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Versions: 00 01 02 03 04 05 06 rfc2716                                  
     PPPEXT Working Group                                     Bernard Aboba
     INTERNET-DRAFT                                               Microsoft
     Category: Informational                                      Dan Simon
     <draft-ietf-pppext-eaptls-04.txt>                            Microsoft
     9 October 1998
     
     
                      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   ftp.ietf.org   (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-04.txt>, and expires May 1, 1999. 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,
     integrity-protected ciphersuite negotiation and key  exchange  between
     two  endpoints.   This  document describes how EAP-TLS, which includes
     support for fragmentation and reassembly, provides for these TLS mech-
     anisms within EAP.
     
     
     
     
     
     
     
     
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     INTERNET-DRAFT   PPP EAP TLS Authentication Protocol    9 October 1998
     
     
     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
     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,  it  may  be  desirable to support mutual authentication, and
     since PPP encryption protocols such as [9] and [10]  assume  existence
     of  a  session  key,  it is useful to have a mechanism for session key
     establishment. Since design of secure key management protocols is non-
     trivial,  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 protected ciphersuite negotiation, mutual authentica-
     tion and key management capabilities of the TLS protocol, described in
     [12].
     
     
     3.1.  Requirements language
     
     In   this   document,  the  key  words  "MAY",  "MUST,   "MUST   NOT",
     "optional", "recommended",  "SHOULD",  and  "SHOULD  NOT",  are to  be
     interpreted as described in [11].
     
     
     4.  Protocol overview
     
     
     
     4.1.  Overview of the EAP-TLS conversation
     
     As  described  in  [5],  the EAP-TLS conversation will typically begin
     with the authenticator and the peer negotiating EAP.  The  authentica-
     tor  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 PPP authenticator and the peer, 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 conversing with the peer.
     
     Once  having received the peer's Identity, the EAP server MUST respond
     with an EAP-TLS/Start packet, which is an EAP-Request packet with EAP-
     Type=EAP-TLS, the Start (S) bit set, and no data.  The EAP-TLS conver-
     sation 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
     
     
     
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     current cipher spec remains the same until the change_cipher_spec mes-
     sage  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
     server MUST correspond to TLS v1.0 or later.
     
     If the client's sessionId is null or unrecognized by the  server,  the
     server  MUST  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.
     
     
     
     
     
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     INTERNET-DRAFT   PPP EAP TLS Authentication Protocol    9 October 1998
     
     
     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
     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 MUST respond to the EAP-Request with an  EAP-Response  packet
     of  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  finished  handshake  message,  and possibly certificate, certifi-
     cate_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 hand-
     shake messages.  The finished message contains the peer's  authentica-
     tion 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 SHOULD
     send an EAP-Request packet with EAP-Type=EAP-TLS, encapsulating a  TLS
     record  containing  the appropriate TLS alert message.  The EAP server
     SHOULD send a TLS alert message  rather  immediately  terminating  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.
     
     
     
     
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     INTERNET-DRAFT   PPP EAP TLS Authentication Protocol    9 October 1998
     
     
     To ensure that the peer receives the TLS alert message, the EAP server
     MUST  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 MUST
     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  MUST  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  handshke messages.  The latter con-
     tains the EAP server's authentication response to the peer.  The  peer
     will then verify the hash in order to authenticate the EAP server.
     
     If  the  EAP server authenticates unsuccessfully, the peer MAY send an
     EAP-Response packet of EAP-Type=EAP-TLS containing a TLS Alert message
     identifying  the  reason  for  the failed authentication. The peer MAY
     send a TLS alert message rather than immediately terminating the  con-
     versation  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
     MUST 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 MUST send an
     EAP-Response packet of EAP-Type=EAP-TLS, and no data.  The  EAP-Server
     then MUST respond 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 MUST resend the EAP-Request for which it has not yet
     received an EAP-Response. However,  the  peer  MUST  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  send  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.
     
     
     
     
     
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     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.  Fragmentation
     
     A single TLS record may be up to 16384 octets in  length,  but  a  TLS
     message  may  span multiple TLS records, and a TLS certificate message
     may in principle be as long as 16MB. The  group  of  EAP-TLS  messages
     sent  in  a single round may thus be larger than the PPP MTU size, the
     maximum RADIUS packet size of 4096 octets, or even the Multilink Maxi-
     mum Received Reconstructed Unit (MRRU).  As described in [2], the mul-
     tilink MRRU is negotiated via the Multilink  MRRU  LCP  option,  which
     includes  an  MRRU  length  field  of two octets, and thus can support
     MRRUs as large as 64 KB.
     
     However, note that in order to protect against reassembly  lockup  and
     denial  of  service attacks, it may be desirable for an implementation
     to set a maximum size for one such group of TLS messages. Since a typ-
     ical  certificate  chain  is rarely longer than a few thousand octets,
     and no other field is likely to be anwhere near as long, a  reasonable
     choice of maximum acceptable message length might be 64 KB.
     
     If  this  value  is  chosen, then fragmentation can be handled via the
     multilink PPP fragmentation mechanisms described in [2]. While this is
     desirable,  there  may  be  cases  in  which multilink or the MRRU LCP
     option cannot be negotiated. As a result,  an  EAP-TLS  implementation
     MUST provide its own support for fragmentation and reassembly.
     
     Since  EAP  is a simple ACK-NAK protocol, fragmentation support can be
     added in a simple manner. In EAP, fragments that are lost  or  damaged
     in  transit will be retransmitted, and since sequencing information is
     provided by the Identifier field in EAP, there is no need for a  frag-
     ment offset field as is provided in IPv4.
     
     EAP-TLS  fragmentation support is provided through addition of a flags
     octet within the EAP-Response and EAP-Request packets, as  well  as  a
     TLS  Message  Length  field  of  four octets. Flags include the Length
     included (L), More fragments (M), and EAP-TLS Start (S)  bits.  The  L
     flag  is  set  to  indicate the presence of the four octet TLS Message
     Length field, and MUST be set for the first fragment of  a  fragmented
     TLS  message or set of messages. The M flag is set on all but the last
     fragment. The S flag is set only within the EAP-TLS start message sent
     from  the EAP server to the peer. The TLS Message Length field is four
     octets, and provides the total length of the TLS  message  or  set  of
     messages  that is being fragmented; this simplifies buffer allocation.
     
     When an EAP-TLS peer receives an EAP-Request packet  with  the  M  bit
     set, it MUST respond with an EAP-Response with EAP-Type=EAP-TLS and no
     data.  This serves as a fragment ACK. The EAP server MUST  wait  until
     it receives the EAP-Response before sending another fragment. In order
     to prevent errors in processing of  fragments,  the  EAP  server  MUST
     increment  the  Identifier field for each fragment contained within an
     
     
     
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     EAP-Request, and the peer MUST include this Identifier  value  in  the
     fragment ACK contained within the EAP-Reponse. Retransmitted fragments
     will contain the same Identifier value.
     
     Similarly, when the EAP server receives an EAP-Response with the M bit
     set,  it MUST respond with an EAP-Request with EAP-Type=EAP-TLS and no
     data. This serves as a fragment ACK. The EAP peer MUST wait  until  it
     receives the EAP-Request before sending another fragment.  In order to
     prevent errors in the processing of fragments, the EAP server MUST use
     increment  the Identifier value for each fragment ACK contained within
     an EAP-Request, and the peer MUST include this Identifier value in the
     subsequent fragment contained within an EAP-Reponse.
     
     
     4.4.  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.
     
     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.
     
     
     4.5.  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
     
     
     
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     length the first 32 bytes  of  the  first  PRF  of  these  two  output
     strings.   TheEAP  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 [9] and [10].
     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.
     
     
     4.6.  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 [6]), 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  onversation  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 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  cor-
     rectly.
     
     
     
     
     
     
     
     
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     4.7.  CCP negotiation
     
     TLS  as  described in [12] 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 [13] should  be  used
     to negotiate the desired compression scheme.
     
     
     4.8.  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)
     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
     
     
     
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     Phase complete,
     NCP Phase starts
     
     ECP negotiation
     
     CCP negotiation
     
     In the case where the EAP-TLS mutual authentication is successful, and
     fragmentation is required, 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, S bit set)
     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)
                      (Fragment 1: L, M bits set)
     PPP EAP-Response/
     EAP-Type=EAP-TLS ->
                             <- PPP EAP-Request/
                                EAP-Type=EAP-TLS
                             (Fragment 2: M bit set)
     PPP EAP-Response/
     EAP-Type=EAP-TLS ->
                             <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
                             (Fragment 3)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
     (TLS certificate,
      TLS client_key_exchange,
     [TLS certificate_verify,]
      TLS change_cipher_spec,
      TLS inished)(Fragment 1:
      L, M bits set)->
                              <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
     
     
     
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     PPP EAP-Response/
     EAP-Type=EAP-TLS
     (Fragment 2)->
                            <- 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
                             (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
     
     
     
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                             (TLS change_cipher_spec,
                             TLS finished)
     PPP EAP-Response/
     EAP-Type=EAP-TLS ->
                             <- 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/
                             EAP-Type=EAP-TLS
                             (TLS change_cipher_spec,
                              TLS finished)
     PPP EAP-Response/
     EAP-Type=EAP-TLS
     (TLS change_cipher_spec,
     TLS finished)
                             <- PPP EAP-Request/
     
     
     
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                             EAP-Type=EAP-TLS
     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
     -------------------     -------------
                             <- PPP LCP Request-EAP
     
     
     
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     INTERNET-DRAFT   PPP EAP TLS Authentication Protocol    9 October 1998
     
     
                             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 EA-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)->
                             <- PPP EAP-Request/
                             EAP-Type=EAP-TLS
     
     
     
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     INTERNET-DRAFT   PPP EAP TLS Authentication Protocol    9 October 1998
     
     
                             (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
     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
     
        13 - EAP TLS
     
     
     
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     INTERNET-DRAFT   PPP EAP TLS Authentication Protocol    9 October 1998
     
     
     Data
     
        The format of the Data field is determined by the Code field.
     
     
     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      |     Flags     |      TLS Message Length
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     TLS Message Length        |       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
     
        13 - EAP TLS
     
     Flags
     
        0 1 2 3 4 5 6 7 8
        +-+-+-+-+-+-+-+-+
        |L M S R R R R R|
        +-+-+-+-+-+-+-+-+
     
        L = Length included
        M = More fragments
        S = EAP-TLS start
        R = Reserved
     
        The L bit (length included) is set to indicate the presence of  the
        four  octet TLS Message Length field, and MUST be set for the first
     
     
     
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     INTERNET-DRAFT   PPP EAP TLS Authentication Protocol    9 October 1998
     
     
        fragment of a fragmented TLS message or set of messages. The M  bit
        (more  fragments)  is  set  on all but the last fragment. The S bit
        (EAP-TLS start) is set in an EAP-TLS Start message. This  differen-
        tiates the EAP-TLS Start message from a fragment acknowledgement.
     
     TLS Message Length
     
        The TLS Message Length field is four octets, and is present only if
        the L bit is set.  This field provides the total length of the  TLS
        message or set of messages that is being fragmented.
     
     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      |     Flags     |      TLS Message Length
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     TLS Message Length        |       TLS Data...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     
     Code
     
        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, Identifir, Length, Type, and TLS data
        fields.
     
     Type
     
        13 - EAP TLS
     
     Flags
     
        0 1 2 3 4 5 6 7 8
     
     
     
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     INTERNET-DRAFT   PPP EAP TLS Authentication Protocol    9 October 1998
     
     
        +-+-+-+-+-+-+-+-+
        |L M S R R R R R|
        +-+-+-+-+-+-+-+-+
     
        L = Length included
        M = More fragments
        S = EAP-TLS start
        R = Reserved
     
        The L bit (length included) is set to indicate the presence of  the
        four  octet TLS Message Length field, and MUST be set for the first
        fragment of a fragmented TLS message or set of messages. The M  bit
        (more  fragments)  is  set  on all but the last fragment. The S bit
        (EAP-TLS start) is set in an EAP-TLS Start message.  This differen-
        tiates the EAP-TLS Start message from a fragment acknowledgement.
     
     TLS Message Length
     
        The TLS Message Length field is four octets, and is present only if
        the L bit is set. This field provides the total length of  the  TLS
        message or set of messages that is being fragmented.
     
     TLS data
     
        The  TLS data consists of the encapsulated TLS packet in TLS record
        format.
     
     
     
     6.  References
     
     [1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)." STD  51,
     RFC 1661, July 1994.
     
     [2]  Sklower, K., Lloyd, B., McGregor, G., Carr, D., and T. Coradetti,
     "The PPP Multilink Protocol (MP)." RFC 1990, August 1996.
     
     [3] Simpson, W., Editor, "PPP LCP Extensions." RFC 1570, January 1994.
     
     [4]  Rivest,  R.,  Dusse,  S., "The MD5 Message-Digest Algorithm", RFC
     1321, April 1992.
     
     [5] Blunk, L., Vollbrecht, J., "PPP Extensible Authentication Protocol
     (EAP)", RFC 2284, March 1998.
     
     [6]  Meyer,  G.,  "The  PPP Encryption Protocol (ECP)." RFC 1968, June
     1996
     
     [7] National Bureau of Standards, "Data Encryption Standard", FIPS PUB
     46 (January 1977).
     
     [8]  National  Bureau of Standards, "DES Modes of Operation", FIPS PUB
     81 (December 1980).
     
     
     
     
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     INTERNET-DRAFT   PPP EAP TLS Authentication Protocol    9 October 1998
     
     
     [9] Sklower, K., Meyer, G., "The PPP DES Encryption Protocol,  Version
     2 (DESE-bis)", RFC 2419, September 1998.
     
     [10]  Hummert,  K.,  "The PPP Triple-DES Encryption Protocol (3DESE)",
     RFC 2420, September 1998.
     
     [11]  Bradner, S., "Key words for use in RFCs to Indicate  Requirement
     Levels", BCP 14, RFC 2119, March 1997.
     
     [12]  Dierks,  T., Allen, C., "The TLS Protocol Version 1.0", Internet
     draft  (work  in  progress)  draft-ietf-tls-protocol-05.txt,  November
     1997.
     
     [13]  D. Rand.  "The PPP Compression Control Protocol." RFC 1962, Nov-
     ell, June 1996.
     
     
     7.  Security Considerations
     
     
     7.1.  Certificate revocation
     
     Since the EAP server is on the Internet during the  EAP  conversation,
     the  server  is  capable of following a certificate chain or verifying
     whether the peer's certificate has been revoked. In contrast, the peer
     may or may not have Internet connectivity, and thus while it can vali-
     date 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.
     
     
     7.2.  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
     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, which may
     or may not reside on the same machine as the EAP server. In  the  case
     
     
     
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     INTERNET-DRAFT   PPP EAP TLS Authentication Protocol    9 October 1998
     
     
     where  the  EAP  server  and  PPP  authenticator  reside  on different
     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.
     
     The  second  issue 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.
     
     
     7.3.  Relationship of PPP encryption to other security mechanisms
     
     It is envisaged that EAP-TLS will be used primarily  with  dialup  PPP
     connections.  However,  there  are  also  circumstances  in  which PPP
     encryption may be used along with Layer 2 tunneling protocols such  as
     PPTP and L2TP.
     
     In  compulsory  layer  2 tunneling, a PPP peer makes a connection to a
     NAS or router which tunnels the PPP packets to a tunnel server.  Since
     with  compulsory  tunneling a PPP peer cannot tell whether its packets
     are being tunneled, let alone whether the network device  is  securing
     the  tunnel, if security 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, PPP
     encryption provides a convenient means to ensure the privacy of  pack-
     ets transiting between the client and the tunnel server.
     
     
     8.  Acknowledgments
     
     Thanks  to  Terence Spies, Glen Zorn and Narendra Gidwani of Microsoft
     for useful discussions of this problem space.
     
     
     9.  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
     
     
     
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     INTERNET-DRAFT   PPP EAP TLS Authentication Protocol    9 October 1998
     
     
     Phone: 425-936-6711
     EMail: dansimon@microsoft.com
     
     
     
     10.  Expiration Date
     
     This memo is filed as <draft-ietf-pppext-eaptls-04.txt>,  and  expires
     May 1, 1999.
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
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