ABFAB J. Howlett
Internet-Draft JANET(UK)
Intended status: Informational S. Hartmann
Expires: June 24, 2011 Painless Security
H. Tschofenig
Nokia Siemens Networks
E. Lear
Cisco Systems GmbH
December 21, 2010
Application Bridging for Federated Access Beyond Web (ABFAB)
Architecture
draft-lear-abfab-arch-01.txt
Abstract
Over the last decade a substantial amount of work has occurred in the
space of federated authentication and authorization. Most of this
effort has focused on two common use cases: network and web-based
access, with few common building blocks within the architecture.
This memo describes an architecture that makes use of extensions to
the commonly used mechanisms for both federated and non-federated
authentication and authorization, including Radius/Diameter, GSS/GS2,
and SAML, to primarily address non-web based authentication, in a
that will scale to large numbers of federations.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on June 24, 2011.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Federation Description . . . . . . . . . . . . . . . . . . 3
1.2. Design Goals . . . . . . . . . . . . . . . . . . . . . . . 7
1.3. Use of Radius . . . . . . . . . . . . . . . . . . . . . . 8
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 9
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1. Federation Substrate . . . . . . . . . . . . . . . . . . . 10
3.2. Subject To Identity Provider . . . . . . . . . . . . . . . 12
3.3. Application to Service . . . . . . . . . . . . . . . . . . 13
3.4. Personalization Layer . . . . . . . . . . . . . . . . . . 14
3.5. Tieing Layers Together . . . . . . . . . . . . . . . . . . 14
4. Application Security Services . . . . . . . . . . . . . . . . 16
4.1. Server (Mutual) Authentication . . . . . . . . . . . . . . 16
4.2. GSS-API Channel Binding . . . . . . . . . . . . . . . . . 17
4.3. Host-Based Service Names . . . . . . . . . . . . . . . . . 18
4.4. Per-Message Tokens . . . . . . . . . . . . . . . . . . . . 19
5. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 20
6. Deployment Considerations . . . . . . . . . . . . . . . . . . 21
6.1. EAP Channel Binding . . . . . . . . . . . . . . . . . . . 21
6.2. AAA Proxy Behavior . . . . . . . . . . . . . . . . . . . . 21
7. Security Considerations . . . . . . . . . . . . . . . . . . . 22
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
10.1. Normative References . . . . . . . . . . . . . . . . . . . 25
10.2. Informative References . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28
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1. Introduction
XXX This document is a first draft. Comments and contributions are
requested.
The Internet makes uses of numerous authentication methods to grant
access to various resources. These mechanisms have been generalized
and scaled over the last decade through mechanisms such as GS2,
Security Assertion Markup Language (SAML) [OASIS.saml-core-2.0-os],
Radius, and Diameter. So-called "federated" access has evolved over
the last decade between web servers through such standards as SAML,
OpenID, and OAUTH, allowing entire domains of individuals to be
authorized for resources. The key scaling points that have been
addressed are the following:
o An Internet service need not copy manually authentication
information from a domain to allow for authentication and
authorization.
o Individual users are able to make use of a single credential to
authenticate to such services.
As the number of such federated services has proliferated, however,
the role of the individual has become ambiguous in certain
circumstances. For example, a school might provide online access to
grades to a parent who is also a teacher. She must clearly
distinguish her role upon access. After all, she is probably not
allowed to edit her own child's grades.
Similarly, as the number of federations proliferates, it becomes
increasingly difficult to discover which identity provider a user is
associated with. This is true for both the web and non-web case, but
particularly acute for the latter ans many non-web authentication
systems are not semantically rich enough on their own to allow for
such ambiguities. For instance, in the case of an email provider,
the use of SMTP and IMAP protocols does not on its own provide for a
way to select a federation. However, the building blocks do exist to
add this functionality.
1.1. Federation Description
The typical setup for a three party protocol involves the following
entities:
o the End Host,
o the Identity Provider, and
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o the Relying Party.
These entities are illustrated graphically in Figure 1.
-----
/- -\
// \\
/ \
| |
,----------\ | | ,---------\
| Identity | | | | Relying |
| Provider +----+ Federation +---+ Party |
`----------' | | '---------'
< | | >
\ | | /
\ \ / /
\ \\ // /
\ \- -/ /
\ ----- /
\ /
\ +------------+ /
\ | | /
v| End Host |v
| |
+------------+
Figure 1: Three Party Authentication Framework
Figure 1 also shows the logical entity 'Federation'. In a
federation, policy is agreed upon by some form of administrative
management, and then instantiated through an operational framework
that the members use, and where compliance is measured in some
fashion. Some deployments may be required to deploy message routing
intermediaries, such as application layer relays or proxies, to offer
the required technical functionality while in other deployments those
are missing.
Often a real world entity is associated with the end host and
responsible for interacting with the identity provider, even if it is
only as weak as completing a web form and confirming the verification
email. The outcome of this initial registration step is that
credentials are made available to the identity provider and to the
end host. It is important to highlight that in some scenarios there
might indeed be a human behind the device denoted as end host and in
other cases there is no human involved in the actual protocol
execution.
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To support the more generic deployment case, we assume that the
identity provider and the relying party belong to different
administrative domains. 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
authorization decisions and when the identity provider does not want
to release information to every relying party (or only under certain
conditions). While it is possible to have a bilateral agreement
between every identity provider and every relying party; on an
Internet scale this setup requires the introduction of a federation
concept, as the management of such pair-wise relationships would
otherwise prove burdensome. While many of the non-technical aspects
of such a federation, such as business practices and operational
arrangements, are outside the scope of the IETF they still impact the
architecture setup on how to ensure the dynamic establishment of
trust.
The steps taken generally in an ABFAB federated authentication/
authorization exchange are as follows (XXX not complete):
1. Principal provides NAI to Application: Somehow the client is
configured with at least the realm portion of an NAI, which
represents the IdP to be discovered.
2. Authentication mechanism selection: this is the step necessary
to indicate that the GSS-EAP SASL/GS2 mechanism will be used for
authentication/authorization.
3. Client Application provides NAI to RP: At the conclusion of
mechanism selection the NAI must be provided to the RP for
discovery.
4. Discovery of federated IdP: This is discussed in detail below.
Either the RP is configured with authorized IdPs, or it makes
use of a federation proxy.
5. Request from Relying Party to IdP: Once the RP knows who the IdP
is, it or its agent will forward RADIUS request that
encapsulates a GSS/EAP access request to an IdP. This may or
may not contain a SAML request as a series of attributes.. At
this stage, the RP will likely have no idea who the principal
is. The RP claims its identity to the IdP in AAA attributes.
6. IdP informs the principal of which EAP method to use: The
available and appropriate methods are discussed below in this
memo.
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7. A bunch of EAP messages happen between the endpoints: Messages
are exchanged between the principal and the IdP until a result
is determined. The number and content of those messages will
depend on the EAP method. If the IdP is unable to authenticate
the principal, the process concludes here. As part of this
process, the principal will, under protection of EAP, assert the
identity of the RP to which it intends to authenticate.
8. Successful Authentication: At the very least the EAP server /
IdP has authenticated the principal, and the principal has
authenticated the IdP. As a result of this step, the principal
and the EAP server hold two cryptographic keys- a Master Session
Key (MSK), and an Extended MSK (EMSK). If the asserted identity
of the RP by the principal matches the identity the RP itself
asserted, there is some confidence that the RP is now
authenticated to the IdP.
9. Local IdP Policy Check: At this stage, the IdP checks local
policy to determine whether the RP and principal are authorized
for the assertion to be made. Additional policy checks will
likely have been made earlier just through the process of
discovery (see later discussion).
10. Response from the IdP to the Relying Party: Once the IdP has
made a determination of whether and how to authenticate or
authorize the principal to the RP, it returns either a negative
answer to the RP, or it returns the identity of the principal to
the RP, as well as an optional set of attributes associated with
the principal. XXX XXX XXX this needs work!!!
11. Return results to principal: Once the RP has a response it must
inform the client application of the result. If all has gone
well, all are authenticated, and the application proceeds with
appropriate authorization levels.
An example communication flow is given below:
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Relying Party Client App IdP
| (1) | Client App gets NAI (somehow)
| | |
|<-----(2)----->| | Mechanism Selection
| | |
|<-----(3)-----<| | NAI transmitted to RP
| | |
|<=====(4)====================>| Discovery
| | |
|>=====(5)====================>| Access request from RP to IdP
| | |
| |< - - (6) - -<| EAP method to Principal
| | |
| |< - - (7) - ->| EAP Exchange to authenticate
| | | Principal
| | |
| | (8 & 9) Local Policy Check
| | |
|<====(10)====================<| IdP Assertion to RP
| | |
|>----(11)----->| | Results to client app.
----- = Between Client App and RP
===== = Between RP and IdP
- - - = Between Client App and IdP
1.2. Design Goals
Our key design goals are as follows:
o Each party of a transaction will be authenticated, and the
principal will be authorized for access to a specific resource .
o Means of authentication is decoupled so as to allow for multiple
authentication methods.
o Hence, the architecture requires no sharing of long term private
keys.
o The system will scale to large numbers of identity providers,
relying parties, and users.
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o The system will be designed primarily for non-Web-based
authentication.
o The system will build upon existing standards, components, and
operational practices.
Designing new three party authentication and authorization protocols
is hard and frought with risk of cryptographic flaws. Achieving
widespead deployment is even more difficult. A lot of attention on
federated access has been devoted to the Web. This document instead
focuses on a non-Web-based environment and focuses on those protocols
where HTTP is not used. Despite the increased excitement for
layering every protocol on top of HTTP there are still a number of
protocols available that do not use HTTP-based transports. Many of
these protocols are lacking a native authentication and authorization
framework of the style shown in Figure 1.
1.3. Use of Radius
Interestingly, for network access authentication the usage of the AAA
framework with RADIUS [RFC2865] and Diameter [RFC3588] was quite
successful from a deployment point of view. To map the terminology
used in Figure 1 to the AAA framework the identity provider
corresponds to the AAA server, the relying party 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 [1].
Is it possible to design a system that builds on top of successful
protocols to offer non-Web-based protocols with a solid starting
point for authentication and authorization in a distributed system?
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2. Terminology
This document uses identity management and privacy terminology from
[I-D.hansen-privacy-terminology].
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3. Architecture
Section 1 already introduced the federated access architecture, with
the illustration of the different actors that need to interact, but
it did not expand on the specifics of providing support for non-Web
based applications. This section details this aspect and motivates
design decisions. The main theme of the work described in this
document is focused on re-using existing building blocks that have
been deployed already and to re-arrange them in a novel way.
Although this architecture assumes updates to both the relying party
as well as to the end host for application integration, those changes
are kept at a minimum. A mechanism that can demonstrate deployment
benefits (based on ease of update of existing software, low
implementation effort, etc.)is preferred and there may be a need to
specify multiple mechanisms to support the range of different
deployment scenarios.
There are a number of ways for encapsulating EAP into an application
protocol. For ease of integration with a wide range of non-Web based
application protocols the usage of the GSS-API was chosen.
Encapsulating EAP into the GSS-API also allows EAP to be used in
SASL. A description of the technical specification can be found in
[I-D.ietf-abfab-gss-eap]. Other alternatives exist as well and may
be considered later, such as "TLS using EAP Authentication"
[I-D.nir-tls-eap].
There are several architectural layers in the system; this section
discusses the individual layers.
3.1. Federation Substrate
The federation substrate is responsible for the connunication between
the relying party and the identity provider. This layer is
responsible for the inter-domain communication and for the technical
mechanisms necessary to establish inter-domain trust.
A key design goal is the re-use of an existing infrastructure, we
build upon the AAA framework as utilized by RADIUS [RFC2138] and
Diameter [RFC3588]. Since this document does not aim to re-describe
the AAA framework the interested reader is referred to [RFC2904].
Building on the AAA infrastructure, and RADIUS and Diameter as
protocols, modifications to that infrastructure is to be avoided.
Also, modifications to AAA servers should be kept at a minimum.
One demand that the AAA substrate must make of the upper layers is
that they must properly identify the end points of the communication.
That is- it must be possible for the AAA server at the RP to
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determine where to send each radius or diameter message. Otherwise,
it is the RP's responsibility to determine the identity of the
principal on its own, without the assistance of an IdP. This
architecture makes use of the Network Access Identifier (NAI), where
the IdP is indicated in the realm component [RFC4282]. The NAI is
represented and consumed by the GSS-API layer as GSS_C_NT_USER_NAME
as specified in [RFC2743]. XXX Where is EAP here?
Once an IdP has been determined by the RP, it or its proxy agent must
determine whether or not the IdP itself is authorized to make
assertions, as it will likely not blindly accept any old provider.
Federations serve this purpose. This architecture provides for three
approaches to resolve whether an IdP is authorized:
Static Configuration: In this case, the federation provides the RP
or its proxy agent with a static list of IdPs that it may trust.
Federation Dynamic Referral In this case, the federation provides a
proxy of its own that will in some way authorize the IdP to the
RP, and visa versa, as not all RPs may be authorized to use all
IdPs for all purposes within a federation. N.B., because the
identity of the principal is likely unknown at this point, it will
not be possible for a federation to authorize an IdP to an RP
based on the identity of the principal.
Federation Proxy: In this case, the authentication request is
forwarded to a federation proxy, who then further forwards the
request to the IdP.
In the first two cases, it is expected that RPs will be configured to
consult multiple federations, as a matter of practice. The first
successful query is sufficient for the RP to then contact the IdP's
AAA server.
The astute reader will notice that RADIUS and Diameter have
substantially similar characteristics. Why not pick one? A key
difference is that today RADIUS is largely transported upon UDP, and
its use is largely, though not exclusively, intra-domain. Diameter
itself was designed to scale to broader uses. We leave as a
deployment decision, which protocol will be appropriate.
Through the integrity protection mechanisms in the AAA framework, the
relying party can establish technical trust that messages are being
sent by the appropriate relying party. Any given interaction will be
associated with one federation at the policy level. The legal or
business relationship defines what statements the identity provider
is trusted to make and how these statements are interpreted by the
relying party. The AAA framework also permits the relying party or
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elements between the relying party and identity provider to make
statements about the relying party.
The AAA framework provides transport for attributes. Statements made
about the subject by the identity provider, statements made about the
relying party and other information is transported as attributes.
3.2. Subject To Identity Provider
Traditional web federation does not describe how a subject
communicates with an identity provider. As a result, this
communication is not standardized. There are several disadvantages
to this approach. It is difficult to have subjects that are machines
rather than humans that use some sort of programatic credential. In
addition, use of browsers for authentication restricts the deployment
of more secure forms of authentication beyond plaintext username and
password known by the server. In a number of cases the
authentication interface may be presented before the subject has
adequately validated they are talking to the intended server. By
giving control of the authentication interface to a potential
attacker, then the security of the system may be reduced and phishing
opportunities introduced.
As a result, it is desirable to choose some standardized approach for
communication between the subject's end-host and the identity
provider. There are a number of requirements this approach must
meet.
Experience has taught us one key security and scalability
requirement: it is important that the relying party not get in
possession of the long-term secret of the entity being authenticated
by the AAA server. Aside from a valuable secret being exposed, a
synchronization problem can also often develop. 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 identity providers may require hardware tokens while
others may use passwords. A service provider would want to support
both sorts of federations, and others.
Fortunately, these requirements can be met by utilizing standardized
and successfully deployed technology, namely by the Extensible
Authentication Protocol (EAP) framework [RFC3748]. Figure 2
illustrates the integration graphically.
EAP is an end-to-end framework; it provides for two-way communication
between a peer (i.e,service client or principal) through the
authenticator (i.e., service provider) to the back-end (i.e.,
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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: one is to carry EAP payloads
from the end host to the relying party. Another is to verify
statements the relying party has made to the subject, confirm these
statements are consistent with statements made to the identity
provider and confirm all the above are consistent with the federation
and any federation-specific policy or configuration. Another
challenge is choosing which identity provider to use for which
service.
3.3. Application to Service
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 e 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 [RFC5801]. In effect, using a
GSS-API mechanism with SASL simply requires placing some headers on
the front of the mechanism and constraining certain GSS-API options.
GSS-API is specified in terms of an abstract set of operations which
can be mapped into a programming language to form an API. When
people are first introduced to GSS-API, they focus on it as an API.
However, from the prospective of authentication for non-web
applications, GSS-API should be thought of as a protocol not an API.
It consists of some abstract operations such as the initial context
exchange, which includes two sub-operations (gss_init_sec_context and
gss_accept_sec_context). An application defines which abstract
operations it is going to use and where messages produced by these
operations fit into the application architecture. A GSS-API
mechanism will define what actual protocol messages result from that
abstract message for a given abstract operation. So, since this work
is focusing on a particular GSS-API mechanism, we generally focus on
protocol elements rather than the API view of GSS-API.
The API view has significant value. Since the abstract operations
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are well defined, the set of information that a mechanism gets from
the application is well defined. Also, the set of assumptions the
application is permitted to make is generally well defined. As a
result, an application protocol that supports GSS-API or SASL is very
likely to be usable with a new approach to authentication including
this one with no required modifications. In some cases, support for
a new authentication mechanism has been added using plugin interfaces
to applications without the application being modified at all. Even
when modifications are required, they can often be limited to
supporting a new naming and authorization model. For example, this
work focuses on privacy; an application that assumes it will always
obtain an identifier for the principal will need to be modified to
support anonymity, unlinkability or pseudonymity.
So, we use GSS-API and SASL because a number of the application
protocols we wish to federate support these strategies for security
integration. What does this mean from a protocol standpoint and how
does this relate to other layers? This means we need to design a
concrete GSS-API mechanism. We have chosen to use a GSS-API
mechanism that encapsulates EAP authentication. So, GSS-API (and
SASL) encapsulate EAP between the end-host and the service. The AAA
framework encapsulates EAP between the relying party and the identity
provider. The GSS-API mechanism includes rules about how principals
and services are named as well as per-message security and other
facilities required by the applications we wish to support.
3.4. Personalization Layer
The AAA framework provides a way to transport statements from the
identity provider to the relying party. However, we also need to say
more about the content of these statements. In simple cases,
attributes particular to the AAA protocol can be defined. However in
more complicated situations it is strongly desirable to re-use an
existing protocol for asking questions and receiving information
about subjects. SAML is used for this.
SAML usage may be as simple as the identity provider including a SAML
Response message in the AAA response. Alternatively the relying
party may generate a SAML request.
3.5. Tieing Layers Together
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+--------------+
| AAA Server |
| (Identity |
| Provider) |
+-^----------^-+
* EAP | RADIUS/
* | Diameter
--v----------v--
/// \\\
// \\ ***
| Federation | back-
| | end
\\ // ***
\\\ ///
--^----------^--
* EAP | RADIUS/
Application * | Diameter
+-------------+ Data +-v----------v--+
| |<---------------->| |
| Client | EAP/EAP Method | Server Side |
| Application |<****************>| Application |
| @ End Host | GSS-API |(Relying Party)|
| |<---------------->| |
| | Application | |
| | Protocol | |
| |<================>| |
+-------------+ +---------------+
*** front-end ***
Legend:
<****>: End-to-end exchange
<---->: Hop-by-hop exchange
<====>: Protocol through which GSS-API/GS2 exchanges are tunnelled
Figure 2: Architecture for Federated Access of non-Web based
Applications
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4. Application Security Services
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.
4.1. Server (Mutual) Authentication
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 does not explicitly talk about what mutual authentication
means. Within the GSS-API community successful mutual authentication
has come to mean:
o If a target name is supplied by the initiator, then the initiator
trusts that the supplied target name describes the acceptor. This
implies both that appropriate cryptographic exchanges took place
for the initiator to make such a trust decision, and that after
evaluating the results of these exchanges, the initiator's policy
trusts that the target name is accurate.
o The initiator trusts that its idea of the acceptor name correctly
names the entity it is communicating with.
o Both the initiator and acceptor have the same key material for
per-message keys and both parties have confirmed they actually
have the key material. In EAP terms, there is a protected
indication of success.
Mutual authentication is an important defense against certain aspects
of phishing. Intuitively, users would like to assume that if some
party asks for their credentials as part of authentication,
successfully gaining access to the resource means that they are
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talking to the expected party. Without mutual authentication, the
acceptor could "grant access" regardless of what credentials are
supplied. Mutual authentication better matches this user intuition.
The GSS-EAP mechanism MUST implement mutual authentication. That is,
an initiator needs to be able to request mutual authentication. When
mutual authentication is requested, only EAP methods capabale of
providing the necessary service can be used, and appropriate steps
need to be taken to provide mutual authentication. A broader set of
EAP methods could be supported when a particular application does not
request mutual authentication. It is an open question whether the
mechanism will permit this.
4.2. GSS-API Channel Binding
[RFC5056] defines a concept of channel binding to prevent man-in-the-
middle attacks. It is common to provide SASL and GSS-API with
another layer to provide transport security; Transport Layer Security
(TLS) is the most common such layer. TLS provides its own server
authentication. However there are a variety of situations where this
authentication is not checked for policy or usability reasons. Even
when it is checked, if the trust infrastructure behind the TLS
authentication is different from the trust infrastructure behind the
GSS-API mutual authentication. If the endpoints of the GSS-API
authentication are different than the endpoints of the lower layer,
this is a strong indication of a problem such as a man-in-the-middle
attack. Channel binding provides a facility to determine whether
these endpoints are the same.
The GSS-EAP mechanism needs to support channel binding. When an
application provides channel binding data, the mechanism needs to
confirm this is the same on both sides consistent with the GSS-API
specification. XXXThere is an open question here as to the details;
today RFC 5554 governs. We could use that and the current draft
assumes we will. However in Beijing we became aware of some changes
to these details that would make life much better for GSS
authentication of HTTP. We should resolve this with kitten and
replace this note with a reference to the spec we're actually
following.
Typically when considering channel binding, people think of channel
binding in combination with mutual authentication. This is
sufficiently common that without additional qualification channel
binding should be assumed to imply mutual authentication. Without
mutual authentication, only one party knows that the endpoints are
correct. That's sometimes useful. Consider for example a user who
wishes to access a protected resource from a shared whiteboard in a
conference room. The whiteboard is the initiator; it does not need
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to actually authenticate that it is talking to the correct resource
because the user will be able to recognize whether the displayed
content is correct. If channel binding were used without mutual
authentication, it would in effect be a request to only disclose the
resource in the context of a particular channel. Such an
authentication would be similar in concept to a holder-of-key SAML
assertion. However, also note that while it is not happening in the
protocol, mutual authentication is happening in the overall system:
the user is able to visually authenticate the content. This is
consistent with all uses of channel binding without protocol level
mutual authentication found so far.
RFC 5056 channel binding (also called GSS-API channel binding when
GSS-API is involved) is not the same thing as EAP channel binding.
EAP channel binding is also used in the ABFAB context in order to
implement acceptor naming and mutual authentication. Details are
discussed in the mechanisms specification [I-D.ietf-abfab-gss-eap].
4.3. Host-Based Service Names
IETF security mechanisms typically take the name of a service entered
by a user and make some trust decision about whether the remote party
in an interaction is the intended party. GSS-API has a relatively
flexible naming architecture. However most of the IETF applications
that use GSS-API, including SSH, NFS, IMAP, LDAP and XMPP, have
chosen to use host-based service names when they use GSS-API. In
this model, the initiator names an acceptor based on a service such
as "imap" or "host" (for login services such as SSH) and a host name.
Using host-based service names leads to a challenging trust
delegation problem. Who is allowed to decide whether a particular
hostname maps to an entity. The public-key infrastructure (PKI) used
by the web has chosen to have a number of trust anchors (root
certificate authorities) each of wich can map any name to a public
key. A number of GSS-API mechanisms suchs as Kerberos [RFC1964]
split the problem into two parts. A new concept called a realm is
introduced. Then the mechanism decides what realm is responsible for
a given name. That realm is responsible for deciding if the acceptor
entity is allowed to claim the name. ABFAB needs to adopt this
approach.
Host-based service names do not work ideally when different instances
of a service are running on different ports. Also, these do not work
ideally when SRV record or other insecure referrals are used.
The GSS-EAP mechanism needs to support host-based service names in
order to work with existing IETF protocols.
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4.4. Per-Message Tokens
GSs-API provides per-message security services that can provide
confidentiality and integrity. Some IETF protocols such as NFS and
SSH take advantage of these services. As a result GSS-EAP needs to
support these services. As with mutual authentication, per-message
services will limit the set of EAP methods that are available. Any
method that produces a Master Session Key (MSK) should be able to
support per-message security services.
GSS-API provides a pseudo-random function. While the pseudo-random
function does not involve sending data over the wire, it provides an
algorithm that both the initiator and acceptor can run in order to
arrive at the same key value. This is useful for designs where a
successful authentication is used to key some other function. This
is similar in concept to the TLS extractor. No current IETF
protocols require this. However GSS-EAP supports this service
because it is valuable for the future and easy to do given per-
message services. Non-IETF protocols are expected to take advantage
of this in the near future.
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5. Privacy Considerations
Sharing identity information may lead to privacy violations. A
future verison of this document will provide a discussion of privacy
considerations in a federated access environment.
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6. Deployment Considerations
6.1. EAP Channel Binding
Discuss the implications of needing EAP channel binding.
6.2. AAA Proxy Behavior
Discuss deployment implications of our proxy requirements.
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7. Security Considerations
This entire document is about security. A future version of the
document will highlight some important security concepts.
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8. IANA Considerations
This document does not require actions by IANA.
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9. Acknowledgments
We would like to thank Mayutan Arumaithurai and Klaas Wierenga for
their feedback. Additionally, we would like to thank Eve Maler,
Nicolas Williams, Bob Morgan, Scott Cantor, Jim Fenton, and Luke
Howard for their feedback on the federation terminology question.
Furthermore, we would like to thank Klaas Wierenga for his review of
the pre-00 draft version.
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10. References
10.1. Normative References
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000.
[RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
Arkko, "Diameter Base Protocol", RFC 3588, September 2003.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)",
RFC 3748, June 2004.
[RFC3579] Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
Dial In User Service) Support For Extensible
Authentication Protocol (EAP)", RFC 3579, September 2003.
[RFC4072] Eronen, P., Hiller, T., and G. Zorn, "Diameter Extensible
Authentication Protocol (EAP) Application", RFC 4072,
August 2005.
[RFC4282] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
Network Access Identifier", RFC 4282, December 2005.
[I-D.hansen-privacy-terminology]
Pfitzmann, A., Hansen, M., and H. Tschofenig, "Terminology
for Talking about Privacy by Data Minimization: Anonymity,
Unlinkability, Undetectability, Unobservability,
Pseudonymity, and Identity Management",
draft-hansen-privacy-terminology-01 (work in progress),
August 2010.
[I-D.ietf-abfab-gss-eap]
Hartman, S. and J. Howlett, "A GSS-API Mechanism for the
Extensible Authentication Protocol",
draft-ietf-abfab-gss-eap-00 (work in progress),
October 2010.
10.2. Informative References
[I-D.nir-tls-eap]
Nir, Y., Sheffer, Y., Tschofenig, H., and P. Gutmann, "TLS
using EAP Authentication", draft-nir-tls-eap-08 (work in
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progress), July 2010.
[RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
RFC 1964, June 1996.
[RFC2203] Eisler, M., Chiu, A., and L. Ling, "RPCSEC_GSS Protocol
Specification", RFC 2203, September 1997.
[RFC3645] Kwan, S., Garg, P., Gilroy, J., Esibov, L., Westhead, J.,
and R. Hall, "Generic Security Service Algorithm for
Secret Key Transaction Authentication for DNS (GSS-TSIG)",
RFC 3645, October 2003.
[RFC2138] Rigney, C., Rigney, C., Rubens, A., Simpson, W., and S.
Willens, "Remote Authentication Dial In User Service
(RADIUS)", RFC 2138, April 1997.
[RFC4462] Hutzelman, J., Salowey, J., Galbraith, J., and V. Welch,
"Generic Security Service Application Program Interface
(GSS-API) Authentication and Key Exchange for the Secure
Shell (SSH) Protocol", RFC 4462, May 2006.
[RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and
Security Layer (SASL)", RFC 4422, June 2006.
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, November 2007.
[RFC5801] Josefsson, S. and N. Williams, "Using Generic Security
Service Application Program Interface (GSS-API) Mechanisms
in Simple Authentication and Security Layer (SASL): The
GS2 Mechanism Family", RFC 5801, July 2010.
[OASIS.saml-core-2.0-os]
Cantor, S., Kemp, J., Philpott, R., and E. Maler,
"Assertions and Protocol for the OASIS Security Assertion
Markup Language (SAML) V2.0", OASIS Standard saml-core-
2.0-os, March 2005.
[RFC2904] Vollbrecht, J., Calhoun, P., Farrell, S., Gommans, L.,
Gross, G., de Bruijn, B., de Laat, C., Holdrege, M., and
D. Spence, "AAA Authorization Framework", RFC 2904,
August 2000.
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URIs
[1] <http://www.eduroam.org>
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Authors' Addresses
Josh Howlett
JANET(UK)
Phone:
Email: Josh.Howlett@ja.net
Sam Hartman
Painless Security
Phone:
Email: hartmans-ietf@mit.edu
Hannes Tschofenig
Nokia Siemens Networks
Linnoitustie 6
Espoo 02600
Finland
Phone: +358 (50) 4871445
Email: Hannes.Tschofenig@gmx.net
URI: http://www.tschofenig.priv.at
Eliot Lear
Cisco Systems GmbH
Richtistrasse 7
Wallisellen, ZH CH-8304
Switzerland
Phone: +41 44 878 9200
Email: lear@cisco.com
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