Distribution of EAP-Based Keys for Handover and Re-Authentication
RFC 5749

Internet Engineering Task Force (IETF)                    K. Hoeper, Ed.
Request for Comments: 5749                                   M. Nakhjiri
Category: Standards Track                                       Motorola
ISSN: 2070-1721                                             Y. Ohba, Ed.
                                                                 Toshiba
                                                              March 2010

   Distribution of EAP-Based Keys for Handover and Re-Authentication

Abstract

   This document describes an abstract mechanism for delivering root
   keys from an Extensible Authentication Protocol (EAP) server to
   another network server that requires the keys for offering security
   protected services, such as re-authentication, to an EAP peer.  The
   distributed root key can be either a usage-specific root key (USRK),
   a domain-specific root key (DSRK), or a domain-specific usage-
   specific root key (DSUSRK) that has been derived from an Extended
   Master Session Key (EMSK) hierarchy previously established between
   the EAP server and an EAP peer.  This document defines a template for
   a key distribution exchange (KDE) protocol that can distribute these
   different types of root keys using a AAA (Authentication,
   Authorization, and Accounting) protocol and discusses its security
   requirements.  The described protocol template does not specify
   message formats, data encoding, or other implementation details.  It
   thus needs to be instantiated with a specific protocol (e.g., RADIUS
   or Diameter) before it can be used.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5749.

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Copyright Notice

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Key Delivery Architecture  . . . . . . . . . . . . . . . . . .  5
   4.  Key Distribution Exchange (KDE)  . . . . . . . . . . . . . . .  6
     4.1.  Context and Scope for Distributed Keys . . . . . . . . . .  7
     4.2.  Key Distribution Exchange Scenarios  . . . . . . . . . . .  8
   5.  KDE Used in the EAP Re-Authentication Protocol (ERP) . . . . .  8
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
     6.1.  Requirements on AAA Key Transport Protocols  . . . . . . .  9
     6.2.  Distributing RK without Peer Consent . . . . . . . . . . . 10
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 10
   8.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 10
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 11

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1.  Introduction

   The Extensible Authentication Protocol (EAP) [RFC3748] is an
   authentication framework supporting authentication methods that are
   specified in EAP methods.  By definition, any key-generating EAP
   method derives a Master Session Key (MSK) and an Extended Master
   Session Key (EMSK).  [RFC5295] reserves the EMSK for the sole purpose
   of deriving root keys that can be used for specific purposes called
   usages.  In particular, [RFC5295] defines how to create a usage-
   specific root key (USRK) for bootstrapping security in a specific
   application, a domain-specific root key (DSRK) for bootstrapping
   security of a set of services within a domain, and a usage-specific
   DSRK (DSUSRK) for a specific application within a domain.  [RFC5296]
   defines a re-authentication root key (rRK) that is a USRK designated
   for re-authentication.

   The MSK and EMSK may be used to derive further keying material for a
   variety of security mechanisms [RFC5247].  For example, the MSK has
   been widely used for bootstrapping the wireless link security
   associations between the peer and the network attachment points.
   However, performance as well as security issues arise when using the
   MSK and the current bootstrapping methods in mobile scenarios that
   require handovers, as described in [RFC5169].  To address handover
   latencies and other shortcomings, [RFC5296] specifies an EAP re-
   authentication protocol (ERP) that uses keys derived from the EMSK or
   DSRK to enable efficient re-authentications in handover scenarios.
   Neither [RFC5295] nor [RFC5296] specifies how root keys are delivered
   to the network server requiring the key.  Such a key delivery
   mechanism is essential because the EMSK cannot leave the EAP server
   ([RFC5295]), but root keys are needed by other network servers
   disjoint with the EAP server.  For example, in order to enable an EAP
   peer to re-authenticate to a network during a handover, certain root
   keys need to be made available by the EAP server to the server
   carrying out the re-authentication.

   This document specifies an abstract mechanism for the delivery of the
   EMSK child keys from the server holding the EMSK or a root key to
   another network server that requests a root key for providing
   protected services (such as re-authentication and other usage and
   domain-specific services) to EAP peers.  In the remainder of this
   document, a server delivering root keys is referred to as a Key
   Delivering Server (KDS), and a server authorized to request and
   receive root keys from a KDS is referred to as a Key Requesting
   Server (KRS).  The Key Distribution Exchange (KDE) mechanism defined
   in this document runs over a AAA (Authentication, Authorization, and
   Accounting) protocol, e.g., RADIUS ([RFC2865], [RFC3579]) or Diameter
   [RFC3588], and has several variants depending on the type of key that
   is requested and delivered (i.e., DRSK, USRK, or DSUSRK).  The

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   presented KDE mechanism is a protocol template that must be
   instantiated for a particular protocol, such as RADIUS or Diameter,
   to specify the format and encoding of the abstract protocol messages.
   Only after such an instantiation can the KDE mechanism described in
   this document be implemented.  This document also describes security
   requirements for the secure key delivery over AAA.

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 [RFC2119].

   The following acronyms are used.

   AAA
      Authentication, Authorization and Accounting.  AAA protocols with
      EAP support include RADIUS ([RFC2865], [RFC3579]) and Diameter
      [RFC3588].

   USRK
      Usage-Specific Root Key.  A root key that is derived from the
      EMSK; see [RFC5295].

   USR-KH
      USRK Holder.  A network server that is authorized to request and
      receive a USRK from the EAP server.  The USR-KH can be a AAA
      server or dedicated service server.

   DSRK
      Domain-Specific Root Key.  A root key that is derived from the
      EMSK; see [RFC5295].

   DSR-KH
      DSRK Holder.  A network server that is authorized to request and
      receive a DSRK from the EAP server.  The most likely
      implementation of a DSR-KH is a AAA server in a domain, enforcing
      the policies for the usage of the DSRK within this domain.

   DSUSRK
      Domain-Specific Usage-Specific Root Key.  A root key that is
      derived from the DSRK; see [RFC5295].

   DSUSR-KH
      DSUSRK holder.  A network server authorized to request and receive
      a DSUSRK from the DSR-KH.  The most likely implementation of a
      DSUSR-KH is a AAA server in a domain, responsible for a particular
      service offered within this domain.

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   RK
      Root Key.  An EMSK child key, i.e., a USRK, DSRK, or DSUSRK.

   KDS
      Key Delivering Server.  A network server that holds an EMSK or
      DSRK and delivers root keys to a KRS requesting root keys.  The
      EAP server (together with the AAA server to which it exports the
      keys for delivery) and the DSR-KH can both act as KDS.

   KRS
      Key Requesting Server.  A network server that shares an interface
      with a KDS and is authorized to request root keys from the KDS.  A
      USR-KH, DSR-KH, and DSUSR-KH can all act as a KRS.

   HOKEY
      Handover Keying.

3.  Key Delivery Architecture

   An EAP server carries out normal EAP authentications with EAP peers
   but is typically not involved in potential handovers and re-
   authentication attempts by the same EAP peer.  Other servers are
   typically in place to offer these requested services.  These servers
   can be AAA servers or other service network servers.  Whenever EAP-
   based keying material is used to protect a requested service, the
   respective keying material has to be available to the server
   providing the requested service.  For example, the first time a peer
   requests a service from a network server, this server acts as a KRS.
   The KRS requests the root keys needed to derive the keys for
   protecting the requested service from the respective KDS.  In
   subsequent requests from the same peer and as long as the root key
   has not expired, the KRS can use the same root keys to derive fresh
   keying material to protect the requested service.  These kinds of key
   requests and distributions are necessary because an EMSK cannot leave
   the EAP server ([RFC5295]).  Hence, any root key that is directly
   derived from an EMSK can only be derived by the EAP server itself.
   The EAP server then exports these keys to a server that can
   distribute the keys to the KRS.  In the remainder of this document,
   the KDS consisting of the EAP server that derives the root keys
   together with the AAA server that distributes these keys is denoted
   EAP/AAA server.  Root keys derived from EMSK child keys, such as a
   DSUSRK, can be requested from the respective root key holder.  Hence,
   a KDS can be either the EAP/AAA server or a DSRK holder (DSR-KH),
   whereas a KRS can be either a USRK holder (USR-KH), a DSR-KH, or a
   DSUSRK holder (DSUSR-KH).

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   The KRS needs to share an interface with the KDS to be able to send
   all necessary input data to derive the requested key and to receive
   the requested key.  The provided data includes the Key Derivation
   Function (KDF) that should be used to derive the requested key.  The
   KRS uses the received root key to derive further keying material in
   order to secure its offered services.  Every KDS is responsible for
   storing and protecting the received root key as well as the
   derivation and distribution of any child key derived from the root
   key.  An example of a key delivery architecture is illustrated in
   Figure 1 showing the different types of KRS and their interfaces to
   the KDS.

                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                 |             EAP/AAA server              |
                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  /             |             |          \
                 /              |             |           \
                /               |             |            \
        +-+-+-+-+-+-+-+   +-+-+-+-+-+-+  +-+-+-+-+-+  +-+-+-+-+-+
        |   USR-KH1   |   |  USR-KH2  |  | DSR-KH1 |  | DSR-KH2 |
        | HOKEY server|   | XYZ server|  |Domain 1 |  | Domain 2|
        +-+-+-+-+-+-+-+   +-+-+-+-+-+-+  +-+-+-+-+-+  +-+-+-+-+-+
                                             /             |
                                            /              |
                                           /               |
                                    +-+-+-+-+-+-+-+  +-+-+-+-+-+-+-+-+
                                    |  DSUSR-KH   |  |  DSUSR-KH2    |
                                    |  Domain 1   |  |   Domain 2    |
                                    |Home domain  |  |Visited domain |
                                    |HOKEY server |  |HOKEY server   |
                                    +-+-+-+-+-+-+-+  +-+-+-+-+-+-+-+-+

   Figure 1: Example Key Delivery Architecture for the Different KRS and
                                    KDS

4.  Key Distribution Exchange (KDE)

   In this section, a generic mechanism for a key distribution exchange
   (KDE) over AAA is described in which a root key (RK) is distributed
   from a KDS to a KRS.  It is required that the communication path
   between the KDS and the KRS is protected by the use of an appropriate
   AAA transport security mechanism (see Section 6 for security
   requirements).  Here, it is assumed that the KRS and the KDS are
   separate entities, logically if not physically, and the delivery of
   the requested RK is specified accordingly.

   The key distribution exchange consists of one round-trip, i.e., two
   messages between the KRS and the KDS, as illustrated in Figure 2.

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   First, the KRS sends a KDE-Request carrying a Key Request Token
   (KRT).  As a response, the KDS sends a KDE-Response carrying a Key
   Delivery Token (KDT).  Both tokens are encapsulated in AAA messages.
   The definition of the AAA attributes depends on the implemented AAA
   protocol and is out of scope of this document.  However, the security
   requirements for AAA messages carrying KDE messages are discussed in
   Section 6.  The contents of KRT and KDT are defined in the following.

     KRS                                        KDS
   --------                                   -------
       |                                          |
       |       KDE-Request: AAA{KRT}              |
       |----------------------------------------->|
       |       KDE-Response: AAA{KDT}             |
       |<-----------------------------------------|

                        Figure 2: KDE Message Flow

   KRT : (PID, KT, KL)

      KRT carries the identifiers of the peer (PID), the key type (KT)
      and the key label (KL).  The key type specifies which type of root
      key is requested, e.g., DSRK, USRK and DSUSRK.  The encoding rules
      for each key type are left to the protocol developers who define
      the instantiation of the KDE mechanism for a particular protocol.
      For the specification of key labels and the associated IANA
      registries, please refer to [RFC5295], which specifies key labels
      for USRKs and establishes an IANA registry for them.  The same
      specifications can be applied to other root keys.

   KDT : (KT, KL, RK, KN_RK, LT_RK)

      KDT carries the root key (RK) to be distributed to the KRS, as
      well as the key type (KT) of the key, the key label (KL), the key
      name (KN_RK), and the lifetime of RK (LT_RK).  The key lifetime of
      each distributed key MUST NOT be greater than that of its parent
      key.

4.1.  Context and Scope for Distributed Keys

   The key context of each distributed key is determined by the sequence
   of KTs in the key hierarchy.  The key scope of each distributed key
   is determined by the sequence of (PID, KT, KL)-tuples in the key
   hierarchy and the identifier of the KRS.  The KDF used to generate
   the requested keys includes context and scope information, thus,
   binding the key to the specific channel [RFC5295].

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4.2.  Key Distribution Exchange Scenarios

   Given the three types of KRS, there are three scenarios for the
   distribution of the EMSK child keys.  For all scenarios, the trigger
   and mechanism for key delivery may involve a specific request from an
   EAP peer and/or another intermediary (such as an authenticator).  For
   simplicity, it is assumed that USR-KHs reside in the same domain as
   the EAP server.

   Scenario 1: EAP/AAA server to USR-KH:  In this scenario, the EAP/AAA
      server delivers a USRK to a USR-KH.

   Scenario 2: EAP/AAA server to DSR-KH:  In this scenario, the EAP/AAA
      server delivers a DSRK to a DSR-KH.

   Scenario 3: DSR-KH to DSUSR-KH:  In this scenario, a DSR-KH in a
      specific domain delivers keying material to a DSUSR-KH in the same
      domain.

   The key distribution exchanges for Scenario 3 can be combined with
   the key distribution exchanges for Scenario 2 into a single round-
   trip exchange as shown in Figure 3.  Here, KDE-Request and KDE-
   Response are messages for Scenarios 2, whereas KDE-Request' and KDE-
   Response' are messages for Scenarios 3.

   DSUSR-KH                   DSR-KH                    EAP/AAA Server
   --------                   ------                     ------------
      |  KDE-Request'(KRT')     |   KDE-Request(KRT)        |
      |------------------------>|-------------------------->|
      |  KDE-Response'(KDT')    |   KDE-Response(KDT)       |
      |<----------------------- |<--------------------------|
      |                         |                           |

                    Figure 3: Combined Message Exchange

5.  KDE Used in the EAP Re-Authentication Protocol (ERP)

   This section describes how the presented KDE mechanism should be used
   to request and deliver the root keys used for re-authentication in
   the EAP Re-authentication Protocol (ERP) defined in [RFC5296].  ERP
   supports two forms of bootstrapping, implicit as well as explicit
   bootstrapping, and KDE is discussed for both cases in the remainder
   of this section.

   In implicit bootstrapping, the local EAP Re-authentication (ER)
   server requests the DSRK from the home AAA server during the initial
   EAP exchange.  Here, the local ER server acts as the KRS and the home

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   AAA server as the KDS.  In this case, the local ER server requesting
   the DSRK includes a KDE-Request in the AAA packet encapsulating the
   first EAP-Response message from the peer.  Here, a AAA User-Name
   attribute is used as the PID.  If the EAP exchange is successful, the
   home AAA server includes a KDE-Response in the AAA message that
   carries the EAP-Success message.

   Explicit bootstrapping is initiated by peers that do not know the
   domain.  Here, the peer sends an EAP-Initiate message with the
   bootstrapping flag turned on.  The local ER server (acting as KRS)
   includes a KDE-Request message in the AAA message that carries the
   peer's EAP-Initiate message and sends it to the peer's home AAA
   server.  Here, a AAA User-Name attribute is used as the PID.  In its
   response, the home AAA server (acting as KDS) includes a KDE-Response
   in the AAA message that carries the EAP-Finish message with the
   bootstrapping flag set.

6.  Security Considerations

   This section provides security requirements and a discussion of
   distributing RK without peer consent.

6.1.  Requirements on AAA Key Transport Protocols

   Any KDE attribute that is exchanged as part of a KDE-Request or KDE-
   Response MUST be integrity-protected and replay-protected by the
   underlying AAA protocol that is used to encapsulate the attributes.
   Additionally, a secure key wrap algorithm MUST be used by the AAA
   protocol to protect the RK in a KDE-Response.  Other confidential
   information as part of the KDE messages (e.g., identifiers if privacy
   is a requirement) SHOULD be encrypted by the underlying AAA protocol.

   When there is an intermediary, such as a AAA proxy, on the path
   between the KRS and the KDS, there will be a series of hop-by-hop
   security associations along the path.  The use of hop-by-hop security
   associations implies that the intermediary on each hop can access the
   distributed keying material.  Hence, the use of hop-by-hop security
   SHOULD be limited to an environment where an intermediary is trusted
   not to abuse the distributed key material.  If such a trusted AAA
   infrastructure does not exist, other means must be applied at a
   different layer to ensure the end-to-end security (i.e., between KRS
   and KDS) of the exchanged KDE messages.  The security requirements
   for such a protocol are the same as previously outlined for AAA
   protocols and MUST hold when encapsulated in AAA messages.

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6.2.  Distributing RK without Peer Consent

   When a KDE-Request is sent as a result of explicit ERP bootstrapping
   [RFC5296], cryptographic verification of peer consent on distributing
   an RK is provided by the integrity checksum of the EAP-Initiate
   message with the bootstrapping flag turned on.

   On the other hand, when a KDE-Request is sent as a result of implicit
   ERP bootstrapping [RFC5296], cryptographic verification of peer
   consent on distributing an RK is not provided.  A peer is not
   involved in the process and, thus, not aware of key delivery requests
   for root keys derived from its established EAP keying material.
   Hence, a peer has no control where keys derived from its established
   EAP keying material are distributed.  A possible consequence of this
   is that a KRS may request and obtain an RK from the home server even
   if the peer does not support ERP.  EAP-Initiate/Re-auth-Start
   messages send to the peer will be silently dropped by the peer
   causing further waste of resources.

7.  Acknowledgments

   The editors would like to thank Dan Harkins, Chunqiang Li, Rafael
   Marin Lopez, and Charles Clancy for their valuable comments.

8.  Contributors

   The following people contributed to this document: Kedar Gaonkar,
   Lakshminath Dondeti, Vidya Narayanan, and Glen Zorn.

9.  References

9.1.  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.

   [RFC5295]  Salowey, J., Dondeti, L., Narayanan, V., and M. Nakhjiri,
              "Specification for the Derivation of Root Keys from an
              Extended Master Session Key (EMSK)", RFC 5295,
              August 2008.

   [RFC5296]  Narayanan, V. and L. Dondeti, "EAP Extensions for EAP Re-
              authentication Protocol (ERP)", RFC 5296, August 2008.

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9.2.  Informative References

   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2865, June 2000.

   [RFC3579]  Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
              Dial In User Service) Support For Extensible
              Authentication Protocol (EAP)", RFC 3579, September 2003.

   [RFC3588]  Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
              Arkko, "Diameter Base Protocol", RFC 3588, September 2003.

   [RFC5169]  Clancy, T., Nakhjiri, M., Narayanan, V., and L. Dondeti,
              "Handover Key Management and Re-Authentication Problem
              Statement", RFC 5169, March 2008.

   [RFC5247]  Aboba, B., Simon, D., and P. Eronen, "Extensible
              Authentication Protocol (EAP) Key Management Framework",
              RFC 5247, August 2008.

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Authors' Addresses

   Katrin Hoeper (editor)
   Motorola, Inc.
   1295 E Algonquin Road
   Schaumburg, IL  60196
   USA

   Phone: +1 847 576 4714
   EMail: khoeper@motorola.com

   Madjid F. Nakhjiri
   Motorola, Inc.
   6450 Sequence Drive
   San Diego, CA  92121
   USA

   EMail: madjid.nakhjiri@motorola.com

   Yoshihiro Ohba (editor)
   Toshiba Corporate Research and Development Center
   1 Komukai-Toshiba-cho
   Saiwai-ku, Kawasaki, Kanagawa  212-8582
   Japan

   Phone: +81 44 549 2230
   EMail: yoshihiro.ohba@toshiba.co.jp

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