Network Working Group                                            B. Aboba
Requests for Commments: 2716                                     D. Simon
Category: Experimental                                          Microsoft
                                                             October 1999


                  PPP EAP TLS Authentication Protocol

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

1.  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
   mechanisms within EAP.

2.  Introduction

   The Extensible Authentication Protocol (EAP), described in [5],
   provides 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 methods 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
   authentication and key management capabilities of the TLS protocol,
   described in [12].

2.1.  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].

3.  Protocol overview

3.1.  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
   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 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
   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
   message, possibly followed by TLS certificate, server_key_exchange,
   certificate_request, server_hello_done and/or finished handshake
   messages, and/or a TLS change_cipher_spec message.  The server_hello
   handshake message contains a TLS version number, another random
   number, 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;
   otherwise, the sessionId  will  match  that  offered by the client,
   indicating 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
   protocol 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
   sessionId chosen by the peer, and the time elapsed since the previous
   authentication, the EAP server will decide whether to allow the
   continuation, 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
   connecting 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
   finished 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 authenticate 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,
   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
   message, which completes the exchange of a shared master secret
   between the peer and the EAP server.  If the EAP server sent a
   certificate_request message in the preceding EAP-Request packet, then
   the peer MUST send, in addition, certificate and certificate_verify
   handshake messages.  The former contains a certificate for the peer's
   signature public key, while the latter contains the peer's signed
   authentication 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.

   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
   contains 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 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 MUST wait for the EAP-Server to reply before terminating the
   conversation.  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.

3.2.  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.

   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.

3.3.  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
   Maximum Received Reconstructed Unit (MRRU).  As described in [2], the
   multilink 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
   typical 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
   fragment 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 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.

3.4.  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
   certificate, 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 certificate is signed by a CA controlling the
   intended destination and whether the EAP server exists within a
   target sub-domain.

3.5.  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 handshake 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.  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 authentication key (the one used for
   computing MACs for messages from EAP server to peer), if used, and if
   different from the peer authentication key, is obtained by truncating
   to the correct length the fourth 32 bytes of this same PRF output
   string.  The peer initialization vector (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 messages 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.

3.6.  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
   negotiation and the subsequent ECP negotiation (described in [6]),
   during ECP negotiation the PPP peer MUST offer only the ciphersuite
   negotiated inEAP-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 ciphersuite, 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
   correctly.

3.7.  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
   endpoints 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.

3.8.  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
   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

   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
                           (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
   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
   (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)

   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
                           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
                           (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)

4.  Detailed description of the EAP-TLS protocol

4.1.  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
      reception.

   Type

      13 - EAP TLS

   Data

      The format of the Data field is determined by the Code field.

4.2.  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
      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
      differentiates 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.

4.3.  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
      +-+-+-+-+-+-+-+-+
      |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
      differentiates 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.  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. and S. Dusse, "The MD5 Message-Digest Algorithm", RFC
        1321, April 1992.

   [5]  Blunk, L. and J. Vollbrecht, "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).

   [9]  Sklower, K. amd G. Meyer, "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. and  C. Allen, "The TLS Protocol Version 1.0", RFC
        2246, November 1998.

   [13] Rand, D., "The PPP Compression Control Protocol", RFC 1962, June
        1996.

6.  Security Considerations

6.1.  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
   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
   interface and Internet connectivity established at the time of tunnel
   initiation.  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
   conversation, it will not have Internet connectivity and is therefore
   not capable of checking for certificate revocation until after NCP
   negotiation 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
   mutually 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
   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.

6.3.  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 packets transiting between the client and the tunnel
   server.

7.  Acknowledgments

   Thanks to Terence Spies, Glen Zorn and Narendra Gidwani of Microsoft
   for useful discussions of this problem space.

8.  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

9.  Full Copyright Statement

   Copyright (C) The Internet Society (1999).  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 implementation 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
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet 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
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.