Network Working Group M. Wasserman
Internet-Draft Painless Security
Intended status: Standards Track H. Tschofenig
Expires: January 12, 2012 Nokia Siemens Networks
S. Hartman
Painless Security
July 11, 2011
Multihop Federations for Application Bridging for Federation Beyond the
Web (ABFAB)
draft-mrw-abfab-multihop-fed-01.txt
Abstract
This document describes a mechanism for establishing trust across a
multihop federation within the Application Bridging for Federation
Beyond the Web (ABFAB) framework.
This document introduces a new entity, the Trust Router. Trust
Routers exchange information about the availability of Trust Paths
across a multihop federation. They can be queried by a Relying Party
to obtain the best Trust Path to reach an Identity Provider. They
also provide temporary identities that can be used by a Relying Party
to traverse a Trust Path.
Status of this Memo
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This Internet-Draft will expire on January 12, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Multihop Federation Example . . . . . . . . . . . . . . . . . 7
5. Trust Router Protocol . . . . . . . . . . . . . . . . . . . . 9
6. Trust Path Query . . . . . . . . . . . . . . . . . . . . . . . 10
7. Temporary Identity Request . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8.1. Threat Model . . . . . . . . . . . . . . . . . . . . . . . 12
8.2. Security Requirements . . . . . . . . . . . . . . . . . . 13
8.3. Data Origin validation and signatures . . . . . . . . . . 13
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
11.1. Normative References . . . . . . . . . . . . . . . . . . . 13
11.2. Informative References . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Introduction
This document describes a mechanism for establishing trust across a
multihop federation within the Application Bridging for Federation
Beyond the Web (ABFAB) framework [I-D.lear-abfab-arch].
This document introduces a new ABFAB entity, the Trust Router. Trust
Routers exchange information about the availability of Trust Paths
across a multihop federation. ABFAB entity, the Trust Router. These
paths are used by RPs to contruct transitive trust chains across a
federation to a Radius or Diameter server within a target IdP.
A Trust Path consists of one or more Trust Links. A Trust Link is an
assertion that a specific Trust Router is capable of providing
temporary identies that can be used to access another entity in the
ABFAB system. At this point, we anticipate that there will be two
types of Trust Links in ABFAB: a Trust Link that indicates that one
Trust Router can be used to reach another Trust Router, and a Trust
Link that indicates that a Trust Router can be used to reach a Radius
or Diameter Server. The first type (Trust Router Links) are shown as
A->B(T), which indicates that the Trust Router in realm A can create
identities to reach the trust router in Realm B. The second type
(Radius/Diameter Links) are shown as A->B(R), to indicate that a
trust router in Realm A can be used to reach a Radius, RadSec or
Diameter server in Realm B.
Trust Routers exchange information about available Trust Links within
a federation, and each Trust Router maintains a tree of available
paths to reach all of the IdPs within the federation that can be
reached from the local realm of the Trust Router.
When an RP receives a request from a party within a realm that not
known directly to the RP, the RP will query its local Trust Router to
obtain the best Trust Path to reach that IdP. Note that we use the
term 'best' here to highlight that there may well be multiple paths
to reach an IdP from a given RP, and the selection of the 'best' path
may involve several factors in addition to the length of the path,
such as security and privacy practices, or monetary costs.
The RP will travers the Trust Path obtained from it's local Trust
Router. At each step, the RP will request a temporary identity to
access the next step in the Trust Path, contructing a transitive
chain of trust to a Radius or Diameter server within the target IdP.
To summarize, the Trust Router performs three functions:
o Trust Routers peer with other Trust Routers to exchange
information about available Trust Links, and Trust Paths. This
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information is exchanged between Trust Routers using the Trust
Router Protocol. The Trust Router Protocol is described in more
detail in Section 5.
o Trust Routers respond to queries from Relying Parties to make
information about Trust Paths available. This exchange is
referred to as a Trust Path Query Protocol, which is described in
Section 6.
o To follow the Trust Path across a federation, the RP will use KNP
to ask each Trust Router along the path to provision a temporary
identity that can be used to gain access to the next step in the
path. This mechanism is called a Temporary Identity Request,
which is described in Section 7.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
This document introduces the following terms:
Trust Router:
This is a logical ABFAB entity that exchanges information about
Trust Paths that Relying Parties can use to create transtitive
chains of trust across multihop ABFAB federations.
Trust Link:
A Trust Link is an assertion that a given Trust Router is capable
of providing a temporary identity to communicate with another
ABFAB entity (either another Trust Router, or a Radius/Diameter
server within an IdP).
Trust Path:
A Trust Path is a concatenation of Trust Links that can be used by
an RP to contruct a transitive trust chain across a federation to
a target IdP.
Trust Router Protocol:
The Trust Router Protocol is the mechanism used by two Trust
Routers to exchange information about Trust Links and Trust Paths.
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The terms Identity Provider (IdP), Relying Party (RP), Subject, and
Federation are used as defined in [I-D.lear-abfab-arch].
3. Motivation
Figure 1 shows an example federation where the Relying Party Foo, has
established relationships with various Identity Providers.
+---------------+
| Identity |
| Provider | `..
| Example-A.org | `-._
+---------------+ `..
`-._
+---------------+ `._ +-----------+
| Identity | `- | Relying |
| Provider | ------------------ | Party Foo |
| Example-B.org | _.- +-----------+
+---------------+ _,-'
,,'
+---------------+ _.-' o
| Identity | _,-' \|/
| Provider | ' |
| Example-C.org | / \
+---------------+ Subject
Figure 1: One-to-many Federation Example
When an RP receives a request to access a protected resource (or
requires authentication and authorization for other purposes) the
request includes a realm name that indicates the IdP the Subject has
selected for this exchange. Offering the Subject the ability to
choose among many different IdPs is necessary because a Subject may
have, and want to maintain, uncorrelated identities in several
different realms within a single federation (i.e. work, school,
social networking, etc.). However, this also places a burden on the
RPs to establish and maintain business agreements and exchange
security credentials with a potentially large number of Identity
Providers.
In order for a single-hop federation to function, each IdP needs to
maintain business agreements and exchange credentials with every RP
that its Subjects are authorized to access. Figure 2, shows the
likely outcome, which is that a single-hop federation will come to
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resemble a dense mesh topology.
+---------------+
| Identity |
| Provider |-.._
| Example-A.org |`. ``-.._
+---------------+ `-. ``-..__ +-----------+
`. `--.| Relying |
+---------------+ `. __..--| Party Foo |
| Identity | __:.--'' .-'+-----------+
| Provider |_..--'' `. .-'
| Example-B.org | .-'.
+---------------+ .' '. +-----------+
.-' -. | Relying |
+---------------+ .-' `-.| Party Bar |
| Identity |.-' ____....---''+-----------+
| Provider |.----'''
| Example-C.org |
+---------------+ o
\|/
|
/ \
Subject
Figure 2: Mesh Federation Example
As discussed in section 2.1.1 of [I-D.lear-abfab-arch], as the number
of organizations involved in a ABFAB federation increase, static
configuration may not scale sufficiently. Also, using a Trust Broker
to establish keys between entities near the RP and entities near the
IDP with improve the security and privacy of an ABFAB federation.
Figure 3 shows the structure of a federation where each IdP and RP
has a single connection to the Trust Router infrastructure.
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+---------------+
| Identity |
| Provider |\
| Example-A.org | `.
+---------------+ \ +-----------+
\ .-'| Relying |
+---------------+ `. +---------------+ .' | Party Foo |
| Identity | \| Trust |.-' +-----------+
| Provider |........| Broker |
| Example-B.org | /| |`-.
+---------------+ .' +---------------+ `. +-----------+
/ `-.| Relying |
+---------------+ / | Party Bar |
| Identity | .' +-----------+
| Provider |/ O
| Example-C.org | \|/
+---------------+ |
/ \
Subject
Figure 3: Federation Broker
To improve the operational scalability and security of large ABFAB
federations, this document proposes a Trust Broker solution
consisting of of a set of Trust Routers, as described in this
document, and the Key Negotiation Protocol (KNP), as described in
[I-D.howlett-radsec-knp].
4. Multihop Federation Example
The diagram below shows an example of a successful exchange in a
multihop federation using the Trust Router Protocol and KNP:
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Realm D | Realm C | Realm B | Realm A
| | |
+----------+ +----------+ +----------+ +----------+
| Trust | | | Trust | | | Trust | | | Trust |
| Router D |<-1->| Router C |<-1->| Router B |<-1->| Router A |
+----------+ | +----------+ | +----------+ | +----------+
^ ^ ^ ^
| | | | | | |
| | +---4------------ + |
| | | | | | |
| +----------------5---------------+ | 3
| | | | | | |
+----------------6------------------------------+ | | |
| | | | | | |
v v v v
+----------+ | | | +----------+
| Identity |<---------7--------------------------->| Relying |
| Provider | | | | | Party |
+----------+ +----------+
^ | | | ^
1 |
| | | | |
v |
+----------+ | | | |
| Subject |----------2---------------------------------+
| | | | |
+----------+
| | |
A multihop federation exchange matching the above diagram can be
summarized as follows:
1. We start with a single federation including four realms, each
containing a single Trust Router. The Trust Routers are peered,
such that their interconnections form a multihop federation.
2. A Subject (with an identity in Realm D) attempts to access a
service provided by a Relying Party in Realm A.
3. The Relying Party does not have direct access to a Radius or
Diameter server in Realm D that it can use to authenticate the
Subject, so it asks its local Trust Router for a Trust Path to
reach Realm D. The Trust Router in Realm A returns the path
A->B(T)->C(T)->D(T)->D(R), which indicates that the Relying Party
should use the Trust Routers in Realms B, C and D to reach a
RADIUS or RADSEC server in Realm D, which could then be used to
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authenticate the Subject.
4. The Relying Party contacts a Trust Router in Realm B (using its
permanent identity in Realm A), and requests the creation of a
temporary identity that can be used to communicate with the Trust
Router in Realm C.
5. The Relying Party then contacts the Trust Router in Realm C
(using the temporary identity returned in the previous step), and
asks for a temporary identity that can be used to communicate
with the Trust Router in realm D.
6. The Relying Party then contacts the Trust Router in Realm D
(using the temporary identity returned in the previous step), and
asks the Trust Router to provision an identity that it can use to
speak to the Radius or Diameter server in Realm D (which is part
of Realm D's Identity Provider).
7. At this point, the Relying Party can reach the Subject's Identity
provider, and the rest of the ABFAB exchange can continue, as
described in [I-D.lear-abfab-arch].
5. Trust Router Protocol
Trust Routers use the Trust Router Protocol to exchange information
about available Trust Links, and Trust Paths across a federation.
The Trust Router Protocol differs from an Internet Routing Protocol
in a couple of important ways:
o Trust Links are unidirectional. It can not be assumed that the
fact that a Trust Router in Realm A is authorized to create
temporary identities to access a Trust Router in realm B, that the
opposite is also true (A -> B(T) does not imply B->A(T)).
o Realm names are not necessarily hierarchical. Although
aggregation might be possible as a later optimization, the ability
to aggregate realm names based on shared roots is not currently
assumed.
In addition to the existence of the links themselves, Trust Links
have a set of associated attributes that can be used for filtering
and tree computation, including:
o The cost of the link.
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o Any security and privacy characteristics associated with the link.
o Information indicating how/if the link should be propagated across
the federation.
Current thinking is that we will use a BGP-based algorithm for
computation of the local tree at each Trust Router, and that we will
communicate a similar set of information between Trust Routers as
would be communicated between Internet Routers running BGP.
6. Trust Path Query
A Trust Path Query is generated by a RP to request a Trust Path to
reach a specific realm within a given Policy Regime. If possible,
the Trust Router will reply with a Trust Path that consists of zero
or more Trust Router steps and ends with a Radius or Diameter server
within the IdP for the indicated realm.
The Trust Path Query is initiated by the RP, and the initial query
message will contain the destination realm and Policy Regime.
When a Trust Path Query is received by a Trust Router, the router
will first authenticate the RP, and check local policy information to
determine whether or not to reply.
Assuming that the RP is successfully authenticated and the request
passes local policy checks, the Trust Router will search it's tree of
Trust Path information to determine whether a Trust Path exists that
will reach the destination Realm within the indicated Policy Regime.
If so, the shortest/best Trust Path will be returned to the Relying
Party.
A Trust Path will consist of a list of steps, each of which will
contain: The type of the step (Trust Router or Radius/Diameter), the
Policy Regime associated with each step, information needed to reach
the indicated Trust Router or server (domain name or IP address), and
any special attributes associated with that step.
7. Temporary Identity Request
A Temporary Identity Request is issued by a Relying Party in order to
obtain an identity that can be used to traverse each step in the
Trust Path. When a Temporary Identity is requested, a Trust Router
will provision a new identity in its local Radius or Diameter
infrastructure that can be used by the Relying Party to communicate
with the Trust Router or Radus/Diameter server that represents the
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next step in the Trust Path. Current thinking is that KNP will be
used as the protocol mechanism for these requests.
These Temporary Identities will have a finite lifetime and, when
authenticated, will include a Radius Attribute/Diameter AVP
indicating that they were generated based on a Temporary Identity
Request. This attribute will inlcude the chain of identities that
preceeded the current identity in the traversal of the Trust Path.
The details of how these messages will be encoded has not yet been
determined. However, it is expected that, for each Trust Router step
in the Trust Path, the following actions will take place:
1. The Relying Party will send a Temporary Identity Request message
to the Trust Router, containing the identity of the next step in
the Trust Path, the destination realm that it is trying to reach,
and the Policy Regime in use. This request will be sent using
the identity that the Trust Router obtained from the previous
step in the Trust Path (or the Trust Router's permanent identity
in it's home realm, if this is the first step).
2. The Trust Router will authenticate the Relying Party.
3. If the authentication is successful, the Trust Router will check
local policy to determine whether it should provision an identity
for the Relying Party for the indicated purpose (details of this
check may be implementation dependent).
4. If the request passes any policy requirements, the Trust Router
will provision a temporary identity for the Relying Party within
the Trust Router's local realm that can be used to access the
next-hop Trust Router or RADIUS/RADSEC server in the Trust Path.
8. Security Considerations
As discussed in [I-D.lear-abfab-arch], the trust broker architecture
is a mechanism for establishing technical trust in an ABFAB
federation. Technical trust mechanisms have three primary
responsibilities in ABFAB. They are responsible for integrity
protection of AAA traffic. They are responsible for constraining the
naming of ABFAB entities: for example the technical trust mechanism
assures that the entity claiming to be the IDP is authenticated and
authorized to act as the IDP for the realm containing the subject.
The technical trust mechanism also determines where AAA messages are
routed.
The trust broker architecture described in this document is designed
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to meet the security and operational requirements of federations and
groups of federations with large numbers of organizations. In these
environments depending on any common credentials or trust mechanism
does not make sense. While federations are expected to interconnect,
they are not expected to have a common set of trust anchors for a
public-key infrastructure. Each realm needs to be able to choose the
appropriate credentials and security policies to use when
establishing a relationship with another realm.
by design, this approach provides flexibility. Parts of the
interconnected set of realms can use high-assurance processes and
mechanisms including strong authentication mechanisms and rigorous
credentialing and enrollment processes. Other realms can use lower-
assurance mechanisms and processes, balancing cost and speed against
security. However this flexibility complicates the security policy.
Just because the local realm has a high-assurance trust link does not
mean that the path is high-assurance. Operational mechanisms are
required in order for RPs to express their security requirements and
for the trust routers to make sure that resulting trust paths meet
these requirements. Similarly, trust routers need to make sure that
paths to a given IDP are not announced unless that IDP's security
requirements will be met.
8.1. Threat Model
Like all Internet protocols, the trust router protocols and KNP need
to have strong protection against parties who are not authorized to
be part of an exchange. Such attackers do not start out knowing
credentials necessary to participate in the system. However these
attackers can be assumed to observe trust router, KNP, AAA and ABFAB
exchanges. The system needs to maintain integrity of all data,
confidentiality of keys and in some cases confidentiality of other
data even when these attackers can insert, suppress, modify or replay
packets. Reasonable defenses against attacks on the availability of
the system are required, although obviously there are limits to these
defenses. An attacker who can disrupt connectivity with a realm can
impact availability.
The interesting threat model surrounds malicious participants
authorized to participate in the system. The threat model is similar
to that of routing protocols [I-D.ietf-karp-threats-reqs]. Defending
against a compromised actor announcing a trust link that actor would
be permitted to announce were it functioning correctly is out of
scope. Similarly, defending against an compromised actor performing
some action that actor is authorized to take is out of scope for this
threat model.
However, it is a requirement that the system needs to provide tools
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to limit the authorization of actors. For example if a particular
session between two trust routers is not authorized to announce a
trust link to a given realm or with certain properties, then attacks
permitting such a link to be announced are in scope. Similarly an
attack permitting a temporary identity with properties inconsistent
with administrative limits would be in scope.
The system must permit zones of more or less trust to be created. An
attack that permits insiders in the zones of less trust to compromise
a zone of higher trust beyond what the zone of lesser trust is
permitted is within the scope of threats. However, trust can only
decrease as distance across the transitive network of trust routers
increases. A peer two hops away cannot be permitted to make any
statement that a peer one hop away cannot make. In general, it is
unknown whether the peer two hops away actually made the statement.
8.2. Security Requirements
TBD
8.3. Data Origin validation and signatures
TBD
9. IANA Considerations
There are no IANA actions required for this document at this time.
10. Acknowledgements
This document was written using the xml2rfc tool described in RFC
2629 [RFC2629].
11. References
11.1. Normative References
[I-D.howlett-radsec-knp]
Howlett, J. and S. Hartman, "Key Negotiation Protocol for
RadSec (KNP)", draft-howlett-radsec-knp-01 (work in
progress), March 2011.
[I-D.lear-abfab-arch]
Howlett, J., Hartman, S., Tschofenig, H., and E. Lear,
"Application Bridging for Federated Access Beyond Web
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(ABFAB) Architecture", draft-lear-abfab-arch-02 (work in
progress), March 2011.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
11.2. Informative References
[I-D.ietf-karp-threats-reqs]
Lebovitz, G., Bhatia, M., and R. White, "The Threat
Analysis and Requirements for Cryptographic Authentication
of Routing Protocols' Transports",
draft-ietf-karp-threats-reqs-03 (work in progress),
June 2011.
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
June 1999.
Authors' Addresses
Margaret Wasserman
Painless Security
356 Abbott Street
North Andover, MA 01845
USA
Phone: +1 781 405 7464
Email: mrw@painless-security.com
URI: http://www.painless-security.com
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
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Sam Hartman
Painless Security
356 Abbott Street
North Andover, MA 01845
USA
Email: hartmans@painless-security.com
URI: http://www.painless-security.com
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