Network Working Group B. Aboba
Requests for Commments: 2716 D. Simon
Category: EXPerimental Microsoft
October 1999
PPP EAP TLS Authentication Protocol
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
1. Abstract
The Point-to-Point Protocol (PPP) provides a standard method for
transporting multi-protocol datagrams over point-to-point links. PPP
also defines an extensible Link Control Protocol (LCP), which can be
used to negotiate authentication methods, as well as an Encryption
Control Protocol (ECP), used to negotiate data encryption over PPP
links, and a Compression Control Protocol (CCP), used to negotiate
compression methods. The Extensible Authentication Protocol (EAP) is
a PPP extension that provides support for additional authentication
methods within PPP.
Transport Level Security (TLS) provides for mutual authentication,
integrity-protected ciphersuite negotiation and key exchange between
two endpoints. This document describes how EAP-TLS, which includes
support for fragmentation and reassembly, provides for these TLS
mechanisms within EAP.
2. IntrodUCtion
The Extensible Authentication Protocol (EAP), described in [5],
provides a standard mechanism for support of additional
authentication methods within PPP. Through the use of EAP, support
for a number of authentication schemes may be added, including smart
cards, Kerberos, Public Key, One Time PassWords, and others. To date
however, EAP methods such as [6] have focussed on authenticating a
client to a server.
However, it may be desirable to support mutual authentication, and
since PPP encryption protocols such as [9] and [10] assume existence
of a session key, it is useful to have a mechanism for session key
establishment. Since design of secure key management protocols is
non-trivial, it is desirable to avoid creating new mechanisms for
this. The EAP protocol described in this document allows a PPP peer
to take advantage of the protected ciphersuite negotiation, mutual
authentication and key management capabilities of the TLS protocol,
described in [12].
2.1. Requirements language
In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
"recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
described in [11].
3. Protocol overview
3.1. Overview of the EAP-TLS conversation
As described in [5], the EAP-TLS conversation will typically begin
with the authenticator and the peer negotiating EAP. The
authenticator will then typically send an EAP-Request/Identity packet
to the peer, and the peer will respond with an EAP-Response/Identity
packet to the authenticator, containing the peer"s userId.
From this point forward, while nominally the EAP conversation occurs
between the PPP authenticator and the peer, the authenticator MAY act
as a passthrough device, with the EAP packets received from the peer
being encapsulated for transmission to a RADIUS server or backend
security server. In the discussion that follows, we will use the term
"EAP server" to denote the ultimate endpoint conversing with the
peer.
Once having received the peer"s Identity, the EAP server MUST respond
with an EAP-TLS/Start packet, which is an EAP-Request packet with
EAP-Type=EAP-TLS, the Start (S) bit set, and no data. The EAP-TLS
conversation will then begin, with the peer sending an EAP-Response
packet with EAP-Type=EAP-TLS. The data field of that packet will
encapsulate one or more TLS records in TLS record layer format,
containing a TLS client_hello handshake message. The current cipher
spec for the TLS records will be TLS_NULL_WITH_NULL_NULL and null
compression. This current cipher spec remains the same until the
change_cipher_spec message signals that subsequent records will have
the negotiated attributes for the remainder of the handshake.
The client_hello message contains the client"s TLS version number, a
sessionId, a random number, and a set of ciphersuites supported by
the client. The version offered by the client MUST correspond to TLS
v1.0 or later.
The EAP server will then respond with an EAP-Request packet with
EAP-Type=EAP-TLS. The data field of this packet will encapsulate one
or more TLS records. These will contain a TLS server_hello handshake
message, possibly followed by TLS certificate, server_key_exchange,
certificate_request, server_hello_done and/or finished handshake
messages, and/or a TLS change_cipher_spec message. The server_hello
handshake message contains a TLS version number, another random
number, a sessionId, and a ciphersuite. The version offered by the
server MUST correspond to TLS v1.0 or later.
If the client"s sessionId is null or unrecognized by the server, the
server MUST choose the sessionId to establish a new session;
otherwise, the sessionId will match that offered by the client,
indicating a resumption of the previously established session with
that sessionID. The server will also choose a ciphersuite from those
offered by the client; if the session matches the client"s, then the
ciphersuite MUST match the one negotiated during the handshake
protocol execution that established the session.
The purpose of the sessionId within the TLS protocol is to allow for
improved efficiency in the case where a client repeatedly attempts to
authenticate to an EAP server within a short period of time. While
this model was developed for use with HTTP authentication, it may
also have application to PPP authentication (e.g. multilink).
As a result, it is left up to the peer whether to attempt to continue
a previous session, thus shortening the TLS conversation. Typically
the peer"s decision will be made based on the time elapsed since the
previous authentication attempt to that EAP server. Based on the
sessionId chosen by the peer, and the time elapsed since the previous
authentication, the EAP server will decide whether to allow the
continuation, or whether to choose a new session.
In the case where the EAP server and authenticator reside on the same
device, then client will only be able to continue sessions when
connecting to the same NAS or tunnel server. Should these devices be
set up in a rotary or round-robin then it may not be possible for the
peer to know in advance the authenticator it will be connecting to,
and therefore which sessionId to attempt to reuse. As a result, it is
likely that the continuation attempt will fail. In the case where the
EAP authentication is remoted then continuation is much more likely
to be successful, since multiple NAS devices and tunnel servers will
remote their EAP authentications to the same RADIUS server.
If the EAP server is resuming a previously established session, then
it MUST include only a TLS change_cipher_spec message and a TLS
finished handshake message after the server_hello message. The
finished message contains the EAP server"s authentication response to
the peer. If the EAP server is not resuming a previously established
session, then it MUST include a TLS server_certificate handshake
message, and a server_hello_done handshake message MUST be the last
handshake message encapsulated in this EAP-Request packet.
The certificate message contains a public key certificate chain for
either a key exchange public key (such as an RSA or Diffie-Hellman
key exchange public key) or a signature public key (such as an RSA or
DSS signature public key). In the latter case, a TLS
server_key_exchange handshake message MUST also be included to allow
the key exchange to take place.
The certificate_request message is included when the server desires
the client to authenticate itself via public key. While the EAP
server SHOULD require client authentication, this is not a
requirement, since it may be possible that the server will require
that the peer authenticate via some other means.
The peer MUST respond to the EAP-Request with an EAP-Response packet
of EAP-Type=EAP-TLS. The data field of this packet will encapsulate
one or more TLS records containing a TLS change_cipher_spec message
and finished handshake message, and possibly certificate,
certificate_verify and/or client_key_exchange handshake messages. If
the preceding server_hello message sent by the EAP server in the
preceding EAP-Request packet indicated the resumption of a previous
session, then the peer MUST send only the change_cipher_spec and
finished handshake messages. The finished message contains the
peer"s authentication response to the EAP server.
If the preceding server_hello message sent by the EAP server in the
preceeding EAP-Request packet did not indicate the resumption of a
previous session, then the peer MUST send, in addition to the
change_cipher_spec and finished messages, a client_key_exchange
message, which completes the exchange of a shared master secret
between the peer and the EAP server. If the EAP server sent a
certificate_request message in the preceding EAP-Request packet, then
the peer MUST send, in addition, certificate and certificate_verify
handshake messages. The former contains a certificate for the peer"s
signature public key, while the latter contains the peer"s signed
authentication response to the EAP server. After receiving this
packet, the EAP server will verify the peer"s certificate and digital
signature, if requested.
If the peer"s authentication is unsuccessful, the EAP server SHOULD
send an EAP-Request packet with EAP-Type=EAP-TLS, encapsulating a TLS
record containing the appropriate TLS alert message. The EAP server
SHOULD send a TLS alert message rather immediately terminating the
conversation so as to allow the peer to inform the user of the cause
of the failure and possibly allow for a restart of the conversation.
To ensure that the peer receives the TLS alert message, the EAP
server MUST wait for the peer to reply with an EAP-Response packet.
The EAP-Response packet sent by the peer MAY encapsulate a TLS
client_hello handshake message, in which case the EAP server MAY
allow the EAP-TLS conversation to be restarted, or it MAY contain an
EAP-Response packet with EAP-Type=EAP-TLS and no data, in which case
the EAP-Server MUST send an EAP-Failure packet, and terminate the
conversation. It is up to the EAP server whether to allow restarts,
and if so, how many times the conversation can be restarted. An EAP
Server implementing restart capability SHOULD impose a limit on the
number of restarts, so as to protect against denial of service
attacks.
If the peers authenticates successfully, the EAP server MUST respond
with an EAP-Request packet with EAP-Type=EAP-TLS, which includes, in
the case of a new TLS session, one or more TLS records containing TLS
change_cipher_spec and finished handshke messages. The latter
contains the EAP server"s authentication response to the peer. The
peer will then verify the hash in order to authenticate the EAP
server.
If the EAP server authenticates unsuccessfully, the peer MAY send an
EAP-Response packet of EAP-Type=EAP-TLS containing a TLS Alert
message identifying the reason for the failed authentication. The
peer MAY send a TLS alert message rather than immediately terminating
the conversation so as to allow the EAP server to log the cause of
the error for examination by the system administrator.
To ensure that the EAP Server receives the TLS alert message, the
peer MUST wait for the EAP-Server to reply before terminating the
conversation. The EAP Server MUST reply with an EAP-Failure packet
since server authentication failure is a terminal condition.
If the EAP server authenticates successfully, the peer MUST send an
EAP-Response packet of EAP-Type=EAP-TLS, and no data. The EAP-Server
then MUST respond with an EAP-Success message.
3.2. Retry behavior
As with other EAP protocols, the EAP server is responsible for retry
behavior. This means that if the EAP server does not receive a reply
from the peer, it MUST resend the EAP-Request for which it has not
yet received an EAP-Response. However, the peer MUST NOT resend EAP-
Response packets without first being prompted by the EAP server.
For example, if the initial EAP-TLS start packet sent by the EAP
server were to be lost, then the peer would not receive this packet,
and would not respond to it. As a result, the EAP-TLS start packet
would be resent by the EAP server. Once the peer received the EAP-TLS
start packet, it would send an EAP-Response encapsulating the
client_hello message. If the EAP-Response were to be lost, then the
EAP server would resend the initial EAP-TLS start, and the peer would
resend the EAP-Response.
As a result, it is possible that a peer will receive duplicate EAP-
Request messages, and may send duplicate EAP-Responses. Both the
peer and the EAP-Server should be engineered to handle this
possibility.
3.3. Fragmentation
A single TLS record may be up to 16384 octets in length, but a TLS
message may span multiple TLS records, and a TLS certificate message
may in principle be as long as 16MB. The group of EAP-TLS messages
sent in a single round may thus be larger than the PPP MTU size, the
maximum RADIUS packet size of 4096 octets, or even the Multilink
Maximum Received Reconstructed Unit (MRRU). As described in [2], the
multilink MRRU is negotiated via the Multilink MRRU LCP option, which
includes an MRRU length field of two octets, and thus can support
MRRUs as large as 64 KB.
However, note that in order to protect against reassembly lockup and
denial of service attacks, it may be desirable for an implementation
to set a maximum size for one such group of TLS messages. Since a
typical certificate chain is rarely longer than a few thousand
octets, and no other field is likely to be anwhere near as long, a
reasonable choice of maximum acceptable message length might be 64
KB.
If this value is chosen, then fragmentation can be handled via the
multilink PPP fragmentation mechanisms described in [2]. While this
is desirable, there may be cases in which multilink or the MRRU LCP
option cannot be negotiated. As a result, an EAP-TLS implementation
MUST provide its own support for fragmentation and reassembly.
Since EAP is a simple ACK-NAK protocol, fragmentation support can be
added in a simple manner. In EAP, fragments that are lost or damaged
in transit will be retransmitted, and since sequencing information is
provided by the Identifier field in EAP, there is no need for a
fragment offset field as is provided in IPv4.
EAP-TLS fragmentation support is provided through addition of a flags
octet within the EAP-Response and EAP-Request packets, as well as a
TLS Message Length field of four octets. Flags include the Length
included (L), More fragments (M), and EAP-TLS Start (S) bits. The L
flag is set to indicate the presence of the four octet TLS Message
Length field, and MUST be set for the first fragment of a fragmented
TLS message or set of messages. The M flag is set on all but the last
fragment. The S flag is set only within the EAP-TLS start message
sent from the EAP server to the peer. The TLS Message Length field is
four octets, and provides the total length of the TLS message or set
of messages that is being fragmented; this simplifies buffer
allocation.
When an EAP-TLS peer receives an EAP-Request packet with the M bit
set, it MUST respond with an EAP-Response with EAP-Type=EAP-TLS and
no data. This serves as a fragment ACK. The EAP server MUST wait
until it receives the EAP-Response before sending another fragment.
In order to prevent errors in processing of fragments, the EAP server
MUST increment the Identifier field for each fragment contained
within an EAP-Request, and the peer MUST include this Identifier
value in the fragment ACK contained within the EAP-Reponse.
Retransmitted fragments will contain the same Identifier value.
Similarly, when the EAP server receives an EAP-Response with the M
bit set, it MUST respond with an EAP-Request with EAP-Type=EAP-TLS
and no data. This serves as a fragment ACK. The EAP peer MUST wait
until it receives the EAP-Request before sending another fragment.
In order to prevent errors in the processing of fragments, the EAP
server MUST use increment the Identifier value for each fragment ACK
contained within an EAP-Request, and the peer MUST include this
Identifier value in the subsequent fragment contained within an EAP-
Reponse.
3.4. Identity verification
As part of the TLS negotiation, the server presents a certificate to
the peer, and if mutual authentication is requested, the peer
presents a certificate to the server.
Note that since the peer has made a claim of identity in the EAP-
Response/Identity (MyID) packet, the EAP server SHOULD verify that
the claimed identity corresponds to the certificate presented by the
peer. Typically this will be accomplished either by placing the
userId within the peer certificate, or by providing a mapping between
the peer certificate and the userId using a Directory service.
Similarly, the peer MUST verify the validity of the EAP server
certificate, and SHOULD also examine the EAP server name presented in
the certificate, in order to determine whether the EAP server can be
trusted. Please note that in the case where the EAP authentication is
remoted that the EAP server will not reside on the same machine as
the authenticator, and therefore the name in the EAP server"s
certificate cannot be expected to match that of the intended
destination. In this case, a more appropriate test might be whether
the EAP server"s certificate is signed by a CA controlling the
intended destination and whether the EAP server exists within a
target sub-domain.
3.5. Key derivation
Since the normal TLS keys are used in the handshake, and therefore
should not be used in a different context, new encryption keys must
be derived from the TLS master secret for use with PPP encryption.
For both peer and EAP server, the derivation proceeds as follows:
given the master secret negotiated by the TLS handshake, the
pseudorandom function (PRF) defined in the specification for the
version of TLS in use, and the value random defined as the
concatenation of the handshake message fields client_hello.random and
server_hello.random (in that order), the value PRF(master secret,
"client EAP encryption", random) is computed up to 128 bytes, and the
value PRF("", "client EAP encryption", random) is computed up to 64
bytes (where "" is an empty string). The peer encryption key (the
one used for encrypting data from peer to EAP server) is oBTained by
truncating to the correct length the first 32 bytes of the first PRF
of these two output strings. TheEAP server encryption key (the one
used for encrypting data from EAP server to peer), if different from
the client encryption key, is obtained by truncating to the correct
length the second 32 bytes of this same PRF output string. The
client authentication key (the one used for computing MACs for
messages from peer to EAP server), if used, is obtained by truncating
to the correct length the third 32 bytes of this same PRF output
string. The EAP server authentication key (the one used for
computing MACs for messages from EAP server to peer), if used, and if
different from the peer authentication key, is obtained by truncating
to the correct length the fourth 32 bytes of this same PRF output
string. The peer initialization vector (IV), used for messages from
peer to EAP server if a block cipher has been specified, is obtained
by truncating to the cipher"s block size the first 32 bytes of the
second PRF output string mentioned above. Finally, the server
initialization vector (IV), used for messages from peer to EAP server
if a block cipher has been specified, is obtained by truncating to
the cipher"s block size the second 32 bytes of this second PRF
output.
The use of these encryption and authentication keys is specific to
the PPP encryption mechanism used, such as those defined in [9] and
[10]. Additional keys or other non-secret values (such as IVs) can
be obtained as needed for future PPP encryption methods by extending
the outputs of the PRF beyond 128 bytes and 64 bytes, respectively.
3.6. ECP negotiation
Since TLS supports ciphersuite negotiation, peers completing the TLS
negotiation will also have selected a ciphersuite, which includes key
strength, encryption and hashing methods. As a result, a subsequent
Encryption Control Protocol (ECP) conversation, if it occurs, has a
predetermined result.
In order to ensure agreement between the EAP-TLS ciphersuite
negotiation and the subsequent ECP negotiation (described in [6]),
during ECP negotiation the PPP peer MUST offer only the ciphersuite
negotiated inEAP-TLS. This ensures that the PPP authenticator MUST
accept the EAP-TLS negotiated ciphersuite in order for the
onversation to proceed. Should the authenticator not accept the
EAP-TLS negotiated ciphersuite, then the peer MUST send an LCP
terminate and disconnect.
Please note that it cannot be assumed that the PPP authenticator and
EAP server are located on the same machine or that the authenticator
understands the EAP-TLS conversation that has passed through it. Thus
if the peer offers a ciphersuite other than the one negotiated in
EAP-TLS there is no way for the authenticator to know how to respond
correctly.
3.7. CCP negotiation
TLS as described in [12] supports compression as well as ciphersuite
negotiation. However, TLS only provides support for a limited number
of compression types which do not overlap with the compression types
used in PPP. As a result, during the EAP-TLS conversation the EAP
endpoints MUST NOT request or negotiate compression. Instead, the PPP
Compression Control Protocol (CCP), described in [13] should be used
to negotiate the desired compression scheme.
3.8. Examples
In the case where the EAP-TLS mutual authentication is successful,
the conversation will appear as follows:
Authenticating Peer Authenticator
------------------- -------------
<- PPP LCP Request-EAP
auth
PPP LCP ACK-EAP
auth ->
<- PPP EAP-Request/
Identity
PPP EAP-Response/
Identity (MyID) ->
<- PPP EAP-Request/
EAP-Type=EAP-TLS
(TLS Start)
PPP EAP-Response/
EAP-Type=EAP-TLS
(TLS client_hello)->
<- PPP EAP-Request/
EAP-Type=EAP-TLS
(TLS server_hello,
TLS certificate,
[TLS server_key_exchange,]
[TLS certificate_request,]
TLS server_hello_done)
PPP EAP-Response/
EAP-Type=EAP-TLS
(TLS certificate,
TLS client_key_exchange,
[TLS certificate_verify,]
TLS change_cipher_spec,
TLS finished) ->
<- PPP EAP-Request/
EAP-Type=EAP-TLS
(TLS change_cipher_spec,
TLS finished)
PPP EAP-Response/
EAP-Type=EAP-TLS ->
<- PPP EAP-Success
PPP Authentication
Phase complete,
NCP Phase starts
ECP negotiation
CCP negotiation
In the case where the EAP-TLS mutual authentication is successful,
and fragmentation is required, the conversation will appear as
follows:
Authenticating Peer Authenticator
------------------- -------------
<- PPP LCP Request-EAP
auth
PPP LCP ACK-EAP
auth ->
<- PPP EAP-Request/
Identity
PPP EAP-Response/
Identity (MyID) ->
<- PPP EAP-Request/
EAP-Type=EAP-TLS
(TLS Start, S bit set)
PPP EAP-Response/
EAP-Type=EAP-TLS
(TLS client_hello)->
<- PPP EAP-Request/
EAP-Type=EAP-TLS
(TLS server_hello,
TLS certificate,
[TLS server_key_exchange,]
[TLS certificate_request,]
TLS server_hello_done)
(Fragment 1: L, M bits set)
PPP EAP-Response/
EAP-Type=EAP-TLS ->
<- PPP EAP-Request/
EAP-Type=EAP-TLS
(Fragment 2: M bit set)
PPP EAP-Response/
EAP-Type=EAP-TLS ->
<- PPP EAP-Request/
EAP-Type=EAP-TLS
(Fragment 3)
PPP EAP-Response/
EAP-Type=EAP-TLS
(TLS certificate,
TLS client_key_exchange,
[TLS certificate_verify,]
TLS change_cipher_spec,
TLS inished)(Fragment 1:
L, M bits set)->
<- PPP EAP-Request/
EAP-Type=EAP-TLS
PPP EAP-Response/
EAP-Type=EAP-TLS
(Fragment 2)->
<- PPP EAP-Request/
EAP-Type=EAP-TLS
(TLS change_cipher_spec,
TLS finished)
PPP EAP-Response/
EAP-Type=EAP-TLS ->
<- PPP EAP-Success
PPP Authentication
Phase complete,
NCP Phase starts
ECP negotiation
CCP negotiation
In the case where the server authenticates to the client
successfully, but the client fails to authenticate to the server, the
conversation will appear as follows:
Authenticating Peer Authenticator
------------------- -------------
<- PPP LCP Request-EAP
auth
PPP LCP ACK-EAP
auth ->
<- PPP EAP-Request/
Identity
PPP EAP-Response/
Identity (MyID) ->
<- PPP EAP-Request/
EAP-Type=EAP-TLS
(TLS Start)
PPP EAP-Response/
EAP-Type=EAP-TLS
(TLS client_hello)->
<- PPP EAP-Request/
EAP-Type=EAP-TLS
(TLS server_hello,
TLS certificate,
[TLS server_key_exchange,]
TLS certificate_request,
TLS server_hello_done)
PPP EAP-Response/
EAP-Type=EAP-TLS
(TLS certificate,
TLS client_key_exchange,
TLS certificate_verify,
TLS change_cipher_spec,
TLS finished) ->
<- PPP EAP-Request/
EAP-Type=EAP-TLS
(TLS change_cipher_spec,
TLS finished)
PPP EAP-Response/
EAP-Type=EAP-TLS ->
<- PPP EAP-Request
EAP-Type=EAP-TLS
(TLS Alert message)
PPP EAP-Response/
EAP-Type=EAP-TLS ->
<- PPP EAP-Failure
(User Disconnected)
In the case where server authentication is unsuccessful, the
conversation will appear as follows:
Authenticating Peer Authenticator
------------------- -------------
<- PPP LCP Request-EAP
auth
PPP LCP ACK-EAP
auth ->
<- PPP EAP-Request/
Identity
PPP EAP-Response/
Identity (MyID) ->
<- PPP EAP-Request/
EAP-Type=EAP-TLS
(TLS Start)
PPP EAP-Response/
EAP-Type=EAP-TLS
(TLS client_hello)->
<- PPP EAP-Request/
EAP-Type=EAP-TLS
(TLS server_hello,
TLS certificate,
[TLS server_key_exchange,]
[TLS certificate_request,]
TLS server_hello_done)
PPP EAP-Response/
EAP-Type=EAP-TLS
(TLS certificate,
TLS client_key_exchange,
[TLS certificate_verify,]
TLS change_cipher_spec,
TLS finished) ->
<- PPP EAP-Request/
EAP-Type=EAP-TLS
(TLS change_cipher_spec,
TLS finished)
PPP EAP-Response/
EAP-Type=EAP-TLS
(TLS change_cipher_spec,
TLS finished)
<- PPP EAP-Request/
EAP-Type=EAP-TLS
PPP EAP-Response/
EAP-Type=EAP-TLS
(TLS Alert message) ->
<- PPP EAP-Failure
(User Disconnected)
In the case where a previously established session is being resumed,
and both sides authenticate successfully, the conversation will
appear as follows:
Authenticating Peer Authenticator
------------------- -------------
<- PPP LCP Request-EAP
auth
PPP LCP ACK-EAP
auth ->
<- PPP EAP-Request/
Identity
PPP EAP-Response/
Identity (MyID) ->
<- PPP EAP-Request/
EAP-Request/
EAP-Type=EAP-TLS
(TLS Start)
PPP EAP-Response/
EAP-Type=EAP-TLS
(TLS client_hello)->
<- PPP EAP-Request/
EAP-Type=EAP-TLS
(TLS server_hello,
TLS change_cipher_spec
TLS finished)
PPP EAP-Response/
EAP-Type=EAP-TLS
(TLS change_cipher_spec,
TLS finished) ->
<- PPP EAP-Success
PPP Authentication
Phase complete,
NCP Phase starts
ECP negotiation
CCP negotiation
In the case where a previously established session is being resumed,
and the server authenticates to the client successfully but the
client fails to authenticate to the server, the conversation will
appear as follows:
Authenticating Peer Authenticator
------------------- -------------
<- PPP LCP Request-EAP
auth
PPP LCP ACK-EAP
auth ->
<- PPP EAP-Request/
Identity
PPP EAP-Response/
Identity (MyID) ->
<- PPP EAP-Request/
EAP-Request/
EAP-Type=EAP-TLS
(TLS Start)
PPP EAP-Response/
EAP-Type=EAP-TLS
(TLS client_hello) ->
<- PPP EAP-Request/
EAP-Type=EAP-TLS
(TLS server_hello,
TLS change_cipher_spec,
TLS finished)
PPP EA-Response/
EAP-Type=EAP-TLS
(TLS change_cipher_spec,
TLS finished) ->
<- PPP EAP-Request
EAP-Type=EAP-TLS
(TLS Alert message)
PPP EAP-Response
EAP-Type=EAP-TLS ->
<- PPP EAP-Failure
(User Disconnected)
In the case where a previously established session is being resumed,
and the server authentication is unsuccessful, the conversation will
appear as follows:
Authenticating Peer Authenticator
------------------- -------------
<- PPP LCP Request-EAP
auth
PPP LCP ACK-EAP
auth ->
<- PPP EAP-Request/
Identity
PPP EAP-Response/
Identity (MyID) ->
<- PPP EAP-Request/
EAP-Request/
EAP-Type=EAP-TLS
(TLS Start)
PPP EAP-Response/
EAP-Type=EAP-TLS
(TLS client_hello)->
<- PPP EAP-Request/
EAP-Type=EAP-TLS
(TLS server_hello,
TLS change_cipher_spec,
TLS finished)
PPP EAP-Response/
EAP-Type=EAP-TLS
(TLS change_cipher_spec,
TLS finished)
<- PPP EAP-Request/
EAP-Type=EAP-TLS
PPP EAP-Response/
EAP-Type=EAP-TLS
(TLS Alert message) ->
<- PPP EAP-Failure
(User Disconnected)
4. Detailed description of the EAP-TLS protocol
4.1. PPP EAP TLS Packet Format
A summary of the PPP EAP TLS Request/Response packet format is shown
below. The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code Identifier Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type Data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
1 - Request
2 - Response
Identifier
The identifier field is one octet and aids in matching responses
with requests.
Length
The Length field is two octets and indicates the length of the EAP
packet including the Code, Identifier, Length, Type, and Data
fields. Octets outside the range of the Length field should be
treated as Data Link Layer padding and should be ignored on
reception.
Type
13 - EAP TLS
Data
The format of the Data field is determined by the Code field.
4.2. PPP EAP TLS Request Packet
A summary of the PPP EAP TLS Request packet format is shown below.
The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code Identifier Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type Flags TLS Message Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TLS Message Length TLS Data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
1
Identifier
The Identifier field is one octet and aids in matching responses
with requests. The Identifier field MUST be changed on each
Request packet.
Length
The Length field is two octets and indicates the length of the EAP
packet including the Code, Identifier, Length, Type, and TLS
Response fields.
Type
13 - EAP TLS
Flags
0 1 2 3 4 5 6 7 8
+-+-+-+-+-+-+-+-+
L M S R R R R R
+-+-+-+-+-+-+-+-+
L = Length included
M = More fragments
S = EAP-TLS start
R = Reserved
The L bit (length included) is set to indicate the presence of the
four octet TLS Message Length field, and MUST be set for the first
fragment of a fragmented TLS message or set of messages. The M bit
(more fragments) is set on all but the last fragment. The S bit
(EAP-TLS start) is set in an EAP-TLS Start message. This
differentiates the EAP-TLS Start message from a fragment
acknowledgement.
TLS Message Length
The TLS Message Length field is four octets, and is present only
if the L bit is set. This field provides the total length of the
TLS message or set of messages that is being fragmented.
TLS data
The TLS data consists of the encapsulated TLS packet in TLS record
format.
4.3. PPP EAP TLS Response Packet
A summary of the PPP EAP TLS Response packet format is shown below.
The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code Identifier Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type Flags TLS Message Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TLS Message Length TLS Data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
2
Identifier
The Identifier field is one octet and MUST match the Identifier
field from the corresponding request.
Length
The Length field is two octets and indicates the length of the EAP
packet including the Code, Identifir, Length, Type, and TLS data
fields.
Type
13 - EAP TLS
Flags
0 1 2 3 4 5 6 7 8
+-+-+-+-+-+-+-+-+
L M S R R R R R
+-+-+-+-+-+-+-+-+
L = Length included
M = More fragments
S = EAP-TLS start
R = Reserved
The L bit (length included) is set to indicate the presence of the
four octet TLS Message Length field, and MUST be set for the first
fragment of a fragmented TLS message or set of messages. The M bit
(more fragments) is set on all but the last fragment. The S bit
(EAP-TLS start) is set in an EAP-TLS Start message. This
differentiates the EAP-TLS Start message from a fragment
acknowledgement.
TLS Message Length
The TLS Message Length field is four octets, and is present only
if the L bit is set. This field provides the total length of the
TLS message or set of messages that is being fragmented.
TLS data
The TLS data consists of the encapsulated TLS packet in TLS record
format.
5. References
[1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD
51, RFC1661, July 1994.
[2] Sklower, K., Lloyd, B., McGregor, G., Carr, D. and T. Coradetti,
"The PPP Multilink Protocol (MP)", RFC1990, August 1996.
[3] Simpson, W., Editor, "PPP LCP Extensions", RFC1570, 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)", RFC2284, March 1998.
[6] Meyer, G., "The PPP Encryption Protocol (ECP)", RFC1968, 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)", RFC2419, September 1998.
[10] Hummert, K., "The PPP Triple-DES Encryption Protocol (3DESE)",
RFC2420, September 1998.
[11] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, 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", RFC1962, June
1996.
6. Security Considerations
6.1. Certificate revocation
Since the EAP server is on the Internet during the EAP conversation,
the server is capable of following a certificate chain or verifying
whether the peer"s certificate has been revoked. In contrast, the
peer may or may not have Internet connectivity, and thus while it can
validate the EAP server"s certificate based on a pre-configured set
of CAs, it may not be able to follow a certificate chain or verify
whether the EAP server"s certificate has been revoked.
In the case where the peer is initiating a voluntary Layer 2 tunnel
using PPTP or L2TP, the peer will typically already have a PPP
interface and Internet connectivity established at the time of tunnel
initiation. As a result, during the EAP conversation it is capable
of checking for certificate revocation.
However, in the case where the peer is initiating an intial PPP
conversation, it will not have Internet connectivity and is therefore
not capable of checking for certificate revocation until after NCP
negotiation completes and the peer has Access to the Internet. In
this case, the peer SHOULD check for certificate revocation after
connecting to the Internet.
6.2. Separation of the EAP server and PPP authenticator
As a result of the EAP-TLS conversation, the EAP endpoints will
mutually authenticate, negotiate a ciphersuite, and derive a session
key for subsequent use in PPP encryption. Since the peer and EAP
client reside on the same machine, it is necessary for the EAP client
module to pass the session key to the PPP encryption module.
The situation may be more complex on the PPP authenticator, which may
or may not reside on the same machine as the EAP server. In the case
where the EAP server and PPP authenticator reside on different
machines, there are several implications for security. Firstly, the
mutual authentication defined in EAP-TLS will occur between the peer
and the EAP server, not between the peer and the authenticator. This
means that as a result of the EAP-TLS conversation, it is not
possible for the peer to validate the identity of the NAS or tunnel
server that it is speaking to.
The second issue is that the session key negotiated between the peer
and EAP server will need to be transmitted to the authenticator.
Therefore a mechanism needs to be provided to transmit the session
key from the EAP server to the authenticator or tunnel server that
needs to use the key. The specification of this transit mechanism is
outside the scope of this document.
6.3. Relationship of PPP encryption to other security mechanisms
It is envisaged that EAP-TLS will be used primarily with dialup PPP
connections. However, there are also circumstances in which PPP
encryption may be used along with Layer 2 tunneling protocols such as
PPTP and L2TP.
In compulsory layer 2 tunneling, a PPP peer makes a connection to a
NAS or router which tunnels the PPP packets to a tunnel server.
Since with compulsory tunneling a PPP peer cannot tell whether its
packets are being tunneled, let alone whether the network device is
securing the tunnel, if security is required then the client must
make its own arrangements. In the case where all endpoints cannot be
relied upon to implement IPSEC, TLS, or another suitable security
protocol, PPP encryption provides a convenient means to ensure the
privacy of packets transiting between the client and the tunnel
server.
7. Acknowledgments
Thanks to Terence Spies, Glen Zorn and Narendra Gidwani of Microsoft
for useful discussions of this problem space.
8. Authors" Addresses
Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
Phone: 425-936-6605
EMail: bernarda@microsoft.com
Dan Simon
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
Phone: 425-936-6711
EMail: dansimon@microsoft.com
9. Full Copyright Statement
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