We have already introduced the federated access architecture, with
the illustration of the different actors that need to interact. This
section expands on the specifics of providing support for
non-web-based applications and provides motivations for design
decisions. The main theme of the work described in this document is
focused on reusing existing building blocks that have been deployed
already and to rearrange them in a novel way.
Although this architecture assumes updates to the RP, the client, and
the IdP, those changes are kept at a minimum. A mechanism that can
demonstrate deployment benefits (based on ease of updates to 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, as 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
| Federation |
| Substrate |
* EAP o RADIUS
| | | |
| Client | EAP/EAP Method | Relying Party |
| Application |<****************>| (RP) |
| | GSS-API | |
| |<---------------->| |
| | Application | |
| | Protocol | |
| |<================>| |
<****>: 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 Instantiation2.1. Relying Party to Identity Provider
Communication between the RP and the IdP is done by the Federation
Substrate. This communication channel is responsible for:
o Establishing the trust relationship between the RP and the IdP.
o Determining the rules governing the relationship.
o Conveying authentication packets from the client to the IdP
o Providing the means of establishing a trust relationship between
the RP and the client.
o Providing a means for the RP to obtain attributes about the client
from the IdP.
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:
o The AAA backbone supplies the trust relationship between the RP
and the IdP.
o The agreements governing a specific AAA backbone contain the rules
governing the relationships within the AAA federation.
o A method exists for carrying EAP packets within RADIUS [RFC3579]
and Diameter [RFC4072].
o The use of EAP channel binding [RFC6677] along with the core ABFAB
protocol provide the pieces necessary to establish the identities
of the RP and the client, while EAP provides the cryptographic
methods for the RP and the client to validate that they are
talking to each other.
o A method exists for carrying SAML packets within RADIUS [RFC7833];
this method allows the RP to query attributes about the client
from the IdP.
Protocols that support the same framework but do different routing
are expected to be defined and used in the future. One such effort,
called the Trust Router, is to set up a framework that creates a
trusted point-to-point channel on the fly
2.1.1. AAA, RADIUS, and Diameter
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 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
clearinghouses). In the case of network access authentication, the
front end, i.e., the communication path between the end host and the
AAA client, is 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 that already exist; the agreements can then merely be
expanded to cover the ABFAB architecture. The AAA framework has
already addressed some of the problems outlined above. For example,
o It already has a method for routing requests based on a domain.
o It already has an extensible architecture allowing for new
attributes to be defined and transported.
o Pre-existing relationships can be reused.
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; RADIUS is also used by
operators of fixed networks. Diameter, on the other hand, is
deployed by operators of mobile networks. Another key difference is
that today RADIUS is largely transported over UDP. The decision
regarding which protocol will be appropriate to deploy is left to
implementers. 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 that convert from RADIUS to Diameter and back.
This makes it possible for both to be deployed in a single Federation
Through the integrity-protection mechanisms in the AAA framework, the
IdP can establish technical trust that messages are being sent by the
appropriate RP. Any given interaction will be associated with one
federation at the policy level. The legal or business relationship
defines what statements the IdP is trusted to make and how these
statements are interpreted by the RP. The AAA framework also permits
the RP or elements between the RP and IdP to make statements about
The AAA framework provides transport for attributes. Statements made
about the client by the IdP, statements made about the RP, 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 endpoints 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 [RFC7542]. 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.
At the time of this writing, no profiles for the use of Diameter have
2.1.2. Discovery and Rules Determination
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 federation selected must
o be able to communicate with the IdP.
o match the business rules and technical policies required for the
RP security requirements.
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
criterion 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 a 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
At the time of this 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; at the present time, the route selection is based on manual
2.1.3. Routing and Technical Trust
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 the 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:
The simplest model is that an RP is a AAA client and can send the
request directly to a AAA proxy. The hop-by-hop integrity
protection of the AAA fabric provides technical trust. An RP can
submit a request directly to the correct federation.
Alternatively, a federation disambiguation fabric can be used.
Such a fabric takes information about what federations the RP is
part of and what federations the IdP is part of, and it routes a
message to the appropriate federation. The routing of messages
across the fabric, plus attributes added to requests and
responses, together provide rules determination. For example,
when a disambiguation fabric routes a message to a given
federation, that federation's rules are chosen. Name validation
is enforced as messages travel across the fabric. The entities
near the RP confirm its identity and validate names it claims.
The fabric routes the message towards the appropriate IdP,
validating the name of the IdP in the process. The routing can be
statically configured. Alternatively, a routing protocol could be
developed to exchange reachability information about a given IdP
and to apply policy across the AAA fabric. Such a routing
protocol could flood naming constraints to the appropriate points
in the fabric.
Instead of routing messages through AAA proxies, some trust broker
could establish keys between entities near the RP and entities
near the IdP. The advantage of this approach is efficiency of
message handling. Fewer entities are needed to be involved for
each message. Security may be improved by sending individual
messages over fewer hops. Rules determination involves decisions
made by trust brokers about what keys to grant. Also, associated
with each credential is context about rules and about other
aspects of technical trust, including names that may be claimed.
A routing protocol similar to the one for AAA proxies is likely to
be useful to trust brokers in flooding rules and naming
A global credential such as a public key and certificate in a
public key infrastructure can be used to establish technical
trust. A directory or distributed database such as the Domain
Name System is used by the RP to discover the endpoint to contact
for a given NAI. Either the database or certificates can provide
a place to store information about rules determination and naming
constraints. Provided that no intermediates are required (or
appear to be required) and that the RP and IdP are sufficient to
enforce and determine rules, rules determination is reasonably
simple. However, applying certain rules is likely to be quite
complex. For example, if multiple sets of rules are possible
between an IdP and RP, confirming that the correct set is used may
be difficult. This is particularly true if intermediates are
involved in making the decision. Also, to the extent that
directory information needs to be trusted, rules determination may
be more complex.
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 above 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.
2.1.4. AAA 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 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 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
Transport Layer Security (TLS) [RFC6614] and/or Datagram Transport
Layer Security (DTLS) [RFC7360] addresses these attacks. The same
level of security is included in the base Diameter specifications.
2.1.5. SAML Assertions
For the traditional use of AAA frameworks, i.e., granting access to a
network, an affirmative response from the IdP is sufficient. 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.
The Security Assertion 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 [RFC7833]. As
SAML statements will frequently be large, RADIUS servers and clients
that deal with SAML statements will need to implement [RFC7499].
There are several issues that need to be highlighted:
o The security of SAML Assertions.
o Namespaces and mapping of SAML attributes.
o Subject naming of entities.
o Making multiple queries about the subject(s).
o Level of assurance for authentication.
SAML Assertions have an optional signature that can be used to
protect and provide the 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 originator's 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
Attributes in a SAML Assertion are identified by a name string. The
name string is either assigned by the SAML issuer context or 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 cases, but not all, 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 to 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 Assertion, as changing the content
of the SAML Assertion 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.
[RFC7833] has defined a new SAML name format that corresponds to the
NAI name form defined by [RFC7542]. 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, [RFC7833] 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 the Tunnel Extensible Authentication Protocol
(TEAP) specification [RFC7170]. 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 that follow [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. A level of assurance is communicated to the RP from the
EAP server by using a SAML Authentication Request, using the
Authentication Profile described in [RFC7833]. When crossing
boundaries between different federations, (1) the policy specified
will need to be shared between the two federations, (2) the policy
will need to be mapped by the proxy server on the boundary, or
(3) the proxy server on the boundary will need to supply information
to the EAP server so that the EAP server 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
2.2. Client to Identity Provider
Looking at the communications between the client and the IdP, the
following items need to be dealt with:
o The client and the IdP need to mutually authenticate each other.
o The client and the IdP need to mutually agree on the identity of
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 [RFC6677] that allows
the IdP to check the identity of the RP provided by the AAA framework
against the identity provided by the client.
2.2.1. Extensible Authentication Protocol (EAP)
Traditional web federation does not describe how a client interacts
with an IdP 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 the credentials are to be entered. It is much easier for humans
to correctly deal with these problems. The use of browsers for
authentication restricts the deployment of more secure forms of
authentication beyond plaintext usernames and passwords known by the
server. In a number of cases, the authentication interface may be
presented before the client has adequately validated that 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 opportunities for phishing may be
As a result, it is desirable to choose some standardized approach for
communication between the client's end host and the IdP. There are a
number of requirements this approach must meet, as noted below.
Experience has taught us one key security and scalability
requirement: it is important that the RP 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, another associated requirement is that 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 also for other methods from IdPs
not yet seen.
These requirements can be met by utilizing standardized and
successfully deployed technology, namely the 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., RP) to the back end (i.e., IdP). This is
precisely -- and conveniently -- the communication path that is
needed for federated identity. Although EAP support is already
integrated in AAA systems (see [RFC3579] and [RFC4072]), several
o The first is how to carry EAP payloads from the end host to
o Another is to verify statements the RP has made to the client,
confirm that these statements are consistent with statements made
to the IdP, and confirm that all of the above are consistent with
the federation and any federation-specific policy or
o Another challenge is choosing which IdP to use for which service.
The EAP method used for ABFAB needs to meet the following
o It needs to provide mutual authentication of the client and IdP.
o It needs to support channel binding.
As of this writing, the only EAP method that meets these criteria is
TEAP [RFC7170], either alone (if client certificates are used) or
with an inner EAP method that does mutual authentication.
2.2.2. EAP Channel Binding
EAP channel binding is easily confused with a facility in GSS-API
that is also called "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. 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
Since the time that [RFC6677] was published, a number of possible
attacks were found. Methods to address these attacks have been
outlined in [RFC7029].
2.3. Client to Relying Party
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:
o Running the protocol that implements the service that is provided
by the RP and desired by the client.
o Authenticating the client to the RP and the RP to the client.
o Providing the necessary security services to the service protocol
that it needs, beyond authentication.
o Dealing with client re-authentication where desired.
One of the remaining layers is responsible for integration of
federated authentication with the application. Applications have
adopted a number of approaches for providing security, so multiple
strategies for integration of federated authentication with
applications may be needed. To this end, we start with a strategy
that provides integration with a large number of application
Many applications, such as Secure Shell (SSH) [RFC4462], NFS
[RFC7530], DNS [RFC3645], and several non-IETF applications, support
GSS-API [RFC2743]. Many applications, such as IMAP, SMTP, the
Extensible Messaging and Presence Protocol (XMPP), and the
Lightweight Directory Access Protocol (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 before the
mechanism's messages and constraining certain GSS-API options.
GSS-API is specified in terms of an abstract set of operations that
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 perspective 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)
[RFC2743]. 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 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 the authentication mechanism defined in
this document, 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 that it will always obtain an identifier for the client will
need to be modified to support anonymity, unlinkability, or
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 that 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 RP and the IdP. 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.
2.3.2. Protocol Transport
The transport of data between the client and the RP 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
In addition, we highlight the following:
o The transport does not need to provide either confidentiality or
integrity. After GSS-EAP has finished negotiation, GSS-API can be
used to provide both services. If the negotiation process itself
needs protection from eavesdroppers, then the transport would need
to provide the necessary services.
o The transport needs to provide reliable transport of the messages.
o The transport needs to ensure that tokens are delivered in order
during the negotiation process.
o GSS-API messages need to be delivered atomically. If the
transport breaks up a message, it must also reassemble the message
There are circumstances where the RP will want to have the client
re-authenticate 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
re-authentication, as all authentication operations start from the
client. If a protocol using GSS-EAP needs to support
re-authentication that is initiated by the server, then a request
from the server to the client for the re-authentication to start
needs to be placed in the protocol.
Clients can reuse the existing secure connection established by
GSS-API, and run the new authentication in that connection, by
calling GSS_Init_sec_context. At this point, a full
re-authentication will be done.