunnel-method] allows for other EAP methods to be used while hiding the contents of those EAP exchanges from the RP and the AAA framework. When considering inner EAP methods the considerations outlined in [RFC7029] about binding the inner and outer EAP methods needs to be considered. Finally, one wants to have the ability to support channel binding in those cases where the client needs to validate that it is talking to the correct RP.
In those places where SAML statements are used, RPs will generally be unable to validate signatures on the SAML statement, either because it is stripped off by the IdP or because it is unable to validate the binding between the signer, the key used to sign and the realm represented by the IdP. For these reasons it is required that IdPs do the necessary trust checking on the SAML statements and RPs can trust the AAA infrastructure to keep the SAML statement valid.
When a pseudonym is generated as a unique long term identifier for a client by an IdP, care must be taken in the algorithm that it cannot easily be reverse engineered by the service provider. If it can be reversed then the service provider can consult an oracle to determine if a given unique long term identifier is associated with a different known identifier.
This document does not require actions by IANA.
We would like to thank Mayutan Arumaithurai, Klaas Wierenga and Rhys Smith for their feedback. Additionally, we would like to thank Eve Maler, Nicolas Williams, Bob Morgan, Scott Cantor, Jim Fenton, Paul Leach, and Luke Howard for their feedback on the federation terminology question.
Furthermore, we would like to thank Klaas Wierenga for his review of the pre-00 draft version.