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rfc:rfc2716

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.

Aboba & Simon Experimental [Page 1] RFC 2716 PPP EAP TLS Authentication Protocol October 1999

 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.

Aboba & Simon Experimental [Page 2] RFC 2716 PPP EAP TLS Authentication Protocol October 1999

 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.

Aboba & Simon Experimental [Page 3] RFC 2716 PPP EAP TLS Authentication Protocol October 1999

 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.

Aboba & Simon Experimental [Page 4] RFC 2716 PPP EAP TLS Authentication Protocol October 1999

 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.

Aboba & Simon Experimental [Page 5] RFC 2716 PPP EAP TLS Authentication Protocol October 1999

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.

Aboba & Simon Experimental [Page 6] RFC 2716 PPP EAP TLS Authentication Protocol October 1999

 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

Aboba & Simon Experimental [Page 7] RFC 2716 PPP EAP TLS Authentication Protocol October 1999

 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

Aboba & Simon Experimental [Page 8] RFC 2716 PPP EAP TLS Authentication Protocol October 1999

 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.

Aboba & Simon Experimental [Page 9] RFC 2716 PPP EAP TLS Authentication Protocol October 1999

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

Aboba & Simon Experimental [Page 10] RFC 2716 PPP EAP TLS Authentication Protocol October 1999

 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

Aboba & Simon Experimental [Page 11] RFC 2716 PPP EAP TLS Authentication Protocol October 1999

 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,

Aboba & Simon Experimental [Page 12] RFC 2716 PPP EAP TLS Authentication Protocol October 1999

  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,]

Aboba & Simon Experimental [Page 13] RFC 2716 PPP EAP TLS Authentication Protocol October 1999

  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)

Aboba & Simon Experimental [Page 14] RFC 2716 PPP EAP TLS Authentication Protocol October 1999

 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)

Aboba & Simon Experimental [Page 15] RFC 2716 PPP EAP TLS Authentication Protocol October 1999

 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)

Aboba & Simon Experimental [Page 16] RFC 2716 PPP EAP TLS Authentication Protocol October 1999

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.

Aboba & Simon Experimental [Page 17] RFC 2716 PPP EAP TLS Authentication Protocol October 1999

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

Aboba & Simon Experimental [Page 18] RFC 2716 PPP EAP TLS Authentication Protocol October 1999

    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.

Aboba & Simon Experimental [Page 19] RFC 2716 PPP EAP TLS Authentication Protocol October 1999

 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.

Aboba & Simon Experimental [Page 20] RFC 2716 PPP EAP TLS Authentication Protocol October 1999

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.

Aboba & Simon Experimental [Page 21] RFC 2716 PPP EAP TLS Authentication Protocol October 1999

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

Aboba & Simon Experimental [Page 22] RFC 2716 PPP EAP TLS Authentication Protocol October 1999

 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

Aboba & Simon Experimental [Page 23] RFC 2716 PPP EAP TLS Authentication Protocol October 1999

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.

Aboba & Simon Experimental [Page 24]

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