path: root/doc/rfc/rfc5281.txt

Network Working Group                                            P. Funk
Request for Comments: 5281                                  Unaffiliated
Category: Informational                                  S. Blake-Wilson
                                                                 SafeNet
                                                             August 2008


  Extensible Authentication Protocol Tunneled Transport Layer Security
             Authenticated Protocol Version 0 (EAP-TTLSv0)

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Abstract

   EAP-TTLS is an EAP (Extensible Authentication Protocol) method that
   encapsulates a TLS (Transport Layer Security) session, consisting of
   a handshake phase and a data phase.  During the handshake phase, the
   server is authenticated to the client (or client and server are
   mutually authenticated) using standard TLS procedures, and keying
   material is generated in order to create a cryptographically secure
   tunnel for information exchange in the subsequent data phase.  During
   the data phase, the client is authenticated to the server (or client
   and server are mutually authenticated) using an arbitrary
   authentication mechanism encapsulated within the secure tunnel.  The
   encapsulated authentication mechanism 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 attacks.  The data phase
   may also be used for additional, arbitrary data exchange.
















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Table of Contents

   1. Introduction ....................................................4
   2. Motivation ......................................................5
   3. Requirements Language ...........................................7
   4. Terminology .....................................................7
   5. Architectural Model .............................................9
      5.1. Carrier Protocols .........................................10
      5.2. Security Relationships ....................................10
      5.3. Messaging .................................................11
      5.4. Resulting Security ........................................12
   6. Protocol Layering Model ........................................12
   7. EAP-TTLS Overview ..............................................13
      7.1. Phase 1: Handshake ........................................14
      7.2. Phase 2: Tunnel ...........................................14
      7.3. EAP Identity Information ..................................15
      7.4. Piggybacking ..............................................15
      7.5. Session Resumption ........................................16
      7.6. Determining Whether to Enter Phase 2 ......................17
      7.7. TLS Version ...............................................18
      7.8. Use of TLS PRF ............................................18
   8. Generating Keying Material .....................................19
   9. EAP-TTLS Protocol ..............................................20
      9.1. Packet Format .............................................20
      9.2. EAP-TTLS Start Packet .....................................21
           9.2.1. Version Negotiation ................................21
           9.2.2. Fragmentation ......................................22
           9.2.3. Acknowledgement Packets ............................22
   10. Encapsulation of AVPs within the TLS Record Layer .............23
      10.1. AVP Format ...............................................23
      10.2. AVP Sequences ............................................25
      10.3. Guidelines for Maximum Compatibility with AAA Servers ....25
   11. Tunneled Authentication .......................................26
      11.1. Implicit Challenge .......................................26
      11.2. Tunneled Authentication Protocols ........................27
           11.2.1. EAP ...............................................27
           11.2.2. CHAP ..............................................29
           11.2.3. MS-CHAP ...........................................30
           11.2.4. MS-CHAP-V2 ........................................30
           11.2.5. PAP ...............................................32
      11.3. Performing Multiple Authentications ......................33
      11.4. Mandatory Tunneled Authentication Support ................34
      11.5. Additional Suggested Tunneled Authentication Support .....34
   12. Keying Framework ..............................................35
      12.1. Session-Id ...............................................35
      12.2. Peer-Id ..................................................35
      12.3. Server-Id ................................................35
   13. AVP Summary ...................................................35



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   14. Security Considerations .......................................36
      14.1. Security Claims ..........................................36
           14.1.1. Authentication Mechanism ..........................36
           14.1.2. Ciphersuite Negotiation ...........................37
           14.1.3. Mutual Authentication .............................37
           14.1.4. Integrity Protection ..............................37
           14.1.5. Replay Protection .................................37
           14.1.6. Confidentiality ...................................37
           14.1.7. Key Derivation ....................................37
           14.1.8. Key Strength ......................................37
           14.1.9. Dictionary Attack Protection ......................38
           14.1.10. Fast Reconnect ...................................38
           14.1.11. Cryptographic Binding ............................38
           14.1.12. Session Independence .............................38
           14.1.13. Fragmentation ....................................38
           14.1.14. Channel Binding ..................................38
      14.2. Client Anonymity .........................................38
      14.3. Server Trust .............................................39
      14.4. Certificate Validation ...................................39
      14.5. Certificate Compromise ...................................40
      14.6. Forward Secrecy ..........................................40
      14.7. Negotiating-Down Attacks .................................40
   15. Message Sequences .............................................41
      15.1. Successful Authentication via Tunneled CHAP ..............41
      15.2. Successful Authentication via Tunneled
            EAP/MD5-Challenge ........................................43
      15.3. Successful Session Resumption ............................46
   16. IANA Considerations ...........................................47
   17. Acknowledgements ..............................................48
   18. References ....................................................48
      18.1. Normative References .....................................48
      18.2. Informative References ...................................49



















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1.  Introduction

   Extensible Authentication Protocol (EAP) [RFC3748] defines a standard
   message exchange that allows a server to authenticate a client using
   an authentication method agreed upon by both parties.  EAP may be
   extended with additional authentication methods by registering such
   methods with IANA or by defining vendor-specific methods.

   Transport Layer Security (TLS) [RFC4346] is an authentication
   protocol that provides for client authentication of a server or
   mutual authentication of client and server, as well as secure
   ciphersuite negotiation and key exchange between the parties.  TLS
   has been defined as an authentication protocol for use within EAP
   (EAP-TLS) [RFC5216].

   Other authentication protocols are also widely deployed.  These are
   typically password-based protocols, and there is a large installed
   base of support for these protocols in the form of credential
   databases that may be accessed by RADIUS [RFC2865], Diameter
   [RFC3588], or other AAA servers.  These include non-EAP protocols
   such as PAP [RFC1661], CHAP [RFC1661], MS-CHAP [RFC2433], or MS-
   CHAP-V2 [RFC2759], as well as EAP protocols such as MD5-Challenge
   [RFC3748].

   EAP-TTLS is an EAP method that provides functionality beyond what is
   available in EAP-TLS.  EAP-TTLS has been widely deployed and this
   specification documents what existing implementations do.  It has
   some limitations and vulnerabilities, however.  These are addressed
   in EAP-TTLS extensions and ongoing work in the creation of
   standardized tunneled EAP methods at the IETF.  Users of EAP-TTLS are
   strongly encouraged to consider these in their deployments.

   In EAP-TLS, a TLS handshake is used to mutually authenticate a client
   and server.  EAP-TTLS extends this authentication negotiation by
   using the secure connection established by the TLS handshake to
   exchange additional information between client and server.  In EAP-
   TTLS, the TLS authentication may be mutual; or it may be one-way, in
   which only the server is authenticated to the client.  The secure
   connection established by the handshake may then be used to allow the
   server to authenticate the client using existing, widely deployed
   authentication infrastructures.  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 attacks.



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   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.

   In EAP-TTLS, client and server communicate using attribute-value
   pairs encrypted within TLS.  This generality allows arbitrary
   functions beyond authentication and key exchange to be added to the
   EAP negotiation, in a manner compatible with the AAA infrastructure.

   The main limitation of EAP-TTLS is that its base version lacks
   support for cryptographic binding between the outer and inner
   authentication.  Please refer to Section 14.1.11 for details and the
   conditions where this vulnerability exists.  It should be noted that
   an extension for EAP-TTLS [TTLS-EXT] fixed this vulnerability.  Users
   of EAP-TTLS are strongly encouraged to adopt this extension.

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.





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   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
      client's username in the clear.  In the context of eavesdropping
      on the wireless channel, this can compromise the client'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
      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 [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 against surveillance, undesired acquisition of
      marketing information, and the like.




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   -  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 access
      domains with which the user has no relationship and which will
      have limited capabilities for routing authentication requests.

3.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

4.  Terminology

   AAA

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

   AAA protocol

      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.  Since the access point
      terminates the server side of the EAP conversation, for the



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      purposes of this document it is therefore equivalent to the
      "authenticator", as used in the EAP specification [RFC3748].
      Since the access point acts as a client to a AAA server, for the
      purposes of this document it is therefore also equivalent to the
      "Network Access Server (NAS)", as used in AAA specifications such
      as [RFC2865].

   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.  Since it terminates the client side of the EAP
      conversation, for the purposes of this document, it is therefore
      equivalent to the "peer", as used in the EAP specification
      [RFC3748].

   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

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

   NAI

      A Network Access Identifier [RFC4282], normally consisting of the
      name of the user and, optionally, the user's home realm.

   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.



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   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 is
      possessed of 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.

5.  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



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   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.

5.1.  Carrier Protocols

   The entities shown above communicate with each other using carrier
   protocols capable of encapsulating EAP.  The client and access point
   communicate typically using a link layer carrier protocol such as PPP
   or EAPOL (EAP over LAN).  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 carrier
   protocol between client and access point.  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 the
   EAP-TTLS negotiation.  This keying material must be communicated to
   the access point by the TTLS server using the AAA carrier protocol.

5.2.  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.




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5.3.  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 need not include the
   user's actual identity in this EAP-Response/Identity packet other
   than for routing purposes (e.g., realm information; see Section 7.3
   and [RFC3748], Section 5.1); the user's actual identity need 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,
   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 authentication (one-
   way or mutual), validation of client integrity and configuration,
   provisioning of information required for data connectivity, etc.

   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 issues 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 (Master Session Key / Extended Master Session Key
   (MSK/EMSK); see Section 8) is generated based on secret information
   developed during the TLS handshake 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 that they can
   use to encrypt data traffic between them.



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5.4.  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 generates keying material to allow network data
   subsequent to authentication to be securely transmitted between
   client and access point.

6.  Protocol Layering Model

   EAP-TTLS packets are encapsulated within EAP, and EAP in turn
   requires a carrier protocol to transport it.  EAP-TTLS packets
   themselves encapsulate TLS, which is then used to encapsulate
   attribute-value pairs (AVPs) which may carry user authentication or
   other information.  Thus, EAP-TTLS messaging can be described using a
   layered model, where each layer is encapsulated by the layer beneath
   it.  The following diagram clarifies the relationship between
   protocols:

   +-----------------------------------------------------------+
   | AVPs, including authentication (PAP, CHAP, MS-CHAP, etc.) |
   +-----------------------------------------------------------+
   |                            TLS                            |
   +-----------------------------------------------------------+
   |                         EAP-TTLS                          |
   +-----------------------------------------------------------+
   |                            EAP                            |
   +-----------------------------------------------------------+
   |   Carrier Protocol (PPP, EAPOL, RADIUS, Diameter, etc.)   |
   +-----------------------------------------------------------+

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

   +-----------------------------------------------------------+
   |              EAP Method (MD-Challenge, etc.)              |
   +-----------------------------------------------------------+
   |                    AVPs, including EAP                    |
   +-----------------------------------------------------------+
   |                            TLS                            |
   +-----------------------------------------------------------+
   |                         EAP-TTLS                          |
   +-----------------------------------------------------------+
   |                            EAP                            |
   +-----------------------------------------------------------+
   |   Carrier Protocol (PPP, EAPOL, RADIUS, Diameter, etc.)   |
   +-----------------------------------------------------------+




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   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 [RFC2865] or
   Diameter [RFC3588] are used to transport EAP between access point and
   TTLS server.

7.  EAP-TTLS Overview

   A EAP-TTLS negotiation comprises two phases: the TLS handshake phase
   and the TLS tunnel phase.

   During phase 1, TLS is used to authenticate the TTLS server to the
   client and, optionally, the client to the TTLS server.  Phase 1
   results in the activation of a cipher suite, allowing phase 2 to
   proceed securely using the TLS record layer.  (Note that the type and
   degree of security in phase 2 depends on the cipher suite negotiated
   during phase 1; if the null cipher suite is negotiated, there will be
   no security!)

   During phase 2, the TLS record layer is used to tunnel information
   between client and TTLS server to perform any of a number of
   functions.  These might include user authentication, client integrity
   validation, negotiation of data communication security capabilities,
   key distribution, communication of accounting information, etc.
   Information between client and TTLS server is exchanged via
   attribute-value pairs (AVPs) compatible with RADIUS and Diameter;
   thus, any type of function that can be implemented via such AVPs may
   easily be performed.

   EAP-TTLS specifies how user authentication may be performed during
   phase 2.  The user authentication may itself be EAP, or it may be a
   legacy protocol such as PAP, CHAP, MS-CHAP, or MS-CHAP-V2.  Phase 2
   user authentication may not always be necessary, since the user may
   already have been authenticated via the mutual authentication option
   of the TLS handshake protocol.

   Functions other than authentication MAY also be performed during
   phase 2.  This document does not define any such functions; however,
   any organization or standards body is free to specify how additional
   functions may be performed through the use of appropriate AVPs.

   EAP-TTLS specifies how keying material for the data connection
   between client and access point is generated.  The keying material is
   developed implicitly between client and TTLS server based on the
   results of the TLS handshake; the TTLS server will communicate the
   keying material to the access point over the carrier protocol.





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7.1.  Phase 1: Handshake

   In phase 1, the TLS handshake protocol is used to authenticate the
   TTLS server to the client and, optionally, to authenticate the client
   to the TTLS server.

   The TTLS server initiates the EAP-TTLS method with an EAP-TTLS/Start
   packet, which is an EAP-Request with Type = EAP-TTLS and the S
   (Start) bit set.  This indicates to the client that it should begin
   the TLS handshake by sending a ClientHello message.

   EAP packets continue to be exchanged between client and TTLS server
   to complete the TLS handshake, as described in [RFC5216].  Phase 1 is
   completed when the client and TTLS server exchange ChangeCipherSpec
   and Finished messages.  At this point, additional information may be
   securely tunneled.

   As part of the TLS handshake protocol, the TTLS server will send its
   certificate along with a chain of certificates leading to the
   certificate of a trusted CA.  The client will need to be configured
   with the certificate of the trusted CA in order to perform the
   authentication.

   If certificate-based authentication of the client is desired, the
   client must have been issued a certificate and must have the private
   key associated with that certificate.

7.2.  Phase 2: Tunnel

   In phase 2, the TLS record layer is used to securely tunnel
   information between client and TTLS server.  This information is
   encapsulated in sequences of attribute-value pairs (AVPs), whose use
   and format are described in later sections.

   Any type of information may be exchanged during phase 2, according to
   the requirements of the system.  (It is expected that applications
   utilizing EAP-TTLS will specify what information must be exchanged
   and therefore which AVPs must be supported.)  The client begins the
   phase 2 exchange by encoding information in a sequence of AVPs,
   passing this sequence to the TLS record layer for encryption, and
   sending the resulting data to the TTLS server.

   The TTLS server recovers the AVPs in clear text from the TLS record
   layer.  If the AVP sequence includes authentication information, it
   forwards this information to the AAA/H server using the AAA carrier
   protocol.  Note that the EAP-TTLS and AAA/H servers may be one and
   the same; in which case, it simply processes the information locally.




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   The TTLS server may respond with its own sequence of AVPs.  The TTLS
   server passes the AVP sequence to the TLS record layer for encryption
   and sends the resulting data to the client.  For example, the TTLS
   server may forward an authentication challenge received from the
   AAA/H.

   This process continues until the AAA/H either accepts or rejects the
   client, resulting in the TTLS server completing the EAP-TTLS
   negotiation and indicating success or failure to the encapsulating
   EAP protocol (which normally results in a final EAP-Success or EAP-
   Failure being sent to the client).

   The TTLS server distributes data connection keying information and
   other authorization information to the access point in the same AAA
   carrier protocol message that carries the final EAP-Success or other
   success indication.

7.3.  EAP Identity Information

   The identity of the user is provided during phase 2, where it is
   protected by the TLS tunnel.  However, prior to beginning the EAP-
   TTLS authentication, the client will typically issue an EAP-
   Response/Identity packet as part of the EAP protocol, containing a
   username in clear text.  To preserve user anonymity against
   eavesdropping, this packet specifically SHOULD NOT include the actual
   name of the user; instead, it SHOULD use a blank or placeholder such
   as "anonymous".  However, this privacy constraint is not intended to
   apply to any information within the EAP-Response/Identity that is
   required for routing; thus, the EAP-Response/Identity packet MAY
   include the name of the realm of a trusted provider to which EAP-TTLS
   packets should be forwarded; for example, "anonymous@myisp.com".

   Note that at the time the initial EAP-Response/Identity packet is
   sent the EAP method is yet to be negotiated.  If, in addition to EAP-
   TTLS, the client is willing to negotiate use of EAP methods that do
   not support user anonymity, then the client MAY include the name of
   the user in the EAP-Response/Identity to meet the requirements of the
   other candidate EAP methods.

7.4.  Piggybacking

   While it is convenient to describe EAP-TTLS messaging in terms of two
   phases, it is sometimes required that a single EAP-TTLS packet
   contain both phase 1 and phase 2 TLS messages.

   Such "piggybacking" occurs when the party that completes the
   handshake also has AVPs to send.  For example, when negotiating a
   resumed TLS session, the TTLS server sends its ChangeCipherSpec and



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   Finished messages first, then the client sends its own
   ChangeCipherSpec and Finished messages to conclude the handshake.  If
   the client has authentication or other AVPs to send to the TTLS
   server, it MUST tunnel those AVPs within the same EAP-TTLS packet
   immediately following its Finished message.  If the client fails to
   do this, the TTLS server will incorrectly assume that the client has
   no AVPs to send, and the outcome of the negotiation could be
   affected.

7.5.  Session Resumption

   When a client and TTLS server that have previously negotiated an
   EAP-TTLS session begin a new EAP-TTLS 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, causing a new session to 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).

   Tunneled authentication is specifically not performed for resumed
   sessions; the presumption is that the knowledge of the master secret
   (as evidenced by the ability to resume the session) is authentication
   enough.  This allows session resumption to occur without any
   messaging between the TTLS server and the AAA/H.  If periodic
   reauthentication to the AAA/H is desired, the AAA/H must indicate
   this to the TTLS server when the original session is established, for
   example, using the RADIUS Session-Timeout attribute.

   The client MAY send other AVPs in its first phase 2 message of a
   session resumption, to initiate non-authentication functions.  If it
   does not, the TTLS server, at its option, MAY send AVPs to the client
   to initiate non-authentication functions, or MAY simply complete the
   EAP-TTLS negotiation and indicate success or failure to the
   encapsulating EAP protocol.



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   The TTLS server MUST retain authorization information returned by the
   AAA/H for use in resumed sessions.  A resumed session MUST operate
   under the same authorizations as the original session, and the TTLS
   server must be prepared to send the appropriate information back to
   the access point.  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 authentication of the user during
   phase 2.  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 during phase 2 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 during phase 2.]

7.6.  Determining Whether to Enter Phase 2

   Entering phase 2 is optional, and may be initiated by either client
   or TTLS server.  If no further authentication or other information
   exchange is required upon completion of phase 1, it is possible to
   successfully complete the EAP-TTLS negotiation without ever entering
   phase 2 or tunneling any AVPs.

   Scenarios in which phase 2 is never entered include:

   -  Successful session resumption, with no additional information
      exchange required,

   -  Authentication of the client via client certificate during phase
      1, with no additional authentication or information exchange
      required.

   The client always has the first opportunity to initiate phase 2 upon
   completion of phase 1.  If the client has no AVPs to send, it either
   sends an Acknowledgement (see Section 9.2.3) if the TTLS server sends
   the final phase 1 message, or simply does not piggyback a phase 2
   message when it issues the final phase 1 message (as will occur
   during session resumption).




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   If the client does not initiate phase 2, the TTLS server, at its
   option, may either complete the EAP-TTLS negotiation without entering
   phase 2 or initiate phase 2 by tunneling AVPs to the client.

   For example, suppose a successful session resumption occurs in phase
   1.  The following sequences are possible:

   -  Neither the client nor TTLS server has additional information to
      exchange.  The client completes phase 1 without piggybacking phase
      2 AVPs, and the TTLS server indicates success to the encapsulating
      EAP protocol without entering phase 2.

   -  The client has no additional information to exchange, but the TTLS
      server does.  The client completes phase 1 without piggybacking
      phase 2 AVPs, but the TTLS server extends the EAP-TTLS negotiation
      into phase 2 by tunneling AVPs in its next EAP-TTLS message.

   -  The client has additional information to exchange, and piggybacks
      phase 2 AVPs with its final phase 1 message, thus extending the
      negotiation into phase 2.

7.7.  TLS Version

   TLS version 1.0 [RFC2246], 1.1 [RFC4346], or any subsequent version
   MAY be used within EAP-TTLS.  TLS provides for its own version
   negotiation mechanism.

   For maximum interoperability, EAP-TTLS implementations SHOULD support
   TLS version 1.0.

7.8.  Use of TLS PRF

   EAP-TTLSv0 utilizes a pseudo-random function (PRF) to generate keying
   material (Section 8) and to generate implicit challenge material for
   certain authentication methods (Section 11.1).  The PRF used in these
   computations is the TLS PRF used in the TLS handshake negotiation
   that initiates the EAP-TTLS exchange.

   TLS versions 1.0 [RFC2246] and 1.1 [RFC4346] define the same PRF
   function, and any EAP-TTLSv0 implementation based on these versions
   of TLS must use the PRF defined therein.  It is expected that future
   versions of or extensions to the TLS protocol will permit alternative
   PRF functions to be negotiated.  If an alternative PRF function is
   specified for the underlying TLS version or has been negotiated
   during the TLS handshake negotiation, then that alternative PRF
   function must be used in EAP-TTLSv0 computations instead of the TLS
   1.0/1.1 PRF.




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   The TLS PRF function used in this specification is denoted as
   follows:

         PRF-nn(secret, label, seed)

   where:

         nn is the number of generated octets

         secret is a secret key

         label is a string (without null-terminator)

         seed is a binary sequence.

   The TLS 1.0/1.1 PRF has invariant output regardless of how many
   octets are generated.  However, it is possible that alternative PRF
   functions will include the size of the output sequence as input to
   the PRF function; this means generating 32 octets and generating 64
   octets from the same input parameters will no longer result in the
   first 32 octets being identical.  For this reason, the PRF is always
   specified with an "nn", indicating the number of generated octets.

8.  Generating Keying Material

   Upon successful conclusion of an EAP-TTLS negotiation, 128 octets of
   keying material are generated and exported for use in securing the
   data connection between client and access point.  The first 64 octets
   of the keying material constitute the MSK, the second 64 octets
   constitute the EMSK.

   The keying material is generated using the TLS PRF function
   [RFC4346], with inputs consisting of the TLS master secret, the
   ASCII-encoded constant string "ttls keying material", the TLS client
   random, and the TLS server random.  The constant string is not null-
   terminated.

      Keying Material = PRF-128(SecurityParameters.master_secret, "ttls
                keying material", SecurityParameters.client_random +
                SecurityParameters.server_random)

      MSK = Keying Material [0..63]

      EMSK = Keying Material [64..127]







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   Note that the order of client_random and server_random for EAP-TTLS
   is reversed from that of the TLS protocol [RFC4346].  This ordering
   follows the key derivation method of EAP-TLS [RFC5216].  Altering the
   order of randoms avoids namespace collisions between constant strings
   defined for EAP-TTLS and those defined for the TLS protocol.

   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 [RFC2548]
   may be used to distribute the first 32 octets and second 32 octets of
   the MSK, respectively.

9.  EAP-TTLS Protocol

9.1.  Packet Format

   The EAP-TTLS packet format is shown below.  The fields are
   transmitted 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     |        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)







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   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 (000 for EAP-TTLSv0)

      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 field is set
      to the version of EAP-TTLS, and is set to 000 for EAP-TTLSv0.

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

9.2.  EAP-TTLS Start Packet

   The S bit MUST be set on the first packet sent by the server to
   initiate the EAP-TTLS 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
   four-octet boundary relative to the first AVP in the sequence.  If an
   AVP is not a multiple of four octets, it must be padded with zeros to
   the next four-octet boundary.

9.2.1.  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-TTLSv0, this is 0).  In its first EAP message in
   response, the client sets the V field to the highest version number



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   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 MUST set the V field to that version
   number in all subsequent EAP messages.

9.2.2.  Fragmentation

   Each EAP-TTLS message contains a single leg of a half-duplex
   conversation.  The EAP carrier protocol (e.g., PPP, EAPOL, RADIUS)
   may impose constraints on the length of an EAP message.  Therefore it
   may be necessary to fragment an EAP-TTLS 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.  Unfragmented messages MAY have the L bit set and include
   the length of the message (though this information is redundant).

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

9.2.3.  Acknowledgement Packets

   An Acknowledgement packet is an EAP-TTLS 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:

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

   -  When the final EAP packet of the EAP-TTLS negotiation is sent by
      the TTLS server, the client must respond with an Acknowledgement
      packet, to allow the TTLS server to proceed with the EAP protocol
      upon completion of EAP-TTLS (typically by sending or causing to be
      sent a final EAP-Success or EAP-Failure to the client).





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10.  Encapsulation of AVPs within the TLS Record Layer

   Subsequent to the TLS handshake, information may be tunneled between
   client and TTLS server through the use of attribute-value pairs
   (AVPs) encrypted within the TLS record layer.

   The AVP format chosen for EAP-TTLS is compatible with the Diameter
   AVP format.  This does not represent a requirement that Diameter be
   supported by any of the devices or servers participating in an EAP-
   TTLS negotiation.  Use of this format is merely a convenience.
   Diameter is a superset of RADIUS and includes the RADIUS attribute
   namespace by definition, though it does not limit the size of an AVP
   as does RADIUS; RADIUS, in turn, is a widely deployed AAA protocol
   and attribute definitions exist for all commonly used password
   authentication protocols, including EAP.

   Thus, Diameter is not considered normative except as specified in
   this document.  Specifically, the representation of the Data field of
   an AVP in EAP-TTLS is identical to that of Diameter.

   Use of the RADIUS/Diameter namespace allows a TTLS server to easily
   translate between AVPs it uses to communicate to clients and the
   protocol requirements of AAA servers that are widely deployed.  Plus,
   it provides a well-understood mechanism to allow vendors to extend
   that namespace for their particular requirements.

   It is expected that the AVP Codes used in EAP-TTLS will carry roughly
   the same meaning in EAP-TTLS as they do in Diameter and, by
   extension, RADIUS.  However, although EAP-TTLS uses the same AVP
   Codes and syntax as Diameter, the semantics may differ, and most
   Diameter AVPs do not have any well-defined semantics in EAP-TTLS.  A
   separate "EAP-TTLS AVP Usage" registry lists the AVPs that can be
   used within EAP-TTLS and their semantics in this context (see Section
   16 for details).  A TTLS server copying AVPs between an EAP-TTLS
   exchange and a Diameter or RADIUS exchange with a backend MUST NOT
   make assumptions about AVPs whose usage in either EAP-TTLS or the
   backend protocol it does not understand.  Therefore, a TTLS server
   MUST NOT copy an AVP between an EAP-TTLS exchange and a Diameter or
   RADIUS exchange unless the semantics of the AVP are understood and
   defined in both contexts.

10.1.  AVP Format

   The format of an AVP is shown below.  All items are in network, or
   big-endian, order; that is, they have the most significant octet
   first.





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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           AVP Code                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |V M r r r r r r|                  AVP Length                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Vendor-ID (opt)                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Data ...
   +-+-+-+-+-+-+-+-+

   AVP Code
      The AVP Code is four octets and, combined with the Vendor-ID field
      if present, identifies the attribute uniquely.  The first 256 AVP
      numbers represent attributes defined in RADIUS [RFC2865].  AVP
      numbers 256 and above are defined in Diameter [RFC3588].

   AVP Flags

      The AVP Flags field is one octet and provides the receiver with
      information necessary to interpret the AVP.

      The 'V' (Vendor-Specific) bit indicates whether the optional
      Vendor-ID field is present.  When set to 1, the Vendor-ID field is
      present and the AVP Code is interpreted according to the namespace
      defined by the vendor indicated in the Vendor-ID field.

      The 'M' (Mandatory) bit indicates whether support of the AVP is
      required.  If this bit is set to 0, this indicates that the AVP
      may be safely ignored if the receiving party does not understand
      or support it.  If set to 1, this indicates that the receiving
      party MUST fail the negotiation if it does not understand the AVP;
      for a TTLS server, this would imply returning EAP-Failure, for a
      client, this would imply abandoning the negotiation.

      The 'r' (reserved) bits are unused and MUST be set to 0 by the
      sender and MUST be ignored by the receiver.

   AVP Length

      The AVP Length field is three octets and indicates the length of
      this AVP including the AVP Code, AVP Length, AVP Flags, Vendor-ID
      (if present), and Data.







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   Vendor-ID

      The Vendor-ID field is present if the V bit is set in the AVP
      Flags field.  It is four octets and contains the vendor's IANA-
      assigned "SMI Network Management Private Enterprise Codes"
      [RFC3232] value.  Vendors defining their own AVPs must maintain a
      consistent namespace for use of those AVPs within RADIUS,
      Diameter, and EAP-TTLS.

      A Vendor-ID value of zero is equivalent to absence of the Vendor-
      ID field altogether.

   Note that the M bit provides a means for extending the functionality
   of EAP-TTLS while preserving backward compatibility when desired.  By
   setting the M bit of the appropriate AVP(s) to 0 or 1, the party
   initiating the function indicates that support of the function by the
   other party is either optional or required.

10.2.  AVP Sequences

   Data encapsulated within the TLS record layer must consist entirely
   of a sequence of zero or more AVPs.  Each AVP must begin on a four-
   octet boundary relative to the first AVP in the sequence.  If an AVP
   is not a multiple of four octets, it must be padded with zeros to the
   next four-octet boundary.

   Note that the AVP Length does not include the padding.

10.3.  Guidelines for Maximum Compatibility with AAA Servers

   For maximum compatibility with AAA servers, the following guidelines
   for AVP usage are suggested:

   -  Non-vendor-specific AVPs intended for use with AAA servers should
      be selected from the set of attributes defined for RADIUS; that
      is, attributes with codes less than 256.  This provides
      compatibility with both RADIUS and Diameter.

   -  Vendor-specific AVPs intended for use with AAA servers should be
      defined in terms of RADIUS.  Vendor-specific RADIUS attributes
      translate to Diameter (and, hence, to EAP-TTLS) automatically; the
      reverse is not true.  RADIUS vendor-specific attributes use RADIUS
      attribute 26 and include Vendor-ID, vendor-specific attribute
      code, and length; see [RFC2865] for details.







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11.  Tunneled Authentication

   EAP-TTLS permits user authentication information to be tunneled
   within the TLS record layer between client and TTLS server, ensuring
   the security of the authentication information against active and
   passive attack between the client and TTLS server.  The TTLS server
   decrypts and forwards this information to the AAA/H over the AAA
   carrier protocol.

   Any type of password or other authentication may be tunneled.  Also,
   multiple tunneled authentications may be performed.  Normally,
   tunneled authentication is used when the client has not been issued a
   certificate, and the TLS handshake provides only one-way
   authentication of the TTLS server to the client; however, in certain
   cases it may be desired to perform certificate authentication of the
   client during the TLS handshake as well as tunneled user
   authentication afterwards.

11.1.  Implicit Challenge

   Certain authentication protocols that use a challenge/response
   mechanism rely on challenge material that is not generated by the
   authentication server, and therefore the material requires special
   handling.

   In CHAP, MS-CHAP, and MS-CHAP-V2, for example, the access point
   issues a challenge to the client, the client then hashes the
   challenge with the password and forwards the response to the access
   point.  The access point then forwards both challenge and response to
   a AAA server.  But because the AAA server did not itself generate the
   challenge, such protocols are susceptible to replay attack.

   If the client were able to create both challenge and response, anyone
   able to observe a CHAP or MS-CHAP exchange could pose as that user,
   even using EAP-TTLS.

   To make these protocols secure under EAP-TTLS, it is necessary to
   provide a mechanism to produce a challenge that the client cannot
   control or predict.  This is accomplished using the same technique
   described above for generating data connection keying material.

   When a challenge-based authentication mechanism is used, both client
   and TTLS server use the pseudo-random function to generate as many
   octets as are required for the challenge, using the constant string
   "ttls challenge", based on the master secret and random values
   established during the handshake:





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      EAP-TTLS_challenge = PRF-nn(SecurityParameters.master_secret,
                             "ttls challenge",
                             SecurityParameters.client_random +
                             SecurityParameters.server_random);

   The number of octets to be generated (nn) depends on the
   authentication method, and is indicated below for each authentication
   method requiring implicit challenge generation.

11.2.  Tunneled Authentication Protocols

   This section describes the methods for tunneling specific
   authentication protocols within EAP-TTLS.

   For the purpose of explication, it is assumed that the TTLS server
   and AAA/H use RADIUS as a AAA carrier protocol between them.
   However, this is not a requirement, and any AAA protocol capable of
   carrying the required information may be used.

   The client determines which authentication protocol will be used via
   the initial AVPs it sends to the server, as described in the
   following sections.

   Note that certain of the authentication protocols described below
   utilize vendor-specific AVPs originally defined for RADIUS.  RADIUS
   and Diameter differ in the encoding of vendor-specific AVPs: RADIUS
   uses the vendor-specific attribute (code 26), while Diameter uses
   setting of the V bit to indicate the presence of Vendor-ID.  The
   RADIUS form of the vendor-specific attribute is always convertible to
   a Diameter AVP with V bit set.  All vendor-specific AVPs described
   below MUST be encoded using the preferred Diameter V bit mechanism;
   that is, the AVP Code of 26 MUST NOT be used to encode vendor-
   specific AVPs within EAP-TTLS.

11.2.1.  EAP

   When EAP is the tunneled authentication protocol, each tunneled EAP
   packet between the client and TTLS server is encapsulated in an EAP-
   Message AVP, prior to tunneling via the TLS record layer.

   Note that because Diameter AVPs are not limited to 253 octets of
   data, as are RADIUS attributes, the RADIUS mechanism of concatenating
   multiple EAP-Message attributes to represent a longer-than-253-octet
   EAP packet is not appropriate in EAP-TTLS.  Thus, a tunneled EAP
   packet within a single EAP-TTLS message MUST be contained in a single
   EAP-Message AVP.





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   The client initiates EAP by tunneling EAP-Response/Identity to the
   TTLS server.  Depending on the requirements specified for the inner
   method, the client MAY now place the actual username in this packet;
   the privacy of the user's identity is now guaranteed by the TLS
   encryption.  This username is typically a Network Access Identifier
   (NAI) [RFC4282]; that is, it is typically in the following format:

      username@realm

   The @realm portion is optional, and is used to allow the TTLS server
   to forward the EAP packet to the appropriate AAA/H.

   Note that the client has two opportunities to specify realms.  The
   first, in the initial, untunneled EAP-Response/Identity packet prior
   to starting EAP-TTLS, indicates the realm of the TTLS server.  The
   second, occurring as part of the EAP exchange within the EAP-TTLS
   tunnel, indicates the realm of the client's home network.  Thus, the
   access point need only know how to route to the realm of the TTLS
   server; the TTLS server is assumed to know how to route to the
   client's home realm.  This serial routing architecture is anticipated
   to be useful in roaming environments, allowing access points or AAA
   proxies behind access points to be configured only with a small
   number of realms.  (Refer to Section 7.3 for additional information
   distinguishing the untunneled and tunneled versions of the EAP-
   Response/Identity packets.)

   Note that TTLS processing of the initial identity exchange is
   different from plain EAP.  The state machine of TTLS is different.
   However, it is expected that the server side is capable of dealing
   with client initiation, because even normal EAP protocol runs are
   client-initiated over AAA.  On the client side, there are various
   implementation techniques to deal with the differences.  Even a
   TTLS-unaware EAP protocol run could be used, if TTLS makes it appear
   as if an EAP-Request/Identity message was actually received.  This is
   similar to what authenticators do when operating between a client and
   a AAA server.

   Upon receipt of the tunneled EAP-Response/Identity, the TTLS server
   forwards it to the AAA/H in a RADIUS Access-Request.

   The AAA/H may immediately respond with an Access-Reject; in which
   case, the TTLS server completes the negotiation by sending an EAP-
   Failure to the access point.  This could occur if the AAA/H does not
   recognize the user's identity, or if it does not support EAP.

   If the AAA/H does recognize the user's identity and does support EAP,
   it responds with an Access-Challenge containing an EAP-Request, with
   the Type and Type-Data fields set according to the EAP protocol with



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   which the AAA/H wishes to authenticate the client; for example MD5-
   Challenge, One-Time Password (OTP), or Generic Token Card.

   The EAP authentication between client and AAA/H proceeds normally, as
   described in [RFC3748], with the TTLS server acting as a passthrough
   device.  Each EAP-Request sent by the AAA/H in an Access-Challenge is
   tunneled by the TTLS server to the client, and each EAP-Response
   tunneled by the client is decrypted and forwarded by the TTLS server
   to the AAA/H in an Access-Request.

   This process continues until the AAA/H issues an Access-Accept or
   Access-Reject.

   Note that EAP-TTLS does not impose special rules on EAP Notification
   packets; such packets MAY be used within a tunneled EAP exchange
   according to the rules specified in [RFC3748].

   EAP-TTLS provides a reliable transport for the tunneled EAP exchange.
   However, [RFC3748] assumes an unreliable transport for EAP messages
   (see Section 3.1), and provides for silent discard of any EAP packet
   that violates the protocol or fails a method-specific integrity
   check, on the assumption that such a packet is likely a counterfeit
   sent by an attacker.  But since the tunnel provides a reliable
   transport for the inner EAP authentication, errors that would result
   in silent discard according to [RFC3748] presumably represent
   implementation errors when they occur within the tunnel, and SHOULD
   be treated as such in preference to being silently discarded.
   Indeed, silently discarding an EAP message within the tunnel
   effectively puts a halt to the progress of the exchange, and will
   result in long timeouts in cases that ought to result in immediate
   failures.

11.2.2.  CHAP

   The CHAP algorithm is described in [RFC1661]; RADIUS attribute
   formats are described in [RFC2865].

   Both client and TTLS server generate 17 octets of challenge material,
   using the constant string "ttls challenge" as described above.  These
   octets are used as follows:

      CHAP-Challenge    [16 octets]
      CHAP Identifier   [1 octet]

   The client initiates CHAP by tunneling User-Name, CHAP-Challenge, and
   CHAP-Password AVPs to the TTLS server.  The CHAP-Challenge value is
   taken from the challenge material.  The CHAP-Password consists of




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   CHAP Identifier, taken from the challenge material; and CHAP
   response, computed according to the CHAP algorithm.

   Upon receipt of these AVPs from the client, the TTLS server must
   verify that the value of the CHAP-Challenge AVP and the value of the
   CHAP Identifier in the CHAP-Password AVP are equal to the values
   generated as challenge material.  If either item does not match
   exactly, the TTLS server must reject the client.  Otherwise, it
   forwards the AVPs to the AAA/H in an Access-Request.

   The AAA/H will respond with an Access-Accept or Access-Reject.

11.2.3.  MS-CHAP

   The MS-CHAP algorithm is described in [RFC2433]; RADIUS attribute
   formats are described in [RFC2548].

   Both client and TTLS server generate 9 octets of challenge material,
   using the constant string "ttls challenge" as described above.  These
   octets are used as follows:

      MS-CHAP-Challenge  [8 octets]
      Ident              [1 octet]

   The client initiates MS-CHAP by tunneling User-Name, MS-CHAP-
   Challenge and MS-CHAP-Response AVPs to the TTLS server.  The MS-
   CHAP-Challenge value is taken from the challenge material.  The MS-
   CHAP-Response consists of Ident, taken from the challenge material;
   Flags, set according the client preferences; and LM-Response and NT-
   Response, computed according to the MS-CHAP algorithm.

   Upon receipt of these AVPs from the client, the TTLS server MUST
   verify that the value of the MS-CHAP-Challenge AVP and the value of
   the Ident in the client's MS-CHAP-Response AVP are equal to the
   values generated as challenge material.  If either item does not
   match exactly, the TTLS server MUST reject the client.  Otherwise, it
   forwards the AVPs to the AAA/H in an Access-Request.

   The AAA/H will respond with an Access-Accept or Access-Reject.

11.2.4.  MS-CHAP-V2

   The MS-CHAP-V2 algorithm is described in [RFC2759]; RADIUS attribute
   formats are described in [RFC2548].

   Both client and TTLS server generate 17 octets of challenge material,
   using the constant string "ttls challenge" as described above.  These
   octets are used as follows:



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      MS-CHAP-Challenge  [16 octets]
      Ident              [1 octet]

   The client initiates MS-CHAP-V2 by tunneling User-Name, MS-CHAP-
   Challenge, and MS-CHAP2-Response AVPs to the TTLS server.  The MS-
   CHAP-Challenge value is taken from the challenge material.  The MS-
   CHAP2-Response consists of Ident, taken from the challenge material;
   Flags, set to 0; Peer-Challenge, set to a random value; and Response,
   computed according to the MS-CHAP-V2 algorithm.

   Upon receipt of these AVPs from the client, the TTLS server MUST
   verify that the value of the MS-CHAP-Challenge AVP and the value of
   the Ident in the client's MS-CHAP2-Response AVP are equal to the
   values generated as challenge material.  If either item does not
   match exactly, the TTLS server MUST reject the client.  Otherwise, it
   forwards the AVPs to the AAA/H in an Access-Request.

   If the authentication is successful, the AAA/H will respond with an
   Access-Accept containing the MS-CHAP2-Success attribute.  This
   attribute contains a 42-octet string that authenticates the AAA/H to
   the client based on the Peer-Challenge.  The TTLS server tunnels this
   AVP to the client.  Note that the authentication is not yet complete;
   the client must still accept the authentication response of the
   AAA/H.

   Upon receipt of the MS-CHAP2-Success AVP, the client is able to
   authenticate the AAA/H.  If the authentication succeeds, the client
   sends an EAP-TTLS packet to the TTLS server containing no data (that
   is, with a zero-length Data field).  Upon receipt of the empty EAP-
   TTLS packet from the client, the TTLS server considers the MS-CHAP-
   V2 authentication to have succeeded.

   If the authentication fails, the AAA/H will respond with an Access-
   Challenge containing the MS-CHAP-Error attribute.  This attribute
   contains a new Ident and a string with additional information such as
   the error reason and whether a retry is allowed.  The TTLS server
   tunnels this AVP to the client.  If the error reason is an expired
   password and a retry is allowed, the client may proceed to change the
   user's password.  If the error reason is not an expired password or
   if the client does not wish to change the user's password, it simply
   abandons the EAP-TTLS negotiation.

   If the client does wish to change the password, it tunnels MS-CHAP-
   NT-Enc-PW, MS-CHAP2-CPW, and MS-CHAP-Challenge AVPs to the TTLS
   server.  The MS-CHAP2-CPW AVP is derived from the new Ident and
   Challenge received in the MS-CHAP-Error AVP.  The MS-CHAP-Challenge
   AVP simply echoes the new Challenge.




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   Upon receipt of these AVPs from the client, the TTLS server MUST
   verify that the value of the MS-CHAP-Challenge AVP and the value of
   the Ident in the client's MS-CHAP2-CPW AVP match the values it sent
   in the MS-CHAP-Error AVP.  If either item does not match exactly, the
   TTLS server MUST reject the client.  Otherwise, it forwards the AVPs
   to the AAA/H in an Access-Request.

   If the authentication is successful, the AAA/H will respond with an
   Access-Accept containing the MS-CHAP2-Success attribute.  At this
   point, the negotiation proceeds as described above; the TTLS server
   tunnels the MS-CHAP2-Success to the client, and the client
   authenticates the AAA/H based on this AVP.  Then, the client either
   abandons the negotiation on failure or sends an EAP-TTLS packet to
   the TTLS server containing no data (that is, with a zero-length Data
   field), causing the TTLS server to consider the MS-CHAP-V2
   authentication to have succeeded.

   Note that additional AVPs associated with MS-CHAP-V2 may be sent by
   the AAA/H; for example, MS-CHAP-Domain.  The TTLS server MUST tunnel
   such authentication-related attributes along with the MS-CHAP2-
   Success.

11.2.5.  PAP

   The client initiates PAP by tunneling User-Name and User-Password
   AVPs to the TTLS server.

   Normally, in RADIUS, User-Password is padded with nulls to a multiple
   of 16 octets, then encrypted using a shared secret and other packet
   information.

   An EAP-TTLS client, however, does not RADIUS-encrypt the password
   since no such RADIUS variables are available; this is not a security
   weakness since the password will be encrypted via TLS anyway.  The
   client SHOULD, however, null-pad the password to a multiple of 16
   octets, to obfuscate its length.

   Upon receipt of these AVPs from the client, the TTLS server forwards
   them to the AAA/H in a RADIUS Access-Request.  (Note that in the
   Access-Request, the TTLS server must encrypt the User-Password
   attribute using the shared secret between the TTLS server and AAA/H.)

   The AAA/H may immediately respond with an Access-Accept or Access-
   Reject.  The TTLS server then completes the negotiation by sending an
   EAP-Success or EAP-Failure to the access point using the AAA carrier
   protocol.





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   The AAA/H may also respond with an Access-Challenge.  The TTLS server
   then tunnels the AVPs from the AAA/H's challenge to the client.  Upon
   receipt of these AVPs, the client tunnels User-Name and User-
   Password again, with User-Password containing new information in
   response to the challenge.  This process continues until the AAA/H
   issues an Access-Accept or Access-Reject.

   At least one of the AVPs tunneled to the client upon challenge MUST
   be Reply-Message.  Normally, this is sent by the AAA/H as part of the
   challenge.  However, if the AAA/H has not sent a Reply-Message, the
   TTLS server MUST issue one, with null value.  This allows the client
   to determine that a challenge response is required.

   Note that if the AAA/H includes a Reply-Message as part of an
   Access-Accept or Access-Reject, the TTLS server does not tunnel this
   AVP to the client.  Rather, this AVP and all other AVPs sent by the
   AAA/H as part of Access-Accept or Access-Reject are sent to the
   access point via the AAA carrier protocol.

11.3.  Performing Multiple Authentications

   In some cases, it is desirable to perform multiple user
   authentications.  For example, a AAA/H may want first to authenticate
   the user by password, then by token card.

   The AAA/H may perform any number of additional user authentications
   using EAP, simply by issuing a EAP-Request with a new EAP type once
   the previous authentication completes.  Note that each new EAP method
   is subject to negotiation; that is, the client may respond to the EAP
   request for a new EAP type with an EAP-Nak, as described in
   [RFC3748].

   For example, a AAA/H wishing to perform an MD5-Challenge followed by
   Generic Token Card would first issue an EAP-Request/MD5-Challenge and
   receive a response.  If the response is satisfactory, it would then
   issue an EAP-Request/Generic Token Card and receive a response.  If
   that response were also satisfactory, it would accept the user.

   The entire inner EAP exchange comprising multiple authentications is
   considered a single EAP sequence, in that each subsequent request
   MUST contain distinct a EAP Identifier from the previous request,
   even as one authentication completes and another begins.

   The peer identity indicated in the original EAP-Response/Identity
   that initiated the EAP sequence is intended to apply to each of the
   sequential authentications.  In the absence of an application profile
   standard specifying otherwise, additional EAP-Identity exchanges
   SHOULD NOT occur.



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   The conditions for overall success or failure when multiple
   authentications are used are a matter of policy on client and server;
   thus, either party may require that all inner authentications
   succeed, or that at least one inner authentication succeed, as a
   condition for success of the overall authentication.

   Each EAP method is intended to run to completion.  Should the TTLS
   server abandon a method and start a new one, client behavior is not
   defined in this document and is a matter of client policy.

   Note that it is not always feasible to use the same EAP method twice
   in a row, since it may not be possible to determine when the first
   authentication completes and the new authentication begins if the EAP
   type does not change.  Certain EAP methods, such as EAP-TLS, use a
   Start bit to distinguish the first request, thus allowing each new
   authentication using that type to be distinguished from the previous.
   Other methods, such as EAP-MS-CHAP-V2, terminate in a well-defined
   manner, allowing a second authentication of the same type to commence
   unambiguously.  While use of the same EAP method for multiple
   authentications is relatively unlikely, implementers should be aware
   of the issues and avoid cases that would result in ambiguity.

   Multiple authentications using non-EAP methods or a mixture of EAP
   and non-EAP methods is not defined in this document, nor is it known
   whether such an approach has been implemented.

11.4.  Mandatory Tunneled Authentication Support

   To ensure interoperability, in the absence of an application profile
   standard specifying otherwise, an implementation compliant with this
   specification MUST implement EAP as a tunneled authentication method
   and MUST implement MD5-Challenge as an EAP type.  However, such an
   implementation MAY allow the use of EAP, any EAP type, or any other
   tunneled authentication method to be enabled or disabled by
   administrative action on either client or TTLS server.

   In addition, in the absence of an application profile standard
   specifying otherwise, an implementation compliant with this
   specification MUST allow an administrator to configure the use of
   tunneled authentication without the M (Mandatory) bit set on any AVP.

11.5.  Additional Suggested Tunneled Authentication Support

   The following information is provided as non-normative guidance based
   on the experience of the authors and reviewers of this specification
   with existing implementations of EAP-TTLSv0.





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   The following authentication methods are commonly used, and servers
   wishing for broad interoperability across multiple media should
   consider implementing them:

   -  PAP (both for password and token authentication)

   -  MS-CHAP-V2

   -  EAP-MS-CHAP-V2

   -  EAP-GTC

12.  Keying Framework

   In compliance with [RFC5247], Session-Id, Peer-Id, and Server-Id are
   here defined.

12.1.  Session-Id

   The Session-Id uniquely identifies an authentication exchange between
   the client and TTLS server.  It is defined as follows:

      Session-Id = 0x15 || client.random || server.random

12.2.  Peer-Id

   The Peer-Id represents the identity to be used for access control and
   accounting purposes.  When the client presents a certificate as part
   of the TLS handshake, the Peer-Id is determined based on information
   in the certificate, as specified in Section 5.2 of [RFC5216].
   Otherwise, the Peer-Id is null.

12.3.  Server-Id

   The Server-Id identifies the TTLS server.  When the TTLS server
   presents a certificate as part of the TLS handshake, the Server-Id is
   determined based on information in the certificate, as specified in
   Section 5.2 of [RFC5216].  Otherwise, the Server-Id is null.

13.  AVP Summary

   The following table lists each AVP defined in this document, whether
   the AVP may appear in a packet from server to client ("Request")
   and/or in a packet from client to server ("Response"), and whether
   the AVP MUST be implemented ("MI").






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   Name              Request  Response    MI
   ---------------------------------------------------
   User-Name                     X
   User-Password                 X
   CHAP-Password                 X
   Reply-Message        X
   CHAP-Challenge                X
   EAP-Message          X        X         X
   MS-CHAP-Response              X
   MS-CHAP-Error        X
   MS-CHAP-NT-Enc-PW             X
   MS-CHAP-Domain       X
   MS-CHAP-Challenge             X
   MS-CHAP2-Response             X
   MS-CHAP2-Success     X
   MS-CHAP2-CPW                  X

14.  Security Considerations

14.1.  Security Claims

   Pursuant to RFC 3748, security claims for EAP-TTLSv0 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:             Up to 384 bits
   Dictionary attack prot.:  Yes
   Fast reconnect:           Yes
   Cryptographic binding:    No
   Session independence:     Yes
   Fragmentation:            Yes
   Channel binding:          No

14.1.1.  Authentication Mechanism

   EAP-TTLSv0 utilizes negotiated underlying authentication protocols,
   both in the phase 1 TLS handshake and the phase 2 tunneled
   authentication.  In a typical deployment, at a minimum the TTLS
   server authenticates to the client in phase 1, and the client
   authenticates to the AAA/H server in phase 2.  Phase 1 authentication
   of the TTLS server to the client is typically by certificate; the
   client may optionally authenticate to the TTLS server by certificate



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   as well.  Phase 2 authentication of the client to the AAA/H server is
   typically by password or security token via an EAP or supported non-
   EAP authentication mechanism; this authentication mechanism may
   provide authentication of the AAA/H server to the client as well
   (mutual authentication).

14.1.2.  Ciphersuite Negotiation

   Ciphersuite negotiation is inherited from TLS.

14.1.3.  Mutual Authentication

   In the recommended minimum configuration, the TTLS server is
   authenticated to the client in phase 1, and the client and AAA/H
   server mutually authenticate in phase 2.

14.1.4.  Integrity Protection

   Integrity protection is inherited from TLS.

14.1.5.  Replay Protection

   Replay protection is inherited from TLS.

14.1.6.  Confidentiality

   Confidentiality is inherited from TLS.  Note, however, that EAP-
   TTLSv0 contains no provision for encryption of success or failure EAP
   packets.

14.1.7.  Key Derivation

   Both MSK and EMSK are derived.  The key derivation PRF is inherited
   from TLS, and cryptographic agility of this mechanism depends on the
   cryptographic agility of the TLS PRF.

14.1.8.  Key Strength

   Key strength is limited by the size of the TLS master secret, which
   for versions 1.0 and 1.1 is 48 octets (384 bits).  Effective key
   strength may be less, depending on the attack resistance of the
   negotiated Diffie-Helman (DH) group, certificate RSA/DSA group, etc.
   BCP 86 [RFC3766], Section 5, offers advice on the required RSA or DH
   module and DSA subgroup size in bits, for a given level of attack
   resistance in bits.  For example, a 2048-bit RSA key is recommended
   to provide 128-bit equivalent key strength.  The National Institute
   for Standards and Technology (NIST) also offers advice on appropriate
   key sizes in [SP800-57].



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14.1.9.  Dictionary Attack Protection

   Phase 2 password authentication is protected against eavesdropping
   and therefore against offline dictionary attack by TLS encryption.

14.1.10.  Fast Reconnect

   Fast reconnect is provided by TLS session resumption.

14.1.11.  Cryptographic Binding

   [MITM] describes a vulnerability that is characteristic of tunneled
   authentication protocols, in which an attacker authenticates as a
   client via a tunneled protocol by posing as an authenticator to a
   legitimate client using a non-tunneled protocol.  When the same proof
   of credentials can be used in both authentications, the attacker
   merely shuttles the credential proof between them.  EAP-TTLSv0 is
   vulnerable to such an attack.  Care should be taken to avoid using
   authentication protocols and associated credentials both as inner
   TTLSv0 methods and as untunneled methods.

   Extensions to EAP-TTLSv0 or a future version of EAP-TTLS should be
   defined to perform a cryptographic binding of keying material
   generated by inner authentication methods and the keying material
   generated by the TLS handshake.  This avoids the man-in-the-middle
   problem when used with key-generating inner methods.  Such an
   extension mechanism has been proposed [TTLS-EXT].

14.1.12.  Session Independence

   TLS guarantees the session independence of its master secret, from
   which the EAP-TTLSv0 MSK/EMSK is derived.

14.1.13.  Fragmentation

   Provision is made for fragmentation of lengthy EAP packets.

14.1.14.  Channel Binding

   Support for channel binding may be added as a future extension, using
   appropriate AVPs.

14.2.  Client Anonymity

   Unlike other EAP methods, 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



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   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
   during phase 1 of EAP-TTLS, 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 EAP-TTLS server and may be included in the clear in AAA
   messages between the access point, the EAP-TTLS server, and the AAA/H
   server.

   Note that if a client authenticating with a certificate wishes to
   shield its certificate, and hence its identity, from eavesdroppers,
   it may use the technique described in Section 2.1.4 ("Privacy") of
   [RFC5216], in which the client sends an empty certificate list, the
   TTLS server issues a ServerHello upon completion of the TLS handshake
   to begin a second, encrypted handshake, during which the client will
   send its certificate list.  Note that for this feature to work the
   client must know in advance that the TTLS server supports it.

14.3.  Server Trust

   Trust of the server by the client is established via a server
   certificate conveyed during the TLS handshake.  The client should
   have a means of determining which server identities are authorized to
   act as a TTLS server and may be trusted, and should refuse to
   authenticate with servers it does not trust.  The consequence of
   pursuing authentication with a hostile server is exposure of the
   inner authentication to attack; e.g., offline dictionary attack
   against the client password.

14.4.  Certificate Validation

   When either client or server presents a certificate as part of the
   TLS handshake, it should include the entire certificate chain minus
   the root to facilitate certificate validation by the other party.

   When either client or server receives a certificate as part of the
   TLS handshake, it should validate the certification path to a trusted
   root.  If intermediate certificates are not provided by the sender,
   the receiver may use cached or pre-configured copies if available, or
   may retrieve them from the Internet if feasible.

   Clients and servers should implement policies related to the Extended
   Key Usage (EKU) extension [RFC5280] of certificates it receives, to
   ensure that the other party's certificate usage conforms to the
   certificate's purpose.  Typically, a client EKU, when present, would



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   be expected to include id-kp-clientAuth; a server EKU, when present,
   would be expected to include id-kp-serverAuth.  Note that absence of
   the EKU extension or a value of anyExtendedKeyUsage implies absence
   of constraint on the certificate's purpose.

14.5.  Certificate Compromise

   Certificates should be checked for revocation to reduce exposure to
   imposture using compromised certificates.

   Checking a server certificate against the most recent revocation list
   during authentication is not always possible for a client, as it may
   not have network access until completion of the authentication.  This
   problem can be alleviated through the use of the Online Certificate
   Status Protocol (OCSP) [RFC2560] during the TLS handshake, as
   described in [RFC4366].

14.6.  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.

14.7.  Negotiating-Down Attacks

   EAP-TTLS negotiates its own protocol version prior to, and therefore
   outside the security established by the TLS tunnel.  In principle,
   therefore, it is subject to a negotiating-down attack, in which an
   intermediary modifies messages in transit to cause a lower version of
   the protocol to be agreed upon, each party assuming that the other
   does not support as high a version as it actually does.

   The version of the EAP-TTLS protocol described in this document is 0,
   and is therefore not subject to such an attack.  However, any new
   version of the protocol using a higher number than 0 should define a
   mechanism to ensure against such an attack.  One such mechanism might
   be the TTLS server's reiteration of the protocol version that it
   proposed in an AVP within the tunnel, such AVP to be inserted with M
   bit clear even when version 0 is agreed upon.





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15.  Message Sequences

   This section presents EAP-TTLS message sequences for various
   negotiation scenarios.  These examples do not attempt to exhaustively
   depict all possible scenarios.

   It is assumed that RADIUS is the AAA carrier protocol both between
   access point and TTLS server, and between TTLS server and AAA/H.

   EAP packets that are passed unmodified between client and TTLS server
   by the access point are indicated as "passthrough".  AVPs that are
   securely tunneled within the TLS record layer are enclosed in curly
   braces ({}).  Items that are optional are suffixed with question mark
   (?).  Items that may appear multiple times are suffixed with plus
   sign (+).

15.1.  Successful Authentication via Tunneled CHAP

   In this example, the client performs one-way TLS authentication of
   the TTLS server.  CHAP is used as a tunneled user authentication
   mechanism.

   client          access point           TTLS server             AAA/H
   ------          ------------           -----------             -----

     EAP-Request/Identity
     <--------------------

     EAP-Response/Identity
     -------------------->

                           RADIUS Access-Request:
                             EAP-Response passthrough
                           -------------------->

                           RADIUS Access-Challenge:
                             EAP-Request/TTLS-Start
                           <--------------------

     EAP-Request passthrough
     <--------------------

     EAP-Response/TTLS:
       ClientHello
     -------------------->






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                           RADIUS Access-Request:
                             EAP-Response passthrough
                           -------------------->

                           RADIUS Access-Challenge:
                             EAP-Request/TTLS:
                               ServerHello
                               Certificate
                               ServerKeyExchange
                               ServerHelloDone
                           <--------------------

     EAP-Request passthrough
     <--------------------

     EAP-Response/TTLS:
       ClientKeyExchange
       ChangeCipherSpec
       Finished
     -------------------->

                           RADIUS Access-Request:
                             EAP-Response passthrough
                           -------------------->

                           RADIUS Access-Challenge:
                             EAP-Request/TTLS:
                               ChangeCipherSpec
                               Finished
                           <--------------------

     EAP-Request passthrough
     <--------------------

     EAP-Response/TTLS:
       {User-Name}
       {CHAP-Challenge}
       {CHAP-Password}
     -------------------->

                           RADIUS Access-Request:
                             EAP-Response passthrough
                           -------------------->








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                                             RADIUS Access-Request:
                                               User-Name
                                               CHAP-Challenge
                                               CHAP-Password
                                             -------------------->

                                             RADIUS Access-Accept
                                             <--------------------

                           RADIUS Access-Accept:
                             EAP-Success
                           <--------------------

     EAP-Success
     <--------------------

15.2.  Successful Authentication via Tunneled EAP/MD5-Challenge

   In this example, the client performs one-way TLS authentication of
   the TTLS server and EAP/MD5-Challenge is used as a tunneled user
   authentication mechanism.

   client          access point           TTLS server             AAA/H
   ------          ------------           -----------             -----

     EAP-Request/Identity
     <--------------------

     EAP-Response/Identity
     -------------------->

                           RADIUS Access-Request:
                             EAP-Response passthrough
                           -------------------->

                           RADIUS Access-Challenge:
                             EAP-Request/TTLS-Start
                           <--------------------

     EAP-Request passthrough
     <--------------------

     EAP-Response/TTLS:
       ClientHello
     -------------------->






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                           RADIUS Access-Request:
                             EAP-Response passthrough
                           -------------------->

                           RADIUS Access-Challenge:
                             EAP-Request/TTLS:
                               ServerHello
                               Certificate
                               ServerKeyExchange
                               ServerHelloDone
                           <--------------------

     EAP-Request passthrough
     <--------------------

     EAP-Response/TTLS:
       ClientKeyExchange
       ChangeCipherSpec
       Finished
     -------------------->

                           RADIUS Access-Request:
                             EAP-Response passthrough
                           -------------------->

                           RADIUS Access-Challenge:
                             EAP-Request/TTLS:
                               ChangeCipherSpec
                               Finished
                           <--------------------

     EAP-Request passthrough
     <--------------------

     EAP-Response/TTLS:
       {EAP-Response/Identity}
     -------------------->

                           RADIUS Access-Request:
                             EAP-Response passthrough
                           -------------------->

                                             RADIUS Access-Request:
                                               EAP-Response/Identity
                                             -------------------->






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                                             RADIUS Access-Challenge
                                               EAP-Request/
                                                   MD5-Challenge
                                             <--------------------

                           RADIUS Access-Challenge:
                             EAP-Request/TTLS:
                               {EAP-Request/MD5-Challenge}
                           <--------------------

     EAP-Request passthrough
     <--------------------

     EAP-Response/TTLS:
       {EAP-Response/MD5-Challenge}
     -------------------->

                           RADIUS Access-Request:
                             EAP-Response passthrough
                           -------------------->

                                             RADIUS Access-Challenge
                                               EAP-Response/
                                                   MD5-Challenge
                                             -------------------->

                                             RADIUS Access-Accept
                                             <--------------------

                           RADIUS Access-Accept:
                             EAP-Success
                           <--------------------

     EAP-Success
     <--------------------
















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15.3.  Successful Session Resumption

   In this example, the client and server resume a previous TLS session.
   The ID of the session to be resumed is sent as part of the
   ClientHello, and the server agrees to resume this session by sending
   the same session ID as part of ServerHello.

   client          access point           TTLS server             AAA/H
   ------          ------------           -----------             -----

     EAP-Request/Identity
     <--------------------

     EAP-Response/Identity
     -------------------->

                           RADIUS Access-Request:
                             EAP-Response passthrough
                           -------------------->

                           RADIUS Access-Challenge:
                             EAP-Request/TTLS-Start
                           <--------------------

     EAP-Request passthrough
     <--------------------

     EAP-Response/TTLS:
       ClientHello
     -------------------->

                           RADIUS Access-Request:
                             EAP-Response passthrough
                           -------------------->

                           RADIUS Access-Challenge:
                             EAP-Request/TTLS:
                               ServerHello
                               ChangeCipherSpec
                               Finished
                           <--------------------

     EAP-Request passthrough
     <--------------------







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     EAP-Response/TTLS:
       ChangeCipherSpec
       Finished
     -------------------->

                           RADIUS Access-Request:
                             EAP-Response passthrough
                           -------------------->

                           RADIUS Access-Accept:
                             EAP-Success
                           <--------------------

     EAP-Success
     <--------------------

16.  IANA Considerations

   IANA has assigned the number 21 (decimal) as the method type of the
   EAP-TTLS protocol.  Mechanisms for defining new RADIUS and Diameter
   AVPs and AVP values are outlined in [RFC2865] and [RFC3588],
   respectively.  No additional IANA registrations are specifically
   contemplated in this document.

   Section 11 of this document specifies how certain authentication
   mechanisms may be performed within the secure tunnel established by
   EAP-TTLS.  New mechanisms and other functions MAY also be performed
   within this tunnel.  Where such extensions use AVPs that are not
   vendor-specific, their semantics must be specified in new RFCs; that
   is, there are TTLS-specific processing rules related to the use of
   each individual AVP, even though such AVPs have already been defined
   for RADIUS or DIAMETER.

   This specification requires the creation of a new registry -- EAP-
   TTLS AVP Usage -- to be managed by IANA, listing each non-vendor-
   specific RADIUS/Diameter AVP that has been defined for use within
   EAP-TTLS, along with a reference to the RFC or other document that
   specifies its semantics.  The initial list of AVPs shall be those
   listed in Section 13 of this document.  The purpose of this registry
   is to avoid potential ambiguity resulting from the same AVP being
   utilized in different functional contexts.  This registry does not
   assign numbers to AVPs, as the AVP numbers are assigned out of the
   RADIUS and Diameter namespaces as outlined in [RFC2865] and
   [RFC3588].  Only top-level AVPs -- that is, AVPs not encapsulated
   within Grouped AVPs -- will be registered.  AVPs should be added to
   this registry based on IETF Review as defined in [RFC5226].





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17.  Acknowledgements

   Thanks to Bernard Aboba, Jari Arkko, Lakshminath Dondeti, Stephen
   Hanna, Ryan Hurst, Avi Lior, and Gabriel Montenegro for careful
   reviews and useful comments.

18.  References

18.1.  Normative References

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

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

   [RFC2246]   Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
               RFC 2246, January 1999.

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

   [RFC5226]   Narten, T. and H. Alvestrand, "Guidelines for Writing an
               IANA Considerations Section in RFCs", BCP 26, RFC 5226,
               May 2008.

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

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

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

   [RFC3232]   Reynolds, J., Ed., "Assigned Numbers: RFC 1700 is
               Replaced by an On-line Database", RFC 3232, January 2002.

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

   [RFC3748]   Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
               Levkowetz, Ed., "Extensible Authentication Protocol
               (EAP)", RFC 3748, June 2004.





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   [RFC4282]   Aboba, B., Beadles, M., Arkko, J. and P. Eronen, "The
               Network Access Identifier", RFC 4282, December 2005.

   [RFC4346]   Dierks, T. and E. Rescorla, "The Transport Layer Security
               (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [RFC5216]   Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS
               Authentication Protocol", RFC 5216, March 2008.

   [RFC5247]   Aboba, B., Simon, D., and P. Eronen, "Extensible
               Authentication Protocol (EAP) Key Management Framework",
               RFC 5247, August 2008.

18.2.  Informative References

   [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.

   [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, 2007.

   [TTLS-EXT]  Hanna, S. and P. Funk, "Key Agility Extensions for EAP-
               TTLSv0", Work in Progress, September 2007.

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

   [RFC5280]   Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
               Housley, R., and W. Polk, "Internet X.509 Public Key
               Infrastructure Certificate and Certificate Revocation
               List (CRL) Profile", RFC 5280, May 2008.

   [RFC3766]   Orman, H. and P. Hoffman, "Determining Strengths For
               Public Keys Used For Exchanging Symmetric Keys", BCP 86,
               RFC 3766, April 2004.

   [RFC4366]   Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen,
               J., and T. Wright, "Transport Layer Security (TLS)
               Extensions", RFC 4366, April 2006.





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

   [SP800-57]  National Institute of Standards and Technology,
               "Recommendation for Key Management", Special Publication
               800-57, May 2006.

Authors' Addresses

   Paul Funk
   43 Linnaean St.
   Cambridge, MA 02138
   EMail: PaulFunk@alum.mit.edu

   Simon Blake-Wilson
   SafeNet
   Amstelveenseweg 88-90
   1054XV, Amsterdam
   The Netherlands
   EMail: sblakewilson@nl.safenet-inc.com





























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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   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.

   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, THE IETF TRUST AND
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   The IETF invites any interested party to bring to its attention any
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   ietf-ipr@ietf.org.












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