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PPPEXT Working Group                                       Bernard Aboba
INTERNET-DRAFT                                                 Microsoft
Category: Informational                                        Dan Simon
<draft-ietf-pppext-eaptls-06.txt>                              Microsoft
12 August 1999

                  PPP EAP TLS Authentication Protocol

1.  Status of this Memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups.  Note that other groups
may also distribute working documents as Internet-Drafts.  Internet-
Drafts are draft documents valid for a maximum of six months and may be
updated, replaced, or obsoleted by other documents at any time.  It is
inappropriate to use Internet-Drafts as reference material or to cite
them other than as "work in progress."


The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

The distribution of this memo is unlimited.  It is filed as <draft-ietf-
pppext-eaptls-06.txt>, and expires March 1, 2000. Please send comments
to the authors.

2.  Copyright Notice

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

3.  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

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for fragmentation and reassembly, provides for these TLS mechanisms
within EAP.

4.  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].

4.1.  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].

5.  Protocol overview

5.1.  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

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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.

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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,

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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.

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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.

5.2.  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.

5.3.  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.

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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.

5.4.  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

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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.

5.5.  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.

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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.

5.6.  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 in
EAP-TLS.  This ensures that the PPP authenticator MUST accept the EAP-
TLS negotiated ciphersuite in order for the onversation to proceed.
Should the authenticator not accept the EAP-TLS negotiated 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.

5.7.  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.

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5.8.  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

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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/

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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

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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)

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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)

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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

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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)

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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)

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6.  Detailed description of the EAP-TLS protocol

6.1.  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.

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6.2.  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

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   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.

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6.3.  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

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   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.

7.  References

[1]  Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD 51,
     RFC 1661, July 1994.

[2]  Sklower, K., Lloyd, B., McGregor, G., Carr, D., and T. Coradetti,
     "The PPP Multilink Protocol (MP)", RFC 1990, August 1996.

[3]  Simpson, W., Editor, "PPP LCP Extensions", RFC 1570, January 1994.

[4]  Rivest, R., Dusse, S., "The MD5 Message-Digest Algorithm", RFC
     1321, April 1992.

[5]  Blunk, L., Vollbrecht, J., "PPP Extensible Authentication Protocol
     (EAP)", RFC 2284, March 1998.

[6]  Meyer, G., "The PPP Encryption Protocol (ECP)", RFC 1968, June
     1996.

[7]  National Bureau of Standards, "Data Encryption Standard", FIPS PUB
     46 (January 1977).

[8]  National Bureau of Standards, "DES Modes of Operation", FIPS PUB 81
     (December 1980).

[9]  Sklower, K., Meyer, G., "The PPP DES Encryption Protocol, Version 2
     (DESE-bis)", RFC 2419, September 1998.

[10] Hummert, K., "The PPP Triple-DES Encryption Protocol (3DESE)", RFC
     2420, September 1998.

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[11] Bradner, S., "Key words for use in RFCs to Indicate Requirement
     Levels", BCP 14, RFC 2119, March 1997.

[12] Dierks, T., Allen, C., "The TLS Protocol Version 1.0", RFC 2246,
     November 1998.

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

8.  Security Considerations

8.1.  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.

8.2.  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

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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.

8.3.  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.

9.  Acknowledgments

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

10.  Author Addresses

Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052

Phone: 425-936-6605
EMail: bernarda@microsoft.com

Dan Simon
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052

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Phone: 425-936-6711
EMail: dansimon@microsoft.com

11.  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 implmentation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are included
on all such copies and derivative works.  However, this 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 inthe 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."

12.  Expiration Date

This memo is filed as <draft-ietf-pppext-eaptls-06.txt>,  and  expires
March 1, 2000.

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