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Versions: 00 01 02 03 04 05 06 rfc2716                                  
     PPP Extensions Working Group                             Bernard Aboba
     INTERNET-DRAFT                                               Microsoft
     Category: Standards Track                                    Dan Simon
     <draft-ietf-pppext-eaptls-01.txt>                            Microsoft
     14 October 1997
     
     
                      PPP EAP TLS Authentication Protocol
     
     
     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-pppext-eaptls-01.txt>, and expires May 1, 1998. Please send  com-
     ments to the authors.
     
     
     2.  Abstract
     
     The  Point-to-Point  Protocol  (PPP)  provides  a  standard method for
     transporting multi-protocol datagrams over point-to-point links.   PPP
     also  defines  an extensible Link Control Protocol (LCP), which can be
     used to negotiate authentication methods, as  well  as  an  Encryption
     Control  Protocol  (ECP),  used  to negotiate data encryption over PPP
     links, and a Compression Control Protocol  (CCP),  used  to  negotiate
     compression  methods.  The Extensible Authentication Protocol (EAP) is
     a PPP extension that provides support  for  additional  authentication
     methods within PPP.
     
     Transport  Level  Security  (TLS)  provides for mutual authentication,
     ciphersuite negotiation and key exchange between two endpoints.   This
     document describes how these TLS mechanisms may be used within EAP.
     
     
     3.  Introduction
     
     The  Extensible  Authentication Protocol (EAP), described in [5], pro-
     vides a standard mechanism for support  of  additional  authentication
     methods  within  PPP.  Through the use of EAP, support for a number of
     
     
     
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     authentication schemes may be added, including smart cards,  Kerberos,
     Public Key, One Time Passwords, and others. To date however, EAP meth-
     ods such as [6] have focussed on authenticating a client to a  server.
     
     However,  in order to guard against rogue servers, it may be desirable
     to support mutual authentication. In addition,  since  PPP  encryption
     protocols  such as [10] and [11] assume existence of a session key, it
     is useful to have a mechanism for  session  key  establishment.  Since
     design of authentication and key management protocols is a non-trivial
     exercise, it is desirable to avoid creating new mechanisms  for  this.
     The  EAP protocol described in this document allows a PPP peer to take
     advantage of the mutual authentication and key management capabilities
     of the TLS protocol, described in [15].
     
     
     3.1.  Requirements language
     
     This  specification  uses the same words as [12] for defining the sig-
     nificance 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)
     
     
     
     
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     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 "uncondition-
     ally compliant"; one that satisfies all the must and must not require-
     ments but not all the should or should not requirements for its proto-
     cols is said to be "conditionally compliant."
     
     
     4.  Protocol overview
     
     
     
     4.1.  Overview of the EAP-TLS conversation
     
     As described in [5] and [17], the EAP-TLS conversation will  typically
     begin  with  the  authenticator  and  the  peer  negotiating EAP.  The
     authenticator will then typically send an EAP-Request/Identity  packet
     to  the  peer, and the peer will respond with an EAP-Response/Identity
     packet to the authenticator, containing the peer's userId.
     
     From this point forward, while nominally the EAP  conversation  occurs
     between  the  authenticator  and  the  peer,  as described in [17] the
     authenticator MAY act as a passthrough device, with  the  EAP  packets
     received from the peer being encapsulated for transmission to a RADIUS
     server or backend security server. In the discussion that follows,  we
     will  use  the  term "EAP server" to denote the ultimate endpoint con-
     versing with the peer.
     
     Once having received the peer's userId, the EAP  server  will  respond
     with an EAP-TLS/Start packet, which is an EAP-Request packet with EAP-
     Type=EAP-TLS, and no data. The EAP-TLS conversation will  then  begin,
     with  the  peer  sending an EAP-Response packet with EAP-Type=EAP-TLS.
     The data field of that packet will encapsulate one or more TLS records
     in  TLS  record  layer format, containing a TLS client_hello handshake
     message.  The  current  cipher  spec  for  the  TLS  records  will  be
     TLS_NULL_WITH_NULL_NULL  and  null  compression.   This current cipher
     spec remains the same until  the  change_cipher_spec  message  signals
     that  subsequent  records  will have the negotiated attributes for the
     remainder of the handshake.
     
     The client_hello message contains the client's TLS version  number,  a
     sessionId, a random number, and a set of ciphersuites supported by the
     client. The version offered by the client MUST correspond to TLS  v1.0
     or later.
     
     The  EAP server will then respond with an EAP-Request packet with EAP-
     Type=EAP-TLS. The data field of this packet will  encapsulate  one  or
     more TLS records. These will contain a TLS server_hello handshake mes-
     sage, possibly followed by TLS certificate, server_key_exchange,  cer-
     tificate_request,  server_hello_done  and/or  finished  handshake mes-
     sages, and/or a  TLS  change_cipher_spec  message.   The  server_hello
     handshake  message  contains a TLS version number, another random num-
     ber, a sessionId, and a  ciphersuite.   The  version  offered  by  the
     
     
     
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     server MUST correspond to TLS v1.0 or later.
     
     If  the  client's sessionId is null or unrecognized by the server, the
     server will choose the sessionId to establish a  new  session;  other-
     wise,  the  sessionId  will  match  that  offered by the client, indi-
     cating a resumption of the previously established  session  with  that
     sessionID.   The  server  will  also  choose  a ciphersuite from those
     offered by  the client; if the session matches the client's, then  the
     ciphersuite  MUST match the one negotiated during the handshake proto-
     col execution that established the session.
     
     The purpose of the sessionId within the TLS protocol is to  allow  for
     improved  efficiency in the case where a client repeatedly attempts to
     authenticate to an EAP server within a short  period  of  time.  While
     this model was developed for use with HTTP authentication, it may also
     have application to PPP authentication (e.g. multilink).
     
     As a result, it is left up to the peer whether to attempt to  continue
     a  previous  session,  thus shortening the TLS conversation. Typically
     the peer's decision will be made based on the time elapsed  since  the
     previous  authentication attempt to that EAP server. Based on the ses-
     sionId chosen by the peer, and the time  elapsed  since  the  previous
     authentication,  the  EAP server will decide whether to allow the con-
     tinuation, or whether to choose a new session.
     
     In the case where the EAP server and authenticator reside on the  same
     device,  then  client will only be able to continue sessions when con-
     necting to the same NAS or tunnel server. Should these devices be  set
     up in a rotary or round-robin then it may not be possible for the peer
     to know in advance the authenticator it will  be  connecting  to,  and
     therefore  which  sessionId  to  attempt  to reuse. As a result, it is
     likely that the continuation attempt will fail. In the case where  the
     EAP authentication is remoted then continuation is much more likely to
     be successful, since multiple NAS  devices  and  tunnel  servers  will
     remote their EAP authentications to the same RADIUS server.
     
     If  the  EAP server is resuming a previously established session, then
     it MUST include only a TLS change_cipher_spec message and a  TLS  fin-
     ished  handshake message after the server_hello message.  The finished
     message contains the EAP server's authentication response to the peer.
     If  the  EAP  server is not resuming a previously established session,
     then it MUST include a TLS server_certificate handshake message, and a
     server_hello_done handshake message MUST be the last handshake message
     encapsulated in this EAP-Request packet.
     
     The certificate message contains a public key  certificate  chain  for
     either a key exchange public key (such as an RSA or Diffie-Hellman key
     exchange public key) or a signature public key (such as an RSA or  DSS
     signature  public key).  In the latter case, a TLS server_key_exchange
     handshake message MUST also be included to allow the key  exchange  to
     take place.
     
     The  certificate_request  message  is included when the server desires
     the client to authenticate itself via public key. While the EAP server
     
     
     
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     SHOULD require client authentication, this is not a requirement, since
     it may be possible that the server will require that the peer  authen-
     ticate via some other means.
     
     The  peer  will  then  respond  with  an EAP-Response packet with EAP-
     Type=EAP-TLS.  The data field of this packet will encapsulate  one  or
     more  TLS records containing a TLS change_cipher_spec message and fin-
     ished handshake message, and possibly certificate,  certificate_verify
     and/or  client_key_exchange  handshake  messages.   If  the  preceding
     server_hello message sent by the EAP  server  in  the  preceding  EAP-
     Request  packet  indicated  the resumption of a previous session, then
     the peer MUST send only the change_cipher_spec and finished  handshake
     messages.   The  finished  message  contains the peer's authentication
     response to the EAP server.
     
     If the preceding server_hello message sent by the EAP  server  in  the
     preceeding  EAP-Request  packet  did  not indicate the resumption of a
     previous session,  then  the  peer  MUST  send,  in  addition  to  the
     change_cipher_spec  and  finished messages, a client_key_exchange mes-
     sage, which completes the exchange of a shared master  secret  between
     the  peer  and  the  EAP  server.   If  the EAP server sent a certifi-
     cate_request message in the preceding  EAP-Request  packet,  then  the
     peer  MUST send, in addition, certificate and certificate_verify hand-
     shake messages.  The former contains a certificate for the peer's sig-
     nature public key, while the latter contains the peer's signed authen-
     tication response to the EAP server. After receiving this packet,  the
     EAP  server  will verify the peer's certificate and digital signature,
     if requested.
     
     If the peer's authentication is unsuccessful, the EAP server will send
     an  EAP-Request  packet  with  EAP-Type=EAP-TLS,  encapsulating  a TLS
     record containing the appropriate TLS alert message.  It is useful for
     the  EAP server to send a TLS alert message rather than to immediately
     terminate the conversation so as to allow the peer to inform the  user
     of  the  cause  of the failure and possibly allow for a restart of the
     conversation.
     
     To ensure that the peer receives the TLS alert message, the EAP server
     will  wait for the peer to reply with an EAP-Response packet. The EAP-
     Response packet sent by the peer may encapsulate  a  TLS  client_hello
     handshake  message, in which case the EAP server may allow the EAP-TLS
     conversation to be restarted, or it may contain an EAP-Response packet
     with  EAP-Type=EAP-TLS  and no data, in which case the EAP-Server will
     send an EAP-Failure packet, and terminate the conversation. It  is  up
     to the EAP server whether to allow restarts, and if so, how many times
     the conversation can be restarted. An EAP Server implementing  restart
     capability  SHOULD  impose a limit on the number of restarts, so as to
     protect against denial of service attacks.
     
     If the peers authenticates successfully, the EAP server  will  respond
     with  an  EAP-Request packet with EAP-Type=EAP-TLS, which includes, in
     the case of a new TLS session, one or more TLS records containing  TLS
     change_cipher_spec  and  finished handshake messages.  The latter con-
     tains the EAP server's authentication response to the peer.  The  peer
     
     
     
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     will then verify the hash in order to authenticate the EAP server.
     
     If the EAP server authenticates unsuccessfully, the peer sends an EAP-
     Response packet of EAP-Type=EAP-TLS containing  a  TLS  Alert  message
     identifying the reason for the failed authentication. It is useful for
     the peer to send a TLS alert message rather than immediately terminat-
     ing the conversation so as to allow the EAP server to log the cause of
     the error for examination by the system administrator.
     
     To ensure that the EAP Server receives the TLS alert message, the peer
     will wait for the EAP-Server to reply before terminating the conversa-
     tion. The EAP Server MUST  reply  with  an  EAP-Failure  packet  since
     server authentication failure is a terminal condition.
     
     If  the  EAP server authenticates successfully, the peer sends an EAP-
     Response packet of EAP-Type=EAP-TLS, and no data. The EAP-Server  then
     responds with an EAP-Success message.
     
     4.2.  Retry behavior
     
     As  with  other EAP protocols, the EAP server is responsible for retry
     behavior. This means that if the EAP server does not receive  a  reply
     from the peer, it will resend the EAP-Request for which it has not yet
     received an EAP-Response. However,  the  peer  will  not  resend  EAP-
     Response packets without first being prompted by the EAP server.
     
     For  example,  if  the  initial  EAP-TLS  start packet sent by the EAP
     server were to be lost, then the peer would not receive  this  packet,
     and  would  not  respond  to it. As a result, the EAP-TLS start packet
     would be resent by the EAP server. Once the peer received the  EAP-TLS
     start  packet, it would respond with an EAP-Response encapsulating the
     client_hello message. If the EAP-Response were to be  lost,  then  the
     EAP  server would resend the initial EAP-TLS start, and the peer would
     resend the EAP-Response.
     
     As a result, it is possible that a peer will  receive  duplicate  EAP-
     Request  messages, and may send duplicate EAP-Responses. Both the peer
     and the EAP-Server should be engineered to handle this possibility.
     
     
     4.3.  Identity verification
     
     As part of the TLS negotiation, the server presents a  certificate  to
     the peer, and if mutual authentication is requested, the peer presents
     a certificate to the server.
     
     Note that since the peer has made a claim  of  identity  in  the  EAP-
     Response/Identity (MyID) packet, the EAP server SHOULD verify that the
     claimed identity corresponds to the certificate presented by the peer.
     Typically  this  will  be  accomplished  either  by placing the userId
     within the peer certificate, or by providing  a  mapping  between  the
     peer certificate and the userId using a directory service.
     
     
     
     
     
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     Similarly,  the  peer  MUST verify the validity of the EAP server cer-
     tificate, and SHOULD also examine the EAP server name presented in the
     certificate,  in  order  to  determine  whether  the EAP server can be
     trusted. Please note that in the case where the EAP authentication  is
     remoted that the EAP server will not reside on the same machine as the
     authenticator, and therefore the name in the EAP server's  certificate
     cannot  be expected to match that of the intended destination. In this
     case, a more appropriate test might be whether the EAP  server's  cer-
     tificate  is  signed  by a CA controlling the intended destination and
     whether the EAP server exists within a target sub-domain.
     
     
     4.4.  Key derivation
     
     Since the normal TLS keys are used in  the  handshake,  and  therefore
     should not be used in a different context, new encryption keys must be
     derived from the TLS master secret for use with PPP  encryption.   For
     both  peer  and EAP server, the derivation proceeds as follows:  given
     the master secret negotiated by the TLS  handshake,  the  pseudorandom
     function  (PRF) defined in the specification for the version of TLS in
     use, and the value random defined as the concatenation  of  the  hand-
     shake  message  fields client_hello.random and server_hello.random (in
     that order), the value PRF(master  secret,  "client  EAP  encryption",
     random) is computed up to 128 bytes, and the value PRF("", "client EAP
     encryption", random) is computed up to 64 bytes (where "" is an  empty
     string).   The  peer  encryption key (the one used for encrypting data
     from peer to EAP server) is obtained  by  truncating  to  the  correct
     length  the  first  32  bytes  of  the  first  PRF of these two output
     strings.  The EAP server encryption key (the one used  for  encrypting
     data from EAP server to peer), if different from the client encryption
     key, is obtained by truncating to the correct  length  the  second  32
     bytes  of  this same PRF output string.  The client authentication key
     (the one used for  computing  MACs  for  messages  from  peer  to  EAP
     server),  if used, is obtained by truncating to the correct length the
     third 32 bytes of this same PRF output string.  The EAP server authen-
     tication  key  (the  one used for computing MACs for messages from EAP
     server to peer), if used, and if different from the  peer  authentica-
     tion  key,  is obtained by truncating to the correct length the fourth
     32 bytes of this same PRF output string.  The peer initialization vec-
     tor  (IV), used for messages from peer to EAP server if a block cipher
     has been specified, is obtained by truncating to  the  cipher's  block
     size  the  first  32  bytes  of the second PRF output string mentioned
     above.  Finally, the server initialization vector (IV), used for  mes-
     sages from peer to EAP server if a block cipher has been specified, is
     obtained by truncating to the cipher's block size the second 32  bytes
     of this second PRF output.
     
     The use of these encryption and authentication keys is specific to the
     PPP encryption mechanism used, such as those defined in [10] and [11].
     Additional  keys  or  other  non-secret  values  (such  as IVs) can be
     obtained as needed for future PPP encryption methods by extending  the
     outputs of the PRF beyond 128 bytes and 64 bytes, respectively.
     
     
     
     
     
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     4.5.  ECP negotiation
     
     Since  TLS  supports ciphersuite negotiation, peers completing the TLS
     negotiation will also have selected a ciphersuite, which includes  key
     strength,  encryption  and  hashing methods. As a result, a subsequent
     Encryption Control Protocol (ECP) conversation, if it  occurs,  has  a
     predetermined result.
     
     In  order to ensure agreement between the EAP-TLS ciphersuite negotia-
     tion and the subsequent ECP negotiation (described in [7]), during ECP
     negotiation the PPP peer MUST offer only the ciphersuite negotiated in
     EAP-TLS.  This ensures that the PPP authenticator MUST accept the EAP-
     TLS  negotiated  ciphersuite in order for the conversation to proceed.
     Should the authenticator not accept  the  EAP-TLS  negotiated  cipher-
     suite, then the peer MUST send an LCP terminate and disconnect.
     
     Please  note  that as described in [17], it cannot be assumed that the
     PPP authenticator and EAP server are located on the  same  machine  or
     that  the  authenticator understands the EAP-TLS conversation that has
     passed through it. Thus if the peer offers a  ciphersuite  other  than
     the one negotiated in EAP-TLS there is no way for the authenticator to
     know how to respond correctly.
     
     
     4.6.  CCP negotiation
     
     TLS as described in [15] supports compression as well  as  ciphersuite
     negotiation.  However,  TLS only provides support for a limited number
     of compression types which do not overlap with the  compression  types
     used in PPP. As a result, during the EAP-TLS conversation the EAP end-
     points MUST NOT request or negotiate  compression.  Instead,  the  PPP
     Compression  Control  Protocol (CCP), described in [16] should be used
     to negotiate the desired compression scheme.
     
     
     4.7.  Examples
     
     In the case where the EAP-TLS mutual authentication is successful, the
     conversation will appear as follows:
     
     Authenticating Peer     Authenticator
     -------------------     -------------
     
                             <- PPP LCP Request-EAP
                             auth
     PPP LCP ACK-EAP
     auth ->
                             <- PPP EAP-Request/
                             Identity
     PPP EAP-Response/
     Identity (MyID) ->
                             <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS Start)
     
     
     
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     PPP EAP-Response/
     EAP-Type=EAP-TLS
     (TLS client_hello)->
                             <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS server_hello,
                              TLS certificate,
                         [TLS server_key_exchange,]
                         [TLS certificate_request,]
                          TLS server_hello_done)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
     (TLS certificate,
      TLS client_key_exchange,
     [TLS certificate_verify,]
      TLS change_cipher_spec,
      TLS finished) ->
                             <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS change_cipher_spec,
                          TLS finished)
     PPP EAP-Response/
     EAP-Type=EAP-TLS ->
                             <- PPP EAP-Success
     PPP Authentication
     Phase complete,
     NCP Phase starts
     
     ECP negotiation
     
     CCP negotiation
     
     In the case where the server authenticates to the client successfully,
     but the client fails to authenticate to the server,  the  conversation
     will appear as follows:
     
     Authenticating Peer     Authenticator
     -------------------     -------------
                             <- PPP LCP Request-EAP
                             auth
     PPP LCP ACK-EAP
     auth ->
                             <- PPP EAP-Request/
                             Identity
     PPP EAP-Response/
     Identity (MyID) ->
                             <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS Start)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
     (TLS client_hello)->
                             <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
     
     
     
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                             (TLS server_hello,
                              TLS certificate,
                         [TLS server_key_exchange,]
                          TLS certificate_request,
                          TLS server_hello_done)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
     (TLS certificate,
      TLS client_key_exchange,
      TLS certificate_verify,
      TLS change_cipher_spec,
      TLS finished) ->
                             <- PPP EAP-Request
                             EAP-Type=EAP-TLS
                             (TLS Alert message)
     PPP EAP-Response/
     EAP-Type=EAP-TLS ->
                             <- PPP EAP-Failure
                             (User Disconnected)
     
     In the case where server authentication is unsuccessful, the conversa-
     tion will appear as follows:
     
     Authenticating Peer     Authenticator
     -------------------     -------------
                             <- PPP LCP Request-EAP
                             auth
     PPP LCP ACK-EAP
     auth ->
                             <- PPP EAP-Request/
                             Identity
     PPP EAP-Response/
     Identity (MyID) ->
                             <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS Start)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
      (TLS client_hello)->
                             <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS server_hello,
                              TLS certificate,
                         [TLS server_key_exchange,]
                         [TLS certificate_request,]
                          TLS server_hello_done)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
      (TLS certificate,
      TLS client_key_exchange,
     [TLS certificate_verify,]
      TLS change_cipher_spec,
      TLS finished) ->
                             <- PPP EAP-Request/
     
     
     
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                             EAP-Type=EAP-TLS
                             (TLS change_cipher_spec,
                              TLS finished)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
     (TLS Alert message) ->
                             <- PPP EAP-Failure
                             (User Disconnected)
     
     In the case where a previously established session is  being  resumed,
     and both sides authenticate successfully, the conversation will appear
     as follows:
     
     Authenticating Peer     Authenticator
     -------------------     -------------
                             <- PPP LCP Request-EAP
                             auth
     PPP LCP ACK-EAP
     auth ->
                             <- PPP EAP-Request/
                             Identity
     PPP EAP-Response/
     Identity (MyID) ->
                             <- PPP EAP-Request/
                             EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS Start)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
     (TLS client_hello)->
                             <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS server_hello,
                              TLS change_cipher_spec,
                          TLS finished)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
     (TLS change_cipher_spec,
      TLS finished) ->
                             <- PPP EAP-Success
     PPP Authentication
     Phase complete,
     NCP Phase starts
     
     ECP negotiation
     
     CCP negotiation
     
     In the case where a previously established session is  being  resumed,
     and the server authenticates to the client successfully but the client
     fails to authenticate to the server, the conversation will  appear  as
     follows:
     
     Authenticating Peer     Authenticator
     
     
     
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     -------------------     -------------
                             <- PPP LCP Request-EAP
                             auth
     PPP LCP ACK-EAP
     auth ->
                             <- PPP EAP-Request/
                             Identity
     PPP EAP-Response/
     Identity (MyID) ->
                             <- PPP EAP-Request/
                             EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS Start)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
     (TLS client_hello) ->
                             <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS server_hello,
                              TLS change_cipher_spec,
                          TLS finished)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
     (TLS change_cipher_spec,
      TLS finished) ->
                             <- PPP EAP-Request
                             EAP-Type=EAP-TLS
                             (TLS Alert message)
     PPP EAP-Response
     EAP-Type=EAP-TLS ->
                              <- PPP EAP-Failure
                              (User Disconnected)
     
     In  the  case where a previously established session is being resumed,
     and the server authentication is unsuccessful, the  conversation  will
     appear as follows:
     
     Authenticating Peer     Authenticator
     -------------------     -------------
                             <- PPP LCP Request-EAP
                             auth
     PPP LCP ACK-EAP
     auth ->
                             <- PPP EAP-Request/
                             Identity
     PPP EAP-Response/
     Identity (MyID) ->
                             <- PPP EAP-Request/
                             EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS Start)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
     (TLS client_hello)->
     
     
     
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                             <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
                             (TLS server_hello,
                              TLS change_cipher_spec,
                          TLS finished)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
     (TLS Alert message) ->
                             <- PPP EAP-Failure
                             (User Disconnected)
     
     
     5.  Detailed description of the EAP-TLS protocol
     
     
     5.1.  PPP EAP TLS Packet Format
     
     A  summary  of the PPP EAP TLS Request/Response packet 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Code      |   Identifier  |            Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |     Data ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
     
     Code
     
        1 - Request
        2 - Response
     
     Identifier
     
        The identifier field is one octet and aids  in  matching  responses
        with requests.
     
     Length
     
        The  Length field is two octets and indicates the length of the EAP
        packet including the  Code,  Identifier,  Length,  Type,  and  Data
        fields.   Octets  outside  the  range of the Length field should be
        treated as Data Link Layer padding and should be ignored on  recep-
        tion.
     
     Type
     
        ? - EAP TLS
     
     Data
     
        The format of the Data field is determined by the Code field.
     
     
     
     
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     5.2.  PPP EAP TLS Request Packet
     
     A  summary  of  the  PPP EAP TLS Request packet 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Code      |   Identifier  |            Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |   TLS data...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     
     Code
     
        1
     
     Identifier
     
        The identifier field is one octet and aids  in  matching  responses
        with  requests.   The  identifier  field  MUST  be  changed on each
        Request packet.
     
     Length
     
        The Length field is two octets and indicates the length of the  EAP
        packet  including  the  Code,  Identifier,  Length,  Type,  and TLS
        Response fields.
     
     Type
     
        ? - EAP TLS
     
     TLS data
     
        The TLS data consists of the encapsulated TLS packet in TLS  record
        format.
     
     
     5.3.  PPP EAP TLS Response Packet
     
     A  summary  of  the PPP EAP TLS Response packet 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Code      |   Identifier  |            Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      | TLS Data...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     
     Code
     
     
     
     
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     INTERNET-DRAFT                                         14 October 1997
     
     
        2
     
     Identifier
     
        The identifier field is one octet and  MUST  match  the  Identifier
        field from the corresponding request.
     
     Length
     
        The  Length field is two octets and indicates the length of the EAP
        packet including the Code, Identifier, Length, Type, and  TLS  data
        fields.
     
     Type
     
        ? - EAP TLS
     
     TLS data
     
        The  TLS data consists of the encapsulated TLS packet in TLS record
        format.
     
     
     6.  Security issues
     
     
     6.1.  Certificate revocation
     
     Since the EAP server is on the Internet during the  EAP  conversation,
     the  EAP server is capable of following a certificate chain or verify-
     ing whether the peer's certificate has been revoked. In contrast,  the
     peer  may or may not have Internet connectivity, and thus while it can
     validate the EAP server's certificate based on a pre-configured set of
     CAs,  it  may  not  be  able  to  follow a certificate chain or verify
     whether the EAP server's certificate has been revoked.
     
     In the case where the peer is initiating a voluntary  Layer  2  tunnel
     using  PPTP or L2TP, the peer will typically already have a PPP inter-
     face and Internet connectivity established at the time of tunnel  ini-
     tiation.  As  a  result,  during the EAP conversation it is capable of
     checking for certificate revocation.
     
     However, in the case where the peer is initiating an intial  PPP  con-
     versation, it will not have Internet connectivity and is therefore not
     capable of checking for certificate revocation until after NCP negoti-
     ation completes and the peer has access to the Internet. In this case,
     the peer SHOULD check for certificate revocation after  connecting  to
     the Internet.
     
     
     6.2.  Separation of the EAP server and PPP authenticator
     
     As  a result of the EAP-TLS conversation, the EAP endpoints will mutu-
     ally authenticate, negotiate a ciphersuite, and derive a  session  key
     
     
     
     Aboba & Simon                                                [Page 15]


     INTERNET-DRAFT                                         14 October 1997
     
     
     for  subsequent  use  in PPP encryption. Since the peer and EAP client
     reside on the same machine, it is necessary for the EAP client  module
     to pass the session key to the PPP encryption module.
     
     The  situation  may be more complex on the PPP authenticator. As noted
     in [17], the PPP authenticator may or  may  not  reside  on  the  same
     machine  as  the EAP server. For example, when RADIUS/EAP is used, 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,
     the  mutual  authentication  defined in EAP-TLS will occur between the
     peer and the EAP server, not between the peer and  the  authenticator.
     This  means  that  as  a result of the EAP-TLS conversation, it is not
     possible for the peer to validate the identity of the  NAS  or  tunnel
     server that it is speaking to.
     
     As described in [17], when EAP/RADIUS is used to encapsulate EAP pack-
     ets, the Signature attribute is required in EAP/RADIUS Access-Requests
     sent  from  the  NAS  or tunnel server to the RADIUS server. Since the
     Signature attribute involves a keyed-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 a keyed-MD5 hash, enabling the NAS or tunnel
     server  to  determine the integrity of the packet and verify the iden-
     tity of the RADIUS server. Moreover, EAP-TLS packets  in  transit  are
     integrity  protected  and authenticated end-to-end via TLS mechanisms,
     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.
     
     
     6.3.  Relationship of PPP encryption to other security mechanisms
     
     PPP  encryption  currently  plays an important role in the securing of
     Layer 2 tunneling protocols such as PPTP, described in [13], and L2TP,
     described  in [14]. While it may be envisaged that security mechanisms
     such as IPSEC will eventually become ubiquitous,  it  will  take  some
     time  for  vendors  to add IPSEC capabilities to their devices, and in
     any case legacy authenticator devices or routers may not  be  able  to
     support  IPSEC without being upgraded.  As a result, it is likely that
     non-IPSEC capable devices will persist  in  operational  networks  for
     quite some time.
     
     
     
     
     
     Aboba & Simon                                                [Page 16]


     INTERNET-DRAFT                                         14 October 1997
     
     
     In  an environment where IPSEC is not ubiquitous, in Layer 2 tunneling
     protocols a role remains for PPP encryption. Since with mandatory tun-
     neling  a PPP peer cannot tell whether its packets are being tunneled,
     let alone whether the authenticator is securing the tunnel,  if  secu-
     rity  is  required  then the client must make its own arrangements. In
     the case where all endpoints cannot be relied upon to implement IPSEC,
     TLS,  or  another suitable security protocol, then PPP encryption pro-
     vides a very convenient means to ensure the privacy of  packets  tran-
     siting between the client and the tunnel server.
     
     There  also may be circumstances in which PPP encryption may be desir-
     able even if IPSEC is available. Routers implementing Network  Address
     Translation (NAT) are now growing rapidly in popularity.  Where NAT is
     turned on, IPSEC cannot be used to secure the outer layer of a client-
     initiated  layer  2  tunnel,  since the address translated packet will
     then fail the authentication check. By contrast, Layer 2 tunnels  uti-
     lizing PPP encryption may pass unimpeded through a NAT.
     
     
     7.  Copyright notice
     
     Copyright (C) The Internet Society, 1997. All Rights Reserved.
     
     This  document  and  translations of it may be copied and furnished to
     others, and derivative works that comment on or otherwise  explain  it
     or  assist in its implmentation may be prepared, copied, published and
     distributed, in whole or in part, without  restriction  of  any  kind,
     provided  that  the  above  copyright  notice  and  this paragraph are
     included on all such copies and derivative works.  However, this docu-
     ment  itself  may  not be modified in any way, such as by removing the
     copyright notice or references to the Internet Society or other Inter-
     net  organizations,  except  as  needed for the  purpose of developing
     Internet standards in which case the procedures for copyrights defined
     in  the Internet Standards process must be followed, or as required to
     translate it into languages other than English.
     
     The limited permissions granted above are perpetual and  will  not  be
     revoked by the Internet Society or its successors or assigns.
     
     This  document  and the information contained herein is provided on an
     "AS IS" basis and THE INTERNET SOCIETY AND  THE  INTERNET  ENGINEERING
     TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT
     NOT LIMITED TO ANY WARRANTY THAT THE USE  OF  THE  INFORMATION  HEREIN
     WILL  NOT  INFRINGE  ANY  RIGHTS  OR  ANY  IMPLIED  WARRANTIES OF MER-
     CHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
     
     
     8.  Acknowledgments
     
     Thanks to Terence Spies, Glen Zorn and Narendra Gidwani  of  Microsoft
     for useful discussions of this problem space.
     
     
     
     
     
     
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     9.  References
     
     [1]  Simpson, W., Editor, "The Point-to-Point Protocol (PPP)." STD 51,
     RFC 1661, Daydreamer, July 1994.
     
     [2] Sklower, K., Lloyd, B., McGregor, G., Carr, D., and T.  Coradetti,
     "The PPP Multilink Protocol (MP)." RFC 1990, UC Berkeley, August 1996.
     
     [3] Simpson, W., Editor, "PPP LCP Extensions." RFC  1570,  Daydreamer,
     January 1994.
     
     [4]  R.  Rivest,  S.  Dusse.  "The MD5 Message-Digest Algorithm."  RFC
     1321, MIT Laboratory for Computer Science,  RSA  Data  Security  Inc.,
     April 1992.
     
     [5]  L.  J.  Blunk,  J. R. Vollbrecht.  "PPP Extensible Authentication
     Protocol (EAP)." Work in progress,  draft-ietf-pppext-eap-auth-02.txt,
     Merit Network, Inc., June 1996.
     
     [6]  W.  T.  Whelan, "PPP EAP RSA Public Key Authentication Protocol."
     Work in progress, draft-ietf-pppext-eaprsa-04.txt, Cabletron  Systems,
     February 1997.
     
     [7]  Meyer,  G., "The PPP Encryption Protocol (ECP)." RFC 1968, Spider
     Systems. June 1996
     
     [8] National Bureau of Standards, "Data Encryption Standard." FIPS PUB
     46 (January 1977).
     
     [9]  National  Bureau of Standards, "DES Modes of Operation." FIPS PUB
     81 (December 1980).
     
     [10] K. Sklower, G. Meyer.  "The PPP DES Encryption Protocol,  Version
     2   (DESE-bis)"   Work   in  progress,  draft-ietf-pppext-des-encrypt-
     v2-00.txt, University of California, Berkeley, Shiva, July 1997.
     
     [11] K. Hummert.  "The PPP  Triple-DES  Encryption  Protocol  (3DESE)"
     Work  in progress, draft-ietf-pppext-3des-encrypt-00.txt, Nentec GmbH,
     July 1997.
     
     [12] S. Bradner.  "Key words for use in RFCs to  Indicate  Requirement
     Levels."  RFC 2119, Harvard University, March 1997.
     
     [13]  K.  Hamzeh,  G.  S.  Pall, J. Taarud, W. Verthein, W. A. Little.
     "Point-to-Point Tunneling Protocol -- PPTP." Internet draft  (work  in
     progress),   draft-ietf-pppext-pptp-02.txt,   Ascend   Communications,
     Microsoft, Copper Mountain Networks, U.S. Robotics, July 1997.
     
     [14] K. Hamzeh, T. Kolar, M. Littlewood, G. S.  Pall,  B.  Palter,  J.
     Taarud,  W. M. Townsley, A. Valencia, W. Verthein.  "Layer Two Tunnel-
     ing Protocol L2TP." Internet  draft  (work  in  progress)  draft-ietf-
     pppext-l2tp-06.txt,  Ascend  Communications, Cisco Systems, Microsoft,
     Copper Mountain Networks, IBM, U.S. Robotics, August 1997.
     
     
     
     
     Aboba & Simon                                                [Page 18]


     INTERNET-DRAFT                                         14 October 1997
     
     
     [15] T. Dierks, C. Allen.  "The TLS Protocol  Version  1.0."  Internet
     draft  (work  in  progress)  draft-ietf-tls-protocol-03.txt, Consensus
     Development, May 1997.
     
     [16] D. Rand.  "The PPP Compression Control Protocol." RFC 1962,  Nov-
     ell, June 1996.
     
     [17]  P.  Calhoun,  A.C. Rubens, B. Aboba.  "Extensible Authentication
     Protocol Support in RADIUS." Internet draft (work in progress), draft-
     ietf-radius-eap-02.txt,  US  Robotics  Access  Corp.,  Merit  Network,
     Microsoft, May, 1997.
     
     
     10.  Authors' Addresses
     
     Bernard Aboba
     Microsoft Corporation
     One Microsoft Way
     Redmond, WA 98052
     
     Phone: 425-936-6605
     EMail: bernarda@microsoft.com
     
     Dan Simon
     Microsoft Corporation
     One Microsoft Way
     Redmond, WA 98052
     
     Phone: 425-936-6711
     EMail: dansimon@microsoft.com
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     Aboba & Simon                                                [Page 19]