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Versions: 00 01 02 03 04 05                                             
     RADIUS Working Group                                       Pat Calhoun
     INTERNET-DRAFT                                        Sun Microsystems
     Updates: RFC 2138                                      Allan C. Rubens
     Category: Standards Track                          Merit Network, Inc.
     <draft-ietf-radius-eap-04.txt>                           Bernard Aboba
     10 March 1998                                                Microsoft
     
     
              Extensible Authentication Protocol Support in RADIUS
     
     
     1.  Status of this Memo
     
     This document is an Internet-Draft.  Internet-Drafts are working docu-
     ments of the Internet Engineering Task Force (IETF),  its  areas,  and
     its  working groups.  Note that other groups may also distribute work-
     ing documents as Internet-Drafts.
     
     Internet-Drafts are draft documents valid for a maximum of six  months
     and  may  be updated, replaced, or obsoleted by other documents at any
     time.  It is inappropriate to use Internet-Drafts as  reference  mate-
     rial or to cite them other than as ``work in progress.''
     
     To  learn  the  current status of any Internet-Draft, please check the
     ``1id-abstracts.txt'' listing contained in the Internet-Drafts  Shadow
     Directories   on   ds.internic.net   (US  East  Coast),  nic.nordu.net
     (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).
     
     The  distribution  of  this memo is unlimited.  It is filed as <draft-
     ietf-radius-eap-04.txt>, and  expires August 1, 1998. Please send com-
     ments to the authors.
     
     
     2.  Abstract
     
     The  Extensible  Authentication Protocol (EAP) is a PPP extension that
     provides support for additional  authentication  methods  within  PPP.
     This  document  describes how the EAP-Message and Signature attributes
     may be used for providing EAP support within RADIUS.
     
     
     3.  Introduction
     
     The Extensible Authentication Protocol (EAP), described in  [1],  pro-
     vides  a  standard  mechanism for support of additional authentication
     methods within PPP.  Through the use of EAP, support for a  number  of
     authentication  schemes may be added, including smart cards, Kerberos,
     Public Key, One Time Passwords, and others.  In order to  provide  for
     support of EAP within RADIUS, two new attributes, EAP-Message and Sig-
     nature, were introduced as RADIUS extensions  in  [4].  This  document
     describes  how these new attributes may be used for providing EAP sup-
     port within RADIUS.
     
     
     
     
     
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     INTERNET-DRAFT                                           10 March 1998
     
     
     In the proposed scheme, the RADIUS server is used to  shuttle  RADIUS-
     encapsulated  EAP  Packets  between  the  NAS  and  a backend security
     server. While the conversation between  the  RADIUS  server   and  the
     backend  security server will typically occur using a proprietary pro-
     tocol developed by the backend security server vendor, it is also pos-
     sible  to  use  RADIUS-encapsulated EAP via the EAP-Message attribute.
     This has the advantage of allowing the RADIUS server  to  support  EAP
     without  the  need for authentication-specific code, which can instead
     reside on a backend security server.
     
     
     3.1.  Requirements language
     
     This specification uses the same words as [6] for defining the signif-
     icance of each particular requirement.  These words are:
     
     
     MUST      This  word,  or  the adjectives "REQUIRED" or "SHALL", means
               that the definition is an absolute requirement of the speci-
               fication.
     
     MUST NOT  This phrase, or the phrase "SHALL NOT", means that the defi-
               nition is an absolute prohibition of the specification.
     
     SHOULD    This word, or the adjective "RECOMMENDED", means that  there
               may  exist  valid  reasons  in  particular  circumstances to
               ignore a particular item, but the full implications must  be
               understood and carefully weighed before choosing a different
               course.
     
     SHOULD NOT
               This phrase means that there may exist valid reasons in par-
               ticular   circumstances  when  the  particular  behavior  is
               acceptable or even useful, but the full implications  should
               be  understood  and the case carefully weighed before imple-
               menting any behavior described with this label.
     
     MAY       This word, or the adjective "", means that an item is  truly
               optional.  One vendor may choose to include the item because
               a particular marketplace requires it or because  the  vendor
               feels  that it enhances the product while another vendor may
               omit the  same  item.   An  implementation  which  does  not
               include a particular option MUST be prepared to interoperate
               with another implementation which does include  the  option,
               though  perhaps with reduced functionality. In the same vein
               an implementation which does  include  a  particular  option
               MUST be prepared to interoperate with another implementation
               which does not include the option.(except,  of  course,  for
               the feature the option provides)
     
     An  implementation is not compliant if it fails to satisfy one or more
     of the must or must not requirements for the protocols it  implements.
     An  implementation  that  satisfies all the must, must not, should and
     should  not  requirements  for   its   protocols   is   said   to   be
     
     
     
     Calhoun, Rubens & Aboba                                       [Page 2]


     INTERNET-DRAFT                                           10 March 1998
     
     
     "unconditionally  compliant"; one that satisfies all the must and must
     not requirements but not all the should or should not requirements for
     its protocols is said to be "conditionally compliant."
     
     
     4.  Protocol overview
     
     The  EAP  conversation  between the authenticating peer (dial-in user)
     and the NAS begins with the negotiation of EAP within LCP.   Once  EAP
     has been negotiated, the NAS MUST send an EAP-Request/Identity message
     to the authenticating peer, unless identity  is  determined  via  some
     other means such as Called-Station-Id or Calling-Station-Id.  The peer
     will then respond with an EAP-Response/Identity which the the NAS will
     then  forward  to  the RADIUS server in the EAP-Message attribute of a
     RADIUS Access-Request packet. The RADIUS Server will typically use the
     EAP-Response/Identity  to determine which EAP type is to be applied to
     the user.
     
     In order to permit non-EAP aware RADIUS proxies to forward the Access-
     Request  packet,  if  the  NAS sends the EAP-Request/Identity, the NAS
     MUST copy the contents of the EAP-Response/Identity into the User-Name
     attribute  and MUST include the EAP-Response/Identity in the User-Name
     attribute in  every  subsequent  Access-Request.  NAS-Port  SHOULD  be
     included  in  the  attributes  issued by the NAS in the Access-Request
     packet, and either NAS-Identifier or NAS-IP-Address MUST be  included.
     In order to permit forwarding of the Access-Reply by EAP-unaware prox-
     ies, if a User-Name attribute was included in an  Access-Request,  the
     RADIUS  Server  MUST  include  the  User-Name  attribute in subsequent
     Access-Challenge and  Access-Accept  packets.  Without  the  User-Name
     attribute, accounting and billing becomes very difficult to manage.
     
     If  identity is determined via another means such as Called-Station-Id
     or  Calling-Station-Id,  the  NAS  MUST  include   these   identifying
     attributes in every Access-Request, and the RADIUS Server MUST include
     them in every Access-Challenge and Access-Accept.
     
     While this approach will save a round-trip, it cannot  be  universally
     employed.   There  are  circumstances in which the user's identity may
     not be needed (such as when authentication and accounting  is  handled
     based  on  Called-Station-Id  or Calling-Station-Id), and therefore an
     EAP-Request/Identity packet may not necessarily be issued by  the  NAS
     to  the  authenticating  peer.  In cases where an EAP-Request/Identity
     packet will not be sent, the NAS will send  to  the  RADIUS  server  a
     RADIUS  Access-Request packet containing an EAP-Message attribute sig-
     nifying EAP-Start. EAP-Start is indicated by  sending  an  EAP-Message
     attribute  with  a  length of 2 (no data). However, it should be noted
     that since no User-Name attribute is included in  the  Access-Request,
     this  approach  is not compatible with RADIUS as specified in [2], nor
     can it easily be applied in situations  where  proxies  are  deployed,
     such as roaming or shared use networks.
     
     If  the  RADIUS  server  supports EAP, it MUST respond with an Access-
     Challenge packet containing an EAP-Message attribute.  If  the  RADIUS
     server  does  not  support EAP, it MUST respond with an Access-Reject.
     
     
     
     Calhoun, Rubens & Aboba                                       [Page 3]


     INTERNET-DRAFT                                           10 March 1998
     
     
     The EAP-Message attribute includes an encapsulated EAP packet which is
     then  passed on to the authenticating peer.  In the case where the NAS
     does not initially send an EAP-Request/Identity message to  the  peer,
     the  Access-Challenge  typically will contain an EAP-Message attribute
     encapsulating an EAP-Request/Identity message, requesting the  dial-in
     user  to  identify  themself.  The NAS will then respond with a RADIUS
     Access-Request packet containing an EAP-Message attribute  encapsulat-
     ing an EAP-Response.  The conversation continues until either a RADIUS
     Access-Reject or Access-Accept packet is received.
     
     Reception of a RADIUS Access-Reject packet, with or  without  an  EAP-
     Message  attribute  encapsulating  EAP-Failure, MUST result in the NAS
     issuing an LCP Terminate Request to the authenticating peer.  A RADIUS
     Access-Accept  packet with an EAP-Message attribute encapsulating EAP-
     Success successfully ends the authentication phase. The RADIUS Access-
     Accept/EAP-Message/EAP-Success packet MUST contain all of the expected
     attributes which are currently returned in an Access-Accept packet.
     
     The above scenario creates a situation in which the NAS never needs to
     manipulate  an  EAP  packet.  An alternative may be used in situations
     where an EAP-Request/Identity message will always be sent by  the  NAS
     to the authenticating peer.
     
     For  proxied  RADIUS requests there are two methods of processing.  If
     the domain is determined based on the  Called-Station-Id,  the  RADIUS
     Server  may  proxy the initial RADIUS Access-Request/EAP-Start. If the
     domain is determined based on the user's identity,  the  local  RADIUS
     Server   MUST  respond  with  a  RADIUS  Access-Challenge/EAP-Identity
     packet. The response from the authenticating peer MUST be  proxied  to
     the final authentication server.
     
     For  proxied  RADIUS  requests,  the  NAS may receive an Access-Reject
     packet in response to its  Access-Request/EAP-Identity  packet.   This
     would  occur  if the message was proxied to a RADIUS Server which does
     not support the EAP-Message extension. On receiving an  Access-Reject,
     the NAS MUST send an LCP Terminate Request to the authenticating peer,
     and disconnect.
     
     The NAS is not responsible for the retransmission of any AP  messages.
     The  authenticating peer and the RADIUS Server are responsible for any
     retransmissions.
     
     
     4.1.  Examples
     
     The example below shows the conversation  between  the  authenticating
     peer,  NAS,  and  RADIUS  server,  for the case of a One Time Password
     (OTP) authentication.  OTP is used  only  for  illustrative  purposes;
     other  authentication  protocols  could  also have been used, although
     they might show somewhat different behavior.
     
     
     Authenticating Peer     NAS                      RADIUS Server
     -------------------     ---                      -------------
     
     
     
     Calhoun, Rubens & Aboba                                       [Page 4]


     INTERNET-DRAFT                                           10 March 1998
     
     
                             <- PPP LCP Request-EAP
                             auth
     PPP LCP ACK-EAP
     auth ->
                             <- PPP EAP-Request/
                             Identity
     PPP EAP-Response/
     Identity (MyID) ->
                             RADIUS
                             Access-Request/
                             EAP-Message/
                             EAP-Response/
                             (MyID) ->
                                                       <- RADIUS
                                                       Access-Challenge/
                                                       EAP-Message/EAP-Request
                                                       OTP/OTP Challenge
                             <- PPP EAP-Request/
                             OTP/OTP Challenge
     PPP EAP-Response/
     OTP, OTPpw ->
                             RADIUS
                             Access-Request/
                             EAP-Message/
                             EAP-Response/
                             OTP, OTPpw ->
                                                        <- RADIUS
                                                        Access-Accept/
                                                        EAP-Message/EAP-Success
                                                        (other attributes)
                             <- PPP EAP-Success
     PPP Authentication
     Phase complete,
     NCP Phase starts
     
     In the case where the NAS first  sends  an  EAP-Start  packet  to  the
     RADIUS server,  the conversation would appear as follows:
     
     Authenticating Peer     NAS                      RADIUS Server
     -------------------     ---                      -------------
     
                             <- PPP LCP Request-EAP
                             auth
     PPP LCP ACK-EAP
     auth ->
                             RADIUS
                             Access-Request/
                            EAP-Message/Start ->
                                                      <- RADIUS
                                                      Access-Challenge/
                                                      EAP-Message/Identity
                             <- PPP EA-Request/
                             Identity
     PPP EAP-Response/
     
     
     
     Calhoun, Rubens & Aboba                                       [Page 5]


     INTERNET-DRAFT                                           10 March 1998
     
     
     Identity (MyID) ->
                             RADIUS
                             Access-Request/
                             EAP-Message/
                             EAP-Response/
                             (MyID) ->
                                                       <- RADIUS
                                                       Access-Challenge/
                                                       EAP-Message/EAP-Request
                                                       OTP/OTP Challenge
                             <- PPP EAP-Request/
                             OTP/OTP Challenge
     PPP EAP-Response/
     OTP, OTPpw ->
                             RADIUS
                             Access-Request/
                             EAP-Message/
                             EAP-Response/
                             OTP, OTPpw ->
                                                        <- RADIUS
                                                        Access-Accept/
                                                        EAP-Message/EAP-Success
                                                        (other attributes)
                             <- PPP EAP-Success
     PPP Authentication
     Phase complete,
     NCP Phase starts
     
     
     In  the  case where the client fails EAP authentication, the conversa-
     tion would appear as follows:
     
     
     Autheticating Peer     NAS                      RADIUS Server
     -------------------     ---                      -------------
     
                             <- PPP LCP Request-EAP
                             auth
     PPP LCP ACK-EAP
     auth ->
                             Access-Request/
                             EAP-Message/Start ->
                                                      <- RADIUS
                                                      Access-Challenge/
                                                      EAP-Message/Identity
                             <- PPP EAP-Request/
                             Identity
     PPP EAP-Response/
     Identity (MyID) ->
                             RADIUS
                             Access-Request/
                             EAP-Message/
                             EAP-Response/
                             (MyID) ->
     
     
     
     Calhoun, Rubens & Aboba                                       [Page 6]


     INTERNET-DRAFT                                           10 March 1998
     
     
                                                       <- RADIUS
                                                       Access-Challenge/
                                                       EAP-Message/EAP-Request
                                                       OTP/OTP Challenge
                             <- PPP EAP-Request/
                             OTP/OTP Challenge
     PPP EAP-Response/
     OTP, OTPpw ->
                             RADIUS
                             Access-Request/
                             EAP-Message/
                             EAP-Response/
                             OTP, OTPpw ->
                                                        <- RADIUS
                                                        Access-Reject/
                                                        EAP-Message/EAP-Failure
                             <- PPP EAP-Failure
                             (client disconnected)
     
     In the case that the RADIUS server or proxy does not support  EAP-Mes-
     sage, the conversation would appear as follows:
     
     Authenticating Peer     NAS                         RADIUS Server
     -------------------     ---                         -------------
     
                             <- PPP LCP Request-EAP
                             auth
     PPP LCP ACK-EAP
     auth ->
                             RADIUS
                             Access-Request/
                             EAP-Message/Start ->
                                                         <- RADIUS
                                                         Access-Reject
                             <- PPP LCP Terminate
                             (User Disconnected)
     
     In  the  case  where the local RADIUS Server does support EAP-Message,
     but the remote RADIUS Server does not, the conversation  would  appear
     as follows:
     
     Authenticating Peer     NAS                         RADIUS Server
     -------------------     ---                         -------------
     
                             <- PPP LCP Request-EAP
                             auth
     PPP LCP ACK-EAP
     auth ->
                             RADIUS
                             Access-Request/
                             EAP-Message/Start ->
                                                         <- RADIUS
                                                         Access-Challenge/
                                                         EAP-Message/Identity
     
     
     
     Calhoun, Rubens & Aboba                                       [Page 7]


     INTERNET-DRAFT                                           10 March 1998
     
     
                             <- PPP EAP-Request/
                             Identity
     PPP EAP-Response/
     Identity
     (MyID) ->
                             RADIUS
                             Access-Request/
                             EAP-Message/EAP-Response/
                             (MyID) ->
                                                         <- RADIUS
                                                         Access-Reject
                                                         (proxied from remote
                                                         RADIUS Server)
                             <- PPP LCP Terminate
                             (User Disconnected)
     
     In  the  case  where the authenticating peer does not support EAP, but
     where EAP is required for that user, the conversation would appear  as
     follows:
     
     Authenticating Peer     NAS                         RADIUS Server
     -------------------     ---                         -------------
     
                             <- PPP LCP Request-EAP
                             auth
     PPP LCP NAK-EAP
     auth ->
                             <- PPP LCP Request-CHAP
                             auth
     PPP LCP ACK-CHAP
     auth ->
                             <- PPP CHAP Challenge
     PPP CHAP Response ->
                             RADIUS
                             Access-Request/
                             User-Name,
                             CHAP-assword ->
                                                         <- RADIUS
                                                         Access-Reject
                             <-  PPP LCP Terminate
                             (User Disconnected)
     
     In  the  case  where  the  NAS  does not support EAP, but where EAP is
     required for that user, the conversation would appear as follows:
     
     Authenticating Peer     NAS                         RADIUS Server
     ------------------     ---                         -------------
     
                             <- PPP LCP Request-CHAP
                             auth
     PP LCP ACK-CHAP
     auth ->
                             <- PPP CHAP Challenge
     PPP CHAP Response ->
     
     
     
     Calhoun, Rubens & Aboba                                       [Page 8]


     INTERNET-DRAFT                                           10 March 1998
     
     
                             RADIUS
                             Access-Request/
                             User-Name,
                             CHAP-Password ->
                                                         <- RADIUS
                                                         Access-Reject
                             <-  PPP LCP Terminate
                             (User Disconnected)
     
     
     5.  Alternative uses
     
     Currently the conversation between the backend security server and the
     RADIUS  server  is proprietary because of lack of standardization.  In
     order to increase standardization and provide interoperability between
     Radius  vendors  and  backend security vendors, it is recommended that
     RADIUS-encapsulated EAP be used for this conversation.
     
     This has the advantage of allowing the RADIUS server  to  support  EAP
     without  the  need for authentication-specific  code within the RADIUS
     server. Authentication-specific code can  then  reside  on  a  backend
     security server instead.
     
     In  the  case  where RADIUS-encapsulated EAP is used in a conversation
     between a RADIUS server and a backend security  server,  the  security
     server  will  typically  return  an  Access-Accept/EAP-Success message
     without inclusion of the expected attributes currently returned in  an
     Access-Accept.  This  means  that  the  RADIUS  server  MUST add these
     attributes prior to sending an  Access-Accept/EAP-Success  message  to
     the NAS.
     
     
     6.  Attributes
     
     
     6.1.  EAP-Message
     
     Description
     
        This  attribute encapsulates Extensible Authentication Protocol [1]
        packets so as to allow the NAS to authenticate  dial-in  users  via
        EAP without having to understand the protocol.
     
        The  NAS  places EAP messages received from the authenticating peer
        into one or more EAP-Message attributes and forwards  them  to  the
        RADIUS  Server  within an Access-Request message.  If multiple EAP-
        Messages are contained within an Access-Request or Access-Challenge
        packet,  they  MUST  be  in  order  and  they  MUST  be consecutive
        attributes  in  the  Access-Request  or  Access-Challenge   packet.
        Access-Accept  and  Access-Reject packets SHOULD only have ONE EAP-
        Message attribute in them, containing EAP-Success or EAP-Failure.
     
        It is expected that EAP will be used  to  implement  a  variety  of
        authentication   methods,   including   methods   involving  strong
     
     
     
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     INTERNET-DRAFT                                           10 March 1998
     
     
        cryptography. In order to prevent attackers from subverting EAP  by
        attacking RADIUS/EAP, (for example, by modifying the EAP-Success or
        EAP-Failure  packets)  it  is  necessary  that  RADIUS/EAP  provide
        integrity  protection  at  least as strong as those used in the EAP
        methods themselves.
     
        The Signature attribute specified in [4] MUST be  used  to  protect
        all  Access-Request,  Access-Challenge,  Access-Accept, and Access-
        Reject packets containing an  EAP-Message  attribute.  For  Access-
        Accepts  the  Signature  is  calculated  as  specified in [4].  For
        Access-Challenge, Access-Accept,  and  Access-Reject  packets,  the
        Signature is calculated as follows:
     
        Signature = HMAC-MD5 (Type, Identifier, Length, Request Authenticator, Attributes)
     
        The  Signature attribute is zeroed for the purposes of the calcula-
        tion and the shared secret is used as  the  key  for  the  HMAC-MD5
        hash.   The  Signature  is  calculated  and  inserted in the packet
        before the Authenticator is calculated.
     
        Access-Request packets including an EAP-Message attribute without a
        Signature  attribute  SHOULD  be  silently  discarded by the RADIUS
        server. A RADIUS Server supporting EAP-Message MUST  calculate  the
        correct  value  of the Signature and silently discard the packet if
        it does not match the value sent.  A RADIUS Server  not  supporting
        EAP-Message  MUST return an Access-Reject if it receives an Access-
        Request  containing  an  EAP-Message  attribute.  A  RADIUS  Server
        receiving an EAP-Message attribute that it does not understand MUST
        return an Access-Reject.
     
        Access-Challenge, Access-Accept, or Access-Reject packets including
        an  EAP-Message  attribute  without a Signature attribute SHOULD be
        silently discarded by the NAS. A NAS  supporting  EAP-Message  MUST
        calculate  the  correct value of the Signature and silently discard
        the packet if it does not match the value sent.
     
     
     A summary of the EAP-Message attribute format  is  shown  below.   The
     fields are transmitted from left to right.
     
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Type     |    Length     |            String...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     
     Type
     
        79 for EAP-Message
     
     Length
     
        >=3 (EAP packet enclosed)
        =2  (EAP-Start message)
     
     
     
     Calhoun, Rubens & Aboba                                      [Page 10]


     INTERNET-DRAFT                                           10 March 1998
     
     
     String
     
        The  String field includes EAP packets, as defined in [1].  If mul-
        tiple EAP-Message attributes are present in a packet  their  values
        should  be  concatenated;  this  allows EAP packets longer than 253
        octets to be passed by RADIUS.
     
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |      Code     | Identifier    |          Length               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                                                               |
        /                                                               /
        /                        Data                                   /
        |                                                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     
     
     7.  Security considerations
     
     Since the purpose of EAP is  to  provide  enhanced  security  for  PPP
     authentication,  it is critical that RADIUS support for EAP be secure.
     In particular, the following issues must be addressed:
     
        Separation of the EAP server and PPP authenticator
        Connection hijacking
        Man in the middle attacks
        Multiple databases
        Negotiation attacks
     
     
     7.1.  Separation of the EAP server and PPP authenticator
     
     As described in [8] and [9], it is possible for the EAP  endpoints  to
     mutually  authenticate,  negotiate a ciphersuite, and derive a session
     key for subsequent use in PPP encryption.
     
     This does not present an issue on the peer, since  the  peer  and  EAP
     client reside on the same machine; all that is required is for the EAP
     client module to pass the session key to the PPP encryption module.
     
     The situation may be more complex when EAP/RADIUS is used,  since  the
     PPP authenticator will typically not reside on the same machine as the
     EAP server. For example, the EAP server  may  be  a  backend  security
     server, or a module residing on the RADIUS server.
     
     In  the case where the EAP server and PPP authenticator reside on dif-
     ferent  machines,  there  are  several  implications   for   security.
     Firstly, mutual authentication will occur between the peer and the EAP
     server, not between the peer and the authenticator. This means that it
     is  not  possible  for the peer to validate the identity of the NAS or
     tunnel server that it is speaking to.
     
     As described earlier, when EAP/RADIUS is used to encapsulate EAP pack-
     ets, the Signature attribute is required in EAP/RADIUS Access-Requests
     
     
     
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     INTERNET-DRAFT                                           10 March 1998
     
     
     sent from the NAS or tunnel server to the  RADIUS  server.  Since  the
     Signature  attribute  involves a HMAC-MD5 hash, it is possible for the
     RADIUS server to verify the integrity of the Access-Request as well as
     the NAS or tunnel server's identity. Similarly, Access-Challenge pack-
     ets sent from the RADIUS server to the NAS are also authenticated  and
     integrity protected using an HMAC-MD5 hash, enabling the NAS or tunnel
     server to determine the integrity of the packet and verify  the  iden-
     tity  of the RADIUS server.  Morever, EAP packets sent via the methods
     described in [8] and [9] contain their own  integrity  protection,  so
     that  they  cannot  be  successfully modified by a rogue NAS or tunnel
     server.
     
     The second issue that arises in the case of  an  EAP  server  and  PPP
     authenticator  residing  on different machines is that the session key
     negotiated between the peer and EAP server will need to be transmitted
     to  the  authenticator.  Therefore a mechanism needs to be provided to
     transmit the session key from the EAP server to the  authenticator  or
     tunnel  server  that  needs  to use the key. The specification of this
     transit mechanism is outside the scope of this document.
     
     
     7.2.  Connection hijacking
     
     In this form of attack, the attacker attempts to inject  packets  into
     the conversation between the NAS and the RADIUS server, or between the
     RADIUS server and the backend security server. RADIUS does not support
     encryption,  and  as  described  in [2], only Access-Reply and Access-
     Challenge packets are integrity  protected.  Moreover,  the  integrity
     protection mechanism described in [2] is weaker than that likely to be
     used by some EAP methods, making it possible to subvert those  methods
     by attacking EAP/RADIUS.
     
     In  order  to  provide  for  authentication  of all packets in the EAP
     exchange, all EAP/RADIUS packets MUST be authenticated using the  Sig-
     nature attribute, as described previously.
     
     
     7.3.  Man in the middle attacks
     
     Since  RADIUS security is based on shared secrets, end-to-end security
     is not provided in the case where authentication or accounting packets
     are  forwarded  along  a  proxy chain.  As a result, attackers gaining
     control of a RADIUS proxy will be able to modify EAP packets in  tran-
     sit without fear of detection.
     
     This  represents a weakness of RADIUS which cannot be remedied without
     providing end-to-end data object security.
     
     
     7.4.  Multiple databases
     
     In many cases a backend security server will be deployed along with  a
     RADIUS  server  in  order  to provide EAP services. Unless the backend
     security server also functions as a RADIUS server, two  separate  user
     
     
     
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     databases  will  exist, each containing information about the security
     requirements for the user. This represents a weakness, since  security
     may be compromised by a successful attack on either of the servers, or
     their backend databases. With multiple user databases,  adding  a  new
     user  may  require  multiple  operations,  increasing  the chances for
     error.  The problems are further magnified  in  the  case  where  user
     information  is also being kept in an LDAP server. In this case, three
     stores of user information may exist.
     
     In order to address these threats, consolidation of databases is  rec-
     ommended.   This  can be achieved by having both the RADIUS server and
     backend  security  server  store  information  in  the  same   backend
     database;  by having the backend security server provide a full RADIUS
     implementation; or by consolidating both the backend  security  server
     and the RADIUS server onto the same machine.
     
     
     7.5.  Negotiation attacks
     
     In  a  negotiation attack, a rogue NAS, tunnel server, RADIUS proxy or
     RADIUS server causes the authenticating peer to choose a  less  secure
     authentication  method  so  as  to make it easier to obtain the user's
     password. For example, a session that would normally be  authenticated
     with EAP would instead authenticated via CHAP or PAP; alternatively, a
     connection that would normally  be  authenticated  via  one  EAP  type
     occurs  via  a  less secure EAP type, such as MD5. The threat posed by
     rogue devices, once thought to be remote, has  gained  currency  given
     compromises  of  telephone  company  switching  systems, such as those
     described in [7].
     
     Protection against negotiation attacks  requires  the  elimination  of
     downward negotiations. This can be achieved via implementation of per-
     connection policy on the part of the authenticating peer, and per-user
     policy on the part of the RADIUS server.
     
     For  the authenticating peer, authentication policy should be set on a
     per-connection basis. Per-connection policy allows  an  authenticating
     peer to negotiate EAP when calling one service, while negotiating CHAP
     for another service, even if both services are accessible via the same
     phone number.
     
     With  per-connection  policy, an authenticating peer will only attempt
     to negotiate EAP for a session in which EAP support is expected. As  a
     result,  there  is a presumption that an authenticating peer selecting
     EAP requires that level of security. If it cannot be provided,  it  is
     likely  that  there is some kind of misconfiguration, or even that the
     authenticating peer is contacting the wrong server. Should the NAS not
     be able to negotiate EAP, or should the EAP-Request sent by the NAS be
     of a different EAP type than what is expected, the authenticating peer
     MUST disconnect. An authenticating peer expecting EAP to be negotiated
     for a session MUST NOT negotiate CHAP or PAP.
     
     For a NAS, it may not be possible  to  determine  whether  a  user  is
     required  to authenticate with EAP until the user's identity is known.
     
     
     
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     For example, for shared-uses NASes it is possible for one reseller  to
     implement  EAP  while another does not. In such cases, if any users of
     the NAS MUST do EAP, then the NAS MUST attempt to  negotiate  EAP  for
     every  call. This avoids forcing an EAP-capable client to do more than
     one authentication, which weakens security.
     
     If CHAP is negotiated, the NAS will pass the User-Name and  CHAP-Pass-
     word  attributes  to the RADIUS Server in an Access-Request packet. If
     the user is not required to use  EAP,  then  the  RADIUS  Server  will
     respond  with an Access-Accept or Access-Reject packet as appropriate.
     However, if CHAP has been negotiated but EAP is required,  the  RADIUS
     server MUST respond with an Access-Reject, rather than an Access-Chal-
     lenge/EAP-Message/EAP-Request packet.  The  authenticating  peer  MUST
     refuse  to  renegotiate  authentication,  even if the renegotiation is
     from CHAP to EAP.
     
     If EAP is negotiated but is not  supported  by  the  RADIUS  proxy  or
     server,  then  the server or proxy MUST respond with an Access-Reject.
     In these cases, the NAS MUST send an LCP-Terminate and disconnect  the
     user.   This  is the correct behavior since the authenticating peer is
     expecting EAP to be negotiated, and that expectation  cannot  be  ful-
     filled.  An EAP-capable authenticating peer MUST refuse to renegotiate
     the authentication protocol if  EAP  had  initially  been  negotiated.
     Note  that  problems  with  a non-EAP capable RADIUS proxy could prove
     difficult to diagnose, since a user dialing in from one location (with
     an  EAP-capable  proxy) might be able to successfully authenticate via
     EAP, while the same user dialing into another location (and encounter-
     ing an EAP-incapable proxy) might be consistently disconnected.
     
     
     8.  Acknowledgments
     
     Thanks to Dave Dawson and Karl Fox of Ascend, and Glen Zorn and Naren-
     dra Gidwani of Microsoft for useful discussions of this problem space.
     
     
     9.  References
     
     [1]  L.  J.  Blunk,  J. R. Vollbrecht.  "PPP Extensible Authentication
     Protocol (EAP)." Work in progress,  draft-ietf-pppext-eap-auth-03.txt,
     Merit Network, Inc., June, 1996.
     
     [2]   C. Rigney, A. Rubens, W. Simpson, S. Willens.  "Remote Authenti-
     cation Dial In User Service (RADIUS)." RFC  2058,  Livingston,  Merit,
     Daydreamer, January, 1997.
     
     [3]   C.  Rigney.  "RADIUS Accounting." RFC 2059, Livingston, January,
     1997.
     
     [4] C. Rigney, W. Willats.  "RADIUS  Extensions."  Work  in  progress,
     draft-ietf-radius-ext-01.txt, Livingston, June, 1997.
     
     [5]  R.  Rivest,  S.  Dusse.  "The MD5 Message-Digest Algorithm."  RFC
     1321, MIT Laboratory for Computer Science,  RSA  Data  Security  Inc.,
     
     
     
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     April 1992.
     
     [6]  S.  Bradner.   "Key words for use in RFCs to Indicate Requirement
     Levels."  RFC 2119, Harvard University, March, 1997.
     
     [7] M. Slatalla, J. Quittner.  "Masters of Deception."  HarperCollins,
     New York, 1995.
     
     [8]  B.  Aboba, D. Simon.  "PPP EAP TLS Authentication Protocol." Work
     in progress, draft-ietf-pppext-eaptls-02.txt, Microsoft, October 1997.
     
     [9]  G.  Carter.   "PPP  EAP  ISAKMP Authentication Protocol." Work in
     progress, draft-ietf-pppext-eapisakmp-00.txt, Entrust, November  1997.
     
     
     
     10.  Authors' Addresses
     
     Pat R. Calhoun
     Technology Development
     Sun Microsystems, Inc.
     15 Network Circle
     Menlo Park, CA 94025
     
     Phone: 847-548-0926
     Fax:   650-786-6445
     EMail: pcalhoun@toast.net
     
     Allan C. Rubens
     Merit Network, Inc.
     4251 Plymouth Rd.
     Ann Arbor, MI 48105-2785
     
     Phone: 313-647-0417
     EMail: acr@merit.edu
     
     Bernard Aboba
     Microsoft Corporation
     One Microsoft Way
     Redmond, WA 98052
     
     Phone: 425-936-6605
     EMail: bernarda@microsoft.com
     -
     
     
     
     
     
     
     
     
     
     
     
     
     
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