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Internet Draft                                              H. Andersson
                                                            S. Josefsson
                                                            RSA Security
                                                             August 2001

                 Protecting EAP with TLS (EAP-TLS-EAP)
                <draft-josefsson-pppext-eap-tls-eap-00>

                          Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026. Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
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   The list of current Internet-Drafts may be found at
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   The list of Internet-Draft Shadow Directories may be found at
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                                 Abstract

   This document specifies an Extensible Authentication Protocol (EAP)
   mechanism for mutual authentication and session key generation in a
   roaming environment. The server authentication and the negotiation of
   the session key is done using the PPP EAP Transport Layer Security
   (TLS) Authentication Protocol. The user authenticates using a PPP EAP
   mechanism, integrity and privacy protected by TLS. In essence, a
   wrapping of EAP inside TLS inside EAP is specified. An important
   application discussed in this document is to provide authentication
   of access points and stations within an IEEE 802.11 Wireless Local
   Area Network (WLAN), but other applications such as LAN access over
   Bluetooth might also be considered in the future.

1. Introduction

   The PPP Extensible Authentication Protocol [2] defines a general
   authentication framework. This document specifies an EAP mechanism
   for mutual authentication and session key generation, with support
   for a roaming environment. The connection is made, using EAP
   terminology, between a peer and an authenticator. The (public-key)



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   authentication of the authenticator and the negotiation of the
   session key is done using the PPP EAP Transport Layer Security (TLS)
   Authentication Protocol [1]. The user performs authentication using
   any defined PPP EAP mechanism, see [2].

   Section 2 defines the model and some terminology. A overview of this
   EAP mechanism is given in Section 3, and Section 4 gives a detailed
   description of packet formats. In Section 5, the protocol is applied
   to an IEEE 802.11 Wireless Local Area Network (WLAN). Finally,
   Section 6 discusses security issues.

2. Model and Terminology

   The term peer refers to a client, acting on behalf of a user, that
   requests access to a network. The entity contacted by the peer is
   denoted authenticator. The authenticator is in turn connected to an
   entity called back-end server. In our model, the authenticator is
   acting merely as a passthrough device during the authentication
   phase, forwarding each packet received from the peer to the back-end
   server, and vice versa. It should be noted that the back-end server
   may be a logical entity located in the same physical device as the
   authenticator. The realisation of the back-end server and the
   communication between the authenticator and backend server are
   outside the scope of this document.

          +---+
          | B |
          | a |
          | c |
          | k |               +---------------+         +--------+
          |   | <-----------> | Authenticator | <-----> |  Peer  |
          | e |               +---------------+   EAP   +--------+
          | n |                                         .
          | d |                                       .
          |   |                                     .
          | S |               +---------------+   .   EAP
          | e | <-----------> | Authenticator | .
          | r |               +---------------+
          | v |
          | e |
          | r |
          +---+

   An overview of the assumed environment is found in the figure above.
   The peer initially contacts the first authenticator (at the top of
   the figure). The dotted line between the peer and the second
   authenticator symbolizes roaming, i.e. the situation where the peer
   transits from one authenticator to another while still maintaining



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   server and user authentication. It is assumed that the same logical
   back-end server sits behind all of the authenticators contacted by
   the peer. In practice, the back-end server may be distributed over
   several machines, for e.g. fail-over or load-balancing purposes, but
   in this document we regard them as one logical unit.

   This document frequently uses the following terms and abbreviations:

      authenticator

         The end of the link requiring the authentication.

      EAP

         Extensible Authentication Protocol. After a connection link has
         been established between two entities, an authentication phase may
         take place. The PPP EAP protocol [2] is a general authentication
         protocol. The authenticator sends one or more requests to the
         peer, and the peer sends a response in reply to each request. The
         authenticator ends the authentication phase with a success or
         failure message.

      peer

         The other end of the point-to-point link; the end which is
         being authenticated by the authenticator.

      TLS

         Transport Layer Security. Internet security protocol for
         point-to-point connections (enhancement of Secure Sockets Layer,
         SSL). Defined in [3]. Under this protocol, two entities are able
         to authenticate each other and to establish a secure link. TLS
         operates at the transport layer. The protocol PPP EAP TLS [1]
         describes how to provide for TLS mechanisms within EAP.

3. Overview of the conversation

   A peer wishes to set up a connection with an authenticator, for the
   purpose of authenticating itself to e.g. a wireless infrastructure.
   In our model, the authenticators are in connection with an back-end
   server. The following describes each EAP packet that is sent between
   the authenticator and peer during the EAP connection.

 3.1. Initial registration

   The first two steps are described in detail in Section 3.1 of [2], we
   include them here for illustration. Note that as per the EAP



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   specification, this Identity exchange is not required.

      1. The first EAP Request packet sent by the authenticator to the
         peer is of type Identity. The data field may optionally contain a
         displayable message.

      2. The peer responds with an EAP-Response packet of type Identity.

   Note that the data field of the Identity packet, which contains the
   peer identity, cannot be assumed to be integrity or privacy
   protected.  Accordingly, it should not be used instead of the peer
   identity sent inside the TLS channel later on.

   The entities now initiate an EAP-TLS conversation. The following is
   an example of a successful TLS handshake within EAP -- the packets
   are described in detail in Section 4 of [1]. The EAP method defined
   in this document does not terminate the TLS connection once the TLS
   handshake phase is concluded (and thus differs subtly from how TLS is
   used in [1]). The retry behavior and fragmentation concerns of
   section 3.2 and 3.3 of [1] are still applicable (but not illustrated
   in this example).

      3. The authenticator sends an EAP-TLS packet of type Start with empty
         data field. The data field of following packets will encapsulate
         TLS Handshake Protocol messages.

      4. Client hello: The peer sends a preferred TLS protocol version
         number, an empty Session ID field, a list of preferred
         cryptographic algorithms, and a random number to initialize the
         TLS handshake.

      5. Server hello: The authenticator responds with a selected TLS
         protocol version number, a new Session ID, a list of selected
         cryptographic algorithms, and another random number. Server
         certificate: The authenticator then sends a chain of X.509v3
         certificates, starting with its own certificate. The packet may
         optionally include a server key exchange. Server hello
         done: Finally, the authenticator indicates the end of this message
         stream. (Note that the authenticator must NOT send any certificate
         request.)

      6. Client key exchange: The peer generates a premaster secret,
         encrypts it using the public key obtained from the server
         certificate, and sends the result. Change cipher spec: The
         peer selects the cipher(s) to use. Client finished: The peer also
         calculates a master secret from the premaster secret, and sends a
         hash of a message consisting of the master secret; all of the data
         from all previous handshake messages; the string "client



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         finished".

      7. Change cipher spec: The authenticator selects the cipher(s) to
         use. Server finished: Finally, the authenticator itself
         generates the master secret from the premaster secret and
         responds with a hash of a message consisting of the master
         secret; all of the data from all previous handshake messages;
         the string "server finished".

      8. The peer acknowledges the end of the TLS negotiation by sending
         an empty EAP Response packet.

   This concludes the TLS handshake phase and the authentication of the
   authenticator. It remains to perform user authentication. Note that
   it is not until now that we deviate from the TLS EAP specification.
   The authenticator will now intiate a second EAP handshake, within
   TLS, to provide peer authentication in a protected channel. In this
   EAP handshake, any EAP mechanism may be used to provide the peer
   authentication.

   This concludes the mutual authentication, and upon success both
   authenticator and peer may generate any amount of new key material to
   be forwarded to the underlying transport. This is accomplished within
   the TLS Record Protocol by using the so-called PRF (Pseudo-Random
   Function), see Section 3.5 "Key Derivation" of [3].

   It remains to be described what happens upon failures. In case the
   TLS negotiation has failed fatally (after the proper TLS Alert
   messages have been sent), an EAP-Failure messages is transmitted.
   Within the TLS channel, in the second EAP handshake, after any EAP-
   Success and EAP-Failure messages has successfully been sent, the same
   type of packet should be send in the outer EAP channel as well.

 3.2. Roaming

   We now describe the case where the peer is transiting between two
   authenticators during a session. In order to obtain a seamless
   transition to a connection between the peer and the new
   authenticator, we use the connection re-establishment mechanism
   provided by the TLS Handshake Protocol. Note that the new
   authenticator is assumed to use the same back-end server as the old
   one, hence the old security parameters are still available. In the
   case where the back-end server is just a logical entity residing at
   the authenticator, the second authenticator will be required to
   (securely) transfer the security parameters from the first
   authenticator.

   The steps 1-3 above are repeated without change. The following



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   describes a successful TLS handshake:

      4. Client hello: The peer sends the TLS protocol version number, the
         Session ID of the old connection, the previously negotiated
         cryptographic algorithms, and a random number.

      5. Server hello: The authenticator responds with the TLS protocol
         version number, the Session ID, the negotiated cryptographic
         algorithms, and another random number. If the old Session ID has
         expired, then a new Session ID is presented to the peer and full
         authentication takes place, as described in Subsection 3.1.
         Change cipher spec: the server selects the cipher(s) to
         use. Server finished: The authenticator responds with a hash of
         a message consisting of the master secret; data from all
         previous handshake messages; the string "server finished".

      6. Change cipher spec: The peer select cipher(s) to use. Client
         finished: The peer sends a hash of a message consisting of
         the previously calculated master secret; data from all previous
         handshake messages; the string "client finished".

   Note that mutual authentication is achieved, since both peer and
   authenticator have to know the old master secret in order to
   successfully complete the protocol. An alternative to TLS resumption
   has been discussed, whereby a "Roaming ID" is used to identify the
   user moving between authenticators. At a new connection, server
   authentication and generation of new security parameters is
   mandatory. The advantage of this approach is that the authentication
   server does not have to store so much key material, since all data
   except the Roaming ID may be deleted when entities are disconnected.
   This can be an important issue if there are many peers to be served.
   On the other hand, having to generate much new key material could be
   very time consuming for the back-end server, and this potential
   danger has led us to choose TLS resumption as described above.

   Finally, the length of time that a Session ID is valid should be
   limited. The time of validity is application dependent. In some
   environments it may be desirable that the authenticator notify the
   peer that the Session ID is about to expire. No mechanism is defined
   in this document to handle such a scenario, but note that the Session
   ID validity is checked during connection re-establishment (see 5
   above).

4. Packet formats

   It is assumed that underlying transport protocols has set up the
   connection so that it is ready to transfer EAP packets.




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 4.1. TLS in EAP

   The syntax of EAP packets containing TLS messages are per [1], and
   the TLS protocol description is per [3]. Note that [1] does not use
   the negotiated TLS tunnel to transfer any data, while this
   specification does, however this does not affect the EAP protocol
   syntax. We include the EAP syntax in the following figure, referring
   to Sections 4.2 and 4.3 of [1] for the definition of the Request and
   Response packets.

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

      Code

         1 - Request
         2 - Response

      Identifier

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

      Length

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

      Type

         TBA - EAP TLS EAP

      Data

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

 4.2. EAP negotiation inside TLS

   We now assume that the TLS handshake has been successfully completed
   and that a secure TLS connection is available within the TLS Record



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   Protocol. The following packets (protected by TLS Record Protocol and
   sent inside EAP packets) are used to negotiate the peer EAP
   authentication.

   The following figure describes the template packet structure that is
   used during this communication.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  EAP Data as per RFC 2284 ...                 /
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The party acting as authenticator in this second, wrapped, EAP
   channel MUST be the same party that acted as authenticator in the
   original EAP channel.

5. Example: IEEE 802.11 WLAN

   IEEE 802.11 Wireless Local Area Network (WLAN) is a standard for
   wireless computer networks, see [5]. Any device that contains an IEEE
   802.11 conformant medium access control and physical layer interface
   to the wireless medium is called a Station (STA). An entity that has
   station functionality and also provides access to the distribution
   services (e.g. a wired LAN) via the wireless medium for associated
   stations is called an Access Point (AP). The authentication services
   defined within IEEE 802.11 are discussed below, and the need for
   higher level authentication is addressed.

   IEEE 802.11 defines two types of authentication methods -- Open
   system authentication and Shared key authentication. Open system
   authentication is essentially a null authentication. The conversation
   is done in clear, no challenge procedure is performed. The purpose of
   Shared key authentication is to check that both parties share a pre-
   negotiated encryption key. The AP sends a challenge and the STA
   responds by encrypting this challenge. If the AP successfully
   decrypts that message, the authentication is finished. In other
   words, the AP is never required to authenticate itself. This opens up
   for a number of attacks, such as denial of service attacks via rogue
   APs. It is thus crucial to achieve mutual authentication.

   The IEEE 802.1X draft [4] specifies a general method for the
   provision of port based network access control. A port in this
   context is an attachment point to the LAN infrastructure, e.g. an
   association between a STA and an AP. The specification describes the
   architectural framework within which the authentication takes place,
   and establishes the requirements for a higher level authentication



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   protocol between the station and the access point.

   The IEEE 802.1X draft provides a framework, Extensible Authentication
   Protocol Over Local area networks (EAPOL), that makes it possible to
   send EAP packets between IEEE 802.11 entities. In a WLAN environment,
   the "Authenticator" is the AP, and the "Peer" is a STA. An
   Authentication Server is an entity connected with the AP. The server
   is communicating with the STA during the authentication -- the AP is
   sitting in between, acting merely as a passthrough device. In a
   roaming environment, the STA may connect to several APs during a
   session. All the APs are assumed to be connected to the same
   authentication server. The protocol described in this paper may
   therefore be applied to a WLAN environment, providing authentication
   of the AP, strong authentication of the user of the STA, and session
   key negotiation.

   Note that the present protocol is partly based on [1], which in turn
   assumes PPP EAP and not EAPOL as the underlying protocol. However,
   this minor difference will cause no problems whatsoever, since the
   TLS conversation carries over word by word to the new environment.

   Let us finally comment on the Wired Equivalent Privacy (WEP)
   encryption scheme defined in the IEEE 802.11 standard. WEP uses the
   stream cipher RC4 with key obtained as the concatenation of a 24 bit
   IV and a 40 bit WEP key. Four WEP keys can be prestored, but it is
   also possible to use a session key negotiated during the
   authentication phase, i.e. follow the approach outlined in this work.
   WEP suffers from some serious security weaknesses, e.g. the WEP key
   is too short to withstand a brute force attack. Also, the IV is too
   short -- even if a new random IV is used for each packet, collisions
   will start appearing within a few seconds (according to the birthday
   paradox). XORing messages with the same IV results in plaintext
   difference that can be further analyzed. Finally, there is no real
   data integrity since the integrity check value used is just a linear
   checksum. An active attacker wishing to alter the plaintext can
   easily modify the checksum to be valid for the new plaintext. The
   IEEE 802.11 working group recognizes the need to improve security,
   and is currently working on a revision of the standard.

6. Security considerations

   The Transport Layer Security protocol is presumed to be a strong
   security protocol and it is widely accepted. Here we discuss some
   security issues. The Session ID is sent in clear, so an attacker may
   contact an authenticator, pretending to be the legitimate user.
   However, by sending correct Finished messages, the parties prove to
   each other that they know the correct premaster secret. The attacker
   will not be able to finish the handshake properly (unless the



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   protocol has been completely broken).

   An attacker, acting as an active man-in-the-middle, might try to
   influence the choice of encryption algorithm by altering the
   corresponding handshake message. However, this will also be detected
   in the verification of the Finished messages, since each of these
   consists of a hash of all previous messages. The hash functions MD5
   and SHA-1 are used in tandem wherever possible. The TLS designers
   claim that this approach ensures that a serious flaw in one of the
   functions will not lead to failure of the entire TLS protocol.

   Finally, the strength of the user authentication is dependent on the
   EAP mechanism chosen. With the approach described here, the EAP
   packets sent by the peer are not transmitted in clear, which improve
   the security of some EAP mechanisms. This is particularly important
   in a wireless environment where passive eavesdropping is a serious
   threat.

7. Acknowledgements

   We wish to thank Jan-Ove Larsson and Magnus Nystrom for helpful
   discussions and comments during the development of this draft. We
   would also like to thank Glen Zorn and Simon Blake-Wilson for
   comments on the first version of this draft.

References

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

      [2]  Blunk, L., Vollbrecht, J., "PPP Extensible Authentication
           Protocol (EAP)", RFC 2284, March 1998.

      [3]  Dierks, T., Allen, C., "The TLS Protocol", RFC 2246, January
           1999.

      [4]  IEEE Standards for Local and Metropolitan Area Networks: Port
           based Network Access Control, IEEE Draft 802.1X/D10, January
           2001.

      [5]  Information technology -- Telecommunications and information
           exchange between systems -- Local and metropolitan area
           networks -- Specific requirements -- Part 11: Wireless LAN
           Medium Access Control (MAC) and Physical Layer (PHY)
           Specifications, IEEE Std. 802.11, 1999.

Address of the authors




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   Hakan Andersson
   RSA Security
   Box 107 04
   SE-121 29 Stockholm
   Sweden
   E-mail: handersson@rsasecurity.com
   Phone: +46 8 725 9758
   Fax: +46 8 649 4970

   Simon Josefsson
   RSA Security
   Box 107 04
   SE-121 29 Stockholm
   Sweden
   E-mail: sjosefsson@rsasecurity.com
   Phone: +46 8 725 0914
   Fax: +46 8 649 4970


































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