HOKEY Working Group T. Clancy, Editor
Internet-Draft LTS
Intended status: Informational November 2006
Expires: May 5, 2007
Handover Key Management and Re-authentication Problem Statement
draft-clancy-hokey-reauth-ps-00
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Copyright (C) The Internet Society (2006).
Abstract
This document describes the Handover Keying (HOKEY) problem
statement. Current EAP keying protocols are incapable of generic
EAP-level re-keying and handoff, which causes often unacceptable
latency to various mobility protocols. HOKEY plans to address these
problems by implementing a generic mechanism to reuse derived EAP
keying material for handoff.
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Table of Contents
1. Authors . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 5
5. Design Goals . . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Security Goals . . . . . . . . . . . . . . . . . . . . . . . . 6
6.1. Key Context and Domino Effect . . . . . . . . . . . . . . 6
6.2. Key Freshness . . . . . . . . . . . . . . . . . . . . . . 7
6.3. Authentication . . . . . . . . . . . . . . . . . . . . . . 7
6.4. Authorization . . . . . . . . . . . . . . . . . . . . . . 7
6.5. Channel Binding . . . . . . . . . . . . . . . . . . . . . 8
6.6. Transport Aspects . . . . . . . . . . . . . . . . . . . . 8
7. Use Cases and Related Work . . . . . . . . . . . . . . . . . . 8
7.1. IEEE 802.11r Applicability . . . . . . . . . . . . . . . . 8
7.2. CAPWAP Applicability . . . . . . . . . . . . . . . . . . . 9
7.3. Inter-Technology Handoff . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . . . 12
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1. Authors
The following authors contributed to the HOKEY problem statement
draft:
o Julien Bournelle, France Telecom R&D,
julien.bournelle@orange-ftgroup.com
o Lakshminath Dondeti, QUALCOMM, ldondeti@qualcomm.com
o Rafael Marin Lopez, Universidad de Murcia, rafa@dif.um.es
o Madjid Nakhjiri, Huawei, mnakhjiri@huawei.com
o Vidya Narayanan, QUALCOMM, vidyan@qualcomm.com
o Mohan Parthasarathy, Nokia, mohan.parthasarathy@nokia.com
o Hannes Tschofenig, Siemens, Hannes.Tschofenig@siemens.com
2. Introduction
The extensible authentication protocol (EAP), specified in RFC3748
[RFC3748] is a generic framework supporting multiple authentication
methods. The primary purpose of EAP is network access control. It
also supports exporting session keys derived during the
authentication. The EAP keying hierarchy defines two keys that are
derived at the top level, the master session key (MSK) and the
extended MSK (EMSK).
In many common deployment scenario, an EAP peer and EAP server
authenticate each other through a third party known as the
authenticator. The authenticator is responsible for translating EAP
packets from the layer 2 (L2) or layer 3 (L3) network access
technology to the AAA protocol.
After successful authentication, the server transports the MSK to the
authenticator. The underlying L2 or L3 protocol used by the
authenticator and the peer derives transient session keys (TSK) using
the MSK as the authentication key or a key derivation key and use the
TSK for per-packet access enforcement. Figure Figure 1 depicts this
process.
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+--------+ +---------------+ +----------+
| EAP |<----->| Passthrough |<--->| EAP |
| Client | L2/L3 | Authenticator | AAA | Server |
+--------+ +---------------+ +----------+
<------------- EAP Authentication ---------->
<---- MSK Transport ----
<-- TSK Generation -->
Figure 1: Logical diagram of EAP authentication and key derivation
using passthrough authenticator
Note that while the authenticator is one logical device, there can be
many physical devices involved. For example, in the CAPWAP model
[RFC3990] WTPs communicate using L2 protocols with the EAP client and
ACs communicate using AAA to the EAP server, while using CAPWAP
protocols to communicate with each other. Depending on the
configuration, authenticator features can be split in a variety of
ways between physical devices, however from the EAP perspective there
is only one logical authenticator.
The EAP model of authentication is unfortunately not efficient in
case of mobile and wireless networks. When a peer associates with an
authenticator, it is expected to run an EAP method irrespective of
whether it has been authenticated to the network recently and has
unexpired keying material. A full or even a reduced round trip EAP
method execution involves several round trips between the EAP peer
and the server.
There have been attempts to solve the problem of efficient re-
authentication in various ways. However, those solutions are either
EAP-method specific, EAP lower-layer specific, otherwise limited in
scope, or do not conform to the AAA keying requirements specified in
[I-D.housley-aaa-key-mgmt].
This document provides a detailed description of EAP efficient re-
authentication protocol requirements.
3. Terminology
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized. 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 [RFC2119].
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This document follows the terminology that has been defined in
RFC3748 [RFC3748] and the EAP Keying Framework [I-D.ietf-eap-keying].
4. Problem Statement
When a peer moves from one authenticator and reattaches to another
authenticator, it is required to engage in a full EAP exchange with
the authentication server in its home domain [RFC3748].
An EAP conversation with a full EAP method run takes several round
trips and significant time to complete, causing delays in handoff
times. Some methods [RFC4187] specify the use of keys and state from
the initial authentication to finish subsequent authentications in
fewer round trips. However, even in those cases, several round trips
to the EAP server are still involved. Furthermore, many commonly-
used EAP methods do not offer such a fast re-authentication feature.
In summary, it is undesirable to have to run a full EAP method each
time a peer associates with a new authenticator; furthermore, it is
desirable to specify a method-independent, efficient, re-
authentication protocol. Keying material from the full
authentication can be used to enable efficient re-authentication.
Another problem with respect to authentication is when the EAP server
is several hops away from the peer, causing too much delay in
executing the re-authentication. It is desirable to allow a locally
reachable server with EAP efficient re-authentication capability with
which the peer can execute such re-authentication without having to
involve the original EAP server all the time. An EAP re-
authentication solution defined MUST NOT prevent its extension to a
fast re-authentication protocol that operates between EAP servers,
and the defined keying hierarchy MUST be designed such that this
could be supported.
These problems are the primary issue to be resolved. In solving
them, there are a number of constraints to conform to and those
result in some additional work to be done in the area of EAP keying.
5. Design Goals
The following are the goals and constraints in designing the EAP re-
authentication and key management protocol:
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Low latency operation: The protocol MUST be responsive to handover
and re-authentication latency performance objectives within a
mobile access network. A solution that minimizes the number of
packet round trips and/or proactively distributes keying material
will be most favorable.
EAP lower-layer independence: Any keying hierarchy and protocol
defined MUST be lower layer independent in order to provide the
capability over heterogeneous technologies. The defined protocols
MAY require some additional support from the lower layers that use
it. Any keying hierarchy and protocol defined MUST accommodate
inter-technology heterogeneous handover.
EAP method independence: Changes to existing EAP methods MUST NOT be
required as a result of the extensions to EAP itself. Note that
the only EAP methods for which independence is required are those
that conform to the specifications of [I-D.ietf-eap-keying] and
[RFC4017].
AAA protocol compatibility and keying: Any modifications to EAP and
EAP keying MUST be compatible with RADIUS and Diameter.
Extensions to both RADIUS and Diameter to support these EAP
modifications are acceptable. The designs and protocols must
satisfy the AAA key management requirements specified in
[I-D.housley-aaa-key-mgmt].
Compatability: Compatibility and especially co-existence with
current EAP implementations and deployment SHOULD be provided.
Compatibility with other fast transition mechanisms SHOULD also be
provided. The keying hierarchy or protocol extensions MUST NOT
preclude the use of CAPWAP or IEEE 802.11r.
6. Security Goals
The section draws from the guidance provided in
[I-D.housley-aaa-key-mgmt] to further define the security goals to be
achieved by a complete re-authentication keying solution.
6.1. Key Context and Domino Effect
Any key MUST have a well-defined scope and MUST be used in a specific
context and for the intended use. This specifically means the
lifetime and scope of each key MUST be defined clearly so that all
entities that are authorized to have access to the key have the same
context during the validity period. In a hierarchical key structure,
the lifetime of lower level keys MUST NOT exceed the lifetime of
higher level keys. This requirement MAY imply that the context and
the scope parameters have to be exchanged. Furthermore, the
semantics of these parameters MUST be defined to provide proper
channel binding specifications. The definition of exact parameter
syntax definition is part of the design of the transport protocol
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used for the parameter exchange and that may be outside scope of this
protocol.
If a key hierarchy is deployed, compromising lower level keys MUST
NOT result in a compromise of higher level keys which they were used
to derive the lower level keys. The compromise of keys at each level
MUST NOT result in compromise of other keys at the same level. The
same principle applies to entities that hold and manage a particular
key defined in the key hierarchy. Compromising keys on one
authenticator MUST NOT reveal the keys of another authenticator.
Note that the compromise of higher-level keys has security
implications on lower levels.
Guidance on parameters required, caching, storage and deletion
procedures to ensure adequate security and authorization provisioning
for keying procedures MUST be defined in a solution document.
All the keying material MUST be uniquely named so that it can be
managed effectively.
6.2. Key Freshness
As [I-D.housley-aaa-key-mgmt] defines, a fresh key is one that is
generated for the intended use. This would mean the key hierarchy
MUST provide for creation of multiple cryptographically separate
child keys from a root key at higher level. Furthermore, the keying
solution needs to provide mechanisms for authorized refreshing each
of the keys within the key hierarchy.
6.3. Authentication
Each party in the handover keying architecture MUST be authenticated
to any other party with whom it communicates, and securely provide
its identity to any other entity that may require the identity for
defining the key scope. The identity provided MUST be meaningful
according to the protocol over which the two parties communicate.
6.4. Authorization
The EAP Key management document [I-D.ietf-eap-keying] discusses
several vulnerabilities that are common to handover mechanisms. One
important issue arises from the way the authorization decisions might
be handled at the AAA server during network access authentication.
For example, if AAA proxies are involved, they may also influence in
the authorization decision. Furthermore, the reasons for making a
particular authorization decision are not communicated to the
authenticator. In fact, the authenticator only knows the final
authorization result. The proposed solution MUST make efforts to
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document and mitigate authorization attacks.
6.5. Channel Binding
Channel Binding procedures are needed to avoid a compromised
intermediate authenticator providing unverified and conflicting
service information to each of the peer and the EAP server. In the
architecture introduced in this document, there are multiple
intermediate entities between the peer and the back-end EAP server.
Various keys need to be established and scoped between these parties
and some of these keys may be parents to other keys. Hence the
channel binding for this architecture will need to consider layering
intermediate entities at each level to make sure that an entity with
higher level of trust can examine the truthfulness of the claims made
by intermediate parties.
6.6. Transport Aspects
Depending on the physical architecture and the functionality of the
elements involved, there may be a need for multiple protocols to
perform the key transport between entities involved in the handover
keying architecture. Thus, a set of requirements for each of these
protocols, and the parameters they will carry, MUST be developed.
Following the requirement specifications, recommendations will be
provided as to whether new protocols or extensions to existing
protocols are needed.
As mentioned, the use of existing AAA protocols for carrying EAP
messages and keying material between the AAA server and AAA clients
that have a role within the architecture considered for the keying
problem will be carefully examined. Definition of specific
parameters, required for keying procedures and to be transferred over
any of the links in the architecture, are part of the scope. The
relation of the identities used by the transport protocol and the
identities used for keying also needs to be explored.
7. Use Cases and Related Work
In order to further clarify the items listed in scope of the proposed
work, this section provides some background on related work and the
use cases envisioned for the proposed work.
7.1. IEEE 802.11r Applicability
One of the EAP lower layers, IEEE 802.11, provides a mechanism to
avoid the problem of repeated full EAP exchanges in a limited
setting, by introducing a two-level key hierarchy. The EAP
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authenticator is collocated with what is known as an R0 Key Holder
(R0-KH), which receives the MSK from the EAP server. A pairwise
master key (PMK-R0) is derived from the last 32 octets of the MSK.
Subsequently, the R0-KH derives an PMK-R1 to be handed out to the
attachment point of the peer. When the peer moves from one R1-KH to
another, a new PMK-R1 is generated by the R0-KH and handed out to the
new R1-KH. The transport protocol used between the R0-KH and the
R1-KH is not specified at the moment.
In some cases, a mobile may seldom move beyond the domain of the
R0-KH and this model works well. A full EAP authentication will
generally be repeated when the PMK-R0 expires. However, in general
cases mobiles may roam beyond the domain of R0-KHs (or EAP
authenticators), and the latency of full EAP authentication remains
an issue.
Another consideration is that there needs to be a key transfer
protocol between the R0-KH and the R1-KH; in other words, there is
either a star configuration of security associations between the key
holder and a centralized entity that serves as the R0-KH, or if the
first authenticator is the default R0-KH, there will be a full-mesh
of security associations between all authenticators. This is
undesirable.
Furthermore, in the 802.11r architecture, the R0-KH may actually be
located close to the edge, thereby creating a vulnerability: If the
R0-KH is compromised, all PMK-R1s derived from the corresponding PMK-
R0s will also be compromised.
The proposed work on EAP efficient re-authentication protocol aims at
addressing the problem in a lower layer agnostic manner that also can
operate without some of the restrictions or shortcomings of 802.11r
mentioned above.
7.2. CAPWAP Applicability
The IETF CAPWAP WG is developing a protocol between what is termed an
Access Controller (AC) and Wireless Termination Points (WTP). The AC
and WTP can be mapped to a WLAN switch and Access Point respectively.
The CAPWAP model supports both split and integrated MAC
architectures, with the authenticator always being implemented at the
AC.
The proposed work on EAP efficient re-authentication protocol
addresses an inter-authenticator handoff problem from an EAP
perspective, which applies during handoff between ACs. Inter-
controller handoffs is a topic yet to be addressed in great detail
and the re-authentication work can potentially address it in an
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effective manner.
7.3. Inter-Technology Handoff
EAP is used for access authentication by several technologies and is
under consideration for use over several other technologies going
forward. Given that, it should be feasible to support smoother
handoffs across technologies. That is one of the big advantages of
using a common authentication protocol. Authentication procedures
typically add substantial handoff delays.
An EAP peer that has multiple radio technologies (802.11 and GSM, for
instance) must perform the full EAP exchange on each interface upon
every horizontal or vertical handoff. With a method independent EAP
efficient re-authentication, it is feasible to support faster
handoffs even in the vertical handoff cases, when the peer may be
roaming from one technology to another.
8. Security Considerations
This document details the HOKEY problem statement. Since HOKEY is an
authentication protocol, there are a myriad of security-related
issues surrounding its development and deployment.
In this document, we have detailed a variety of security properties
inferred from [I-D.housley-aaa-key-mgmt] to which HOKEY must conform,
including the management of key context, scope, freshness, and
transport; resistance to attacks based on the domino effect; and
authentication and authorization. See section Section 6 for further
details.
9. IANA Considerations
This document does not introduce any new IANA considerations.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)",
RFC 3748, June 2004.
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[RFC4017] Stanley, D., Walker, J., and B. Aboba, "Extensible
Authentication Protocol (EAP) Method Requirements for
Wireless LANs", RFC 4017, March 2005.
[I-D.ietf-eap-keying]
Aboba, B., "Extensible Authentication Protocol (EAP) Key
Management Framework", draft-ietf-eap-keying-15 (work in
progress), October 2006.
[I-D.housley-aaa-key-mgmt]
Housley, R. and B. Aboba, "Guidance for AAA Key
Management", draft-housley-aaa-key-mgmt-06 (work in
progress), November 2006.
10.2. Informative References
[RFC3990] O'Hara, B., Calhoun, P., and J. Kempf, "Configuration and
Provisioning for Wireless Access Points (CAPWAP) Problem
Statement", RFC 3990, February 2005.
[RFC4187] Arkko, J. and H. Haverinen, "Extensible Authentication
Protocol Method for 3rd Generation Authentication and Key
Agreement (EAP-AKA)", RFC 4187, January 2006.
Author's Address
T. Charles Clancy, Editor
DoD Laboratory for Telecommunication Sciences
8080 Greenmead Drive
College Park, MD 20740
USA
Email: clancy@LTSnet.net
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