PPP Extensions Working Group Bernard Aboba
INTERNET-DRAFT Microsoft
Category: Informational Dan Simon
<draft-ietf-pppext-eaptls-03.txt> Microsoft
21 April 1998
PPP EAP TLS Authentication Protocol
1. Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working docu-
ments of the Internet Engineering Task Force (IETF), its areas, and
its working groups. Note that other groups may also distribute work-
ing documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference mate-
rial or to cite them other than as ``work in progress.''
To view the entire list of current Internet-Drafts, please check
the "1id-abstracts.txt" listing contained in the Internet-Drafts
Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net
(Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au
(Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu
(US West Coast).
The distribution of this memo is unlimited. It is filed as <draft-
ietf-pppext-eaptls-03.txt>, and expires November 1, 1998. Please send
comments to the authors.
2. Abstract
The Point-to-Point Protocol (PPP) provides a standard method for
transporting multi-protocol datagrams over point-to-point links. PPP
also defines an extensible Link Control Protocol (LCP), which can be
used to negotiate authentication methods, as well as an Encryption
Control Protocol (ECP), used to negotiate data encryption over PPP
links, and a Compression Control Protocol (CCP), used to negotiate
compression methods. The Extensible Authentication Protocol (EAP) is
a PPP extension that provides support for additional authentication
methods within PPP.
Transport Level Security (TLS) provides for mutual authentication,
integrity-protected ciphersuite negotiation and key exchange between
two endpoints. This document describes how EAP-TLS, which includes
support for fragmentation and reassembly, provides for these TLS mech-
anisms within EAP.
Aboba & Simon [Page 1]
INTERNET-DRAFT 21 April 1998
3. Introduction
The Extensible Authentication Protocol (EAP), described in [5], pro-
vides a standard mechanism for support of additional authentication
methods within PPP. Through the use of EAP, support for a number of
authentication schemes may be added, including smart cards, Kerberos,
Public Key, One Time Passwords, and others. To date however, EAP meth-
ods such as [6] have focussed on authenticating a client to a server.
However, it may be desirable to support mutual authentication, and
since PPP encryption protocols such as [10] and [11] assume existence
of a session key, it is useful to have a mechanism for session key
establishment. Since design of secure key management protocols is non-
trivial, it is desirable to avoid creating new mechanisms for this.
The EAP protocol described in this document allows a PPP peer to take
advantage of the protected ciphersuite negotiation, mutual authentica-
tion and key management capabilities of the TLS protocol, described in
[13].
3.1. Requirements language
This specification uses the same words as [12] for defining the sig-
nificance of each particular requirement. These words are:
MUST This word, or the adjectives "REQUIRED" or "SHALL", means
that the definition is an absolute requirement of the speci-
fication.
MUST NOT This phrase, or the phrase "SHALL NOT", means that the defi-
nition is an absolute prohibition of the specification.
SHOULD This word, or the adjective "RECOMMENDED", means that there
may exist valid reasons in particular circumstances to
ignore a particular item, but the full implications must be
understood and carefully weighed before choosing a different
course.
SHOULD NOT
This phrase means that there may exist valid reasons in par-
ticular circumstances when the particular behavior is
acceptable or even useful, but the full implications should
be understood and the case carefully weighed before imple-
menting any behavior described with this label.
MAY This word, or the adjective "", means that an item is truly
optional. One vendor may choose to include the item because
a particular marketplace requires it or because the vendor
feels that it enhances the product while another vendor may
omit the same item. An implementation which does not
include a particular option MUST be prepared to interoperate
with another implementation which does include the option,
though perhaps with reduced functionality. In the same vein
Aboba & Simon [Page 2]
INTERNET-DRAFT 21 April 1998
an implementation which does include a particular option
MUST be prepared to interoperate with another implementation
which does not include the option.(except, of course, for
the feature the option provides)
An implementation is not compliant if it fails to satisfy one or more
of the must or must not requirements for the protocols it implements.
An implementation that satisfies all the must, must not, should and
should not requirements for its protocols is said to be "uncondition-
ally compliant"; one that satisfies all the must and must not require-
ments but not all the should or should not requirements for its proto-
cols is said to be "conditionally compliant."
4. Protocol overview
4.1. Overview of the EAP-TLS conversation
As described in [5] and [15], 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, as described in [15] 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 con-
versing with the peer.
Once having received the peer's Identity, the EAP server MUST respond
with an EAP-TLS/Start packet, which is an EAP-Request packet with EAP-
Type=EAP-TLS, the Start (S) bit set, and no data. The EAP-TLS conver-
sation will then begin, with the peer sending an EAP-Response packet
with EAP-Type=EAP-TLS. The data field of that packet will encapsulate
one or more TLS records in TLS record layer format, containing a TLS
client_hello handshake message. The current cipher spec for the TLS
records will be TLS_NULL_WITH_NULL_NULL and null compression. This
current cipher spec remains the same until the change_cipher_spec mes-
sage signals that subsequent records will have the negotiated
attributes for the remainder of the handshake.
The client_hello message contains the client's TLS version number, a
sessionId, a random number, and a set of ciphersuites supported by the
client. The version offered by the client MUST correspond to TLS v1.0
or later.
The EAP server will then respond with an EAP-Request packet with EAP-
Type=EAP-TLS. The data field of this packet will encapsulate one or
more TLS records. These will contain a TLS server_hello handshake
Aboba & Simon [Page 3]
INTERNET-DRAFT 21 April 1998
message, possibly followed by TLS certificate, server_key_exchange,
certificate_request, server_hello_done and/or finished handshake mes-
sages, and/or a TLS change_cipher_spec message. The server_hello
handshake message contains a TLS version number, another random num-
ber, a sessionId, and a ciphersuite. The version offered by the
server MUST correspond to TLS v1.0 or later.
If the client's sessionId is null or unrecognized by the server, the
server MUST choose the sessionId to establish a new session; other-
wise, the sessionId will match that offered by the client, indi-
cating a resumption of the previously established session with that
sessionID. The server will also choose a ciphersuite from those
offered by the client; if the session matches the client's, then the
ciphersuite MUST match the one negotiated during the handshake proto-
col execution that established the session.
The purpose of the sessionId within the TLS protocol is to allow for
improved efficiency in the case where a client repeatedly attempts to
authenticate to an EAP server within a short period of time. While
this model was developed for use with HTTP authentication, it may also
have application to PPP authentication (e.g. multilink).
As a result, it is left up to the peer whether to attempt to continue
a previous session, thus shortening the TLS conversation. Typically
the peer's decision will be made based on the time elapsed since the
previous authentication attempt to that EAP server. Based on the ses-
sionId chosen by the peer, and the time elapsed since the previous
authentication, the EAP server will decide whether to allow the con-
tinuation, or whether to choose a new session.
In the case where the EAP server and authenticator reside on the same
device, then client will only be able to continue sessions when con-
necting to the same NAS or tunnel server. Should these devices be set
up in a rotary or round-robin then it may not be possible for the peer
to know in advance the authenticator it will be connecting to, and
therefore which sessionId to attempt to reuse. As a result, it is
likely that the continuation attempt will fail. In the case where the
EAP authentication is remoted then continuation is much more likely to
be successful, since multiple NAS devices and tunnel servers will
remote their EAP authentications to the same RADIUS server.
If the EAP server is resuming a previously established session, then
it MUST include only a TLS change_cipher_spec message and a TLS fin-
ished handshake message after the server_hello message. The finished
message contains the EAP server's authentication response to the peer.
If the EAP server is not resuming a previously established session,
then it MUST include a TLS server_certificate handshake message, and a
server_hello_done handshake message MUST be the last handshake message
encapsulated in this EAP-Request packet.
The certificate message contains a public key certificate chain for
either a key exchange public key (such as an RSA or Diffie-Hellman key
exchange public key) or a signature public key (such as an RSA or DSS
signature public key). In the latter case, a TLS server_key_exchange
Aboba & Simon [Page 4]
INTERNET-DRAFT 21 April 1998
handshake message MUST also be included to allow the key exchange to
take place.
The certificate_request message is included when the server desires
the client to authenticate itself via public key. While the EAP server
SHOULD require client authentication, this is not a requirement, since
it may be possible that the server will require that the peer authen-
ticate via some other means.
The peer MUST respond to the EAP-Request with an EAP-Response packet
of EAP-Type=EAP-TLS. The data field of this packet will encapsulate
one or more TLS records containing a TLS change_cipher_spec message
and finished handshake message, and possibly certificate, certifi-
cate_verify and/or client_key_exchange handshake messages. If the
preceding server_hello message sent by the EAP server in the preceding
EAP-Request packet indicated the resumption of a previous session,
then the peer MUST send only the change_cipher_spec and finished hand-
shake messages. The finished message contains the peer's authentica-
tion response to the EAP server.
If the preceding server_hello message sent by the EAP server in the
preceeding EAP-Request packet did not indicate the resumption of a
previous session, then the peer MUST send, in addition to the
change_cipher_spec and finished messages, a client_key_exchange mes-
sage, which completes the exchange of a shared master secret between
the peer and the EAP server. If the EAP server sent a certifi-
cate_request message in the preceding EAP-Request packet, then the
peer MUST send, in addition, certificate and certificate_verify hand-
shake messages. The former contains a certificate for the peer's sig-
nature public key, while the latter contains the peer's signed authen-
tication response to the EAP server. After receiving this packet, the
EAP server will verify the peer's certificate and digital signature,
if requested.
If the peer's authentication is unsuccessful, the EAP server SHOULD
send an EAP-Request packet with EAP-Type=EAP-TLS, encapsulating a TLS
record containing the appropriate TLS alert message. The EAP server
SHOULD send a TLS alert message rather immediately terminating the
conversation so as to allow the peer to inform the user of the cause
of the failure and possibly allow for a restart of the conversation.
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.
Aboba & Simon [Page 5]
INTERNET-DRAFT 21 April 1998
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 handshake messages. The latter con-
tains the EAP server's authentication response to the peer. The peer
will then verify the hash in order to authenticate the EAP server.
If the EAP server authenticates unsuccessfully, the peer MAY send an
EAP-Response packet of EAP-Type=EAP-TLS containing a TLS Alert message
identifying the reason for the failed authentication. The peer MAY
send a TLS alert message rather than immediately terminating the con-
versation so as to allow the EAP server to log the cause of the error
for examination by the system administrator.
To ensure that the EAP Server receives the TLS alert message, the peer
MUST wait for the EAP-Server to reply before terminating the conversa-
tion. The EAP Server MUST reply with an EAP-Failure packet since
server authentication failure is a terminal condition.
If the EAP server authenticates successfully, the peer MUST send an
EAP-Response packet of EAP-Type=EAP-TLS, and no data. The EAP-Server
then MUST respond with an EAP-Success message.
4.2. Retry behavior
As with other EAP protocols, the EAP server is responsible for retry
behavior. This means that if the EAP server does not receive a reply
from the peer, it MUST resend the EAP-Request for which it has not yet
received an EAP-Response. However, the peer MUST NOT resend EAP-
Response packets without first being prompted by the EAP server.
For example, if the initial EAP-TLS start packet sent by the EAP
server were to be lost, then the peer would not receive this packet,
and would not respond to it. As a result, the EAP-TLS start packet
would be resent by the EAP server. Once the peer received the EAP-TLS
start packet, it would send an EAP-Response encapsulating the
client_hello message. If the EAP-Response were to be lost, then the
EAP server would resend the initial EAP-TLS start, and the peer would
resend the EAP-Response.
As a result, it is possible that a peer will receive duplicate EAP-
Request messages, and may send duplicate EAP-Responses. Both the peer
and the EAP-Server should be engineered to handle this possibility.
4.3. Fragmentation
A single TLS record may be up to 16384 octets in length, but a TLS
message may span multiple TLS records, and a TLS certificate message
may in principle be as long as 16MB. The group of EAP-TLS messages
sent in a single round may thus be larger than the PPP MTU size, the
maximum RADIUS packet size of 4096 octets, or even the Multilink Maxi-
mum Received Reconstructed Unit (MRRU). As described in [16], the
Aboba & Simon [Page 6]
INTERNET-DRAFT 21 April 1998
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 typ-
ical certificate chain is rarely longer than a few thousand octets,
and no other field is likely to be anwhere near as long, a reasonable
choice of maximum acceptable message length might be 64 KB.
If this value is chosen, then fragmentation can be handled via the
multilink PPP fragmentation mechanisms described in [16]. While this
is desirable, there may be cases in which multilink or the MRRU LCP
option cannot be negotiated. As a result, an EAP-TLS implementation
MUST provide its own support for fragmentation and reassembly.
Since EAP is a simple ACK-NAK protocol, fragmentation support can be
added in a simple manner. In EAP, fragments that are lost or damaged
in transit will be retransmitted, and since sequencing information is
provided by the Identifier field in EAP, there is no need for a frag-
ment offset field as is provided in IPv4.
EAP-TLS fragmentation support is provided through addition of a flags
octet within the EAP-Response and EAP-Request packets, as well as a
TLS Message Length field of four octets. Flags include the Length
included (L), More fragments (M), and EAP-TLS Start (S) bits. The L
flag is set to indicate the presence of the four octet TLS Message
Length field, and MUST be set for the first fragment of a fragmented
TLS message or set of messages. The M flag is set on all but the last
fragment. The S flag is set only within the EAP-TLS start message sent
from the EAP server to the peer. The TLS Message Length field is four
octets, and provides the total length of the TLS message or set of
messages that is being fragmented; this simplifies buffer allocation.
When an EAP-TLS peer receives an EAP-Request packet with the M bit
set, it MUST respond with an EAP-Response with EAP-Type=EAP-TLS and no
data. This serves as a fragment ACK. The EAP server MUST wait until
it receives the EAP-Response before sending another fragment. In order
to prevent errors in processing of fragments, the EAP server MUST
increment the Identifier field for each fragment contained within an
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.
Aboba & Simon [Page 7]
INTERNET-DRAFT 21 April 1998
4.4. Identity verification
As part of the TLS negotiation, the server presents a certificate to
the peer, and if mutual authentication is requested, the peer presents
a certificate to the server.
Note that since the peer has made a claim of identity in the EAP-
Response/Identity (MyID) packet, the EAP server SHOULD verify that the
claimed identity corresponds to the certificate presented by the peer.
Typically this will be accomplished either by placing the userId
within the peer certificate, or by providing a mapping between the
peer certificate and the userId using a directory service.
Similarly, the peer MUST verify the validity of the EAP server cer-
tificate, and SHOULD also examine the EAP server name presented in the
certificate, in order to determine whether the EAP server can be
trusted. Please note that in the case where the EAP authentication is
remoted that the EAP server will not reside on the same machine as the
authenticator, and therefore the name in the EAP server's certificate
cannot be expected to match that of the intended destination. In this
case, a more appropriate test might be whether the EAP server's cer-
tificate is signed by a CA controlling the intended destination and
whether the EAP server exists within a target sub-domain.
4.5. Key derivation
Since the normal TLS keys are used in the handshake, and therefore
should not be used in a different context, new encryption keys must be
derived from the TLS master secret for use with PPP encryption. For
both peer and EAP server, the derivation proceeds as follows: given
the master secret negotiated by the TLS handshake, the pseudorandom
function (PRF) defined in the specification for the version of TLS in
use, and the value random defined as the concatenation of the hand-
shake message fields client_hello.random and server_hello.random (in
that order), the value PRF(master secret, "client EAP encryption",
random) is computed up to 128 bytes, and the value PRF("", "client EAP
encryption", random) is computed up to 64 bytes (where "" is an empty
string). The peer encryption key (the one used for encrypting data
from peer to EAP server) is obtained by truncating to the correct
length the first 32 bytes of the first PRF of these two output
strings. The EAP server encryption key (the one used for encrypting
data from EAP server to peer), if different from the client encryption
key, is obtained by truncating to the correct length the second 32
bytes of this same PRF output string. The client authentication key
(the one used for computing MACs for messages from peer to EAP
server), if used, is obtained by truncating to the correct length the
third 32 bytes of this same PRF output string. The EAP server authen-
tication key (the one used for computing MACs for messages from EAP
server to peer), if used, and if different from the peer authentica-
tion key, is obtained by truncating to the correct length the fourth
32 bytes of this same PRF output string. The peer initialization vec-
tor (IV), used for messages from peer to EAP server if a block cipher
has been specified, is obtained by truncating to the cipher's block
Aboba & Simon [Page 8]
INTERNET-DRAFT 21 April 1998
size the first 32 bytes of the second PRF output string mentioned
above. Finally, the server initialization vector (IV), used for mes-
sages from peer to EAP server if a block cipher has been specified, is
obtained by truncating to the cipher's block size the second 32 bytes
of this second PRF output.
The use of these encryption and authentication keys is specific to the
PPP encryption mechanism used, such as those defined in [10] and [11].
Additional keys or other non-secret values (such as IVs) can be
obtained as needed for future PPP encryption methods by extending the
outputs of the PRF beyond 128 bytes and 64 bytes, respectively.
4.6. ECP negotiation
Since TLS supports ciphersuite negotiation, peers completing the TLS
negotiation will also have selected a ciphersuite, which includes key
strength, encryption and hashing methods. As a result, a subsequent
Encryption Control Protocol (ECP) conversation, if it occurs, has a
predetermined result.
In order to ensure agreement between the EAP-TLS ciphersuite negotia-
tion and the subsequent ECP negotiation (described in [7]), during ECP
negotiation the PPP peer MUST offer only the ciphersuite negotiated in
EAP-TLS. This ensures that the PPP authenticator MUST accept the EAP-
TLS negotiated ciphersuite in order for the onversation to proceed.
Should the authenticator not accept the EAP-TLS negotiated cipher-
suite, then the peer MUST send an LCP terminate and disconnect.
Please note that as described in [15], 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.
4.7. CCP negotiation
TLS as described in [13] supports compression as well as ciphersuite
negotiation. However, TLS only provides support for a limited number
of compression types which do not overlap with the compression types
used in PPP. As a result, during the EAP-TLS conversation the EAP end-
points MUST NOT request or negotiate compression. Instead, the PPP
Compression Control Protocol (CCP), described in [14] should be used
to negotiate the desired compression scheme.
4.8. Examples
In the case where the EAP-TLS mutual authentication is successful, the
conversation will appear as follows:
Authenticating Peer Authenticator
Aboba & Simon [Page 9]
INTERNET-DRAFT 21 April 1998
------------------- -------------
<- 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/
Aboba & Simon [Page 10]
INTERNET-DRAFT 21 April 1998
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 finished)(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
Aboba & Simon [Page 11]
INTERNET-DRAFT 21 April 1998
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 Alert message)
PPP EAP-Response/
EAP-Type=EAP-TLS ->
<- PPP EAP-Failure
(User Disconnected)
In the case where server authentication is unsuccessful, the conversa-
tion will appear as follows:
Authenticating Peer Authenticator
------------------- -------------
<- PPP LCP Request-EAP
auth
PPP LCP ACK-EAP
auth ->
<- PPP EAP-Request/
Identity
PPP EAP-Response/
Aboba & Simon [Page 12]
INTERNET-DRAFT 21 April 1998
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 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
Aboba & Simon [Page 13]
INTERNET-DRAFT 21 April 1998
(TLS server_hello,
TLS change_cipher_spec,
TLS finished)
PPP EAP-Response/
EAP-Type=EAP-TLS
(TLS change_cipher_spec,
TLS finished) ->
<- PPP EAP-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)
Aboba & Simon [Page 14]
INTERNET-DRAFT 21 April 1998
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 Alert message) ->
<- PPP EAP-Failure
(User Disconnected)
5. Detailed description of the EAP-TLS protocol
5.1. PPP EAP TLS Packet Format
A summary of the PPP EAP TLS Request/Response packet format is shown
below. The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
1 - Request
2 - Response
Aboba & Simon [Page 15]
INTERNET-DRAFT 21 April 1998
Identifier
The identifier field is one octet and aids in matching responses
with requests.
Length
The Length field is two octets and indicates the length of the EAP
packet including the Code, Identifier, Length, Type, and Data
fields. Octets outside the range of the Length field should be
treated as Data Link Layer padding and should be ignored on recep-
tion.
Type
13 - EAP TLS
Data
The format of the Data field is determined by the Code field.
5.2. PPP EAP TLS Request Packet
A summary of the PPP EAP TLS Request packet format is shown below.
The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Flags | TLS Message Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLS Message Length | TLS Data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
1
Identifier
The Identifier field is one octet and aids in matching responses
with requests. The Identifier field MUST be changed on each
Request packet.
Length
The Length field is two octets and indicates the length of the EAP
packet including the Code, Identifier, Length, Type, and TLS
Response fields.
Type
Aboba & Simon [Page 16]
INTERNET-DRAFT 21 April 1998
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 differen-
tiates the EAP-TLS Start message from a fragment acknowledgement.
TLS Message Length
The TLS Message Length field is four octets, and is present only if
the L bit is set. This field provides the total length of the TLS
message or set of messages that is being fragmented.
TLS data
The TLS data consists of the encapsulated TLS packet in TLS record
format.
5.3. PPP EAP TLS Response Packet
A summary of the PPP EAP TLS Response packet format is shown below.
The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Flags | TLS Message Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLS Message Length | TLS Data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
2
Identifier
Aboba & Simon [Page 17]
INTERNET-DRAFT 21 April 1998
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, Identifier, 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 differen-
tiates the EAP-TLS Start message from a fragment acknowledgement.
TLS Message Length
The TLS Message Length field is four octets, and is present only if
the L bit is set. This field provides the total length of the TLS
message or set of messages that is being fragmented.
TLS data
The TLS data consists of the encapsulated TLS packet in TLS record
format.
6. Security issues
6.1. Certificate revocation
Since the EAP server is on the Internet during the EAP conversation,
the EAP server is capable of following a certificate chain or verify-
ing 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
Aboba & Simon [Page 18]
INTERNET-DRAFT 21 April 1998
CAs, it may not be able to follow a certificate chain or verify
whether the EAP server's certificate has been revoked.
In the case where the peer is initiating a voluntary Layer 2 tunnel
using PPTP or L2TP, the peer will typically already have a PPP inter-
face and Internet connectivity established at the time of tunnel ini-
tiation. As a result, during the EAP conversation it is capable of
checking for certificate revocation.
However, in the case where the peer is initiating an intial PPP con-
versation, it will not have Internet connectivity and is therefore not
capable of checking for certificate revocation until after NCP negoti-
ation completes and the peer has access to the Internet. In this case,
the peer SHOULD check for certificate revocation after connecting to
the Internet.
6.2. Separation of the EAP server and PPP authenticator
As a result of the EAP-TLS conversation, the EAP endpoints will mutu-
ally authenticate, negotiate a ciphersuite, and derive a session key
for subsequent use in PPP encryption. Since the peer and EAP client
reside on the same machine, it is necessary for the EAP client module
to pass the session key to the PPP encryption module.
The situation may be more complex on the PPP authenticator. As noted
in [15], the PPP authenticator may or may not reside on the same
machine as the EAP server. For example, when RADIUS/EAP is used, the
EAP server may be a backend security server, or a module residing on
the RADIUS server.
In the case where the EAP server and PPP authenticator reside on dif-
ferent 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.
As described in [15], when EAP/RADIUS is used to encapsulate EAP pack-
ets, the Signature attribute is required in EAP/RADIUS Access-Requests
sent from the NAS or tunnel server to the RADIUS server. Since the
Signature attribute involves a keyed-MD5 hash, it is possible for the
RADIUS server to verify the integrity of the Access-Request as well as
the NAS or tunnel server's identity. Similarly, Access-Challenge pack-
ets sent from the RADIUS server to the NAS are also authenticated and
integrity protected using a keyed-MD5 hash, enabling the NAS or tunnel
server to determine the integrity of the packet and verify the iden-
tity of the RADIUS server. Moreover, EAP-TLS packets in transit are
integrity protected and authenticated end-to-end via TLS mechanisms,
so that they cannot be successfully modified by a rogue NAS or tunnel
server.
Aboba & Simon [Page 19]
INTERNET-DRAFT 21 April 1998
The second issue that arises in the case of an EAP server and PPP
authenticator residing on different machines 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 mandatory 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 mandatory tunneling a PPP peer cannot tell whether its packets
are being tunneled, let alone whether the network device is securing
the tunnel, if security is required then the client must make its own
arrangements. In the case where all endpoints cannot be relied upon to
implement IPSEC, TLS, or another suitable security protocol, PPP
encryption provides a convenient means to ensure the privacy of pack-
ets transiting between the client and the tunnel server.
There also may be circumstances in which PPP encryption may be desir-
able even where voluntary tunneling is being used. In voluntary tun-
neling, the client initiates the tunnel to the tunnel server without
assistance from a network device. Routers implementing Network
Address and Port Translation (NAPT) are now growing rapidly in popu-
larity. Where NAPT is turned on, IPSEC cannot be used to secure the
outer layer of a client-initiated layer 2 tunnel, since the address
and port translated packet will then fail the authentication check. By
contrast, Layer 2 tunnels utilizing PPP encryption may pass unimpeded
through a NAT.
7. Acknowledgments
Thanks to Terence Spies, Glen Zorn and Narendra Gidwani of Microsoft
for useful discussions of this problem space.
8. References
[1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)." STD 51,
RFC 1661, Daydreamer, July 1994.
[2] Sklower, K., Lloyd, B., McGregor, G., Carr, D., and T. Coradetti,
"The PPP Multilink Protocol (MP)." RFC 1990, UC Berkeley, August 1996.
[3] Simpson, W., Editor, "PPP LCP Extensions." RFC 1570, Daydreamer,
January 1994.
Aboba & Simon [Page 20]
INTERNET-DRAFT 21 April 1998
[4] R. Rivest, S. Dusse. "The MD5 Message-Digest Algorithm." RFC
1321, MIT Laboratory for Computer Science, RSA Data Security Inc.,
April 1992.
[5] L. Blunk, J. Vollbrecht. "PPP Extensible Authentication Protocol
(EAP)." RFC 2284, Merit Network, Inc., March 1998.
[6] W. T. Whelan, "PPP EAP RSA Public Key Authentication Protocol."
Work in progress, draft-ietf-pppext-eaprsa-04.txt, Cabletron Systems,
February 1997.
[7] Meyer, G., "The PPP Encryption Protocol (ECP)." RFC 1968, Spider
Systems. June 1996
[8] National Bureau of Standards, "Data Encryption Standard." FIPS PUB
46 (January 1977).
[9] National Bureau of Standards, "DES Modes of Operation." FIPS PUB
81 (December 1980).
[10] K. Sklower, G. Meyer. "The PPP DES Encryption Protocol, Version
2 (DESE-bis)" Work in progress, draft-ietf-pppext-des-encrypt-
v2-00.txt, University of California, Berkeley, Shiva, July 1997.
[11] K. Hummert. "The PPP Triple-DES Encryption Protocol (3DESE)"
Work in progress, draft-ietf-pppext-3des-encrypt-00.txt, Nentec GmbH,
July 1997.
[12] S. Bradner. "Key words for use in RFCs to Indicate Requirement
Levels." RFC 2119, Harvard University, March 1997.
[13] T. Dierks, C. Allen. "The TLS Protocol Version 1.0." Internet
draft (work in progress) draft-ietf-tls-protocol-05.txt, Consensus
Development, November 1997.
[14] D. Rand. "The PPP Compression Control Protocol." RFC 1962, Nov-
ell, June 1996.
[15] P. Calhoun, A.C. Rubens, B. Aboba. "Extensible Authentication
Protocol Support in RADIUS." Internet draft (work in progress), draft-
ietf-radius-eap-04.txt, Sun Microsystems, Merit Network, Microsoft,
March 1998.
[16] K. Sklower, B. Lloyd, G. McGregor, D. Carr, T. Coradetti, "The
PPP Multilink Protocol (MP)", RFC 1990, August 1996.
9. Authors' Addresses
Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
Aboba & Simon [Page 21]
INTERNET-DRAFT 21 April 1998
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
Pc
Aboba & Simon [Page 22]
