rfc5216bis.nr

.\" format this document with "rfc rfc5216bis.nr draft-aboba-emu-rfc5216bis-00.txt"
.nr HY 0
.pl 10.0i
.po 0
.ll 7.2i
.lt 7.2i
.nr LL 7.2i
.nr LT 7.2i
.ds LH INTERNET-DRAFT
.ds CH EAP TLS Authentication Protocol 
.ds RH \*(DY
.ds LF Aboba, Hurst & DeKok
.ds CF Full Standard
.ds RF FORMFEED[Page %]

.tl 'EMU Working Group''Bernard Aboba'
.tl 'Internet-Draft''Microsoft Corporation'
.tl 'Obsoletes: 5216''Ryan Hurst'
.tl 'Category: Full Standard''Google'
.tl 'Expires: July 1, 2018''Alan DeKok'
.tl '''FreeRADIUS Project'
.tl ''''\*(DY'
.ad l
.LP

.ce
The EAP TLS Authentication Protocol
.ce
draft-aboba-emu-rfc5216bis-01.txt

Abstract

.in +0.3i
The Extensible Authentication Protocol (EAP), defined in RFC 3748,
provides support for multiple authentication methods.
Transport Level Security (TLS) provides for mutual authentication,
integrity-protected ciphersuite negotiation and key exchange between
two endpoints.
This document defines EAP-TLS, which includes support for certificate-based
mutual authentication and key derivation.

This document obsoletes RFC 5216. A summary of the changes between
this document and RFC 5216 is available in Appendix A.
.in -0.3i

Status of This Memo

.in +0.3i
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF).  Note that other groups may also distribute
working documents as Internet-Drafts.  The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.

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."

This Internet-Draft will expire on July 1, 2018.
.in -0.3i

Copyright Notice

.in +0.3i
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors.  All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document.  Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.  Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008.  The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
.in -0.3i

Table of Contents
.LP
.nf
1.  Introduction..............................................    3
      1.1    Requirements Language ...........................    3
      1.2    Terminology .....................................    3
2.  Protocol Overview ........................................    4
      2.1    Overview of the EAP-TLS Conversation ............    4
      2.2    Identity Verification ...........................   15
      2.3    Key Hierarchy ...................................   16
      2.4    Ciphersuite and Compression Negotiation .........   18
3.  Detailed Description of the EAP-TLS Protocol .............   19
      3.1     EAP TLS Request Packet .........................   19
      3.2     EAP TLS Response Packet ........................   21
4.  IANA Considerations ......................................   22
5.  Security Considerations ..................................   22
      5.1     Security Claims ................................   22
      5.2     Peer and Server Identities .....................   23 
      5.3     Certificate Validation .........................   24 
      5.4     Certificate Revocation .........................   25
      5.5     Packet Modification Attacks ....................   26
6.  References ...............................................   27
      6.1    Normative references ....... ....................   27
      6.2    Informative references ..........................   28
Acknowledgments ..............................................   29
Authors' Addresses ...........................................   30
Appendix A - Changes from RFC 2716 ...........................   31
Full Copyright Statement .....................................   32
Intellectual Property ........................................   32
.bp
.NH 1
.R
Introduction

.in +0.3i
The Extensible Authentication Protocol (EAP), described in [RFC3748],
provides a standard mechanism for support of multiple
authentication methods.  
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.  
EAP has been defined for use with a variety of lower layers,
including Point-to-Point Protocol (PPP) [RFC1661], Layer 2 tunneling protocols
such as PPTP [RFC2637] or L2TP [RFC2661],
IEEE 802 wired networks [IEEE-802.1X] and wireless technologies such
as IEEE 802.11 [IEEE-802.11] and IEEE 802.16 [IEEE-802.16e].

While the EAP methods defined in [RFC3748] did not support mutual
authentication, the use of EAP with wireless technologies such as
[IEEE-802.11] has resulted in development of a new set of requirements.
As described in "EAP Method Requirements for Wireless LANs" [RFC4017], 
it is desirable for EAP methods used for wireless LAN authentication
to support mutual authentication and key derivation.  
Other link layers can also make use of EAP to enable mutual
authentication and key derivation.

This document defines EAP-Transport Layer Security (EAP-TLS), 
which includes support for certificate-based
mutual authentication and key derivation, utilizing the
version negotiation, protected ciphersuite negotiation, mutual
authentication and key management capabilities of the TLS protocol.
While this document obsoletes RFC 5216 [RFC5216], it remains backward compatible with it. 
A summary of the changes between this document and RFC 5216 is available in Appendix A.
.in -0.3i
.NH 2
.R
Requirements

.in +0.3i
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
.in -0.3i
.NH 2
.R
Terminology

.in +0.3i
 This document frequently uses the following terms:
.in -0.3i
.IP "authenticator"
The entity initiating EAP authentication.  
.IP "peer"
The entity that responds to the authenticator.  In
[IEEE-802.1X], this entity is known as the Supplicant.
.IP "backend authentication server"
A backend authentication server is an entity that provides an
authentication service to an authenticator.  When used, this
server typically executes EAP methods for the authenticator.  This
terminology is also used in [IEEE-802.1X].
.IP "EAP server"
The entity that terminates the EAP authentication method with the
peer.  In the case where no backend authentication server is used,
the EAP server is part of the authenticator.  In the case where
the authenticator operates in pass-through mode, the EAP server is
located on the backend authentication server.
.IP "Master Session Key (MSK)"
Keying material that is derived between the EAP peer and server
and exported by the EAP method.  
.IP "Extended Master Session Key (EMSK)"
Additional keying material derived between the EAP peer and
server that is exported by the EAP method.  
.NH 1
.R
Protocol Overview
.NH 2
.R
Overview of the EAP-TLS Conversation

.in +0.3i
As described in [RFC3748], 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 user-Id.

From this point forward, while nominally the EAP conversation occurs
between the EAP authenticator and the peer, the authenticator can act
as a pass-through device, with the EAP packets received from the peer
being encapsulated for transmission to a backend authentication server.
In the discussion that follows, we will use the term
"EAP server" to denote the ultimate endpoint conversing with the
peer.
.in -0.3i
.NH 3
.R
Base Case
.in +0.3i
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 peer's TLS version number, a
sessionId, a random number, and a set of ciphersuites supported by
the peer.  The version offered by the peer 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 peer'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 peer,
indicating a resumption of the previously established session with
that sessionId.  The server will also choose a ciphersuite from those
offered by the peer.  If the session matches the peer's, then the
ciphersuite MUST match the one negotiated during the handshake
protocol execution that established the session.

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
Digital Signature Standard (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 peer to authenticate itself via public key.  While the EAP server
SHOULD require peer authentication, this is not mandatory, since
there are circumstances in which peer authentication will not be
needed (e.g., emergency services, as described in [UNAUTH]), or where
the peer will authenticate via some other means.

If the peer supports EAP-TLS and is configured to use it, it MUST
respond to the EAP-Request with an EAP-Response packet of EAP-
Type=EAP-TLS.  If the preceding server_hello message sent by the EAP
server in the preceding EAP-Request packet did not indicate the
resumption of a previous session, the data field of this packet MUST
encapsulate one or more TLS records containing a TLS
client_key_exchange, change_cipher_spec, and finished messages.  If
the EAP server sent a certificate_request message in the preceding
EAP-Request packet, then unless the peer is configured for privacy
(see Section 2.1.4) the peer MUST send, in addition, certificate and
certificate_verify 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 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.

In the case where the EAP-TLS mutual authentication is successful,
the conversation will appear as follows:

.nf
Authenticating Peer     Authenticator
-------------------     -------------
                        <- EAP-Request/
                        Identity
EAP-Response/
Identity (MyID) ->
                        <- EAP-Request/
                        EAP-Type=EAP-TLS
                        (TLS Start)
EAP-Response/
EAP-Type=EAP-TLS
(TLS client_hello)->
                        <- EAP-Request/
                        EAP-Type=EAP-TLS
                        (TLS server_hello,
                          TLS certificate,
                 [TLS server_key_exchange,]
                  TLS certificate_request,
                     TLS server_hello_done)
EAP-Response/
EAP-Type=EAP-TLS
(TLS certificate,
 TLS client_key_exchange,
 TLS certificate_verify,
 TLS change_cipher_spec,
 TLS finished) ->
                        <- EAP-Request/
                        EAP-Type=EAP-TLS
                        (TLS change_cipher_spec,
                         TLS finished)
EAP-Response/
EAP-Type=EAP-TLS ->
                        <- EAP-Success
.fi
.in -0.3i
.NH 3
.R
Session Resumption

.in +0.3i
The purpose of the sessionId within the TLS protocol is to allow for
improved efficiency in the case where a peer 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 also
can be used to provide "fast reconnect" functionality as defined in
[RFC3748] Section 7.2.1.

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, the peer will only be able to continue sessions when
connecting to the same authenticator.  Should the authenticators 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 authenticators will
utilize the same backend authentication 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. 

In the case where a previously established session is being resumed, and both
sides authenticate successfully, the conversation will appear as follows:

.nf
Authenticating Peer     Authenticator
-------------------     -------------
                        <- EAP-Request/
                        Identity
EAP-Response/
Identity (MyID) ->
                        <- EAP-Request/
                        EAP-Request/
                        EAP-Type=EAP-TLS 
                        (TLS Start)
EAP-Response/
EAP-Type=EAP-TLS 
(TLS client_hello)->
                        <- EAP-Request/
                        EAP-Type=EAP-TLS 
                        (TLS server_hello,
                        TLS change_cipher_spec
                        TLS finished)
EAP-Response/
EAP-Type=EAP-TLS 
(TLS change_cipher_spec,
 TLS finished) ->
                        <- EAP-Success
.fi
.in -0.3i
.NH 3
.R
Termination

.in +0.3i
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 per-peer limit on the
number of restarts, so as to protect against denial of service
attacks.

If the peer 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
contains the EAP server's authentication response to the peer.  The
peer will then verify the finished message in order to authenticate the EAP
server.

If EAP server authentication is unsuccessful, the peer SHOULD
delete the session from its cache, preventing reuse of the
sessionId. 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.

In the case where the server authenticates to the peer
successfully, but the peer fails to authenticate to the
server, the conversation will appear as follows: 

.nf
Authenticating Peer     Authenticator
-------------------     -------------
                        <- EAP-Request/
                        Identity
EAP-Response/
Identity (MyID) ->
                        <- EAP-Request/
                        EAP-Type=EAP-TLS
                        (TLS Start)
EAP-Response/
EAP-Type=EAP-TLS 
(TLS client_hello)->
                        <- EAP-Request/
                        EAP-Type=EAP-TLS
                        (TLS server_hello,
                          TLS certificate,
                 [TLS server_key_exchange,]
		  TLS certificate_request,
		    TLS server_hello_done)
EAP-Response/
EAP-Type=EAP-TLS 
(TLS certificate,
 TLS client_key_exchange,
 TLS certificate_verify,
 TLS change_cipher_spec,
 TLS finished) ->
                        <- EAP-Request/
                        EAP-Type=EAP-TLS
                        (TLS change_cipher_spec,
                        TLS finished)
EAP-Response/
EAP-Type=EAP-TLS ->
                        <- EAP-Request
                        EAP-Type=EAP-TLS
                        (TLS Alert message)
EAP-Response/
EAP-Type=EAP-TLS ->
                        <- EAP-Failure
                        (User Disconnected)
.fi

In the case where server authentication is unsuccessful, the
conversation will appear as follows:

.nf
Authenticating Peer     Authenticator
-------------------     -------------
                        <- EAP-Request/
                        Identity
EAP-Response/
Identity (MyID) ->
                        <- EAP-Request/
                        EAP-Type=EAP-TLS 
                        (TLS Start)
EAP-Response/
EAP-Type=EAP-TLS 
 (TLS client_hello)->
                        <- EAP-Request/
                        EAP-Type=EAP-TLS 
                        (TLS server_hello,
                         TLS certificate,
			[TLS server_key_exchange,]
			 TLS certificate_request,
			 TLS server_hello_done)
EAP-Response/
EAP-Type=EAP-TLS 
(TLS Alert message) ->
                        <- EAP-Failure
                        (User Disconnected)
.fi
.in -0.3i
.NH 3
.R
Privacy

.in +0.3i
EAP-TLS peer and server implementations MAY support privacy.  Disclosure
of the username is avoided by utilizing a privacy Network Access Identifier
(NAI) [RFC7542] in the EAP-Response/Identity, and transmitting the peer
certificate within a TLS session providing confidentiality.

In order to avoid disclosing the peer username,
an EAP-TLS peer configured for privacy MUST negotiate a TLS ciphersuite
supporting confidentiality and
MUST provide a client certificate list containing no entries
in response to the initial certificate_request from the EAP-TLS server.

An EAP-TLS server supporting privacy MUST NOT treat a certificate list
containing no entries as a terminal condition;  instead it MUST
bring up the TLS session and then
send a hello_request.
The handshake then proceeds normally;
the peer sends a client_hello and the server replies with a server_hello,
certificate, server_key_exchange,
certificate_request, server_hello_done, etc.
For the calculation of exported keying material (see Section 2.3), 
the master_secret derived within the second handshake is used. 

An EAP-TLS peer supporting privacy MUST provide a certificate list
containing at least one entry in response to the subsequent
certificate_request sent by the server.
If the EAP-TLS server supporting privacy does not receive a
client certificate in
response to the subsequent certificate_request, then it MUST abort the session.

EAP-TLS privacy support is designed to allow EAP-TLS peers that do not support
privacy to interoperate with EAP-TLS servers supporting privacy.
EAP-TLS servers supporting privacy MUST request a client certificate, and
MUST be able to accept a client certificate offered by the EAP-TLS peer, in order to
preserve interoperability with EAP-TLS peers that do not support privacy.

However, an EAP-TLS peer configured for privacy typically will not be able to
successfully authenticate with an EAP-TLS server that does not support privacy,
since such
a server will typically treat the refusal to provide a client certificate as
a terminal error.  
As a result, unless authentication failure is
considered preferable to disclosure of the username,
EAP-TLS peers SHOULD only be configured for privacy on networks known to
support it.

This is most easily achieved with EAP lower layers that support network
advertisement, so that the network and appropriate privacy configuration can be
determined.
In order to determine the privacy configuration on
link layers (such as IEEE 802 wired networks) which do not support network
advertisement, it may be desirable to utilize information provided in the
server certificate (such as the subject and subjectAltName fields) or within identity selection hints [RFC4284] 
to determine
the appropriate configuration.

In the case where the peer and server support privacy and mutual
authentication, the conversation will appear as follows:

.nf
Authenticating Peer     Authenticator
-------------------     -------------
                        <- EAP-Request/
                        Identity
EAP-Response/
Identity (Anonymous NAI) ->
                        <- EAP-Request/
                        EAP-Type=EAP-TLS
                        (TLS Start)
EAP-Response/
EAP-Type=EAP-TLS
(TLS client_hello)->
                        <- EAP-Request/
                        EAP-Type=EAP-TLS
                        (TLS server_hello,
                         TLS certificate,
                 [TLS server_key_exchange,]
                  TLS certificate_request,
                     TLS server_hello_done)
EAP-Response/
EAP-Type=EAP-TLS
(TLS certificate (no cert),
 TLS client_key_exchange,
 TLS change_cipher_spec,
 TLS finished) ->
                        <- EAP-Request/
                        EAP-Type=EAP-TLS
                        (TLS change_cipher_spec,
                          finished,
                          hello_request)
EAP-Response/
EAP-Type=EAP-TLS
(TLS client_hello)->
                        <- EAP-Request/
                        EAP-Type=EAP-TLS
                        (TLS server_hello,
                          TLS certificate,
                  TLS server_key_exchange,
                  TLS certificate_request,
                     TLS server_hello_done)
EAP-Response/
EAP-Type=EAP-TLS
(TLS certificate,
 TLS client_key_exchange,
 TLS certificate_verify,
 TLS change_cipher_spec,
 TLS finished) ->
                        <- EAP-Request/
                        EAP-Type=EAP-TLS
                        (TLS change_cipher_spec,
                         TLS finished)
EAP-Response/
EAP-Type=EAP-TLS ->
                        <- EAP-Success
.fi
.in -0.3i
.NH 3
.R
Fragmentation

.in +0.3i
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 MTU size or the
maximum RADIUS packet size of 4096 octets.
As a result, an EAP-TLS implementation
MUST provide its own support for fragmentation and reassembly.
However, in order to ensure interoperability with existing implementations,
TLS handshake messages SHOULD NOT be fragmented into multiple TLS
records if they fit within a single TLS record.

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 single certificate is rarely longer than a few thousand
octets, and no other field is likely to be anywhere near as long, a
reasonable choice of maximum acceptable message length might be 64
KB.

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-Response.
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 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-
Response.

In the case where the EAP-TLS mutual authentication is successful, and 
fragmentation is required, the conversation will appear as follows:

.nf
Authenticating Peer     Authenticator
-------------------     -------------
                        <- EAP-Request/
                        Identity
EAP-Response/
Identity (MyID) ->
                        <- EAP-Request/
                        EAP-Type=EAP-TLS 
                        (TLS Start, S bit set)
EAP-Response/
EAP-Type=EAP-TLS 
(TLS client_hello)->
                        <- 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)
EAP-Response/
EAP-Type=EAP-TLS ->
                        <- EAP-Request/
                           EAP-Type=EAP-TLS 
                        (Fragment 2: M bit set)
EAP-Response/
EAP-Type=EAP-TLS ->
                        <- EAP-Request/
                        EAP-Type=EAP-TLS 
                        (Fragment 3)
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)->
                         <- EAP-Request/
                        EAP-Type=EAP-TLS 
EAP-Response/
EAP-Type=EAP-TLS 
(Fragment 2)->
                       <- EAP-Request/
                        EAP-Type=EAP-TLS 
                        (TLS change_cipher_spec,
                         TLS finished)
EAP-Response/
EAP-Type=EAP-TLS ->
                        <- EAP-Success
.fi
.in +0.3i
.NH 2 
.R
Identity Verification

.in +0.3i
As noted in [RFC3748] Section 5.1:

.nf
   It is RECOMMENDED that the Identity Response be used primarily for
   routing purposes and selecting which EAP method to use.  EAP
   Methods SHOULD include a method-specific mechanism for obtaining
   the identity, so that they do not have to rely on the Identity
   Response.  
.fi

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. EAP-TLS therefore provides 
a mechanism for determining both the peer identity (Peer-Id in [RFC5247])
and server identity (Server-Id in [RFC5247]).  
For details, see Section 5.2. 

Since the identity presented in the EAP-Response/Identity need not be
related to the identity presented in the peer certificate, EAP-TLS
implementations SHOULD NOT require that they be identical.
However, if they are not identical, the identity presented in the
EAP-Response/Identity is unauthenticated information, and SHOULD NOT be
used for access control or accounting purposes.
.in -0.3i 
.NH 2
.R
Key Hierarchy

.in +0.3i
Figure 1 illustrates the TLS Key Hierarchy, described in [RFC4346]
Section 6.3.  The derivation proceeds as follows:

.nf
master_secret = TLS-PRF-48(pre_master_secret, "master secret",
                 client.random || server.random)
key_block     = TLS-PRF-X(master_secret, "key expansion",
                 server.random || client.random)

Where:

TLS-PRF-X =     TLS pseudo-random function defined in [RFC4346],
                computed to X octets.
.fi

In EAP-TLS, the MSK, EMSK and IV are derived from the TLS master
secret via a one-way function.  This ensures that the TLS master
secret cannot be derived from the MSK, EMSK or IV unless the one-way
function (TLS PRF) is broken.  Since the MSK and EMSK are derived from the
TLS master secret, if the TLS master secret is compromised then the
MSK and EMSK are also compromised.

The MSK is divided into two halves,
corresponding to the "Peer to Authenticator Encryption Key" 
(Enc-RECV-Key, 32 octets) and "Authenticator to Peer Encryption Key" 
(Enc-SEND-Key, 32 octets).

The IV is a 64 octet quantity that is a known value; octets
0-31 are known as the "Peer to Authenticator IV" or RECV-IV, and
Octets 32-63 are known as the "Authenticator to Peer IV", or SEND-IV.
.bp
.nf
      |                       | pre_master_secret       |
server|                       |                         | client
Random|                       V                         | Random
      |     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     |
      |     |                                     |     |
      +---->|             master_secret           |<----+
      |     |                                     |     |
      |     |                                     |     |
      |     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     |
      |                       |                         |
      V                       V                         V
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                         |
|                      key_block                          |
|               label == "key expansion"                  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |         |         |         |         |         |
  | client  | server  | client  | server  | client  | server
  | MAC     | MAC     | write   | write   | IV      | IV
  |         |         |         |         |         |
  V         V         V         V         V         V

   Figure 1 - TLS [RFC4346] Key Hierarchy
.fi

EAP-TLS derives exported keying material and parameters as follows:

.nf
Key_Material = TLS-PRF-128(master_secret, "client EAP encryption",
                  client.random || server.random)
MSK          = Key_Material(0,63)
EMSK         = Key_Material(64,127)
IV           = TLS-PRF-64("", "client EAP encryption",
                  client.random || server.random)

Enc-RECV-Key = MSK(0,31) = Peer to Authenticator Encryption Key 
               (MS-MPPE-Recv-Key in [RFC2548]).  Also known as the
               PMK in [IEEE-802.11].
Enc-SEND-Key = MSK(32,63) = Authenticator to Peer Encryption Key
               (MS-MPPE-Send-Key in [RFC2548])
RECV-IV      = IV(0,31) = Peer to Authenticator Initialization Vector
SEND-IV      = IV(32,63) = Authenticator to Peer Initialization 
                           Vector
Session-Id   = 0x0D || client.random || server.random
.fi

Where:

.nf
Key_Material(W,Z) = Octets W through Z inclusive of the key material.
IV(W,Z)           = Octets W through Z inclusive of the IV.
MSK(W,Z)          = Octets W through Z inclusive of the MSK.
EMSK(W,Z)         = Octets W through Z inclusive of the EMSK.
TMS               = TLS master_secret
TLS-PRF-X         = TLS PRF function computed to X octets
client.random     = Nonce generated by the TLS client.
server.random     = Nonce generated by the TLS server.
.fi

.nf
      |                       | pre_master_secret       |
server|                       |                         | client
Random|                       V                         | Random
      |     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     |
      |     |                                     |     |
      +---->|             master_secret           |<----+
      |     |                                     |     |
      |     |                                     |     |
      |     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     |
      |                       |                         |
      V                       V                         V
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                         |
|                        MSK, EMSK                        |
|               label == "client EAP encryption"          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
  |             |             |        
  | MSK(0,31)   | MSK(32,63)  | EMSK(0,63)
  |             |             |    
  |             |             | 
  V             V             V 

   Figure 2 - EAP-TLS Key Hierarchy
.fi

The use of these keys is specific to the lower layer, as described in [RFC5247]
Section 2.1. 
.in +0.3i
.NH 2
.R
Version, Ciphersuite and Compression Negotiation

.in +0.3i
Since EAP-TLS represents the encapsulation of TLS within EAP, implementations
utilize TLS to negotiate protocol versions and ciphersuites.

To ensure backward compatibility, EAP-TLS implementations MUST support TLS 1.0.
To ensure forward compatibility, EAP-TLS implementations MUST support TLS version negotiation.

EAP-TLS implementations inherit the normative requirements of each TLS version they
support.  For each version of TLS they support, EAP-TLS implementations MUST support
the mandatory ciphersuites, SHOULD support the recommended ciphersuites and
MAY support the optional ciphersuites.

However, administrators MAY impose TLS version and ciphersuite requirements
on EAP clients and servers under their control. For example, an administrator
can configure EAP-TLS servers within their organization to only negotiate TLS
versions and ciphersuites conforming to organizational policy.

Since TLS supports ciphersuite negotiation, peers completing the TLS
negotiation will also have selected a ciphersuite, which includes
encryption and hashing methods.  Since the ciphersuite negotiated within
EAP-TLS applies only to the EAP conversation, TLS ciphersuite negotiation
MUST NOT be used to negotiate the ciphersuites used to secure data.

TLS also supports compression as well as ciphersuite negotiation.
However, during the EAP-TLS conversation the EAP peer and server
MUST NOT request or negotiate compression.
.in -0.3i
.NH 1
.R
Detailed description of the EAP-TLS protocol
.NH 2
.R
EAP TLS Request Packet

.in +0.3i
A summary of the EAP TLS Request packet format is shown below.  
The fields are transmitted from left to right.

.nf
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...   
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.fi

Code

.in +0.3i
1
.in -0.3i

Identifier

.in +0.3i
The Identifier field is one octet and aids in
matching responses with requests.  The Identifier
field MUST be changed on each Request packet. 
.in -0.3i

Length

.in +0.3i
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 MUST be
ignored on reception.
.in -0.3i

Type

.in +0.3i
13 - EAP TLS
.in -0.3i

Flags

.in +0.3i
.nf
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
.fi

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 acknowledgment. 
Implementations of this specification MUST set the reserved bits to zero, and
MUST ignore them on reception. 
.in -0.3i

TLS Message Length

.in +0.3i
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. 
.in -0.3i

TLS data

.in +0.3i
The TLS data consists of the encapsulated TLS
packet in TLS record format. 
.in -0.3i
.in -0.3i
.NH 2
.R
EAP TLS Response Packet

.in +0.3i
A summary of the EAP TLS Response packet format is shown below.  
The fields are transmitted from left to right.

.nf
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...   
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.fi

Code

.in +0.3i
2
.in -0.3i

Identifier

.in +0.3i
The Identifier field is one octet and MUST match
the Identifier field from the corresponding
request.
.in -0.3i

Length

.in +0.3i
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 MUST be
ignored on reception.
.in -0.3i

Type

.in +0.3i
13 - EAP TLS
.in -0.3i

Flags

.in +0.3i
.nf
0 1 2 3 4 5 6 7 8
+-+-+-+-+-+-+-+-+
|L M R R R R R R|      
+-+-+-+-+-+-+-+-+

L = Length included
M = More fragments
R = Reserved
.fi

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.  
Implementations of this specification MUST set the reserved bits to zero, and
MUST ignore them on reception.
.in -0.3i

TLS Message Length

.in +0.3i
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. 
.in -0.3i

TLS data

.in +0.3i
The TLS data consists of the encapsulated TLS
packet in TLS record format. 
.in -0.3i
.in -0.3i
.NH 1
.R
IANA Considerations

.in +0.3i
IANA has allocated EAP Type 13 for EAP-TLS.  
The allocation should be updated
to reference this document.  
There are no other IANA actions.
.in -0.3i
.NH 1
.R
Security Considerations
.NH 2
.R
Security Claims

.in +0.3i
EAP security claims are defined in [RFC3748] Section 7.2.1.  The security claims
for EAP-TLS are as follows:

.nf
Auth. mechanism:           Certificates
Ciphersuite negotiation:   Yes [4]
Mutual authentication:     Yes [1]
Integrity protection:      Yes [1]
Replay protection:         Yes [1]
Confidentiality:           Yes [2]
Key derivation:            Yes
Key strength:              [3]
Dictionary attack prot.:   Yes
Fast reconnect:            Yes
Crypt. binding:            N/A
Session independence:      Yes [1]
Fragmentation:             Yes
Channel binding:           No

Notes
-----
.fi

[1] A formal proof of the security of EAP-TLS when used 
with [IEEE-802.11] is provided in [He].  This
proof relies on the assumption that the private 
key pairs used by the EAP peer and server are not
shared with other parties or applications.  
For example, a backend authentication server supporting
EAP-TLS SHOULD NOT utilize the same certificate with https.  

[2] Privacy is an optional feature described in Section 2.1.4.

[3] BCP 86 [RFC3766] Section 5 offers advice on the required RSA or DH
module and DSA subgroup size in bits, for a given level of attack
resistance in bits.  For example, a 2048-bit RSA key is recommended to provide
128-bit equivalent key strength.  The National Institute for Standards and
Technology (NIST) also offers advice on appropriate key sizes in
[SP800-57].

[4] EAP-TLS inherits the secure ciphersuite negotiation features of TLS,
including key derivation function negotiation (for versions of TLS
where this is supported).
.in -0.3i
.NH 2
.R
Peer and Server Identities

.in +0.3i
The EAP-TLS peer name (Peer-Id) represents the identity to be used
for access control and accounting purposes.
The Server-Id represents the identity of the EAP server.
Together the Peer-Id and Server-Id name the entities involved in
deriving the MSK/EMSK. 
 
In EAP-TLS, the Peer-Id and Server-Id are determined from the subject
or subjectAltName fields in the peer and server certificates.  
For details, see [RFC3280] Section 4.1.2.6.
Where the subjectAltName field is present in the peer or server 
certificate, the Peer-Id or Server-Id MUST be set to the contents 
of the subjectAltName. 
If subject naming information is present only in the subjectAltName
extension of a peer or server certificate, then the subject field
MUST be an empty sequence and the subjectAltName extension
MUST be critical.

Where the peer identity represents a host, a subjectAltName of type
dnsName SHOULD be present in the peer certificate.  Where the peer
identity represents a user and not a resource, a subjectAltName of
type rfc822Name SHOULD be used, conforming to the grammar for the
Network Access Identifier (NAI) defined in [RFC7542].
If a dnsName or rfc822Name are not available other field types
(for example a subjectAltName of type ipAddress or uniformResourceIdentifier)
MAY be used.

A server identity will typically represent a host, not a user or
a resource.  As a result, a subjectAltName of type dnsName SHOULD be
present in the server certificate.  If a dnsName is not available
other field types (for example a subjectAltName of type ipAddress or
uniformResourceIdentifier) MAY be used. 
 
Conforming implementations generating new certificates
with Network Access Identifiers (NAIs) MUST use the rfc822Name in the
subject alternative name field to describe such identities.  The
use of the subject name field to contain an emailAddress 
Relative Distinguished Name (RDN)
is deprecated, and MUST NOT be used.
The subject name field
MAY contain other RDNs for representing the subject's identity.

Where it is non-empty, the subject name field MUST contain an
X.500 distinguished name (DN).
If subject naming information is present only in the subject name field
of a peer certificate and the peer identity represents 
a host or device the subject name field SHOULD contain a CommonName (CN) RDN or
serialNumber RDN.
If subject naming information is present only in the subject name
field of a server certificate, then the subject name field 
SHOULD contain a CN RDN or serialNumber RDN. 

It is possible for more than one subjectAltName field to be present
in a peer or server certificate in addition to an empty or non-empty
subject distinguished name. 
EAP-TLS implementations supporting export of the Peer-Id and Server-Id SHOULD export
all the subjectAltName fields within Peer-Ids or Server-Ids, and SHOULD also
export a non-empty subject distinguished name field within the Peer-Ids or Server-Ids. 
All of the exported Peer-Ids and
Server-Ids are considered valid.

EAP-TLS implementations supporting export of the Peer-Id and Server-Id SHOULD export
Peer-Ids and Server-Ids in the same 
order in which they appear within the certificate.
Such canonical ordering would aid in comparison operations 
and would enable using those identifiers for key derivation if that 
is deemed useful.  However, the ordering of fields within
the certificate SHOULD NOT be used for access control.
.in -0.3i
.NH 2
.R
Certificate Validation

.in +0.3i
Since the EAP-TLS server is typically connected to the Internet,
it SHOULD support validating the peer certificate using 
RFC 3280 [RFC3280] compliant path validation, including the 
ability to retrieve intermediate certificates that may be
necessary to validate the peer certificate. For details, 
see [RFC3280] Section 4.2.2.1.
 
Where the EAP-TLS server is unable to retrieve intermediate
certificates, it either will need to be pre-configured with the necessary 
intermediate certificates to complete path validation or 
it will rely on the EAP-TLS peer to provide this information as 
part of the TLS Handshake (see [RFC4346] section 7.4.6).
   
In contrast to the EAP-TLS server, the EAP-TLS peer may
not have Internet connectivity. 
Therefore the EAP-TLS server 
SHOULD provide its entire certificate chain minus the root to 
facilitate certificate validation by the peer.
The EAP-TLS peer SHOULD
support validating the server certificate using
RFC 3280 [RFC3280] compliant path validation.
 
Once a TLS session is established EAP-TLS peer and server 
implementations MUST validate that the identities represented 
in the certificate are appropriate and authorized for 
use with EAP-TLS.  The authorization process makes 
use of the contents of the certificates as well 
as other contextual information.  While authorization 
requirements will vary from deployment to deployment 
it is RECOMMENDED that implementations be able to 
authorize based on the EAP-TLS Peer-Id and Server-Id 
determined as described in Section 5.2. 
 
In the case of the EAP-TLS peer this involves ensuring 
that the certificate presented by the EAP-TLS server was 
intended to be used as a server certificate.
Implementations SHOULD use the Extended Key 
Usage (see [RFC3280] section 4.2.1.13) 
extension and ensure that at least one of the 
following is true:

.nf
1) The certificate issuer included no Extended Key Usage 
   identifiers in the certificate.
2) The issuer included the anyExtendedKeyUsage identifier 
   in the certificate (see [RFC3280] section 4.2.1.13).
3) The issuer included the id-kp-serverAuth identifier 
   in the certificate (See [RFC3280] section 4.2.1.13).
.fi

When performing this comparison implementations MUST follow the 
validation rules specified in [RFC2818] Section 3.1.
In the case of the server this involves ensuring the certificate 
presented by the EAP-TLS peer was intended to be used as a 
client certificate. 
Implementations SHOULD use the Extended 
Key Usage (see [RFC3280] Section 4.2.1.13) extension and 
ensure that at least one of the following is true:
 
.nf
1) The certificate issuer included no Extended Key Usage 
   identifiers in the certificate. 
2) The issuer included the anyExtendedKeyUsage identifier in 
   the certificate (see [RFC3280] section 4.2.1.13).
3) The issuer included the id-kp-clientAuth identifier in 
   the certificate (see [RFC3280] section 4.2.1.13).
.fi
.in -0.3i
.NH 2
.R
Certificate Revocation

.in +0.3i
Certificates are long lived assertions of identity. 
Therefore it is important for EAP-TLS implementations to 
be capable of checking whether these assertions have been 
revoked. 
 
EAP-TLS peer and server implementations MUST support the 
use of Certificate Revocation Lists (CRLs); for details,
see [RFC3280] section 3.3.  EAP-TLS peer and server 
implementations SHOULD also support the Online 
Certificate Status Protocol (OCSP), described in 
"X.509 Internet Public Key Infrastructure Online
Certificate Status Protocol - OCSP" [RFC2560].
OCSP messages are typically much smaller than CRLs which can shorten 
connection times especially in bandwidth constrained environments.
While EAP-TLS servers are typically connected to the Internet during
the EAP conversation, an EAP-TLS peer may not have Internet
connectivity until authentication completes. 

In the case where the peer is initiating a voluntary Layer 2 tunnel
using PPTP [RFC2637] or L2TP [RFC2661], the peer will typically
already have a PPP interface and Internet connectivity established at
the time of tunnel initiation.
 
However, in the case where the EAP-TLS peer is attempting to obtain network
access, it will not have network connectivity and is therefore not
capable of checking for certificate revocation until after
authentication completes and network connectivity is available.  
For this reason EAP-TLS peers and servers SHOULD implement 
Certificate Status Request messages, as described in 
"Transport Layer Security (TLS) Extensions" [RFC4366] 
section 3.6.  To enable revocation checking in situations 
where servers do not support Certificate
Status Request messages and network connectivity is not
available prior to authentication completion,  peer 
implementations MUST also support checking for certificate 
revocation after authentication completes and network connectivity 
is available, and they SHOULD utilize this capability by default.
.in -0.3i
.NH 2
.R
Packet Modification Attacks

.in +0.3i
The integrity protection of EAP-TLS packets does not extend to the 
EAP header fields (Code, Identifier, Length) or the Type or Flags
fields.  As a result, these fields can be modified by an attacker.

In most cases modification of the Code or Identifier fields will
only result in a denial of service attack.  
However, an attacker can
add additional data to an EAP-TLS
packet so as to cause it to be longer than implied by the
Length field.  EAP peers, authenticators or servers that do not check for
this could be vulnerable to a buffer overrun. 

It is also possible for an attacker to modify the Type or Flags fields. 
By modifying the Type field, an attacker could cause one
TLS-based EAP method to be negotiated instead of another.  For
example, the EAP-TLS Type field (13) could be changed to indicate
another TLS-based EAP method. 
Unless the alternative TLS-based EAP method utilizes a different 
key derivation formula, it is possible that an EAP method conversation 
altered by a man-in-the-middle
could run all the way to completion without
detection.  Unless the ciphersuite selection policies are identical for
all TLS-based EAP methods utilizing the same key derivation formula, 
it may be possible for an attacker to mount a
successful downgrade attack, causing the peer to utilize an inferior
ciphersuite or TLS-based EAP method.   
.in -0.3i
.NH 1
.R
References
.NH 2
.R
Normative References
.IP [RFC2119] 15
Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
.IP [RFC2560]
Myers, M., Ankney, R., Malpani, A., Galperin, S. and C. Adams,
"X.509 Internet Public Key Infrastructure Online Certificate Status
Protocol - OCSP", RFC 2560, June 1999.
.IP [RFC2818]
Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
.IP [RFC3268]
Chown, P., "Advanced Encryption Standard (AES) Ciphersuites for Transport Layer
Security (TLS)", RFC 3268, June 2002.
.IP [RFC3280]
Housley, R., Polk, W., Ford, W. and D. Solo,
"Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile",
RFC 3280, April 2002.
.IP [RFC3748] 
Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J. and H.
Levkowetz, "Extensible Authentication Protocol (EAP)", RFC
3748, June 2004.
.IP [RFC4366]
Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J. and T. Wright,
"Transport Layer Security (TLS) Extensions", RFC 4366, April 2006.
.IP [RFC5246bis]
Rescorla, E.
"The Transport Layer Security (TLS) Protocol Version 1.3",
draft-ietf-tls-tls13-21.txt, Internet draft (work in progress), July 2017.
.IP [RFC5705]
Rescorla, E., "Keying Material Exporters for Transport Layer Security (TLS)",
RFC 5705, March 2010.
.IP [RFC7542]
DeKok, A., "The Network Access Identifier",
RFC 7542, May 2015.
.NH 2
.R
Informative References
.IP [IEEE-802.1X]
Institute of Electrical and Electronics Engineers, "Local and
Metropolitan Area Networks: Port-Based Network Access
Control", IEEE Standard 802.1X-2004, December 2004.
.IP [IEEE-802.11]
Information technology - Telecommunications and information
exchange between systems - Local and metropolitan area
networks - Specific Requirements Part 11:  Wireless LAN Medium
Access Control (MAC) and Physical Layer (PHY) Specifications,
IEEE Std. 802.11-2007, 2007.
.IP [IEEE-802.16e]
Institute of Electrical and Electronics Engineers, "IEEE
Standard for Local and Metropolitan Area Networks: Part 16:
Air Interface for Fixed and Mobile Broadband Wireless Access
Systems: Amendment for Physical and Medium Access Control
Layers for Combined Fixed and Mobile Operations in Licensed
Bands", IEEE 802.16e, August 2005.
.IP [He]
He, C., Sundararajan, M., Datta, A., Derek, A. and J. Mitchell, 
"A Modular Correctness Proof of IEEE 802.11i and TLS",
CCS '05, November 7-11, 2005, Alexandria, Virginia, USA
.IP [RFC1661] 
Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD 51,
RFC 1661, July 1994.
.IP [RFC2548]
Zorn, G., "Microsoft Vendor-specific RADIUS Attributes", RFC 2548, 
March 1999.
.IP [RFC2637]        
Hamzeh, K., Pall, G., Verthein, W., Taarud, J.,
Little, W., and G. Zorn, "Point-to-Point Tunneling
Protocol", RFC 2637, July 1999.
.IP [RFC2661]        
Townsley, W., Valencia, A., Rubens, A., Pall, G.,
Zorn, G., and B. Palter, "Layer Two Tunneling
Protocol "L2TP"", RFC 2661, August 1999.
.IP [RFC3766]
Orman. H. and P. Hoffman, "Determining Strengths for Public Keys Used for
Exchanging Symmetric Keys", RFC 3766, April 2004.
.IP [RFC4017]
Stanley, D., Walker, J. and B. Aboba, "Extensible Authentication Protocol (EAP)
Method Requirements for Wireless LANs", RFC 4017, March 2005. 
.IP [RFC4284]
Adrangi, F., Lortz, V., Bari, F. and P. Eronen, "Identity Selection Hints for the Extensible
Authentication Protocol (EAP)", RFC 4284, January 2006.
.IP [RFC5216]
Simon, D., Aboba, B. and R. Hurst, "The EAP-TLS Authentication Protocol",
RFC 5216, March 2008.
.IP [RFC5247]
Aboba, B., Simon, D. and P. Eronen, 
"Extensible Authentication Protocol (EAP) Key Management
Framework", RFC 5247, August 2008.
.IP [SP800-57]
National Institute of Standards and Technology,
"Recommendation for Key Management", Special Publication
800-57, May 2006.
.IP [UNAUTH]
Schulzrinne. H., McCann, S., Bajko, G. and H. Tschofenig,
"Extensions to the Emergency Services Architecture for dealing
with Unauthenticated and Unauthorized Devices", Internet draft
(work in progress), draft-schulzrinne-ecrit-unauthenticated-access-01.txt,
November 2007.
.LP
Acknowledgments

.in +0.3i
Thanks to Terence Spies, Mudit Goel, Anthony Leibovitz and Narendra Gidwani of 
Microsoft, Glen Zorn of NetCube, Joe Salowey of Cisco and Pasi Eronen of Nokia 
for useful discussions of this problem space.
.in -0.3i

Authors' Addresses

.in +0.3i
.nf
Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052-6399

Phone: +1 425 706 6605
Fax:   +1 425 936 7329
EMail: bernard.aboba@gmail.com

Ryan Hurst
Google
EMail: rmh@google.com

Alan DeKok
The FreeRADIUS Server Project
EMail: aland@freeradius.org
.fi
.in -0.3i
.bp
Appendix A - Changes from RFC 5216

.in +0.3i
This Appendix lists the major changes between 
[RFC5216] and this document.  Minor changes, including
style, grammar, spelling, and editorial changes are not
mentioned here.

o The document now cites TLS v1.3 as a normative reference (Sections 1, 6.1). 

o The EAP-TLS key hierarchy is defined using the TLS key extractor from
[RFC5705]. 

o Mandatory, recommended and optional TLS ciphersuites are inherited from
the TLS versions that are supported.
.in -0.3i