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rfc:rfc5216

Network Working Group D. Simon Request for Comments: 5216 B. Aboba Obsoletes: 2716 R. Hurst Category: Standards Track Microsoft Corporation

                                                            March 2008
                The EAP-TLS Authentication Protocol

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Abstract

 The Extensible Authentication Protocol (EAP), defined in RFC 3748,
 provides support for multiple authentication methods.  Transport
 Layer 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 2716.  A summary of the changes between
 this document and RFC 2716 is available in Appendix A.

Simon, et al. Standards Track [Page 1] RFC 5216 EAP-TLS Authentication Protocol March 2008

Table of Contents

 1. Introduction ....................................................2
    1.1. Requirements ...............................................3
    1.2. Terminology ................................................3
 2. Protocol Overview ...............................................4
    2.1. Overview of the EAP-TLS Conversation .......................4
         2.1.1. Base Case ...........................................4
         2.1.2. Session Resumption ..................................7
         2.1.3. Termination .........................................8
         2.1.4. Privacy ............................................11
         2.1.5. Fragmentation ......................................14
    2.2. Identity Verification .....................................16
    2.3. Key Hierarchy .............................................17
    2.4. Ciphersuite and Compression Negotiation ...................19
 3. Detailed Description of the EAP-TLS Protocol ...................20
    3.1. EAP-TLS Request Packet ....................................20
    3.2. EAP-TLS Response Packet ...................................22
 4. IANA Considerations ............................................23
 5. Security Considerations ........................................24
    5.1. Security Claims ...........................................24
    5.2. Peer and Server Identities ................................25
    5.3. Certificate Validation ....................................26
    5.4. Certificate Revocation ....................................27
    5.5. Packet Modification Attacks ...............................28
 6. References .....................................................29
    6.1. Normative References ......................................29
    6.2. Informative References ....................................29
 Acknowledgments ...................................................31
 Appendix A -- Changes from RFC 2716 ...............................32

1. Introduction

 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 the Point-to-Point
 Protocol (PPP) [RFC1661], Layer 2 tunneling protocols such as the
 Point-to-Point Tunneling Protocol (PPTP) [RFC2637] or Layer 2
 Tunneling Protocol (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

Simon, et al. Standards Track [Page 2] RFC 5216 EAP-TLS Authentication Protocol March 2008

 requirements.  As described in "Extensible Authentication Protocol
 (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 protected ciphersuite negotiation, mutual
 authentication and key management capabilities of the TLS protocol,
 described in "The Transport Layer Security (TLS) Protocol
 Version 1.1" [RFC4346].  While this document obsoletes RFC 2716
 [RFC2716], it remains backward compatible with it.  A summary of the
 changes between this document and RFC 2716 is available in Appendix
 A.

1.1. Requirements

 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].

1.2. Terminology

 This document frequently uses the following terms:
 authenticator
   The entity initiating EAP authentication.
 peer
   The entity that responds to the authenticator.  In [IEEE-802.1X],
   this entity is known as the Supplicant.
 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].
 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.

Simon, et al. Standards Track [Page 3] RFC 5216 EAP-TLS Authentication Protocol March 2008

 Master Session Key (MSK)
   Keying material that is derived between the EAP peer and server and
   exported by the EAP method.
 Extended Master Session Key (EMSK)
   Additional keying material derived between the EAP peer and server
   that is exported by the EAP method.

2. Protocol Overview

2.1. Overview of the EAP-TLS Conversation

 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 MAY 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.

2.1.1. Base Case

 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

Simon, et al. Standards Track [Page 4] RFC 5216 EAP-TLS Authentication Protocol March 2008

 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

Simon, et al. Standards Track [Page 5] RFC 5216 EAP-TLS Authentication Protocol March 2008

 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:
 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

Simon, et al. Standards Track [Page 6] RFC 5216 EAP-TLS Authentication Protocol March 2008

2.1.2. Session Resumption

 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
 Section 7.2.1 of [RFC3748].
 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 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 to which it will be
 connecting, 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.

Simon, et al. Standards Track [Page 7] RFC 5216 EAP-TLS Authentication Protocol March 2008

 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
 -------------------     -------------
                         <- 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

2.1.3. Termination

 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 immediately terminating the
 conversation so as to allow the peer to inform the user or log 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 per-peer limit

Simon, et al. Standards Track [Page 8] RFC 5216 EAP-TLS Authentication Protocol March 2008

 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.

Simon, et al. Standards Track [Page 9] RFC 5216 EAP-TLS Authentication Protocol March 2008

 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:
 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)

Simon, et al. Standards Track [Page 10] RFC 5216 EAP-TLS Authentication Protocol March 2008

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

2.1.4. Privacy

 EAP-TLS peer and server implementations MAY support privacy.
 Disclosure of the username is avoided by utilizing a privacy Network
 Access Identifier (NAI) [RFC4282] 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.

Simon, et al. Standards Track [Page 11] RFC 5216 EAP-TLS Authentication Protocol March 2008

 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) that
 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:
 Authenticating Peer     Authenticator
 -------------------     -------------
                         <- EAP-Request/
                         Identity
 EAP-Response/
 Identity (Anonymous NAI) ->
                         <- EAP-Request/
                         EAP-Type=EAP-TLS
                         (TLS Start)

Simon, et al. Standards Track [Page 12] RFC 5216 EAP-TLS Authentication Protocol March 2008

 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

Simon, et al. Standards Track [Page 13] RFC 5216 EAP-TLS Authentication Protocol March 2008

2.1.5. 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 16 MB.  The group of EAP-TLS messages
 sent in a single round may thus be larger than the MTU size or the
 maximum Remote Authentication Dail-In User Service (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.

Simon, et al. Standards Track [Page 14] RFC 5216 EAP-TLS Authentication Protocol March 2008

 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:
 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)

Simon, et al. Standards Track [Page 15] RFC 5216 EAP-TLS Authentication Protocol March 2008

 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

2.2. Identity Verification

 As noted in Section 5.1 of [RFC3748]:
    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.
 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
 [KEYFRAME]) and server identity (Server-Id in [KEYFRAME]).  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.

Simon, et al. Standards Track [Page 16] RFC 5216 EAP-TLS Authentication Protocol March 2008

2.3. Key Hierarchy

 Figure 1 illustrates the TLS Key Hierarchy, described in [RFC4346]
 Section 6.3.  The derivation proceeds as follows:
 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.
 In EAP-TLS, the MSK, EMSK, and Initialization Vector (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.

Simon, et al. Standards Track [Page 17] RFC 5216 EAP-TLS Authentication Protocol March 2008

          |                       | pre_master_secret       |
    server|                       |                         | client
    Random|                       V                         | Random
          |     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     |
          |     |                                     |     |
          +---->|             master_secret           |<----+
          |     |               (TMS)                 |     |
          |     |                                     |     |
          |     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     |
          |                       |                         |
          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
 EAP-TLS derives exported keying material and parameters as follows:
 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

Simon, et al. Standards Track [Page 18] RFC 5216 EAP-TLS Authentication Protocol March 2008

 Where:
 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.
 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.
       |                       | 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
 The use of these keys is specific to the lower layer, as described in
 Section 2.1 of [KEYFRAME].

2.4. Ciphersuite and Compression Negotiation

 EAP-TLS implementations MUST support TLS v1.0.
 EAP-TLS implementations need not necessarily support all TLS
 ciphersuites listed in [RFC4346].  Not all TLS ciphersuites are
 supported by available TLS tool kits, and licenses may be required in
 some cases.

Simon, et al. Standards Track [Page 19] RFC 5216 EAP-TLS Authentication Protocol March 2008

 To ensure interoperability, EAP-TLS peers and servers MUST support
 the TLS [RFC4346] mandatory-to-implement ciphersuite:
    TLS_RSA_WITH_3DES_EDE_CBC_SHA
 EAP-TLS peers and servers SHOULD also support and be able to
 negotiate the following TLS ciphersuites:
    TLS_RSA_WITH_RC4_128_SHA [RFC4346]
    TLS_RSA_WITH_AES_128_CBC_SHA [RFC3268]
 In addition, EAP-TLS servers SHOULD support and be able to negotiate
 the following TLS ciphersuite:
    TLS_RSA_WITH_RC4_128_MD5 [RFC4346]
 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.

3. Detailed Description of the EAP-TLS Protocol

3.1. EAP-TLS Request Packet

 A summary of the 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

Simon, et al. Standards Track [Page 20] RFC 5216 EAP-TLS Authentication Protocol March 2008

 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 Data
    fields.  Octets outside the range of the Length field should be
    treated as Data Link Layer padding and MUST be ignored on
    reception.
 Type
    13 -- EAP-TLS
 Flags
    0 1 2 3 4 5 6 7 8
    +-+-+-+-+-+-+-+-+
    |L M S R R R R R|
    +-+-+-+-+-+-+-+-+
    L = Length included
    M = More fragments
    S = EAP-TLS start
    R = Reserved
    The L bit (length included) is set to indicate the presence of the
    four-octet TLS Message Length field, and MUST be set for the first
    fragment of a fragmented TLS message or set of messages.  The M
    bit (more fragments) is set on all but the last fragment.  The S
    bit (EAP-TLS start) is set in an EAP-TLS Start message.  This
    differentiates the EAP-TLS Start message from a fragment
    acknowledgment.  Implementations of this specification MUST set
    the reserved bits to zero, and MUST ignore them on reception.
 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.

Simon, et al. Standards Track [Page 21] RFC 5216 EAP-TLS Authentication Protocol March 2008

 TLS data
    The TLS data consists of the encapsulated TLS packet in TLS record
    format.

3.2. EAP-TLS Response Packet

    A summary of the EAP-TLS Response packet format is shown below.
    The fields are transmitted from left to right.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Code      |   Identifier  |            Length             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Type      |     Flags     |      TLS Message Length
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     TLS Message Length        |       TLS Data...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Code
    2
 Identifier
    The Identifier field is one octet and MUST match the Identifier
    field from the corresponding request.
 Length
    The Length field is two octets and indicates the length of the EAP
    packet including the Code, 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.
 Type
    13 -- EAP-TLS

Simon, et al. Standards Track [Page 22] RFC 5216 EAP-TLS Authentication Protocol March 2008

 Flags
    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
    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.
 TLS Message Length
    The TLS Message Length field is four octets, and is present only
    if the L bit is set.  This field provides the total length of the
    TLS message or set of messages that is being fragmented.
 TLS data
    The TLS data consists of the encapsulated TLS packet in TLS record
    format.

4. IANA Considerations

 IANA has allocated EAP Type 13 for EAP-TLS.  The allocation has been
 updated to reference this document.

Simon, et al. Standards Track [Page 23] RFC 5216 EAP-TLS Authentication Protocol March 2008

5. Security Considerations

5.1. Security Claims

 EAP security claims are defined in Section 7.2.1 of [RFC3748].  The
 security claims for EAP-TLS are as follows:
 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
 -----
 [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] Section 5 of BCP 86 [RFC3766] offers advice on the required RSA
 or Diffie-Hellman (DH) module and Digital Signature Algorithm (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 of 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 when utilized with
 TLS v1.2 [RFC4346bis].

Simon, et al. Standards Track [Page 24] RFC 5216 EAP-TLS Authentication Protocol March 2008

5.2. Peer and Server Identities

 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 Section 4.1.2.6 of [RFC3280].  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 Section 2.1 of [RFC4282].
 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.

Simon, et al. Standards Track [Page 25] RFC 5216 EAP-TLS Authentication Protocol March 2008

 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.

5.3. Certificate Validation

 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 Section 4.2.2.1 of [RFC3280].
 Where the EAP-TLS server is unable to retrieve intermediate
 certificates, either it 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 Section 7.4.6 of [RFC4346]).
 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.

Simon, et al. Standards Track [Page 26] RFC 5216 EAP-TLS Authentication Protocol March 2008

 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 Section 4.2.1.13 of [RFC3280]) extension and ensure that
 at least one of the following is true:
 1) The certificate issuer included no Extended Key Usage identifiers
    in the certificate.
 2) The issuer included the anyExtendedKeyUsage identifier in the
    certificate (see Section 4.2.1.13 of [RFC3280]).
 3) The issuer included the id-kp-serverAuth identifier in the
    certificate (see Section 4.2.1.13 [RFC3280]).
 When performing this comparison, implementations MUST follow the
 validation rules specified in Section 3.1 of [RFC2818].  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 Section
 4.2.1.13 [RFC3280]) extension and ensure that at least one of the
 following is true:
 1) The certificate issuer included no Extended Key Usage identifiers
    in the certificate.
 2) The issuer included the anyExtendedKeyUsage identifier in the
    certificate (see Section 4.2.1.13 of [RFC3280]).
 3) The issuer included the id-kp-clientAuth identifier in the
    certificate (see Section 4.2.1.13 of [RFC3280]).

5.4. Certificate Revocation

 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 Section 3.3 of
 [RFC3280].  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.

Simon, et al. Standards Track [Page 27] RFC 5216 EAP-TLS Authentication Protocol March 2008

 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", Section 3.6 of [RFC4366].  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.

5.5. Packet Modification Attacks

 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.

Simon, et al. Standards Track [Page 28] RFC 5216 EAP-TLS Authentication Protocol March 2008

6. References

6.1. Normative References

 [RFC2119]      Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.
 [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.
 [RFC2818]      Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
 [RFC3268]      Chown, P., "Advanced Encryption Standard (AES)
                Ciphersuites for Transport Layer Security (TLS)", RFC
                3268, June 2002.
 [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.
 [RFC3748]      Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and
                H. Levkowetz, Ed., "Extensible Authentication Protocol
                (EAP)", RFC 3748, June 2004.
 [RFC4282]      Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
                Network Access Identifier", RFC 4282, December 2005.
 [RFC4346]      Dierks, T. and E. Rescorla, "The Transport Layer
                Security (TLS) Protocol Version 1.1", RFC 4346, April
                2006.
 [RFC4366]      Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen,
                J., and T. Wright, "Transport Layer Security (TLS)
                Extensions", RFC 4366, April 2006.

6.2. Informative References

 [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.

Simon, et al. Standards Track [Page 29] RFC 5216 EAP-TLS Authentication Protocol March 2008

 [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.
 [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.
 [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
 [KEYFRAME]     Aboba, B., Simon, D. and P. Eronen, "Extensible
                Authentication Protocol (EAP) Key Management
                Framework", Work in Progress, November 2007.
 [RFC1661]      Simpson, W., Ed., "The Point-to-Point Protocol (PPP)",
                STD 51, RFC 1661, July 1994.
 [RFC2548]      Zorn, G., "Microsoft Vendor-specific RADIUS
                Attributes", RFC 2548, March 1999.
 [RFC2637]      Hamzeh, K., Pall, G., Verthein, W., Taarud, J.,
                Little, W., and G. Zorn, "Point-to-Point Tunneling
                Protocol (PPTP)", RFC 2637, July 1999.
 [RFC2661]      Townsley, W., Valencia, A., Rubens, A., Pall, G.,
                Zorn, G., and B. Palter, "Layer Two Tunneling Protocol
                "L2TP"", RFC 2661, August 1999.
 [RFC2716]      Aboba, B. and D. Simon, "PPP EAP TLS Authentication
                Protocol", RFC 2716, October 1999.
 [RFC3766]      Orman, H. and P. Hoffman, "Determining Strengths For
                Public Keys Used For Exchanging Symmetric Keys", BCP
                86, RFC 3766, April 2004.
 [RFC4017]      Stanley, D., Walker, J., and B. Aboba, "Extensible
                Authentication Protocol (EAP) Method Requirements for
                Wireless LANs", RFC 4017, March 2005.

Simon, et al. Standards Track [Page 30] RFC 5216 EAP-TLS Authentication Protocol March 2008

 [RFC4284]      Adrangi, F., Lortz, V., Bari, F., and P. Eronen,
                "Identity Selection Hints for the Extensible
                Authentication Protocol (EAP)", RFC 4284, January
                2006.
 [SP800-57]     National Institute of Standards and Technology,
                "Recommendation for Key Management", Special
                Publication 800-57, May 2006.
 [RFC4346bis]   Dierks, T. and E. Rescorla, "The TLS Protocol Version
                1.2", Work in Progress, February 2008.
 [UNAUTH]       Schulzrinne. H., McCann, S., Bajko, G. and H.
                Tschofenig, "Extensions to the Emergency Services
                Architecture for dealing with Unauthenticated and
                Unauthorized Devices", Work in Progress, November
                2007.

Acknowledgments

 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.

Simon, et al. Standards Track [Page 31] RFC 5216 EAP-TLS Authentication Protocol March 2008

Appendix A – Changes from RFC 2716

 This appendix lists the major changes between [RFC2716] and this
 document.  Minor changes, including style, grammar, spelling, and
 editorial changes, are not mentioned here.
 o  As EAP is now in use with a variety of lower layers, not just PPP
    for which it was first designed, mention of PPP is restricted to
    situations relating to PPP-specific behavior and reference is made
    to other lower layers such as IEEE 802.11, IEEE 802.16, etc.
 o  The document now cites TLS v1.1 as a normative reference (Sections
    1 and 6.1).
 o  The terminology section has been updated to reflect definitions
    from [RFC3748] (Section 1.2), and the EAP Key Management Framework
    [KEYFRAME] (Section 1.2).
 o  Use for peer unauthenticated access is clarified (Section 2.1.1).
 o  Privacy is supported as an optional feature (Section 2.1.4).
 o  It is no longer recommended that the identity presented in the
    EAP-Response/Identity be compared to the identity provided in the
    peer certificate (Section 2.2).
 o  The EAP-TLS key hierarchy is defined, using terminology from
    [RFC3748].  This includes formulas for the computation of TEKs as
    well as the MSK, EMSK, IV, and Session-Id (Section 2.3).
 o  Mandatory and recommended TLS ciphersuites are provided.  The use
    of TLS ciphersuite negotiation for determining the lower layer
    ciphersuite is prohibited (Section 2.4).
 o  The Start bit is not set within an EAP-Response packet (Section
    3.2).
 o  A section on security claims has been added and advice on key
    strength is provided (Section 5.1).
 o  The Peer-Id and Server-Id are defined (Section 5.2), and
    requirements for certificate validation (Section 5.3) and
    revocation (Section 5.4) are provided.
 o  Packet modification attacks are described (Section 5.5).

Simon, et al. Standards Track [Page 32] RFC 5216 EAP-TLS Authentication Protocol March 2008

 o  The examples have been updated to reflect typical messages sent in
    the described scenarios.  For example, where mutual authentication
    is performed, the EAP-TLS server is shown to request a client
    certificate and the peer is shown to provide a certificate_verify
    message.  A privacy example is provided, and two faulty examples
    of session resume failure were removed.

Authors' Addresses

 Dan Simon
 Microsoft Corporation
 One Microsoft Way
 Redmond, WA 98052-6399
 Phone: +1 425 882 8080
 Fax:   +1 425 936 7329
 EMail: dansimon@microsoft.com
 Bernard Aboba
 Microsoft Corporation
 One Microsoft Way
 Redmond, WA 98052-6399
 Phone: +1 425 706 6605
 Fax:   +1 425 936 7329
 EMail: bernarda@microsoft.com
 Ryan Hurst
 Microsoft Corporation
 One Microsoft Way
 Redmond, WA 98052-6399
 Phone: +1 425 882 8080
 Fax:   +1 425 936 7329
 EMail: rmh@microsoft.com

Simon, et al. Standards Track [Page 33] RFC 5216 EAP-TLS Authentication Protocol March 2008

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Simon, et al. Standards Track [Page 34]

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