HOKEY Working Group                                       T. Clancy, Ed.
Internet-Draft                                                       LTS
Intended status: Informational                        September 10, 2007
Expires: March 13, 2008

    Handover Key Management and Re-authentication Problem Statement

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   This document describes the Handover Keying (HOKEY) problem
   statement.  The current EAP keying framework is not designed to
   support re-authentication and handovers.  This often cause
   unacceptable latency in various mobile wireless environments.  HOKEY
   plans to address these HOKEY plans to address these problems by
   implementing a generic mechanism to reuse derived EAP keying material
   for handover.

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Table of Contents

   1.  Authors  . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . .  5
   5.  Design Goals . . . . . . . . . . . . . . . . . . . . . . . . .  6
   6.  Security Goals . . . . . . . . . . . . . . . . . . . . . . . .  7
     6.1.  Key Context and Domino Effect  . . . . . . . . . . . . . .  7
     6.2.  Key Freshness  . . . . . . . . . . . . . . . . . . . . . .  8
     6.3.  Authentication . . . . . . . . . . . . . . . . . . . . . .  8
     6.4.  Authorization  . . . . . . . . . . . . . . . . . . . . . .  8
     6.5.  Channel Binding  . . . . . . . . . . . . . . . . . . . . .  8
     6.6.  Transport Aspects  . . . . . . . . . . . . . . . . . . . .  8
   7.  Use Cases and Related Work . . . . . . . . . . . . . . . . . .  9
     7.1.  IEEE 802.11r Applicability . . . . . . . . . . . . . . . .  9
     7.2.  IEEE 802.21 Applicability  . . . . . . . . . . . . . . . . 10
     7.3.  CAPWAP Applicability . . . . . . . . . . . . . . . . . . . 10
     7.4.  Inter-Technology Handover  . . . . . . . . . . . . . . . . 10
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 11
     10.2. Informative References . . . . . . . . . . . . . . . . . . 11
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
   Intellectual Property and Copyright Statements . . . . . . . . . . 13

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1.  Authors

   The following authors contributed to the HOKEY problem statement

   o  Julien Bournelle, France Telecom R&D,
   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]
   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 pass-
   through authenticator (hereafter referred to as simply
   "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.

   According to [RFC3748], after successful authentication, the server
   transports the MSK to the authenticator.  The underlying L2 or L3
   protocol uses the MSK to derive additional keys, including the
   transient session keys (TSK) used per-packet access encryption and
   enforcement.  Figure Figure 1 depicts this process.

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   +--------+       +---------+                     +--------+
   |  EAP   |<----->| Authen- |<------------------->|  EAP   |
   | Client | L2/L3 | ticator |         AAA         | Server |
   +--------+       +---------+                     +--------+
        ^                                               ^
        |                           +---------+         |
        \-------------------------->| Authen- |<--------/
                     L2/L3          | ticator |   AAA

    Initial Authentication:

      <----------------- EAP Authentication ------------->
                       <------ MSK Transport -------------
      <- TSK Generation ->

    Re-Authentication / Handover:

      <----------------- 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 current models of EAP authentication and keying are unfortunately
   not efficient in case of mobile and wireless networks.  When a peer
   arrives at the new authenticator, or is expected to re-affirm its
   access through the current authenticator, the security restraints
   will require the peer to run an EAP method irrespective of whether it
   has been authenticated to the network recently and has unexpired
   keying material.  A full 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, or are otherwise

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   limited in scope.  Furthermore, these solutions do not deal with
   scenarios involving handovers to new authenticators, or do not
   conform to the AAA keying requirements specified in [RFC4962].

   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
   "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document
   are to be interpreted as described in [RFC2119].

   This document follows the terminology that has been defined in

4.  Problem Statement

   When a peer needs to re-affirm access to an authenticator or moves
   from one authenticator and reattaches to another authenticator, the
   current EAP keying model requires the peer 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 re-
   authentication and handover 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 or needs to extend its current association with the
   same 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

   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-

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   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.

   Lastly, a re-authentication protocol should also be capable of
   supporting handover keying.  Handover keying allows re-authentication
   keys to be passed to a different domain.  Execution of the re-
   authentication protocol in that domain will then allow handover from
   one domain to another.

   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:

   Lower latency operation:  The protocol MUST be responsive to handover
      and re-authentication latency performance objectives within a
      mobile access network.  A solution that reduces latency as
      compared to a full EAP authentication 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 re-authentication protocol.  There
      MUST be no requirements imposed on future EAP methods.  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

   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

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   Compatability:  Compatibility and co-existence with compliant
      ([RFC3748] [I-D.ietf-eap-keying]) EAP deployments SHOULD 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 [RFC4962] 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
   used for the parameter exchange and that may be outside scope of this

   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.

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6.2.  Key Freshness

   As [RFC4962] 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 refreshing each of the keys within the key

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
   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

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   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 [IEEE.802-11R-D7.0], is in
   the process of specifying a mechanism to avoid the problem of
   repeated full EAP exchanges in a limited setting, by introducing a
   two-level key hierarchy.  The EAP 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.

   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

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   first authenticator is the default R0-KH, there will be a full-mesh
   of security associations between all authenticators.  This is

   The proposed work on EAP efficient re-authentication protocol aims at
   addressing re-authentication in a lower layer agnostic manner that
   also can fill some of the gaps in IEEE 802.11r.

7.2.  IEEE 802.21 Applicability

   The IEEE 802.21 working group [IEEE.802-21] is standardizing
   mechanisms for media-independent handover.  More specifically, they
   are looking at transitions from one link-layer protocol to another,
   which is currently beyond the scope of the HOKEY charter.

   The techniques developed within HOKEY could be applicable to IEEE
   802.21 if the necessary issues with handover between different lower
   layers can be resolved.  In particular, pre-authentication may be
   more appropriate than re-authentication.

7.3.  CAPWAP Applicability

   The IETF CAPWAP WG [RFC3990] 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 handover problem from an EAP
   perspective, which applies during handover between ACs.  Inter-
   controller handover is a topic yet to be addressed in great detail
   and the re-authentication work can potentially address it in an
   effective manner.

7.4.  Inter-Technology Handover

   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
   handover across technologies.  That is one of the big advantages of
   using a common authentication protocol.  Authentication procedures
   typically add substantial handover 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 handover.  With a method independent EAP

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   efficient re-authentication, it is feasible to support faster
   handover even in the vertical handover 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 [RFC4962] 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.

   [RFC4017]  Stanley, D., Walker, J., and B. Aboba, "Extensible
              Authentication Protocol (EAP) Method Requirements for
              Wireless LANs", RFC 4017, March 2005.

   [RFC4962]  Housley, R. and B. Aboba, "Guidance for Authentication,
              Authorization, and Accounting (AAA) Key Management",
              BCP 132, RFC 4962, July 2007.

10.2.  Informative References

              Aboba, B., "Extensible Authentication Protocol (EAP) Key
              Management Framework", draft-ietf-eap-keying-18 (work in
              progress), February 2007.

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   [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.

              "Information technology - Telecommunications and
              information exchange between systems - Local and
              metropolitan area networks - Specific requirements - Part
              11: Wireless LAN Medium Access Control (MAC) and Physical
              Layer (PHY) specifications - Amendment 2: Fast BSS
              Transition", IEEE Standard 802.11r, June 2007.

              "Media Independent Handover Working Group", IEEE Working
              Group 802.21.

Author's Address

   T. Charles Clancy, Editor
   DoD Laboratory for Telecommunications Sciences
   8080 Greenmead Drive
   College Park, MD  20740

   Email: clancy@LTSnet.net

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