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Versions: 00 01

EAP                                                           Paul Funk
Internet-Draft                                         Juniper Networks
Category: Standards Track                            Simon Blake-Wilson
<draft-funk-eap-ttls-v1-01.txt>                        Basic Commerce &
                                                       Industries, Inc.
                                                             March 2006



          EAP Tunneled TLS Authentication Protocol Version 1
                             (EAP-TTLSv1)



Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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Copyright Notice

   Copyright (C) The Internet Society (2006). All Rights Reserved.

Abstract

   EAP-TTLS is an EAP type that utilizes TLS to establish a secure
   connection between a client and server, through which additional
   information may be exchanged. The initial TLS handshake may mutually
   authenticate client and server; or it may perform a one-way
   authentication, in which only the server is authenticated to the
   client. The secure connection established by the initial handshake
   may then be used to allow the server to authenticate the client
   using existing, widely-deployed authentication infrastructures such

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   as RADIUS. The authentication of the client may itself be EAP, or it
   may be another authentication protocol such as PAP, CHAP, MS-CHAP or
   MS-CHAP-V2.

   Thus, EAP-TTLS allows legacy password-based authentication protocols
   to be used against existing authentication databases, while
   protecting the security of these legacy protocols against
   eavesdropping, man-in-the-middle and other cryptographic attacks.

   EAP-TTLS also allows client and server to exchange other information
   in addition to authentication-related information.

   This document describes EAP-TTLSv1; that is, version 1 of the EAP-
   TTLS protocol. It represents a significant enhancement to the
   original version 0 of the protocol. EAP-TTLSv1 utilizes an extended
   version of TLS, called TLS/IA (TLS/InnerApplication) as its
   underlying protocol [TLS/IA].

Table of Contents

1.  Introduction......................................................3
1.1    EAP-TTLSv1....................................................3
1.2    Differences From Version 0....................................4
2.  Motivation........................................................5
3.  Terminology.......................................................6
4.  Architectural Model...............................................9
4.1    Carrier Protocols.............................................9
4.2    Security Relationships.......................................10
4.3    Messaging....................................................10
4.4    Resulting Security...........................................11
5.  Protocol Layering Model..........................................11
6.  EAP-TTLSv1 Overview..............................................12
6.1    Session Resumption...........................................13
6.1.1      TTLS Server Guidelines for Session Resumption............14
7.  Generating Keying Material.......................................15
8.  EAP-TTLSv1 Protocol..............................................15
8.1    Packet Format................................................15
8.2    EAP-TTLS Start Packet........................................17
8.2.1      Version Negotiation......................................17
8.2.2      Fragmentation............................................17
8.2.3      Acknowledgement Packets..................................18
9.  Security Claims..................................................18
10. Security Considerations..........................................19
11. References.......................................................20
11.1   Normative References.........................................20
11.2   Informative References.......................................21
12. Authors' Addresses...............................................22







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1. Introduction

   EAP-TTLS is an EAP type that utilizes TLS to establish a secure
   connection between a client and server, through which additional
   information may be exchanged. The initial TLS handshake may mutually
   authenticate client and server; or it may perform a one-way
   authentication, in which only the server is authenticated to the
   client. The secure connection established by the initial handshake
   may then be used to allow the server to authenticate the client
   using existing, widely-deployed authentication infrastructures such
   as RADIUS. The authentication of the client may itself be EAP, or it
   may be another authentication protocol such as PAP, CHAP, MS-CHAP or
   MS-CHAP-V2.

   Thus, EAP-TTLS allows legacy password-based authentication protocols
   to be used against existing authentication databases, while
   protecting the security of these legacy protocols against
   eavesdropping, man-in-the-middle and other cryptographic attacks.

   EAP-TTLS also allows client and server to establish keying material
   for use in the data connection between the client and access point.
   The keying material is established implicitly between client and
   server based on the TLS handshake.

   This document describes EAP-TTLSv1; that is, version 1 of the EAP-
   TTLS protocol. It represents a significant enhancement to the
   original version 0 of the protocol. (EAP-TTLSv0).

1.1 EAP-TTLSv1

   EAP-TTLSv1 utilizes TLS with the Inner Application extension
   (TLS/IA), as its underlying protocol. In TLS/IA, the TLS handshake
   is followed by an exchange of messages with record type
   InnerApplication, in which an arbirary exchange of messages between
   client and server is conducted under the confidentiality and
   integrity protection afforded by the TLS handshake.

   The InnerApplication messages that are exchanged between client and
   server are encoded as sequences of Attribute-Value-Pairs (AVPs) from
   the RADIUS/Diameter namespace. Use of the RADIUS/Diameter namespace
   provides natural compatibility between TLS/IA applications and
   widely deployed AAA infrastructures. This namespace is extensible,
   allowing new AVPs and, thus, new applications to be defined as
   needed, either by standards bodies or by vendors wishing to define
   proprietary applications.

   The AVPs exchanged between client and server typically provide for
   client authentication, or mutual client-server authentication.
   However, the AVP exchange accommodates any type of client-server
   exchange, not just authentication, though authentication may often
   be the prerequisite that allows other exchanges to proceed. For



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   example, EAP-TTLSv1 may be used to verify endpoint integrity,
   provision keying material for use in separate data channel
   communications (e.g. IPsec), provide client credentials for single
   sign-on, and so on.

1.2 Differences From Version 0

   Version 1 of EAP-TTLS is similar to version 0 in that a TLS
   handshake is used to protect a subsequent AVP exchange. In version
   0, the handshake portion of TLS is used to establish a tunnel and
   the data portion is used to carry AVPs. This approach is similar to
   that of other tunneled protocols, such as EAP-PEAP and EAP-FAST.

   In version 1, an extension to TLS, called TLS/IA, is utilized;
   TLS/IA already provides for a protected AVP exchange following the
   TLS handshake, in effect producing an "extended" handshake. TLS/IA
   was developed to allow authentication and other client-server
   negotiations to occur within TLS itself. Thus, TLS/IA is suitable
   both as the underlying protocol for EAP methods as well as a means
   of introducing authentication and other client-server exchanges when
   TLS is used to protect data communications such as an HTTP
   conversation.

   Use of TLS/IA in version 1 of EAP-TTLS provides several improvements
   over verion 0:

   -  Inner authentications are confirmed by mixing session keys
      developed from those authentications with the master secret
      developed during the TLS handshake. This guarantees that the TLS
      handshake endpoint and the authentication endpoint are one and
      the same, thus eliminating the Man-in-the-Middle (MitM) attack
      against tunneled protocols for inner authentications that
      generate session keys. See [MITM] for information about this
      attack.

   -  Session keys developed from inner authentications are mixed with
      the TLS master secret to produce an "inner secret", which is
      exported by TLS/IA. The inner secret is used to generate the MSK
      (master session key) exported by EAP-TTLSv1 for use in the
      subsequent data connection. Use of a session key that is bound to
      inner session keys guarantees that the subsequent data connection
      wll not operate except with the authentic client, even if the
      original TLS master secret were compromised and available to an
      eavesdropper.

   -  TLS/IA's multi-phase operation allows a subsequent phase to
      confirm the results of prior phases before proceeding.

   -  A secure final exchange of the result of inner authentication is
      exchanged between client and server to conclude the EAP-TTLSv1
      exchange. This precludes any possibility of truncation attack



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      that could occur when the client relies solely on an unprotected
      EAP-Success message to determine that the server has completed
      its authentication.

2. Motivation

   Most password-based protocols in use today rely on a hash of the
   password with a random challenge. Thus, the server issues a
   challenge, the client hashes that challenge with the password and
   forwards a response to the server, and the server validates that
   response against the user's password retrieved from its database.
   This general approach describes CHAP, MS-CHAP, MS-CHAP-V2, EAP/MD5-
   Challenge and EAP/One-Time Password.

   An issue with such an approach is that an eavesdropper that observes
   both challenge and response may be able to mount a dictionary
   attack, in which random passwords are tested against the known
   challenge to attempt to find one which results in the known
   response. Because passwords typically have low entropy, such attacks
   can in practice easily discover many passwords.

   While this vulnerability has long been understood, it has not been
   of great concern in environments where eavesdropping attacks are
   unlikely in practice. For example, users with wired or dial-up
   connections to their service providers have not been concerned that
   such connections may be monitored. Users have also been willing to
   entrust their passwords to their service providers, or at least to
   allow their service providers to view challenges and hashed
   responses which are then forwarded to their home authentication
   servers using, for example, proxy RADIUS, without fear that the
   service provider will mount dictionary attacks on the observed
   credentials. Because a user typically has a relationship with a
   single service provider, such trust is entirely manageable.

   With the advent of wireless connectivity, however, the situation
   changes dramatically:

   -  Wireless connections are considerably more susceptible to
      eavesdropping and man-in-the-middle attacks. These attacks may
      enable dictionary attacks against low-entropy passwords. In
      addition, they may enable channel hijacking, in which an attacker
      gains fraudulent access by seizing control of the communications
      channel after authentication is complete.

   -  Existing authentication protocols often begin by exchanging the
      clientÂ’s username in the clear. In the context of eavesdropping
      on the wireless channel, this can compromise the clientÂ’s
      anonymity and locational privacy.

   -  Often in wireless networks, the access point does not reside in
      the administrative domain of the service provider with which the



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      user has a relationship. For example, the access point may reside
      in an airport, coffee shop, or hotel in order to provide public
      access via 802.11. Even if password authentications are protected
      in the wireless leg, they may still be susceptible to
      eavesdropping within the untrusted wired network of the access
      point.

   -  In the traditional wired world, the user typically intentionally
      connects with a particular service provider by dialing an
      associated phone number; that service provider may be required to
      route an authentication to the user's home domain. In a wireless
      network, however, the user does not get to choose an access
      domain, and must connect with whichever access point is nearby;
      providing for the routing of the authentication from an arbitrary
      access point to the user's home domain may pose a challenge.

   Thus, the authentication requirements for a wireless environment
   that EAP-TTLS attempts to address can be summarized as follows:

   -  Legacy password protocols must be supported, to allow easy
      deployment against existing authentication databases.

   -  Password-based information must not be observable in the
      communications channel between the client node and a trusted
      service provider, to protect the user against dictionary attacks.

   -  The user's identity must not be observable in the communications
      channel between the client node and a trusted service provider,
      to protect the user's locational privacy against surveillance,
      undesired acquisition of marketing information, and the like.

   -  The authentication process must result in the distribution of
      shared keying information to the client and access point to
      permit encryption and validation of the wireless data connection
      subsequent to authentication, to secure it against eavesdroppers
      and prevent channel hijacking.

   -  The authentication mechanism must support roaming among small
      access domains with which the user has no relationship and which
      will have limited capabilities for routing authentication
      requests.

3. Terminology

   AAA

      Authentication, Authorization and Accounting - functions that are
      generally required to control access to a network and support
      billing and auditing.

   AAA protocol



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      A network protocol used to communicate with AAA servers; examples
      include RADIUS and Diameter.

   AAA server

      A server which performs one or more AAA functions: authenticating
      a user prior to granting network service, providing authorization
      (policy) information governing the type of network service the
      user is to be granted, and accumulating accounting information
      about actual usage.

   AAA/H

      A AAA server in the user's home domain, where authentication and
      authorization for that user are administered.

   access point

      A network device providing users with a point of entry into the
      network, and which may enforce access control and policy based on
      information returned by a AAA server. For the purposes of this
      document, "access point" and "NAS" are architecturally
      equivalent. "Access point" is used throughout because it is
      suggestive of devices used for wireless access; "NAS" is used
      when more traditional forms of access, such as dial-up, are
      discussed.

   access domain

      The domain, including access points and other devices, that
      provides users with an initial point of entry into the network;
      for example, a wireless hot spot.

   client

      A host or device that connects to a network through an access
      point.

   domain

      A network and associated devices that are under the
      administrative control of an entity such as a service provider or
      the user's home organization.

   link layer protocol

      A protocol used to carry data between hosts that are connected
      within a single network segment; examples include PPP and
      Ethernet.

   NAI



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      A Network Access Identifier [RFC2486], normally consisting of the
      name of the user and, optionally, the user's home realm.

   NAS

      A network device providing users with a point of entry into the
      network, and which may enforce access control and policy based on
      information returned by a AAA server. For the purposes of this
      document, "access point" and "NAS" are architecturally
      equivalent. "Access point" is used throughout because it is
      suggestive of devices used for wireless access; "NAS" is used
      when more traditional forms of access, such as dial-up, are
      discussed.

   proxy

      A server that is able to route AAA transactions to the
      appropriate AAA server, possibly in another domain, typically
      based on the realm portion of an NAI.

   realm

      The optional part of an NAI indicating the domain to which a AAA
      transaction is to be routed, normally the user's home domain.

   service provider

      An organization with which a user has a business relationship,
      that provides network or other services. The service provider may
      provide the access equipment with which the user connects, may
      perform authentication or other AAA functions, may proxy AAA
      transactions to the user's home domain, etc.

   TTLS server

      A AAA server which implements EAP-TTLS. This server may also be
      capable of performing user authentication, or it may proxy the
      user authentication to a AAA/H.

   user

      The person operating the client device. Though the line is often
      blurred, "user" is intended to refer to the human being who
      possesses an identity (username), password or other
      authenticating information, and "client" is intended to refer to
      the device which makes use of this information to negotiate
      network access. There may also be clients with no human
      operators; in this case the term "user" is a convenient
      abstraction.





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4. Architectural Model

   The network architectural model for EAP-TTLS usage and the type of
   security it provides is shown below.

   +----------+      +----------+      +----------+      +----------+
   |          |      |          |      |          |      |          |
   |  client  |<---->|  access  |<---->| TTLS AAA |<---->|  AAA/H   |
   |          |      |  point   |      |  server  |      |  server  |
   |          |      |          |      |          |      |          |
   +----------+      +----------+      +----------+      +----------+

   <---- secure password authentication tunnel --->

   <---- secure data tunnel ---->

   The entities depicted above are logical entities and may or may not
   correspond to separate network components. For example, the TTLS
   server and AAA/H server might be a single entity; the access point
   and TTLS server might be a single entity; or, indeed, the functions
   of the access point, TTLS server and AAA/H server might be combined
   into a single physical device. The above diagram illustrates the
   division of labor among entities in a general manner and shows how a
   distributed system might be constructed; however, actual systems
   might be realized more simply.

   Note also that one or more AAA proxy servers might be deployed
   between access point and TTLS server, or between TTLS server and
   AAA/H server. Such proxies typically perform aggregation or are
   required for realm-based message routing. However, such servers play
   no direct role in EAP-TTLS and are therefore not shown.

4.1 Carrier Protocols

   The entities shown above communicate with each other using carrier
   protocols capable of encapsulating EAP. The client and access point
   communicate using a link layer carrier protocol such as PPP or
   EAPOL. The access point, TTLS server and AAA/H server communicate
   using a AAA carrier protocol such as RADIUS or Diameter.

   EAP, and therefore EAP-TTLS, must be initiated via the link layer
   protocol. In PPP or EAPOL, for example, EAP is initiated when the
   access point sends an EAP-Request/Identity packet to the client.

   The keying material used to encrypt and authenticate the data
   connection between the client and access point is developed
   implicitly between the client and TTLS server as a result of EAP-
   TTLS negotiation. This keying material must be communicated to the
   access point by the TTLS server using the AAA carrier protocol.





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   The client and access point must also agree on an
   encryption/validation algorithm to be used based on the keying
   material. In some systems, both these devices may be preconfigured
   with this information, and distribution of the keying material alone
   is sufficient. Or, the link layer protocol may provide a mechanism
   for client and access point to negotiate an algorithm.

   In the most general case, however, it may be necessary for both
   client and access point to communicate their algorithm preferences
   to the TTLS server, and for the TTLS server to select one and
   communicate its choice to both parties. This information would be
   transported between access point and TTLS server via the AAA
   protocol, and between client and TTLS server via EAP-TTLS in
   encrypted form.

4.2 Security Relationships

   The client and access point have no pre-existing security
   relationship.

   The access point, TTLS server and AAA/H server are each assumed to
   have a pre-existing security association with the adjacent entity
   with which it communicates. With RADIUS, for example, this is
   achieved using shared secrets. It is essential for such security
   relationships to permit secure key distribution.

   The client and AAA/H server have a security relationship based on
   the user's credentials such as a password.

   The client and TTLS server may have a one-way security relationship
   based on the TTLS server's possession of a private key guaranteed by
   a CA certificate which the user trusts, or may have a mutual
   security relationship based on certificates for both parties.

4.3 Messaging

   The client and access point initiate an EAP conversation to
   negotiate the client's access to the network. Typically, the access
   point issues an EAP-Request/Identity to the client, which responds
   with an EAP-Response/Identity. Note that the client does not include
   the user's actual identity in this EAP-Response/Identity packet; the
   user's identity will not be transmitted until an encrypted channel
   has been established.

   The access point now acts as a passthrough device, allowing the TTLS
   server to negotiate EAP-TTLS with the client directly.

   During the first phase of the negotiation, the TLS handshake
   protocol is used to authenticate the TTLS server to the client and,
   optionally, to authenticate the client to the TTLS server, based on
   public/private key certificates. As a result of the handshake,



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   client and TTLS server now have shared keying material and an agreed
   upon TLS record layer cipher suite with which to secure subsequent
   EAP-TTLS communication.

   During the second phase of negotiation, client and TTLS server use
   the secure TLS record layer channel established by the TLS handshake
   as a tunnel to exchange information encapsulated in attribute-value
   pairs, to perform additional functions such as client authentication
   and key distribution for the subsequent data connection.

   If a tunneled client authentication is performed, the TTLS server
   de-tunnels and forwards the authentication information to the AAA/H.
   If the AAA/H performs a challenge, the TTLS server tunnels the
   challenge information to the client. The AAA/H server may be a
   legacy device and needs to know nothing about EAP-TTLS; it only
   needs to be able to authenticate the client based on commonly used
   authentication protocols.

   Keying material for the subsequent data connection between client
   and access point may be generated based on secret information
   developed during the TLS handshake and subsequent tunneled
   authentications between client and TTLS server. At the conclusion of
   a successful authentication, the TTLS server may transmit this
   keying material to the access point, encrypted based on the existing
   security associations between those devices (e.g., RADIUS).

   The client and access point now share keying material which they can
   use to encrypt data traffic between them.

   In EAP-TTLSv1, the AVP exchange during the second phase is performed
   using InnerApplication records via the TLS/IA protocol. This AVP
   exchange itself may be be multi-phase, with each phase proceeding
   only if the prior phase resulted in success.

4.4 Resulting Security

   As the diagram above indicates, EAP-TTLS allows user identity and
   password information to be securely transmitted between client and
   TTLS server, and performs key distribution to allow network data
   subsequent to authentication to be securely transmitted between
   client and access point.

5. Protocol Layering Model

   EAP-TTLSv1 packets are encapsulated within EAP, and EAP in turn
   requires a carrier protocol to transport it. EAP-TTLSv1 packets
   themselves encapsulate TLS/IA, which is then used to encapsulate
   user authentication information. TLS/IA, as an extension to TLS, can
   be considered encapsulated by TLS. Thus, EAP-TTLSv1 messaging can be
   described using a layered model, where each layer is encapsulated by




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   the layer beneath it. The following diagram clarifies the
   relationship between protocols:

   +--------------------------------------------------------+
   | User Authentication Protocol (PAP, CHAP, MS-CHAP, etc.)|
   +--------------------------------------------------------+
   |          Inner Application extension to TLS            |
   +--------------------------------------------------------+
   |                       TLS                              |
   +--------------------------------------------------------+
   |                     EAP-TTLS                           |
   +--------------------------------------------------------+
   |                       EAP                              |
   +--------------------------------------------------------+
   | Carrier Protocol (PPP, EAPOL, RADIUS, Diameter, etc.)  |
   +--------------------------------------------------------+

   When the user authentication protocol is itself EAP, the layering is
   as follows:

   +--------------------------------------------------------+
   | User EAP Authentication Protocol (MD-Challenge, etc.)  |
   +--------------------------------------------------------+
   |                       EAP                              |
   +--------------------------------------------------------+
   |          Inner Application extension to TLS            |
   +--------------------------------------------------------+
   |                       TLS                              |
   +--------------------------------------------------------+
   |                     EAP-TTLS                           |
   +--------------------------------------------------------+
   |                       EAP                              |
   +--------------------------------------------------------+
   | Carrier Protocol (PPP, EAPOL, RADIUS, Diameter, etc.)  |
   +--------------------------------------------------------+

   Methods for encapsulating EAP within carrier protocols are already
   defined. For example, PPP [RFC1661] or EAPOL [802.1X] may be used to
   transport EAP between client and access point; RADIUS [RFC2685] or
   Diameter [RFC3588] are used to transport EAP between access point
   and TTLS server.

6. EAP-TTLSv1 Overview

   EAP-TTLSv1 is initiated by the server's transmission of a Start
   packet to the client.

   The EAP exchange proceeds with transmission of TLS/IA message
   sequences alternately by client and server, with each message
   sequence encapsulated in an EAP-TTLSv1 frame. Descriptions of the
   TLS/IA messages can be found in [TLS/IA].



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   A successful authentication will result in the server sending a
   TLS/IA FinalPhaseFinished message and the client responding with
   it's own FinalPhaseFinished message.

   The server then sends an EAP-Success to the client to complete the
   authentication. This message is the standard EAP success message and
   is sent in the clear.

   Client and server each computes the MSK (the Master Sesion Key, as
   defined in [RFC3784]), based on information generated in the TLS/IA
   exchange. The server may then transmit the MSK to the access point
   for use in its data communications with the client.

   If the TLS/IA negotiation fails, the server sends an EAP-Failure to
   the client.

6.1 Session Resumption

   When a client and TTLS server that have previously negotiated a EAP-
   TTLSv1 session begin a new EAP-TTLSv1 negotiation, the client and
   TTLS server may agree to resume the previous session. This
   significantly reduces the time required to establish the new
   session. This could occur when the client connects to a new access
   point, or when an access point requires reauthentication of a
   connected client.

   Session resumption is accomplished using the standard TLS mechanism.
   The client signals its desire to resume a session by including the
   session ID of the session it wishes to resume in the ClientHello
   message; the TTLS server signals its willingness to resume that
   session by echoing that session ID in its ServerHello message.

   If the TTLS server elects not to resume the session, it simply does
   not echo the session ID and a new session will be negotiated. This
   could occur if the TTLS server is configured not to resume sessions,
   if it has not retained the requested session's state, or if the
   session is considered stale. A TTLS server may consider the session
   stale based on its own configuration, or based on session-limiting
   information received from the AAA/H (e.g., the RADIUS Session-
   Timeout attribute).

   Addition messages beyond the TLS handshake may or may not occur
   within a resumed session. TLS/IA provides a negotiation mechanism
   allowing client and server to determine whether InnerApplication
   messages are to ensue upon session resumption. Typically, inner
   authentications would not be required in a resumed session, as the
   ability to resume the session may provide sufficient evidence to
   either party of the identity of the other. However, there may be
   additional information that needs to be refreshed or renegotiated
   during a session resumption.




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   When an inner authentication is not performed during a resumed
   session, the TTLS server will not receive new authorization
   information from the AAA/H. In this case, the TTLS server must
   retain authorization information initially returned by the AAA/H for
   use in resumed sessions. Authorization information might include the
   maximum time for the session, the maximum allowed bandwidth, packet
   filter information and the like. The TTLS server is responsible for
   modifying time values, such as Session-Timeout, appropriately for
   each resumed session.

   A TTLS server MUST NOT permit a session to be resumed if that
   session did not result in a successful completion of the entire
   TLS/IA exchange, and a client MUST NOT propose the session ID of a
   failed session for resumption. The consequence of incorrectly
   implementing this aspect of session resumption would be
   catastrophic; any attacker could easily gain network access by first
   initiating a session that succeeds in the TLS handshake but fails
   the inner authentication, and then resuming that session.

   [Implementation note: Toolkits that implement TLS often cache
   resumable TLS sessions automatically. Implementers must take care to
   override such automatic behavior, and prevent sessions from being
   cached for possible resumption until the user has been positively
   authenticated.]

   A TTLS server MUST NOT permit a session negotiated with different
   tunneled TLS-based EAP protocol to be resumed in an EAP-TTLSv1
   session, and a client MUST NOT propose the session ID resulting from
   such a protocol for resumption in EAP-TTLSv1. Note that previous
   versions of EAP-TTLS are considered different tunneled TLS-based
   protocols for the purposes of this paragraph. Thus, a session
   negotiated using EAP-PEAP, EAP-FAST or EAP-TTLSv0 are not candidate
   sessions for resumption in EAP-TTLSv1.

6.1.1 TTLS Server Guidelines for Session Resumption

   When a domain comprises multiple TTLS servers, a client's attempt to
   resume a session may fail because each EAP-TTLS negotiation may be
   routed to a different TTLS server.

   One strategy to ensure that subsequent EAP-TTLS negotiations are
   routed to the original TTLS server is for each TTLS server to encode
   its own identifying information, for example, IP address, in the
   session IDs that it generates. This would allow any TTLS server
   receiving a session resumption request to forward the request to the
   TTLS server that established the original session.








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7. Generating Keying Material

   Upon successful conclusion of an EAP-TTLSv1 negotiation, a 64-octet
   MSK (Master Session Key) is generated and exported for use in
   securing the data connection between client and access point.

   The MSK is generated using the TLS PRF function [RFC2246], with
   inputs consisting of the inner secret exported by TLS/IA, the ASCII-
   encoded constant string "ttls v1 keying material", the TLS client
   random, and the TLS server random. The constant string is not null-
   terminated. The TLS/IA inner secret, rather than the TLS master
   secret, is used because it binds session keys from inner
   authentications with the TLS master secret and therefore provides
   greater security in the (unlikely) case that an adversary is able to
   compromise the master secret.

      MSK = PRF(inner_secret,
                "ttls v1 keying material",
                SecurityParameters.client_random +
                SecurityParameters.server_random) [0..63]

   Note that the order of client_random and server_random for EAP-TTLS
   is reversed from that of the TLS protocol [RFC2246]. This ordering
   follows the key derivation method of EAP-TLS [RFC2716]. Altering the
   order of randoms avoids namespace collisions between constant
   strings defined for EAP-TTLSv1 and those defined for the TLS
   protocol.

   The inner secret used in the PRF MUST be the one generated at the
   conclusion of the final InnerApplication phase of TLS/IA; the client
   random and server random MUST be those established during the TLS
   handshake. Client and TTLS server generate this keying material
   independently, and the result is guaranteed to be the same for each
   if the TLS/IA exchange succeeds.

   The TTLS server distributes this keying material to the access point
   via the AAA carrier protocol. When RADIUS is the AAA carrier
   protocol, the MPPE-Recv-Key and MPPE-Send-Key attributes may be used
   to distribute the first 32 octets and second 32 octets of the MSK,
   respectively.

8. EAP-TTLSv1 Protocol

8.1 Packet Format

   The EAP-TTLSv1 packet format is shown below. The fields are
   transmitted 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      |     Flags     |        Message Length
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            Message Length         |             Data...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Code
      1 for request, 2 for response.

   Identifier
      The Identifier field is one octet and aids in matching responses
      with requests.  The Identifier field MUST be changed for each
      request packet and MUST be echoed in each response packet.

   Length
      The Length field is two octets and indicates the number of octets
      in the entire EAP packet, from the Code field through the Data
      field.

   Type
      21 (EAP-TTLS, all versions)

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

      L = Length included
      M = More fragments
      S = Start
      R = Reserved
      V = Version (001 for EAP-TTLSv1)

      The L bit is set to indicate the presence of the four octet TLS
      Message Length field. The M bit indicates that more fragments are
      to come. The S bit indicates a Start message. The V bit is set to
      the version of EAP-TTLS, and is set to 001 for EAP-TTLSv1.

   Message Length
      The Message Length field is four octets, and is present only if
      the L bit is set. This field provides the total length of the raw
      data message sequence prior to fragmentation.

   Data
      For all packets other than a Start packet, the Data field
      consists of the raw TLS message sequence or fragment thereof. For



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      a Start packet, the Data field may optionally contain an AVP
      sequence.

8.2 EAP-TTLS Start Packet

   The S bit MUST be set on the first packet sent by the server to
   initiate the EAP-TTLSv1 protocol. It MUST NOT be set on any other
   packet.

   This packet MAY contain additional information in the form of AVPs,
   which may provide useful hints to the client; for example, the
   server identity may be useful to the client to allow it to pick the
   correct TLS session ID for session resumption. Each AVP must begin
   on a 4-octet boundary relative to the first AVP in the sequence. If
   an AVP is not a multiple of 4 octets, it must be padded with 0s to
   the next 4-octet boundary.

8.2.1 Version Negotiation

   The version of EAP-TTLS is negotiated in the first exchange between
   server and client. The server sets the highest version number of
   EAP-TTLS that it supports in the V field of its Start message (in
   the case of EAP-TTLS v1, this is 1). In its first EAP message in
   response, the client sets the V field to the highest version number
   that it supports that is no higher than the version number offered
   by the server. If the client version is not acceptable to the
   server, it sends an EAP-Failure to terminate the EAP session.
   Otherwise, the version sent by the client is the version of EAP-TTLS
   that MUST be used, and both server and client set the V field to
   that version number in all subsequent EAP messages.

8.2.2 Fragmentation

   Each EAP-TTLSv1 message contains a sequence of TLS messages that
   represent a single leg of a half-duplex conversation. The EAP
   carrier protocol (e.g., PPP, EAPOL, RADIUS) may impose constraints
   on the length of of an EAP message. Therefore it may be necessary to
   fragment an EAP-TTLSv1 message across multiple EAP messages.

   Each fragment except for the last MUST have the M bit set, to
   indicate that more data is to follow; the final fragment MUST NOT
   have the M bit set.

   If there are multiple fragments, the first fragment MUST have the L
   bit set and include the length of the entire raw message prior to
   fragmentation. Fragments other than the first MUST NOT have the L
   bit set.

   Upon receipt of a packet with M bit set, the receiver MUST transmit
   an Acknowledgement packet. The receiver is responsible for
   reassembly of fragmented packets.



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8.2.3 Acknowledgement Packets

   An Acknowledgement packet is an EAP-TTLSv1 packet with no additional
   data beyond the Flags octet, and with the L, M and S bits of the
   Flags octet set to 0. (Note, however, that the V field MUST still be
   set to the appropriate version number.)

   An Acknowledgement packet is sent for the following purposes:

   -  Fragment Acknowledgement

      A Fragment Acknowledgement is sent in response to an EAP packet
      with M bit set.

   -  Error Alert Acknowledgement

      Either party may at any time send a TLS error alert to fail the
      TLS/IA handshake.

      If the client sends an error alert to the server, no further EAP-
      TTLS messages are exchanged, and the server sends an EAP-Failure
      to terminate the conversation.

      If the server sends an error alert to the client, the client MUST
      respond with an Acknowledgement packet to allow the conversation
      to continue. Upon receipt of the Acknowledgement packet, the
      server sends an EAP-Failure to terminate the conversation.

   Note that, unlike EAP-TTLSv0, in EAP-TTLSv1 there is no case in
   which a client sends a packet with data as a result of having no
   AVPs to send. In EAP-TTLSv1, if no AVPs are to be sent, there will
   nevertheless be an InnerApplication message carrying zero AVPs,
   which the client must send.

9. Security Claims

   Pursuant to [RFC3748], security claims for EAP-TTLSv1 are as
   follows:

   Authentication mechanism: TLS plus arbitrary additional protected
                              authentication(s)
   Ciphersuite negotiation:  Yes
   Mutual authentication:    Yes, in recommended implementation
   Integrity protection:     Yes
   Replay protection:        Yes
   Confidentiality:          Yes
   Key derivation:           Yes
   Key strength:             384 bits or higher
   Dictionary attack prot.:  Yes
   Fast reconnect:           Yes
   Crypt. binding:           Yes



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   Session independence:     Yes
   Fragmentation:            Yes
   Channel binding:          Supported via AVPs, though optional

10. Security Considerations

   This draft is entirely about security and the security
   considerations associated with the mechanisms employed in this
   document should be considered by implementers.

   The following additional issues are relevant:

   -  Anonymity and privacy. EAP-TTLS does not communicate a username
      in the clear in the initial EAP-Response/Identity. This feature
      is designed to support anonymity and location privacy from
      attackers eavesdropping the network path between the client and
      the TTLS server. However implementers should be aware that other
      factors - both within EAP-TTLS and elsewhere - may compromise a
      user's identity. For example, if a user authenticates with a
      certificate, the subject name in the certificate may reveal the
      user's identity. Outside of EAP-TTLS, the client's fixed MAC
      address, or in the case of wireless connections, the client's
      radio signature, may also reveal information. Additionally,
      implementers should be aware that a user's identity is not hidden
      from the TTLS server and may be included in the clear in AAA
      messages between the TTLS server and the AAA/H server.

   -  Trust in the TTLS server. EAP-TTLS is designed to allow the use
      of legacy authentication methods to be extended to mediums like
      wireless in which eavesdropping the link between the client and
      the access point is easy. However implementers should be aware of
      the possibility of attacks by rogue TTLS servers - for example in
      the event that the inner authentication method is susceptible to
      dictionary attacks. Therefore it is essential that clients be
      properly configured to only proceed with inner authentications
      with trusted TTLS servers, as evidenced by the certificate chain
      presented by the TTLS server in the TTLS handshake. In general,
      cipher suites that allow the TTLS server to remain anonymous
      should be avoided, unless the inner authentication itself
      provides mutual authentication and is resistant to dictionary
      attack.

   -  TTLS server certificate compromise. The use of TTLS server
      certificates within EAP-TTLS makes EAP-TTLS susceptible to attack
      in the event that a TTLS server's certificate is compromised. -
      TTLS servers should therefore take care to protect their private
      key. In addition, certificate revocation methods may be used to
      mitigate against the possibility of key compromise. [RFC3546]
      describes a way to integrate one such method - OCSP [RFC2560] -
      into the TLS handshake - use of this approach may be appropriate
      within EAP-TTLS.



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   -  Listing of data cipher preferences. EAP-TTLS negotiates data
      cipher suites by having the EAP-TTLS server select the first
      cipher suite appearing on the client list that also appears on
      the access point list. In order to maximize security, it is
      therefore recommended that the client order its list according to
      security - most secure acceptable cipher suite first, least
      secure acceptable cipher suite last.

   -  Forward secrecy. With forward secrecy, revelation of a secret
      does not compromise session keys previously negotiated based on
      that secret. Thus, when the TLS key exchange algorithm provides
      forward secrecy, if a TTLS server certificate's private key is
      eventually stolen or cracked, tunneled user password information
      will remain secure as long as that certificate is no longer in
      use. Diffie-Hellman key exchange is an example of an algorithm
      that provides forward secrecy. A forward secrecy algorithm should
      be considered if attacks against recorded authentication or data
      sessions are considered to pose a significant threat.

11. References

11.1 Normative References

   [TLS/IA]   Funk, P., Blake-Wilson, S., Smith, N., Tschofenig, H.
               and T. Hardjono, " TLS Inner Application Extension
               (TLS/IA)", draft-funk-tls-inner-application-extension-
               02.txt, March 2006.

   [RFC1700]  Reynolds, J., and J. Postel, "Assigned Numbers", RFC
               1700, October 1994.

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

   [RFC2246]  Dierks, T., and C. Allen, "The TLS Protocol Version
               1.0", RFC 2246, November 1998.

   [RFC2486]  Aboba, B., and M. Beadles, "The Network Access
               Identifier", RFC 2486, January 1999.

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

   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
               "Remote Authentication Dial In User Service (RADIUS)",
               RFC 2865, June 2000.

   [RFC3546]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen,
               J., and T. Wright, "Transport Layer Security (TLS)
               Extensions", RFC 3546, June 2003.




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   [RFC3579]  Aboba, B., and P.Calhoun, "RADIUS (Remote Authentication
               Dial In User Service) Support For Extensible
               Authentication Protocol (EAP)", RFC 3579, September
               2003.

   [RFC3588]  Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
               Arkko, "Diameter Base Protocol", RFC 3588, July 2003.

   [RFC3784]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and
               H. Levkowetz, "PPP Extensible Authentication Protocol
               (EAP)", RFC 3784, June 2004.

11.2 Informative References

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

   [RFC1994]  Simpson, W., "PPP Challenge Handshake Authentication
               Protocol (CHAP)", RFC 1994, August 1996.

   [RFC2716]  Aboba, B., and D. Simon, "PPP EAP TLS Authentication
               Protocol", RFC 2716, October 1999.

   [RFC2433]  Zorn, G., and S. Cobb, "Microsoft PPP CHAP Extensions",
               RFC 2433, October 1998.

   [RFC2560]  Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
               Adams, "Internet X.509 Public Key Infrastructure: Online
               Certificate Status Protocol - OCSP", RFC 2560, June
               1999.

   [RFC2759]  Zorn, G., "Microsoft PPP CHAP Extensions, Version 2",
               RFC 2759, January 2000.

   [EAP-PEAP] Palekar, A., Simon, D., Salowey, J., Zhou, H., Zorn, G.,
               and S. Josefsson, "Protected EAP Protocol (PEAP) Version
               2", draft-josefsson-pppext-eap-tls-eap-08.txt, July
               2004.

   [TLS-PSK]  Eronen, P., and H. Tschofenig, "Pre-Shared Key
               Ciphersuites for Transport Layer Security (TLS)", draft-
               ietf-tls-psk-01.txt, August 2004.

   [802.1X]   IEEE Standards for Local and Metropolitan Area Networks:
               Port based Network Access Control, IEEE Std 802.1X-2001,
               June 2001.

   [MITM]     Asokan, N., Niemi, V., and K. Nyberg, "Man-in-the-Middle
               in Tunneled Authentication",
               http://www.saunalahti.fi/~asokan/research/mitm.html,
               Nokia Research Center, Finland, October 24 2002.



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   [KEYING]   Aboba, B., Simon, D., Arkko, J. and H. Levkowetz, "EAP
               Key Management Framework", draft-ietf-eap-keying-01.txt
               (work in progress), October 2003.

   [IKEv2]    C.Kaufman, "Internet Key Exchange (IKEv2) Protocol",
               draft-ietf-ipsec-ikev2-16.txt (work in progress),
               September 2004.

   [AAA-EAP]  Eronen, P., Hiller, T. and G. Zorn, "Diameter Extensible
               Authentication Protocol (EAP) Application", draft-ietf-
               aaa-eap-03.txt (work in progress), October 2003.

12. Authors' Addresses

   Questions about this memo can be directed to:

      Paul Funk
      Juniper Networks
      222 Third Street
      Cambridge, MA 02142
      USA
      Phone:  +1 617 497-6339
      E-mail: pfunk@juniper.net

      Simon Blake-Wilson
      Basic Commerce & Industries, Inc.
      304 Harper Drive, Suite 203
      Moorestown, NJ 08057
      Phone: +1 856 778-1660
      E-mail: sblakewilson@bcisse.com

Disclaimer of Validity

   This document and the information contained herein are provided on
   an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
   REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
   INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
   IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Copyright Statement

   Copyright (C) The Internet Society (2006).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.








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