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Versions: (RFC 2716) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 RFC 5216

Network Working Group                                          Dan Simon
INTERNET-DRAFT                                             Bernard Aboba
Category: Proposed Standard                                    Microsoft
<draft-simon-emu-rfc2716bis-04.txt>
22 October 2006


                  The EAP TLS Authentication Protocol

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   This Internet-Draft will expire on May 10, 2007.

Copyright Notice

   Copyright (C) The Internet Society 2006.

Abstract

   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.







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Table of Contents

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    Retry Behavior ..................................    8
      2.3    Fragmentation ...................................    8
      2.4    Identity Verification ...........................    9
      2.5    Key Hierarchy ...................................   10
      2.6    Ciphersuite and Compression Negotiation .........   13
      2.7    Privacy .........................................   13
3.  Detailed Description of the EAP-TLS Protocol .............   14
      3.1     EAP TLS Packet Format ..........................   14
      3.2     EAP TLS Request Packet .........................   16
      3.3     EAP TLS Response Packet ........................   17
4.  Security Considerations ..................................   19
      4.1     Security Claims ................................   19
      4.2     Certificate Usage ..............................   20
      4.3     Certificate Revocation .........................   21
      4.4     Separation of EAP Authenticator and Server .....   21
      4.5     Lower Layer Security Mechanisms ................   22
      4.6     Packet Modification Attacks ....................   22
5.  References ...............................................   23
      5.1    Normative references ....... ....................   23
      5.2    Informative references ..........................   23
Acknowledgments ..............................................   25
Authors' Addresses ...........................................   26
Appendix A - Changes from RFC 2716 ...........................   27
Appendix B - Examples ........................................   28
Intellectual Property Statement ..............................   35
Disclaimer of Validity .......................................   35
Copyright Statement ..........................................   36

















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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.

   While the EAP methods defined in [RFC3748] did not support mutual
   authentication, the use of EAP with wireless technologies such as
   [IEEE-802.11i] 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.  Since PPP encryption protocols such as DESE-bis
   [RFC2419], 3DESE [RFC2420], and MPPE [RFC3078] assume existence of a
   session key, it is useful to have a mechanism for session key
   establishment.  Since design of secure key management protocols is
   non-trivial, it is desirable to avoid creating new mechanisms for
   this.  The EAP protocol described in this document allows a EAP peer
   to take advantage of 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].

1.1.  Requirements

   In this document, several words are used to signify the requirements
   of the specification.  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
   "Key words for use in RFCs to Indicate Requirement Levels" [RFC2119].

1.2.  Terminology

    This document frequently uses the following terms:

authenticator
     The end of the link initiating EAP authentication.  The term
     authenticator is used in [IEEE-802.1X], and has the same meaning in
     this document.

peer The end of the link that responds to the authenticator.  In
     [IEEE-802.1X], this end 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



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     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.

Master Session Key (MSK)
     Keying material that is derived between the EAP peer and server and
     exported by the EAP method.  The MSK is at least 64 octets in
     length.

Extended Master Session Key (EMSK)
     Additional keying material derived between the EAP client and
     server that is exported by the EAP method.  The EMSK is at least 64
     octets in length.

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 userId.

   From this point forward, while nominally the EAP conversation occurs
   between the EAP authenticator and the peer, the authenticator MAY act
   as a passthrough device, with the EAP packets received from the peer
   being encapsulated for transmission to a backend security server. In
   the discussion that follows, we will use the term "EAP server" to
   denote the ultimate endpoint conversing with the peer.

   Once having received the peer's Identity, the EAP server MUST respond
   with an EAP-TLS/Start packet, which is an EAP-Request packet with
   EAP-Type=EAP-TLS, the Start (S) bit set, and no data.  The EAP-TLS
   conversation will then begin, with the peer sending an EAP-Response
   packet with EAP-Type=EAP-TLS.  The data field of that packet will
   encapsulate one or more TLS records in TLS record layer format,
   containing a TLS client_hello handshake message.  The current cipher
   spec for the TLS records will be TLS_NULL_WITH_NULL_NULL and null
   compression.  This current cipher spec remains the same until the
   change_cipher_spec message signals that subsequent records will have
   the negotiated attributes for the remainder of the handshake.



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   The client_hello message contains the client's TLS version number, a
   sessionId, a random number, and a set of ciphersuites supported by
   the client. The version offered by the client MUST correspond to TLS
   v1.0 or later.

   The EAP server will then respond with an EAP-Request packet with EAP-
   Type=EAP-TLS. The data field of this packet will encapsulate one or
   more TLS records. These will contain a TLS server_hello handshake
   message, possibly followed by TLS certificate, server_key_exchange,
   certificate_request, server_hello_done and/or finished handshake
   messages, and/or a TLS change_cipher_spec message.  The server_hello
   handshake message contains a TLS version number, another random
   number, a sessionId, and a ciphersuite.  The version offered by the
   server MUST correspond to TLS v1.0 or later.

   If the client's sessionId is null or unrecognized by the server, the
   server MUST choose the sessionId to establish a new session;
   otherwise, the sessionId  will  match  that  offered by the client,
   indicating a resumption of the previously established session with
   that sessionID.  The server will also choose a ciphersuite from those
   offered by  the client; if the session matches the client's, then the
   ciphersuite MUST match the one negotiated during the handshake
   protocol execution that established the session.

   The purpose of the sessionId within the TLS protocol is to allow for
   improved efficiency in the case where a client repeatedly attempts to
   authenticate to an EAP server within a short period of time.  While
   this model was developed for use with HTTP authentication, it 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, then client will only be able to continue sessions when
   connecting to the same NAS or tunnel server.  Should these devices be
   set up in a rotary or round-robin then it may not be possible for the
   peer to know in advance the authenticator it will be connecting to,
   and therefore which sessionId to attempt to reuse. As a result, it is
   likely that the continuation attempt will fail.  In the case where
   the EAP authentication is remoted then continuation is much more
   likely to be successful, since multiple NAS devices and tunnel



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   servers will remote their EAP authentications to 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.  If the EAP server is not resuming a previously established
   session, then it MUST include a TLS server_certificate handshake
   message, and a server_hello_done handshake message MUST be the last
   handshake message encapsulated in this EAP-Request packet.

   The certificate message contains a public key certificate chain for
   either a key exchange public key (such as an RSA or Diffie-Hellman
   key exchange public key) or a signature public key (such as an RSA or
   DSS signature public key).  In the latter case, a TLS
   server_key_exchange handshake message MUST also be included to allow
   the key exchange to take place.

   The certificate_request message is included when the server desires
   the client to authenticate itself via public key.  While the EAP
   server SHOULD require client authentication, this is not a
   requirement, since it may be possible that the server will require
   that the peer authenticate via some other means.

   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.  The data field of this packet will encapsulate one or
   more TLS records containing a TLS change_cipher_spec message and
   finished handshake message, and possibly certificate,
   certificate_verify and/or client_key_exchange handshake messages.  If
   the preceding server_hello message sent by the EAP server in the
   preceding EAP-Request packet indicated the resumption of a previous
   session, then the peer MUST send only the change_cipher_spec and
   finished handshake messages.  The finished message contains the
   peer's authentication response to the EAP server.

   If the preceding server_hello message sent by the EAP server in the
   preceeding EAP-Request packet did not indicate the resumption of a
   previous session, then the peer MUST send, in addition to the
   change_cipher_spec and finished messages, a client_key_exchange
   message, which completes the exchange of a shared master secret
   between the peer and the EAP server.  If the EAP server sent a
   certificate_request message in the preceding EAP-Request packet, then
   unless the peer is configured for privacy (see Section 2.7) the peer
   MUST send, in addition, certificate and certificate_verify handshake
   messages.  The former contains a certificate for the peer's signature
   public key, while the latter contains the peer's signed



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   authentication response to the EAP server.  After receiving this
   packet, the EAP server will verify the peer's certificate and digital
   signature, if requested.

   If the peer's authentication is unsuccessful, the EAP server SHOULD
   send an EAP-Request packet with EAP-Type=EAP-TLS, encapsulating a TLS
   record containing the appropriate TLS alert message.  The EAP server
   SHOULD send a TLS alert message rather immediately terminating the
   conversation so as to allow the peer to inform the user of the cause
   of the failure and possibly allow for a restart of the conversation.

   To ensure that the peer receives the TLS alert message, the EAP
   server MUST wait for the peer to reply with an EAP-Response packet.
   The EAP-Response packet sent by the peer MAY encapsulate a TLS
   client_hello handshake message, in which case the EAP server MAY
   allow the EAP-TLS conversation to be restarted, or it MAY contain an
   EAP-Response packet with EAP-Type=EAP-TLS and no data, in which case
   the EAP-Server MUST send an EAP-Failure packet, and terminate the
   conversation. It is up to the EAP server whether to allow restarts,
   and if so, how many times the conversation can be restarted. An EAP
   Server implementing restart capability SHOULD impose a limit on the
   number of restarts, so as to protect against denial of service
   attacks.

   If the peers authenticates successfully, the EAP server MUST respond
   with an EAP-Request packet with EAP-Type=EAP-TLS, which includes, in
   the case of a new TLS session, one or more TLS records containing TLS
   change_cipher_spec and finished handshake messages.  The latter
   contains the EAP server's authentication response to the peer.  The
   peer will then verify the hash in order to authenticate the EAP
   server.

   If the EAP server authenticates unsuccessfully, the peer MAY send an
   EAP-Response packet of EAP-Type=EAP-TLS containing a TLS Alert
   message identifying the reason for the failed authentication.  The
   peer MAY send a TLS alert message rather than immediately terminating
   the conversation so as to allow the EAP server to log the cause of
   the error for examination by the system administrator.

   To ensure that the EAP Server receives the TLS alert message, the
   peer MUST wait for the EAP-Server to reply before terminating the
   conversation.  The EAP Server MUST reply with an EAP-Failure packet
   since server authentication failure is a terminal condition.

   If the EAP server authenticates successfully, the peer MUST send an
   EAP-Response packet of EAP-Type=EAP-TLS, and no data.  The EAP-Server
   then MUST respond with an EAP-Success message.




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2.2.  Retry Behavior

   As with other EAP protocols, the EAP authenticator is responsible for
   retry behavior.  This means that if the EAP authenticator does not
   receive a reply from the peer, it MUST resend the EAP-Request for
   which it has not yet received an EAP-Response.  However, the peer
   MUST NOT resend EAP- Response packets without first being prompted by
   the EAP authenticator.

   For example, if the initial EAP-TLS start packet sent by the EAP
   authenticator were to be lost, then the peer would not receive this
   packet, and would not respond to it.  As a result, the EAP-TLS start
   packet would be resent by the EAP authenticator.  Once the peer
   received the EAP-TLS start packet, it would send an EAP-Response
   encapsulating the client_hello message.  If the EAP-Response were to
   be lost, then the EAP authenticator would resend the initial EAP-TLS
   start, and the peer would resend the EAP-Response.

   As a result, it is possible that a peer will receive duplicate EAP-
   Request messages, and may send duplicate EAP-Responses.  Both the
   peer and the EAP server should be engineered to handle this
   possibility.

2.3.  Fragmentation

   A single TLS record may be up to 16384 octets in length, but a TLS
   message may span multiple TLS records, and a TLS certificate message
   may in principle be as long as 16MB.  The group of EAP-TLS messages
   sent in a single round may thus be larger than the PPP MTU size, the
   maximum RADIUS packet size of 4096 octets, or even the Multilink
   Maximum Received Reconstructed Unit (MRRU).  As described in
   [RFC1990], the multilink MRRU is negotiated via the Multilink MRRU
   LCP option, which includes an MRRU length field of two octets, and
   thus can support MRRUs as large as 64 KB.

   However, note that in order to protect against reassembly lockup and
   denial of service attacks, it may be desirable for an implementation
   to set a maximum size for one such group of TLS messages. Since a
   typical certificate chain is rarely longer than a few thousand
   octets, and no other field is likely to be anywhere near as long, a
   reasonable choice of maximum acceptable message length might be 64
   KB.

   If this value is chosen, then fragmentation can be handled via the
   multilink PPP fragmentation mechanisms described in [RFC1990].  While
   this is desirable, there may be cases in which multilink or the MRRU
   LCP option cannot be negotiated.  As a result, an EAP-TLS
   implementation MUST provide its own support for fragmentation and



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   reassembly.

   Since EAP is a simple ACK-NAK protocol, fragmentation support can be
   added in a simple manner.  In EAP, fragments that are lost or damaged
   in transit will be retransmitted, and since sequencing information is
   provided by the Identifier field in EAP, there is no need for a
   fragment offset field as is provided in IPv4.

   EAP-TLS fragmentation support is provided through addition of a flags
   octet within the EAP-Response and EAP-Request packets, as well as a
   TLS Message Length field of four octets. Flags include the Length
   included (L), More fragments (M), and EAP-TLS Start (S) bits.  The L
   flag is set to indicate the presence of the four octet TLS Message
   Length field, and MUST be set for the first fragment of a fragmented
   TLS message or set of messages. The M flag is set on all but the last
   fragment.  The S flag is set only within the EAP-TLS start message
   sent from the EAP server to the peer. The TLS Message Length field is
   four octets, and provides the total length of the TLS message or set
   of messages that is being fragmented; this simplifies buffer
   allocation.

   When an EAP-TLS peer receives an EAP-Request packet with the M bit
   set, it MUST respond with an EAP-Response with EAP-Type=EAP-TLS and
   no data.  This serves as a fragment ACK.  The EAP server MUST wait
   until it receives the EAP-Response before sending another fragment.
   In order to prevent errors in processing of fragments, the EAP server
   MUST increment the Identifier field for each fragment contained
   within an EAP-Request, and the peer MUST include this Identifier
   value in the fragment ACK contained within the EAP-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.

2.4.  Identity Verification

   As noted in [RFC3748] Section 5.1:

      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



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      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 4.2.

   Since the identity presented in the Identity Response need not be
   related to the identity presented in the peer certificate, EAP-TLS
   implementations SHOULD NOT require that they be identical, and SHOULD
   NOT use the identity presented in the Identity Response for access
   control or accounting purposes.

   The peer MUST verify the validity of the EAP server certificate, and
   SHOULD also examine the Server-id in order to determine whether the
   EAP server can be trusted.  For example, just because a server
   certificate can chain to a trust anchor does not necessarily imply
   that it is valid for connection to a particular network.  As a
   result, the peer may also want to test whether the EAP server
   certificate is signed by a CA controlling the destination network and
   whether the Server-Id matches the format expected for that network.
   For example, an EAP peer connecting to the "EXAMPLE" SSID may wish to
   check whether the Server-Id matches the regular expression
   "*.example.com", in addition to checking whether the server
   certificate chains to the example.com CA.

2.5.  Key Hierarchy

   Figure 1 illustrates the TEK key hierarchy for EAP-TLS which is based
   on the TLS key hierarchy described in [RFC4346].  The TLS-negotiated
   ciphersuite is used to set up a protected channel for use in
   protecting the EAP conversation,  keyed by the derived TEKs.  The TEK
   derivation proceeds as follows:

   master_secret = TLS-PRF-48(pre_master_secret, "master secret",
                    client.random || server.random)
   TEK           = 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.





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         |                       | pre_master_secret       |
   server|                       |                         | client
   Random|                       V                         | Random
         |     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     |
         |     |                                     |     |
         +---->|             master_secret           |<----+
         |     |               (TMS)                 |     |
         |     |                                     |     |
         |     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     |
         |                       |                         |
         V                       V                         V
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                         |
   |                    Key Block  (TEKs)                    |
   |               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

   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.

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

   MSK(0,63)    = TLS-PRF-64(TMS, "client EAP encryption",
                    client.random || server.random)
   EMSK(0,63)   = second 64 octets of:
                  TLS-PRF-128(TMS, "client EAP encryption",
                    client.random || server.random)
   IV(0,63)     = TLS-PRF-64("", "client EAP encryption",
                    client.random || server.random)



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   Enc-RECV-Key = MSK(0,31) = Peer to Authenticator Encryption Key
                  (MS-MPPE-Recv-Key in [RFC2548]).  Also known as the
                  PMK in [IEEE-802.11i].
   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   = 0x0C || client.random || server.random

   Where:

   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.

         |                       | pre_master_secret       |
   server|                       |                         | client
   Random|                       V                         | Random
         |     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     |
         |     |                                     |     |
         +---->|             master_secret           |<----+
         |     |               (TMS)                 |     |
         |     |                                     |     |
         |     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     |
         |                       |                         |
         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
   [KEYFRAME] Section 2.1.






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2.6.  Ciphersuite and Compression Negotiation

   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.

   To ensure interoperability, EAP-TLS peers and servers MUST support
   the TLS [RFC4346] mandatory-to-implement ciphersuite:

       TLS_RSA_WITH_3DES_EDE_CBC_SHA.

   In addition, EAP-TLS servers SHOULD support and be able to negotiate
   all of the following TLS ciphersuites:

       TLS_RSA_WITH_RC4_128_MD5
       TLS_RSA_WITH_RC4_128_SHA
       TLS_RSA_WITH_AES_128_CBC_SHA

   In addition, EAP-TLS peers SHOULD support the following TLS
   ciphersuites [RFC3268]:

       TLS_RSA_WITH_AES_128_CBC_SHA
       TLS_RSA_WITH_RC4_128_SHA

   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 SHOULD NOT be used to negotiate the ciphersuites used to
   secure data.

   TLS also supports compression as well as ciphersuite negotiation.
   Therefore during the EAP-TLS conversation the endpoints MAY request
   or negotiate TLS compression.  Since compression negotiated within
   EAP-TLS applies only to the EAP conversation, TLS compression
   negotiation MUST NOT be used to negotiate compression mechanisms to
   be applied to data.

2.7.  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 client certificate within a TLS session providing
   confidentiality.

   In order to avoid disclosing the peer username, an EAP-TLS peer



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   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 server_hello followed by a
   certificate_request.  An EAP-TLS peer supporting privacy MUST provide
   a certificate list containing at least one entry in response to this
   subsequent certificate_request.  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 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 or within identity
   selection hints [RFC4284] to determine the appropriate configuration.

3.  Detailed description of the EAP-TLS protocol

3.1.  EAP TLS Packet Format

   A summary of the EAP TLS Request/Response packet format is shown
   below.  The fields are transmitted from left to right.








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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |   Identifier  |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |        Data...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Code

      1 - Request
      2 - Response

   Identifier

      The Identifier field is one octet and aids in matching responses
      with requests.

   Length

      The Length field is two octets and indicates the length of the EAP
      packet including the Code, Identifier, Length, Type, and Data
      fields.  Octets outside the range of the Length field should be
      treated as Data Link Layer padding and MUST be ignored on
      reception.

   Type

      13 - EAP TLS

   Data

      The format of the Data field is determined by the Code field.


















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3.2.  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

   Identifier

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

   Length

      The Length field is two octets and indicates the length of the EAP
      packet including the Code, Identifier, Length, Type, and TLS
      Response fields.

   Type

      13 - EAP TLS

   Flags

      0 1 2 3 4 5 6 7 8
      +-+-+-+-+-+-+-+-+
      |L M S R R R R R|
      +-+-+-+-+-+-+-+-+

      L = Length included
      M = More fragments
      S = EAP-TLS start
      R = Reserved

      The L bit (length included) is set to indicate the presence of the



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      four octet TLS Message Length field, and MUST be set for the first
      fragment of a fragmented TLS message or set of messages. The M bit
      (more fragments) is set on all but the last fragment.  The S bit
      (EAP-TLS start) is set in an EAP-TLS Start message. This
      differentiates the EAP-TLS Start message from a fragment
      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.

   TLS data

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

3.3.  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 TLS data
      fields.



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   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.

   TLS data

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
















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4.  Security Considerations

4.1.  Security Claims

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

   Auth. mechanism:           Certificates
   Ciphersuite negotiation:   Yes [1]
   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-802.11i] 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 2.7.

   [3] As noted in [RFC3766] Section 5, the effective level of attack
   resistance provided by EAP-TLS is related to the RSA or DH module and
   DSA subgroup size in bits.  Based on the table below, a 2048-bit RSA
   key is required to provide 128-bit equivalent key strength.













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   Attack Resistance     RSA or DH Modulus            DSA subgroup
    (bits)                  size (bits)                size (bits)
   -----------------     -----------------            ------------
   70                         947                        128
   80                        1228                        145
   90                        1553                        153
   100                       1926                        184
   150                       4575                        279
   200                       8719                        373
   250                      14596                        475

4.2.  Certificate Usage

   The EAP-TLS peer name (Peer-Id) represents the Network Access
   Identifier (NAI) [RFC4282] to be used for access control and
   accounting purposes.  The Peer-Id and Server-Id are determined from
   the subject or altSubjectName fields in the peer and server
   certificates.  As noted in [RFC3280] Section 4.1.2.6:

      The subject field identifies the entity associated with the public
      key stored in the subject public key field.  The subject name MAY
      be carried in the subject field and/or the subjectAltName
      extension...  If subject naming information is present only in the
      subjectAltName extension (e.g., a key bound only to an email
      address or URI), then the subject name MUST be an empty sequence
      and the subjectAltName extension MUST be critical.

      Where it is non-empty, the subject field MUST contain an X.500
      distinguished name (DN).

   Where the subject field is not empty, the Peer-Id and Server-Id are
   obtained from the Common Name (CN) field of the DN.  Where subject
   naming information is present only in the subjectAltName extension,
   the Peer-Id and Server-Id are obtained from the subjectAltName.

   A valid EAP-TLS client certificate SHOULD contain an extendedKeyUsage
   value indicating support for Client Authentication
   (1.3.6.1.5.5.7.3.2).  A valid EAP-TLS server certificate SHOULD
   contain an extendedKeyUsage value indicating support for Server
   Authentication (1.3.6.1.5.5.7.3.1).

   As discussed in [He], the security of EAP-TLS can be compromised if
   the same credentials are used for authentication within multiple
   applications.  Certificate extensions for use with EAP-TLS are
   discussed in "Certificate Extensions and Attributes Supporting
   Authentication in Point-to-Point Protocol (PPP) and Wireless Local
   Area Networks (WLAN)" [RFC4334].  These extensions enable certificate
   usage to be restricted to use with lower layers such as PPP or IEEE



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   802.11.  An EAP-TLS implementation MAY make these and other
   certificate fields available to the lower layer.

4.3.  Certificate revocation

   Since the EAP server is on the Internet during the EAP conversation,
   the server is capable of following a certificate chain or verifying
   whether the peer's certificate has been revoked.  In contrast, the
   peer may or may not have Internet connectivity, and thus while it can
   validate the EAP server's certificate based on a pre-configured set
   of CAs, it may not be able to follow a certificate chain or verify
   whether the EAP server's certificate has been revoked.

   In the case where the peer is initiating a voluntary Layer 2 tunnel
   using PPTP [RFC2637] or L2TP [RFC2661], the peer will typically
   already have a PPP interface and Internet connectivity established at
   the time of tunnel initiation.  As a result, during the EAP
   conversation it is capable of checking for certificate revocation.

   However, in the case where the peer is initiating an initial PPP
   conversation, it will not have Internet connectivity and is therefore
   not capable of checking for certificate revocation until after NCP
   negotiation completes and the peer has access to the Internet.  In
   this case, the peer SHOULD check for certificate revocation after
   connecting to the Internet.

4.4.  Separation of the EAP Authenticator and Server

   As a result of the EAP-TLS conversation, the EAP peer and server
   endpoints will mutually authenticate and derive the MSK and EMSK.
   Subsequently the EAP peer and authenticator may negotiate a
   ciphersuite for protection of data and derive a session key for
   subsequent use.  Since the peer and EAP client reside on the same
   machine, it is necessary for the EAP client module to pass the
   required keying material to the lower layer, as described in
   [KEYFRAME].

   The situation may be more complex on the EAP authenticator, which may
   or may not reside on the same machine as the EAP server.  In the case
   where the EAP server and authenticator reside on different machines,
   there are several implications for security.  Firstly, the mutual
   authentication defined in EAP-TLS will occur between the EAP peer and
   server, not between the peer and the authenticator.  This means that
   as a result of the EAP-TLS conversation, it is not possible for the
   EAP peer to validate the identity of the NAS or tunnel server that it
   is speaking to.

   The second issue is that the EAP keying material derived between the



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   peer and EAP server will need to be transported to the authenticator.
   The implications of this are described in more detail in [KEYFRAME];
   however the specification of this transport mechanism is outside the
   scope of this document.

4.5.  Lower Layer Security Mechanisms

   EAP-TLS has been deployed for use with a variety of lower layers,
   including PPP, Layer 2 tunneling protocols such as PPTP and L2TP,
   IEEE 802 wired networks [IEEE-802.1X] and wireless technologies such
   as IEEE 802.11 [IEEE-802.11i] and IEEE 802.16 [IEEE-802.16e].

   In compulsory layer 2 tunneling, a PPP peer makes a connection to a
   NAS or router which tunnels the PPP packets to a tunnel server.
   Since with compulsory tunneling a PPP peer cannot tell whether its
   packets are being tunneled, let alone whether the network device is
   securing the tunnel, if security is required then the client must
   make its own arrangements. In the case where all endpoints cannot be
   relied upon to implement IPSEC, TLS, or another suitable security
   protocol, PPP encryption provides a convenient means to ensure the
   privacy of packets transiting between the client and the tunnel
   server.

4.6.  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 may 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, it may be possible
   for an attacker to 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.



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5.  References

5.1.  Normative References

[RFC2119]      Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

[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.
               Lefkowetz, "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 TLS Protocol Version
               1.1", RFC 4346, April 2006.

5.2.  Informative References

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

[IEEE-802.11]  Institute of Electrical and Electronics Engineers,
               "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 Standard
               802.11-2003, 2003.

[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.

[IEEE-802.11i] Institute of Electrical and Electronics Engineers,
               "Supplement to STANDARD FOR Telecommunications and
               Information Exchange between Systems - LAN/MAN Specific
               Requirements - Part 11: Wireless Medium Access Control
               (MAC) and physical layer (PHY) specifications:



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               Specification for Enhanced Security", IEEE 802.11i,
               December 2004.

[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., Eronen, P. and H. Levkowetz,
               "Extensible Authentication Protocol (EAP) Key Management
               Framework", Internet Draft (work in progress), draft-
               ietf-eap-keying-15.txt, October 2006.

[RFC1321]      Rivest, R. and S. Dusse, "The MD5 Message-Digest
               Algorithm", RFC 1321, April 1992.

[RFC1570]      Simpson, W., Editor, "PPP LCP Extensions", RFC 1570,
               January 1994.

[RFC1661]      Simpson, W., Editor, "The Point-to-Point Protocol (PPP)",
               STD 51, RFC 1661, July 1994.

[RFC1962]      D. Rand, "The PPP Compression Control Protocol", RFC
               1962, Novell, June 1996.

[RFC1968]      Meyer, G., "The PPP Encryption Protocol (ECP)", RFC 1968,
               June 1996.

[RFC1990]      Sklower, K., Lloyd, B., McGregor, G., Carr, D. and T.
               Coradetti, "The PPP Multilink Protocol (MP)", RFC 1990,
               August 1996.

[RFC2419]      Sklower, K. and G. Meyer, "The PPP DES Encryption
               Protocol, Version 2 (DESE-bis)", RFC 2419, September
               1998.

[RFC2420]      Hummert, K., "The PPP Triple-DES Encryption Protocol
               (3DESE)", RFC 2420, September 1998.

[RFC2548]      Zorn, G., "Microsoft Vendor-specific RADIUS Attributes",
               RFC 2548, March 1999.



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[RFC2637]      Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little,
               W., and G. Zorn, "Point-to-Point Tunneling Protocol", 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.

[RFC3078]      Pall, G. and G. Zorn, "Microsoft Point-to-Point
               Encryption (MPPE) Protocol", RFC 3078, March 2001.

[RFC3766]      Orman. H. and P. Hoffman, "Determining Strengths for
               Public Keys Used for Exchanging Symmetric Keys", 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.

[RFC4284]      Adrangi, F., Lortz, V., Bari, F. and P. Eronen, "Identity
               Selection Hints for the Extensible Authentication
               Protocol (EAP)", RFC 4284, January 2006.

[RFC4334]      Housley, R. and T. Moore, "Certificate Extensions and
               Attributes Supporting Authentication in Point-to-Point
               Protocol (PPP) and Wireless Local Area Networks (WLAN)",
               RFC 4334, February 2006.

Acknowledgments

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
















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Authors' Addresses

   Dan Simon
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

   Phone: +1 425 706 6711
   EMail: dansimon@microsoft.com

   Bernard Aboba
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

   Phone: +1 425 706 6605
   EMail: bernarda@microsoft.com


































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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 terminology section has been updated to reflect definitions
      from [RFC3748] (Section 1.2).

      o The responsibility for retry is clarified to reside with the EAP
      authenticator, not the EAP server (Section 2.2).

      o The recommendation that the identity presented in the EAP-
      Response/Identity correspond to the identity provided in the peer
      certificate has been removed.  The Peer-Id and Server-Id are
      defined.  Recommendations on EAP server certificate validation are
      provided (Section 2.4).

      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 2.5).

      o Mandatory and recommended TLS ciphersuites are provided.  The
      use of TLS ciphersuite negotiation for determining the lower layer
      ciphersuite is not recommended, and the prohibition on TLS
      compression negotiated has been removed (Section 2.6).

      o Privacy is supported as an optional feature (Section 2.7).

      o A section on security claims has been added and advice on key
      strength is provided (Section 4.1).

      o Advice on certificate usage is provided (Section 4.2).

      o Packet modification attacks are described (Section 4.6).

      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 client is shown to provide a
      certificate_verify message (Appendix B).





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   Appendix B - Examples

      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

      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 (AnonymousNAI) ->



Simon & Aboba               Proposed Standard                  [Page 28]

INTERNET-DRAFT      EAP TLS Authentication Protocol      22 October 2006


                              <- 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 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 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 & Aboba               Proposed Standard                  [Page 29]

INTERNET-DRAFT      EAP TLS Authentication Protocol      22 October 2006


      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)
      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/



Simon & Aboba               Proposed Standard                  [Page 30]

INTERNET-DRAFT      EAP TLS Authentication Protocol      22 October 2006


                              EAP-Type=EAP-TLS
                              (TLS change_cipher_spec,
                               TLS finished)
      EAP-Response/
      EAP-Type=EAP-TLS ->
                              <- EAP-Success

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

      Authenticating Peer     Authenticator
      -------------------     -------------
                              <- 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 ->



Simon & Aboba               Proposed Standard                  [Page 31]

INTERNET-DRAFT      EAP TLS Authentication Protocol      22 October 2006


                              <- EAP-Failure
                              (User Disconnected)

      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 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
      (TLS change_cipher_spec,
      TLS finished)
                              <- EAP-Request/
                              EAP-Type=EAP-TLS
      EAP-Response/
      EAP-Type=EAP-TLS
      (TLS Alert message) ->
                              <- EAP-Failure
                              (User Disconnected)




Simon & Aboba               Proposed Standard                  [Page 32]

INTERNET-DRAFT      EAP TLS Authentication Protocol      22 October 2006


      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

      In the case where a previously established session is being
      resumed, and the server authenticates to the client successfully
      but the client fails to authenticate to the server, the
      conversation will appear as follows:

      Authenticating Peer     Authenticator
      -------------------     -------------
                              <- 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



Simon & Aboba               Proposed Standard                  [Page 33]

INTERNET-DRAFT      EAP TLS Authentication Protocol      22 October 2006


                              (TLS server_hello,
                               TLS change_cipher_spec,
                               TLS finished)
      EA-Response/
      EAP-Type=EAP-TLS
      (TLS change_cipher_spec,
       TLS finished) ->
                              <- EAP-Request
                              EAP-Type=EAP-TLS
                              (TLS Alert message)
      EAP-Response
      EAP-Type=EAP-TLS ->
                               <- EAP-Failure
                               (User Disconnected)

      In the case where a previously established session is being
      resumed, and the server authentication is unsuccessful, the
      conversation will appear as follows:

      Authenticating Peer     Authenticator
      -------------------     -------------
                              <- 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-Request/
                              EAP-Type=EAP-TLS
      EAP-Response/
      EAP-Type=EAP-TLS
      (TLS Alert message) ->
                              <- EAP-Failure
                              (User Disconnected)



Simon & Aboba               Proposed Standard                  [Page 34]

INTERNET-DRAFT      EAP TLS Authentication Protocol      22 October 2006


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      This document and the information contained herein are provided on
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   Copyright Statement

      Copyright (C) The Internet Society (2006).  This document is
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   Acknowledgment

      Funding for the RFC Editor function is currently provided by the
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Simon & Aboba               Proposed Standard                  [Page 35]


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