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rfc:rfc4746

Network Working Group T. Clancy Request for Comments: 4746 LTS Category: Informational W. Arbaugh

                                                                   UMD
                                                         November 2006
             Extensible Authentication Protocol (EAP)
                  Password Authenticated Exchange

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.

Copyright Notice

 Copyright (C) The IETF Trust (2006).

Copyright Notice

 Copyright (C) The Internet Society (2006).

Abstract

 This document defines an Extensible Authentication Protocol (EAP)
 method called EAP-PAX (Password Authenticated eXchange).  This method
 is a lightweight shared-key authentication protocol with optional
 support for key provisioning, key management, identity protection,
 and authenticated data exchange.

Table of Contents

 1. Introduction ....................................................2
    1.1. Language Requirements ......................................3
    1.2. Terminology ................................................3
 2. Overview ........................................................5
    2.1. PAX_STD Protocol ...........................................6
    2.2. PAX_SEC Protocol ...........................................7
    2.3. Authenticated Data Exchange ................................9
    2.4. Key Derivation ............................................10
    2.5. Verification Requirements .................................11
    2.6. PAX Key Derivation Function ...............................12
 3. Protocol Specification .........................................13
    3.1. Header Specification ......................................13
         3.1.1. Op-Code ............................................13
         3.1.2. Flags ..............................................14

Clancy & Arbaugh Informational [Page 1] RFC 4746 EAP-PAX November 2006

         3.1.3. MAC ID .............................................14
         3.1.4. DH Group ID ........................................14
         3.1.5. Public Key ID ......................................15
         3.1.6. Mandatory to Implement .............................15
    3.2. Payload Formatting ........................................16
    3.3. Authenticated Data Exchange (ADE) .........................18
    3.4. Integrity Check Value (ICV) ...............................19
 4. Security Considerations ........................................19
    4.1. Server Certificates .......................................20
    4.2. Server Security ...........................................20
    4.3. EAP Security Claims .......................................21
         4.3.1. Protected Ciphersuite Negotiation ..................21
         4.3.2. Mutual Authentication ..............................21
         4.3.3. Integrity Protection ...............................21
         4.3.4. Replay Protection ..................................21
         4.3.5. Confidentiality ....................................21
         4.3.6. Key Derivation .....................................21
         4.3.7. Key Strength .......................................22
         4.3.8. Dictionary Attack Resistance .......................22
         4.3.9. Fast Reconnect .....................................22
         4.3.10. Session Independence ..............................22
         4.3.11. Fragmentation .....................................23
         4.3.12. Channel Binding ...................................23
         4.3.13. Cryptographic Binding .............................23
         4.3.14. Negotiation Attack Prevention .....................23
 5. IANA Considerations ............................................23
 6. Acknowledgments ................................................24
 7. References .....................................................24
    7.1. Normative References ......................................24
    7.2. Informative References ....................................25
 Appendix A. Key Generation from Passwords ........................ 27
 Appendix B. Implementation Suggestions ........................... 27
   B.1. WiFi Enterprise Network ................................... 27
   B.2. Mobile Phone Network ...................................... 28

1. Introduction

 EAP-PAX (Password Authenticated eXchange) is an Extensible
 Authentication Protocol (EAP) method [RFC3748] designed for
 authentication using a shared key.  It makes use of two separate
 subprotocols, PAX_STD and PAX_SEC.  PAX_STD is a simple, lightweight
 protocol for mutual authentication using a shared key, supporting
 Authenticated Data Exchange (ADE).  PAX_SEC complements PAX_STD by
 providing support for shared-key provisioning and identity protection
 using a server-side public key.

Clancy & Arbaugh Informational [Page 2] RFC 4746 EAP-PAX November 2006

 The idea motivating EAP-PAX is a desire for device authentication
 bootstrapped by a simple Personal Identification Number (PIN).  If a
 weak key is used or a expiration period has elapsed, the
 authentication server forces a key update.  Rather than using a
 symmetric key exchange, the client and server perform a Diffie-
 Hellman key exchange, which provides forward secrecy.
 Since implementing a Public Key Infrastructure (PKI) can be
 cumbersome, PAX_SEC defines multiple client security policies,
 selectable based on one's threat model.  In the weakest mode, PAX_SEC
 allows the use of raw public keys completely eliminating the need for
 a PKI.  In the strongest mode, PAX_SEC requires that EAP servers use
 certificates signed by a trusted Certification Authority (CA).  In
 the weaker modes, during provisioning PAX_SEC is vulnerable to a
 man-in-the-middle dictionary attack.  In the strongest mode, EAP-PAX
 is provably secure under the Random Oracle model.
 EAP-PAX supports the generation of strong key material; mutual
 authentication; resistance to desynchronization, dictionary, and
 man-in-the-middle attacks; ciphersuite extensibility with protected
 negotiation; identity protection; and the authenticated exchange of
 data, useful for implementing channel binding.  These features
 satisfy the EAP method requirements for wireless LANs [RFC4017],
 making EAP-PAX ideal for wireless environments such as IEEE 802.11
 [IEEE.80211].

1.1. Language Requirements

 In this document, several words are used to signify the requirements
 of the specification.  The key words "MUST", "MUST NOT", "REQUIRED",
 "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
 and "OPTIONAL" in this document are to be interpreted as described in
 [RFC2119].

1.2. Terminology

 This section describes the various variables and functions used in
 the EAP-PAX protocol.  They will be referenced frequently in later
 sections.
 Variables:
 CID
    User-supplied client ID, specified as a Network Access Identifier
    (NAI) [RFC4282], restricted to 65535 octets
 g
    public Diffie-Hellman generator, typically the integer 2 [RFC2631]

Clancy & Arbaugh Informational [Page 3] RFC 4746 EAP-PAX November 2006

 M
    128-bit random integer generated by the server
 N
    128-bit random integer generated by the client
 X
    256-bit random integer generated by the server
 Y
    256-bit random integer generated by the client
 Keys:
 AK
    authentication key shared between the client and EAP server
 AK'
    new authentication key generated during a key update
 CertPK
    EAP server's certificate containing public key PK
 CK
    Confirmation Key generated from the MK and used during
    authentication to prove knowledge of AK
 EMSK
    Extended Master Session Key also generated from the MK and
    containing additional keying material
 IV
    Initialization Vector used to seed ciphers; exported to the
    authenticator
 MID
    Method ID used to construct the EAP Session ID and consequently
    name all the exported keys [IETF.KEY]
 MK
    Master Key between the client and EAP server from which all other
    EAP method session keys are derived
 MSK
    Master Session Key generated from the MK and exported by the EAP
    method to the authenticator

Clancy & Arbaugh Informational [Page 4] RFC 4746 EAP-PAX November 2006

 PK
    EAP server's public key
 Operations:
 enc_X(Y)
    encryption of message Y with public key X
 MAC_X(Y)
    keyed message authentication code computed over message Y with
    symmetric key X
 PAX-KDF-W(X, Y, Z)
    PAX Key Derivation Function computed using secret X, identifier Y,
    and seed Z, and producing W octets of output
 ||
    string or binary data concatenation

2. Overview

 The EAP framework [RFC3748] defines four basic steps that occur
 during the execution of an EAP conversation between client and
 server.  The first phase, discovery, is handled by the underlying
 link-layer protocol.  The authentication phase is defined here.  The
 key distribution and secure association phases are handled
 differently depending on the underlying protocol, and are not
 discussed in this document.
      +--------+                                     +--------+
      |        |                EAP-Request/Identity |        |
      | CLIENT |<------------------------------------| SERVER |
      |        |                                     |        |
      |        | EAP-Response/Identity               |        |
      |        |------------------------------------>|        |
      |        |                                     |        |
      |        |        EAP-PAX (STD or SEC)         |        |
      |        |<----------------------------------->|        |
      |        | ...                             ... |        |
      |        |<----------------------------------->|        |
      |        |                                     |        |
      |        |          EAP-Success or EAP-Failure |        |
      |        |<------------------------------------|        |
      +--------+                                     +--------+
                  Figure 1: EAP-PAX Packet Exchanges

Clancy & Arbaugh Informational [Page 5] RFC 4746 EAP-PAX November 2006

 There are two distinct subprotocols that can be executed.  The first,
 PAX_STD, is used during typical authentications.  The second,
 PAX_SEC, provides more secure features such as key provisioning and
 identity protection.
 PAX_STD and PAX_SEC have two modes of operation.  When an AK update
 is being performed, the client and server exchange Diffie-Hellman
 exponents g^X and g^Y, which are computed modulo prime P or over an
 elliptic curve multiplicative group.  When no update is being
 performed, and only session keys are being derived, X and Y are
 exchanged.  Using Diffie-Hellman during the key update provides
 forward secrecy, and secure key derivation when a weak provisioned
 key is used.
 The main deployment difference between PAX_STD and PAX_SEC is that
 PAX_SEC requires a server-side public key.  More specifically, that
 means a private key known only to the server with corresponding
 public key being transmitted to the client during each PAX_SEC
 session.  For every authentication, the client is required to compute
 computationally intensive public-key operations.  PAX_STD, on the
 other hand, uses purely symmetric operations, other than a possible
 Diffie-Hellman exchange.
 Each of the protocols is now defined.

2.1. PAX_STD Protocol

 PAX_STD is a simple nonce-based authentication using the strong
 long-term key.  The client and server each exchange 256 bits of
 random data, which is used to seed the PAX-KDF for generation of
 session keys.  The randomly exchanged data in the protocol differs
 depending on whether a key update is being performed.  If no key
 update is being performed, then let:
 o  A = X
 o  B = Y
 o  E = X || Y
 Thus, A and B are 256-bit values and E is their 512-bit
 concatenation.  To provide forward secrecy and security, let the
 following be true when a key update is being performed:
 o  A = g^X
 o  B = g^Y
 o  E = g^(XY)
 Here A and B are Diffie-Hellman exponents whose length is determined
 by the Diffie-Hellman group size.  The value A is data transmitted

Clancy & Arbaugh Informational [Page 6] RFC 4746 EAP-PAX November 2006

 from the server to the client, and B is data transmitted from the
 client to the server.  The value E is the entropy computed by each
 that is used in Section 2.4 to perform key derivation.
 The full protocol is as follows:
 o  PAX_STD-1 : client <- server : A
 o  PAX_STD-2 : client -> server : B, CID, MAC_CK(A, B, CID),
    [optional ADE]
 o  PAX_STD-3 : client <- server : MAC_CK(B, CID), [optional ADE]
 o  PAX-ACK : client -> server : [optional ADE]
 See Section 2.3 for more information on the ADE component, and
 Section 2.4 for the key derivation process, including derivation of
 CK.

2.2. PAX_SEC Protocol

 PAX_SEC is the high-security protocol designed to provide identity
 protection and support for provisioning.  PAX_SEC requires a server-
 side public key, and public-key operations for every authentication.
 PAX_SEC can be performed with and without key update.  Let A, B, and
 E be defined as in the previous section.
 The exchanges for PAX_SEC are as follows:
 o  PAX_SEC-1 : client <- server : M, PK or CertPK
 o  PAX_SEC-2 : client -> server : Enc_PK(M, N, CID)
 o  PAX_SEC-3 : client <- server : A, MAC_N(A, CID)
 o  PAX_SEC-4 : client -> server : B, MAC_CK(A, B, CID), [optional
    ADE]
 o  PAX_SEC-5 : client <- server : MAC_CK(B, CID), [optional ADE]
 o  PAX-ACK : client -> server : [optional ADE]
 See Section 2.3 for more information on the ADE component, and
 Section 2.4 for the key derivation process, including derivation of
 CK.
 Use of CertPK is optional in PAX_SEC; however, careful consideration
 should be given before omitting the CertPK.  The following table
 describes the risks involved when using PAX_SEC without a
 certificate.

Clancy & Arbaugh Informational [Page 7] RFC 4746 EAP-PAX November 2006

      Certificate    |    Provisioning     |       Identity
          Mode       |                     |      Protection
   ==================+=====================+======================
     No Certificate  |    MiTM offline     |   ID reveal attack
                     |  dictionary attack  |
   ------------------+---------------------+---------------------
      Self-Signed    |    MiTM offline     |   ID reveal attack
      Certificate    |  dictionary attack  |
   ------------------+---------------------+---------------------
     Certificate/PK  |    MiTM offline     |   ID reveal attack
        Caching      |  dictionary attack  |  during first auth
   ------------------+---------------------+---------------------
       CA-Signed     |   secure mutual     |   secure mutual
      Certificate    |   authentication    |   authentication
              Figure 2: Table of Different Security Modes
 When using PAX_SEC to support provisioning with a weak key, use of a
 CA-signed certificate is RECOMMENDED.  When not using a CA-signed
 certificate, the initial authentication is vulnerable to an offline
 man-in-the-middle (MiTM) dictionary attack.
 When using PAX_SEC to support identity protection, use of either a
 CA-signed certificate or key caching is RECOMMENDED.  Caching
 involves a client recording the public key of the EAP server and
 verifying its consistency between sessions, similar to Secure SHell
 (SSH) Protocol [RFC4252].  Otherwise, an attacker can spoof an EAP
 server during a session and gain knowledge of a client's identity.
 Whenever certificates are used, clients MUST validate that the
 certificate's extended key usage, KeyPurposeID, is either
 "eapOverPPP" or "eapOverLAN" [RFC3280][RFC4334].  If the underlying
 EAP transport protocol is known, then the client MUST differentiate
 between these values.  For example, an IEEE 802.11 supplicant SHOULD
 require KeyPurposeID == eapOverLAN.  By not distinguishing, a client
 could accept as valid an unauthorized server certificate.
 When using EAP-PAX with Wireless LAN, clients SHOULD validate that
 the certificate's wlanSSID extension matches the SSID of the network
 to which it is currently authenticating.
 In order to facilitate discussion of packet validations, three client
 security policies for PAX_SEC are defined.
 open
    Clients support both use of PK and CertPK.  If CertPK is used, the
    client MUST validate the KeyPurposeID.

Clancy & Arbaugh Informational [Page 8] RFC 4746 EAP-PAX November 2006

 caching
    Clients save PK for each EAP server the first time it encounters
    the server, and SHOULD NOT authenticate to EAP servers whose
    public key has been changed.  If CertPK is used, the client MUST
    validate the KeyPurposeID.
 strict
    In strict mode, clients require servers to present a valid
    certificate signed by a trusted CA.  As with the other modes, the
    KeyPurposeID MUST be validated.
 Servers SHOULD support the PAX_SEC mode of operation, and SHOULD
 support both the use of PK and CertPK with PAX_SEC.  Clients MUST
 support PAX_SEC, and MUST be capable of accepting both raw public
 keys and certificates from the server.  Local security policy will
 define which forms of key or certificate authentications are
 permissible.  Default configurations SHOULD require a minimum of the
 caching security policy, and MAY require strict.
 The ability to perform key management on the AK is built in to EAP-
 PAX through the use of AK'.  However, key management of the server
 public key is beyond the scope of this document.  If self-signed
 certificates are used, the deployers should be aware that expired
 certificates may be difficult to replace when the caching security
 mode is used.
 See Section 4 for further discussion on security considerations.

2.3. Authenticated Data Exchange

 Messages PAX_STD-2, PAX_STD-3, PAX_SEC-4, PAX_SEC-5, and PAX_ACK
 contain optional component ADE.  This component is used to convey
 authenticated data between the client and server during the
 authentication.
 The Authenticated Data Exchanged (ADE) can be used in a variety of
 ways, including the implementation of channel bindings.  Channel
 bindings allow link-layer network properties to be securely validated
 by the EAP client and server during the authentication session.
 It is important to note that ADE is not encrypted, so any data
 included will not be confidential.  However, since these packets are
 all protected by the Integrity Check Value (ICV), authenticity is
 guaranteed.

Clancy & Arbaugh Informational [Page 9] RFC 4746 EAP-PAX November 2006

 The ADE element consists of an arbitrary number of subelements, each
 with length and type specified.  If the number and size of
 subelements is too large, packet fragmentation will be necessary.
 Vendor-specific options are supported.  See Section 3.3.
 Note that more than 1.5 round-trips may be necessary to execute a
 particular authenticated protocol within EAP-PAX.  In this case,
 instead of sending an EAP-Success after receiving the PAX_ACK, the
 server can continue sending PAX_ACK messages with attached elements.
 The client responds to these PAX_ACK messages with PAX_ACK messages
 possibly containing more ADE elements.  Such an execution could look
 something like the following:
      +--------+                                     +--------+
      |        |                           PAX_STD-1 |        |
      |        |<------------------------------------|        |
      |        | PAX_STD-2(ADE[1])                   |        |
      |        |------------------------------------>|        |
      |        |                   PAX_STD-3(ADE[2]) |        |
      |        |<------------------------------------|        |
      |        | PAX_ACK(ADE[3])                     |        |
      |        |------------------------------------>|        |
      |        |                     PAX_ACK(ADE[4]) |        |
      |        |<------------------------------------|        |
      |        |                                     |        |
      |        |                 ...                 |        |
      |        |                                     |        |
      |        | PAX_ACK(ADE[i])                     |        |
      |        |------------------------------------>|        |
      |        |                   PAX_ACK(ADE[i+1]) |        |
      |        |<------------------------------------|        |
      |        |                                     |        |
      |        |                 ...                 |        |
      |        |                                     |        |
      |        |          EAP-Success or EAP-Failure |        |
      |        |<------------------------------------|        |
      +--------+                                     +--------+
        Figure 3: Extended Diagram of EAP-PAX Packet Exchanges

2.4. Key Derivation

 Keys are derived independently of which authentication mechanism was
 used.  The process uses the entropy value E computed as described
 above.  Session and authentication keys are computed as follows:
 o  AK' = PAX-KDF-16(AK, "Authentication Key", E)
 o  MK = PAX-KDF-16(AK, "Master Key", E)

Clancy & Arbaugh Informational [Page 10] RFC 4746 EAP-PAX November 2006

 o  CK = PAX-KDF-16(MK, "Confirmation Key", E)
 o  ICK = PAX-KDF-16(MK, "Integrity Check Key", E)
 o  MID = PAX-KDF-16(MK, "Method ID", E)
 o  MSK = PAX-KDF-64(MK, "Master Session Key", E)
 o  EMSK = PAX-KDF-64(MK, "Extended Master Session Key", E)
 o  IV = PAX-KDF-64(0x00^16, "Initialization Vector", E)
 The IV is computed using a 16-octet NULL key.  The value of AK' is
 only used to replace AK if a key update is being performed.  The EAP
 Method ID is represented in ASCII as 32 hexadecimal characters
 without any octet delimiters such as colons or dashes.
 The EAP Key Management Framework [IETF.KEY] recommends specification
 of key names and scope.  The EAP-PAX Method-ID is the MID value
 computed as described above.  The EAP peer name is the CID value
 exchanged in PAX_STD-2 and PAX_SEC-2.  The EAP server name is an
 empty string.

2.5. Verification Requirements

 In order for EAP-PAX to be secure, MACs must be properly verified
 each step of the way.  Any packet with an ICV (see Section 3.4) that
 fails validation must be silently discarded.  After ICV validation,
 the following checks must be performed:
 PAX_STD-2
    The server MUST validate the included MAC, as it serves to
    authenticate the client to the server.  If this validation fails,
    the server MUST send an EAP-Failure message.
 PAX_STD-3
    The client MUST validate the included MAC, as it serves to
    authenticate the server to the client.  If this validation fails,
    the client MUST send an EAP-Failure message.
 PAX_SEC-1
    The client MUST validate PK or CertPK in a manner specified by its
    local security policy (see Section 2.2).  If this validation
    fails, the client MUST send an EAP-Failure message.
 PAX_SEC-2
    The server MUST verify that the decrypted value of M matches the
    value transmitted in PAX_SEC-1.  If this validation fails, the
    server MUST send an EAP-Failure message.

Clancy & Arbaugh Informational [Page 11] RFC 4746 EAP-PAX November 2006

 PAX_SEC-3
    The client MUST validate the included MAC, as it serves to prevent
    replay attacks.  If this validation fails, the client MUST send an
    EAP-Failure message.
 PAX_SEC-4
    The server MUST validate the included MAC, as it serves to
    authenticate the client to the server.  If this validation fails,
    the server MUST send an EAP-Failure message.
 PAX_SEC-5
    The client MUST validate the included MAC, as it serves to
    authenticate the server to the client.  If this validation fails,
    the client MUST send an EAP-Failure message.
 PAX-ACK
    If PAX-ACK is received in response to a message fragment, the
    receiver continues the protocol execution.  If PAX-ACK is received
    in response to PAX_STD-3 or PAX_SEC-5, then the server MUST send
    an EAP-Success message.  This indicates a successful execution of
    PAX.

2.6. PAX Key Derivation Function

 The PAX-KDF is a secure key derivation function used to generate
 various keys from the provided entropy and shared key.
 PAX-KDF-W(X, Y, Z)
 W  length, in octets, of the desired output
 X  secret key used to protect the computation
 Y  public identifier for the key being derived
 Z  exchanged entropy used to seed the KDF
 Let's define some variables and functions:
 o  M_i = MAC_X(Y || Z || i), where i is an 8-bit unsigned integer
 o  L = ceiling(W/16)
 o  F(A, B) = first A octets of binary data B
 We define PAX-KDF-W(X, Y, Z) = F(W, M_1 || M_2 || ... || M_L).
 Consequently for the two values of W used in this document, we have:
 o  PAX-KDF-16(X, Y, Z) = MAC_X(Y || Z || 0x01)
 o  PAX-KDF-64(X, Y, Z) = MAC_X(Y || Z || 0x01) || MAC_X(Y || Z ||
    0x02) || MAC_X(Y || Z || 0x03) || MAC_X(Y || Z || 0x04)

Clancy & Arbaugh Informational [Page 12] RFC 4746 EAP-PAX November 2006

 The MAC used in the PRF is extensible and is the same MAC used in the
 rest of the protocol.  It is specified in the EAP-PAX header.

3. Protocol Specification

 In this section, the packet format and content for the EAP-PAX
 messages are defined.
 EAP-PAX packets have the following structure:
  1. – bit offset —>

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      |    OP-Code    |     Flags     |    MAC ID     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  DH Group ID  | Public Key ID |                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
  |                                                               |
  ...                         Payload                           ...
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  ...                           ICV                             ...
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  Figure 4: EAP-PAX Packet Structure

3.1. Header Specification

 The Code, Identifier, Length, and Type fields are all part of the EAP
 header, and defined in [RFC3748].  IANA has allocated EAP Method Type
 46 for EAP-PAX; thus, the Type field in the EAP header MUST be 46.

3.1.1. Op-Code

 The OP-Code field is one of the following values:
 o  0x01 : PAX_STD-1
 o  0x02 : PAX_STD-2
 o  0x03 : PAX_STD-3
 o  0x11 : PAX_SEC-1
 o  0x12 : PAX_SEC-2
 o  0x13 : PAX_SEC-3
 o  0x14 : PAX_SEC-4

Clancy & Arbaugh Informational [Page 13] RFC 4746 EAP-PAX November 2006

 o  0x15 : PAX_SEC-5
 o  0x21 : PAX-ACK

3.1.2. Flags

 The flags field is broken up into 8 bits each representing a binary
 flag.  The field is defined as the Logical OR of the following
 values:
 o  0x01 : more fragments (MF)
 o  0x02 : certificate enabled (CE)
 o  0x04 : ADE Included (AI)
 o  0x08 - 0x80 : reserved
 The MF flag is set if the current packet required fragmentation, and
 further fragments need to be transmitted.  If a packet does not
 require fragmentation, the MF flag is not set.
 When a payload requires fragmentation, each fragment is transmitted,
 and the receiving party responds with a PAX-ACK packet for each
 received fragment.
 When using PAX_STD, the CE flag MUST be zero.  When using PAX_SEC,
 the CE flag MUST be set if PAX_SEC-1 includes CertPK.  It MUST NOT be
 set if PAX_SEC-1 includes PK.  If CE is set in PAX_SEC-1, it MUST be
 set in PAX_SEC-2, PAX_SEC-3, PAX_SEC-4, and PAX_SEC-5.  If either
 party detects an inconsistent value of the CE flag, he MUST send an
 EAP-Failure message and discontinue the session.
 The AI flag indicates the presence of an ADE element.  AI MUST only
 be set on packets PAX_STD-2, PAX_STD-3, PAX_SEC-4, PAX_SEC-5, and
 PAX_ACK if an ADE element is included.  On packets of other types,
 ADE elements MUST be silently discarded as they cannot be
 authenticated.

3.1.3. MAC ID

 The MAC field specifies the cryptographic hash used to generate the
 keyed hash value.  The following are currently supported:
 o  0x01 : HMAC_SHA1_128 [FIPS198] [FIPS180]
 o  0x02 : HMAC_SHA256_128 [FIPS180]

3.1.4. DH Group ID

 The Diffie-Hellman group field specifies the group used in the
 Diffie-Hellman computations.  The following are currently supported:

Clancy & Arbaugh Informational [Page 14] RFC 4746 EAP-PAX November 2006

 o  0x00 : NONE (iff not performing a key update)
 o  0x01 : 2048-bit MODP Group (IANA DH Group 14) [RFC3526]
 o  0x02 : 3072-bit MODP Group (IANA DH Group 15) [RFC3526]
 o  0x03 : NIST ECC Group P-256 [FIPS186]
 If no key update is being performed, the DH Group ID field MUST be
 zero.  Otherwise, the DH Group ID field MUST NOT be zero.

3.1.5. Public Key ID

 The Public Key ID field specifies the cipher used to encrypt the
 client's EAP-Response in PAX_SEC-2.
 The following are currently supported:
 o  0x00 : NONE (if using PAX_STD)
 o  0x01 : RSAES-OAEP [RFC3447]
 o  0x02 : RSA-PKCS1-V1_5 [RFC3447]
 o  0x03 : El-Gamal Over NIST ECC Group P-256 [FIPS186]
 If PAX_STD is being executed, the Public Key ID field MUST be zero.
 If PAX_SEC is being executed, the Public Key ID field MUST NOT be
 zero.
 When using RSAES-OAEP, the hash algorithm and mask generation
 algorithm used SHALL be the MAC specified by the MAC ID, keyed using
 an all-zero key.  The label SHALL be null.
 The RSA-based schemes specified here do not dictate the length of the
 public keys.  DER encoding rules will specify the key size in the key
 or certificate [X.690].  Key sizes SHOULD be used that reflect the
 desired level of security.

3.1.6. Mandatory to Implement

 The following ciphersuite is mandatory to implement and achieves
 roughly 112 bits of security:
 o  HMAC_SHA1_128
 o  IANA DH Group 14 (2048 bits)
 o  RSA-PKCS1-V1_5 (RECOMMEND 2048-bit public key)
 The following ciphersuite is RECOMMENDED and achieves 128 bits of
 security:
 o  HMAC_SHA256_128
 o  IANA DH Group 15 (3072 bits)
 o  RSAES-OAEP (RECOMMEND 3072-bit public key)

Clancy & Arbaugh Informational [Page 15] RFC 4746 EAP-PAX November 2006

3.2. Payload Formatting

 This section describes how to format the payload field.  Depending on
 the packet type, different values are transmitted.  Sections 2.1 and
 2.2 define the fields, and in what order they are to be concatenated.
 For simplicity and since many field lengths can vary with the
 ciphersuite, each value is prepended with a 2-octet length value
 encoded as an integer as described below.  This length field MUST
 equal the length in octets of the subsequent value field.
  1. – octet offset —>

0 1

             0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
            +---+---------------------
            |len|  value ....
            +---+--------
              Figure 5: Length Encoding for Data Elements
 All integer values are stored as octet arrays in network-byte order,
 with the most significant octet first.  Integers are padded on the
 most significant end to reach octet boundaries.
 Public keys and certificates SHALL be in X.509 format [RFC3280]
 encoded using the Distinguished Encoding Rules (DER) format [X.690].
 Strings are not null-terminated and are encoded using UTF-8.  Binary
 data, such as message authentication codes, are transmitted as-is.
 MACs are computed by concatenating the specified values in the
 specified order.  Note that for MACs, length fields are not included,
 though the resulting MAC will itself have a length field.  Values are
 encoded as described above, except that no length field is specified.
 To illustrate this process, an example is presented.  What follows is
 the encoding of the payload for PAX_STD-2.  The three basic steps
 will be computing the MAC, forming the payload, and encrypting the
 payload.
 To create the MAC, we first need to form the buffer that will be
 MACed.  For this example, assume that no key update is being done and
 HMAC_SHA1_128 is used such that the result will be a 16-octet value.

Clancy & Arbaugh Informational [Page 16] RFC 4746 EAP-PAX November 2006

  1. – octet offset —>

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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       32-octet integer A                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       32-octet integer B                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 ...                    variable length CID                    ...
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                ||
                ||
         CK --> MAC
                ||
                \/
  1. – octet offset —>

0 1

  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      16-octet MAC output      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           Figure 6: Example Encoding of PAX_STD-2 MAC Data
 With this, we can now create the encoded payload:
  1. – octet offset —>

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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |32 |                     32-octet integer B
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | L |                                                       |
 +-+-+-+-+                                                       +
 |                                                               |
 ...                        L-octet CID                        ...
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |16 |       MAC computed above      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            Figure 7: Example Encoding of PAX_STD-2 Packet

Clancy & Arbaugh Informational [Page 17] RFC 4746 EAP-PAX November 2006

 These 52+L octets are then attached to the packet as the payload.
 The ICV is then computed by MACing the packet headers and payload,
 and appended after the payload (see Section 3.4).

3.3. Authenticated Data Exchange (ADE)

 This section describes the formatting of the ADE elements.  ADE
 elements can only occur on packets of type PAX_STD-2, PAX_STD-3,
 PAX_SEC-4, PAX_SEC-5, and PAX_ACK.  Values included in other packets
 MUST be silently ignored.
 The ADE element is preceded by its 2-octet length L.  Each subelement
 has first a 2-octet length Li followed by a 2-octet type Ti.  The
 entire ADE element looks as follows:
  1. – octet offset —>

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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | L |L1 |T1 |                                                   |
 +-+-+-+-+-+-+                                                   +
 |                                                               |
 ...                 subADE-1, type T1, length L1              ...
 |                                                               |
 +                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                   |L2 |T2 |                                   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                   +
 |                                                               |
 ...                 subADE-2, type T2, length L2              ...
 |                                                               |
 +         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         | more subADE elements...                           ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                 Figure 8: Encoding of ADE Components
 The following type values have been allocated:
 o  0x01 : Vendor Specific
 o  0x02 : Client Channel Binding Data
 o  0x03 : Server Channel Binding Data

Clancy & Arbaugh Informational [Page 18] RFC 4746 EAP-PAX November 2006

 The first three octets of a subADE utilizing type code 0x01 must be
 the vendor's Enterprise Number [RFC3232] as registered with IANA.
 The format for such a subADE is as follows:
  1. – octet offset —>

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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |Li | 1 | ENi |                                                 |
 +-+-+-+-+-+-+-+                                                 +
 |                                                               |
 ...   subADE-i, type Vendor Specific, length Li, vendor ENi  ...
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               Figure 9: Encoding of Vendor-specific ADE
 Channel binding subADEs have yet to be defined.  Future IETF
 documents will specify the format for these subADE fields.

3.4. Integrity Check Value (ICV)

 The ICV is computed as the MAC over the entire EAP packet, including
 the EAP header, the EAP-PAX header, and the EAP-PAX payload.  The MAC
 is keyed using the 16-octet ICK, using the MAC type specified by the
 MAC ID in the EAP-PAX header.  For packets of type PAX_STD-1,
 PAX_SEC-1, PAX_SEC-2, and PAX_SEC-3, where the MK has not yet been
 derived, the MAC is keyed using a zero-octet NULL key.
 If the ICV field is incorrect, the receiver MUST silently discard the
 packet.

4. Security Considerations

 Any authentication protocol, especially one geared for wireless
 environments, must assume that adversaries have many capabilities.
 In general, one must assume that all messages between the client and
 server are delivered via the adversary.  This allows passive
 attackers to eavesdrop on all traffic, while active attackers can
 modify data in any way before delivery.
 In this section, we discuss the security properties and requirements
 of EAP-PAX with respect to this threat model.  Also note that the
 security of PAX can be proved using under the Random Oracle model.

Clancy & Arbaugh Informational [Page 19] RFC 4746 EAP-PAX November 2006

4.1. Server Certificates

 PAX_SEC can be used in several configurations.  It can be used with
 or without a server-side certificate.  Section 2.2 details the
 possible modes and the resulting security risk.
 When using PAX_SEC for identity protection and not using a CA-signed
 certificate, an attacker can convince a client to reveal his
 username.  To achieve this, an attacker can simply forge a PAX_SEC-1
 message and send it to the client.  The client would respond with a
 PAX_SEC-2 message containing his encrypted username.  The attacker
 can then use his associated private key to decrypt the client's
 username.  Use of key caching can reduce the risk of identity
 revelation by allowing clients to detect when the EAP server to which
 they are accustom has a different public key.
 When provisioning with PAX_SEC and not using a CA-signed certificate,
 an attacker could first forge a PAX_SEC-1 message and send it to the
 client.  The client would respond with a PAX_SEC-2 message.  Using
 the decrypted value of N, an attacker could forge a PAX_SEC-3
 message.  Once the client responds with a PAX_SEC-4 message, an
 attacker can guess values of the weak AK and compute CK = PAX-KDF(AK,
 "Confirmation Key", g^XY).  Given enough time, the attacker can
 obtain both the old AK and new AK' and forge a responding PAX_SEC-5.

4.2. Server Security

 In order to maintain a reasonable security policy, the server should
 manage five pieces of information concerning each user, most
 obviously, the username and current key.  In addition, the server
 must keep a bit that indicates whether the current key is weak.  Weak
 keys must be updated prior to key derivation.  Also, the server
 should track the date of last key update.  To implement the coarse-
 grained forward secrecy, the authentication key must be updated on a
 regular basis, and this field can be used to expire keys.  Last, the
 server should track the previous key, to prevent attacks where an
 adversary desynchronizes the key state by interfering with PAX-ACK
 packets.  See Appendix B for more suggested implementation strategies
 that prevent key desynchronization attacks.
 Since the client keys are stored in plaintext on the server, special
 care should be given to the overall security of the authentication
 server.  An operating system-level attack yielding root access to an
 intruder would result in the compromise of all client credentials.

Clancy & Arbaugh Informational [Page 20] RFC 4746 EAP-PAX November 2006

4.3. EAP Security Claims

 This section describes EAP-PAX in terms of specific security
 terminology as required by [RFC3748].

4.3.1. Protected Ciphersuite Negotiation

 In the initial packet from the server, the server specifies the
 ciphersuite in the packet header.  The server is in total control of
 the ciphersuite; thus, a client not supporting the specified
 ciphersuite will not be able to authenticate.  In addition, each
 client's local security policy should specify secure ciphersuites the
 client will accept.  The ciphersuite specified in PAX_STD-1 and
 PAX_SEC-1 MUST remain the same in successive packets within the same
 authentication session.  Since later packets are covered by an ICV
 keyed with the ICK, the server can verify that the originally
 transmitted ciphersuite was not altered by an adversary.

4.3.2. Mutual Authentication

 Both PAX_STD and PAX_SEC authenticate the client and the server, and
 consequently achieve explicit mutual authentication.

4.3.3. Integrity Protection

 The ICV described in Section 3.4 provides integrity protection once
 the integrity check key has been derived.  The header values in the
 unprotected packets can be verified when an ICV is received later in
 the session.

4.3.4. Replay Protection

 EAP-PAX is inherently designed to avoid replay attacks by
 cryptographically binding each packet to the previous one.  Also the
 EAP sequence number is covered by the ICV to further strengthen
 resistance to replay attacks.

4.3.5. Confidentiality

 With identity protection enabled, PAX_SEC provides full
 confidentiality.

4.3.6. Key Derivation

 Session keys are derived using the PAX-KDF and fresh entropy supplied
 by both the client and the server.  Since the key hierarchy is
 derived from the shared password, only someone with knowledge of that
 password or the capability of guessing it is capable of deriving the

Clancy & Arbaugh Informational [Page 21] RFC 4746 EAP-PAX November 2006

 session keys.  One of the main benefits of PAX_SEC is that it allows
 you to bootstrap a strong shared secret using a weak password while
 preventing offline dictionary attacks.

4.3.7. Key Strength

 Authentication keys are 128 bits.  The key generation is protected by
 a Diffie-Hellman key exchange.  It is believed that a 3000-bit MODP
 public-key scheme is roughly equivalent [RFC3766] to a 128-bit
 symmetric-key scheme.  Consequently, EAP-PAX requires the use of a
 Diffie-Hellman group with modulus larger than 3000.  Also, the
 exponent used as the private DH parameter must be at least twice as
 large as the key eventually generated.  Consequently, EAP-PAX uses
 256-bit DH exponents.  Thus, the authentication keys contain the full
 128 bits of security.
 Future ciphersuites defined for EAP-PAX MUST contain a minimum of 128
 bits of security.

4.3.8. Dictionary Attack Resistance

 EAP-PAX is resistant to dictionary attacks, except for the case where
 a weak password is initially used and the server is not using a
 certificate for authentication.  See Section 4.1 for more information
 on resistance to dictionary attacks.

4.3.9. Fast Reconnect

 Although a specific fast reconnection option is not included,
 execution of PAX_STD requires very little computation time and is
 therefore bound primarily by the latency of the Authentication,
 Authorization, and Accounting (AAA) server.

4.3.10. Session Independence

 This protocol easily achieves backward secrecy through, among other
 things, use of the PAX-KDF.  Given a current session key, attackers
 can discover neither the entropy used to generate it nor the key used
 to encrypt that entropy as it was transmitted across the network.
 This protocol has coarse-grained forward secrecy.  Compromised
 session keys are only useful on data for that session, and one cannot
 derive AK from them.  If an attacker can discover AK, that value can
 only be used to compromise session keys derived using that AK.
 Reasonably frequent password updates will help mitigate such attacks.
 Session keys are independently generated using fresh nonces for each
 session, and therefore the sessions are independent.

Clancy & Arbaugh Informational [Page 22] RFC 4746 EAP-PAX November 2006

4.3.11. Fragmentation

 Fragmentation and reassembly is supported through the fragmentation
 flag in the header.

4.3.12. Channel Binding

 EAP-PAX can be extended to support channel bindings through the use
 of its subADE fields.

4.3.13. Cryptographic Binding

 EAP-PAX does not include any cryptographic binding.  This is relevant
 only for tunneled methods.

4.3.14. Negotiation Attack Prevention

 EAP is susceptible to an attack where an attacker uses NAKs to
 convince an EAP client and server to use a less secure method, and
 can be prevented using method-specific integrity protection on NAK
 messages.  Since EAP-PAX does not have suitable keys derived for this
 integrity protection at the beginning of a PAX conversation, this is
 not included.

5. IANA Considerations

 This document requires IANA to maintain the namespace for the
 following header fields: MAC ID, DH Group ID, Public Key ID, and ADE
 type.  The initial namespace populations are as follows.
 MAC ID Namespace:
 o  0x01 : HMAC_SHA1_128
 o  0x02 : HMAC_SHA256_128
 DH Group ID Namespace:
 o  0x00 : NONE
 o  0x01 : IANA DH Group 14
 o  0x02 : IANA DH Group 15
 o  0x03 : NIST ECC Group P-256

Clancy & Arbaugh Informational [Page 23] RFC 4746 EAP-PAX November 2006

 Public Key ID Namespace:
 o  0x00 : NONE
 o  0x01 : RSAES-OAEP
 o  0x02 : RSA-PKCS1-V1_5
 o  0x03 : El-Gamal Over NIST ECC Group P-256
 ADE Type Namespace:
 o  0x01 : Vendor Specific
 o  0x02 : Client Channel Binding Data
 o  0x03 : Server Channel Binding Data
 Allocation of values for these namespaces shall be reviewed by a
 Designated Expert appointed by the IESG.  The Designated Expert will
 post a request to the EAP WG mailing list (or a successor designated
 by the Designated Expert) for comment and review, including an
 Internet-Draft.  Before a period of 30 days has passed, the
 Designated Expert will either approve or deny the registration
 request and publish a notice of the decision to the EAP WG mailing
 list or its successor, as well as informing IANA.  A denial notice
 must be justified by an explanation and, in the cases where it is
 possible, concrete suggestions on how the request can be modified so
 as to become acceptable.

6. Acknowledgments

 The authors would like to thank Jonathan Katz for discussion with
 respect to provable security, Bernard Aboba for technical guidance,
 Jari Arkko for his expert review, and Florent Bersani for feedback
 and suggestions.  Finally, the authors would like to thank the
 Defense Information Systems Agency for initially funding this work.

7. References

7.1. Normative References

 [FIPS180]    National Institute for Standards and Technology, "Secure
              Hash Standard", Federal Information Processing Standard
              180-2, August 2002.
 [FIPS186]    National Institute for Standards and Technology,
              "Digital Signature Standard (DSS)", Federal Information
              Processing Standard 186, May 1994.
 [FIPS198]    National Institute for Standards and Technology, "The
              Keyed-Hash Message Authentication Code (HMAC)", Federal
              Information Processing Standard 198, March 2002.

Clancy & Arbaugh Informational [Page 24] RFC 4746 EAP-PAX November 2006

 [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3232]    Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by
              an On-line Database", RFC 3232, January 2002.
 [RFC3280]    Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and
              Certificate Revocation List (CRL) Profile", RFC 3280,
              April 2002.
 [RFC3447]    Jonsson, J. and B. Kaliski, "Public-Key Cryptography
              Standards (PKCS) #1: RSA Cryptography Specifications
              Version 2.1", RFC 3447, February 2003.
 [RFC3526]    Kivinen, T. and M. Kojo, "More Modular Exponential
              (MODP) Diffie-Hellman groups for Internet Key Exchange
              (IKE)", RFC 3526, May 2003.
 [RFC3748]    Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and
              H. Levkowetz, "Extensible Authentication Protocol
              (EAP)", RFC 3748, June 2004.
 [RFC4282]    Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
              Network Access Identifier", RFC 4282, December 2005.
 [RFC4334]    Housley, R. and T. Moore, "Certificate Extensions and
              Attributes Supporting Authentication in Point-to-Point
              Protocol (PPP) and Wireless Local Area Networks (WLAN)",
              RFC 4334, February 2006.
 [X.690]      International Telecommunications Union, "Information
              technology - ASN.1 encoding rules: Specification of
              Basic Encoding Rules (BER), Canonical Encoding Rules
              (CER) and Distinguished Encoding Rules (DER)", Data
              Networks and Open System Communication Recommendation
              X.690, July 2002.

7.2. Informative References

 [IETF.KEY]   Aboba, B., Simon, D., Arkko, J., Eronen, P., and H.
              Levkowetz, "Extensible Authentication Protocol (EAP) Key
              Management Framework", Work in Progress.

Clancy & Arbaugh Informational [Page 25] RFC 4746 EAP-PAX November 2006

 [IEEE.80211] 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
              802.11-1997, 1997.
 [RFC2631]    Rescorla, E., "Diffie-Hellman Key Agreement Method", RFC
              2631, June 1999.
 [RFC3766]    Orman, H. and P. Hoffman, "Determining Strengths For
              Public Keys Used For Exchanging Symmetric Keys", BCP 86,
              RFC 3766, April 2004.
 [RFC4017]    Stanley, D., Walker, J., and B. Aboba, "Extensible
              Authentication Protocol (EAP) Method Requirements for
              Wireless LANs", RFC 4017, March 2005.
 [RFC4252]    Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
              Authentication Protocol", RFC 4252, January 2006.

Clancy & Arbaugh Informational [Page 26] RFC 4746 EAP-PAX November 2006

Appendix A. Key Generation from Passwords

 If a 128-bit key is not available to bootstrap the authentication
 process, then one must be generated from some sort of weak preshared
 key.  Note that the security of the hashing process is unimportant,
 as long as it does not significantly decrease the password's entropy.
 Resistance to dictionary attacks is provided by PAX_SEC.
 Consequently, computing the SHA-1 of the password and truncating the
 output to 128 bits is RECOMMENDED as a means of converting a weak
 password to a key for provisioning.
 When using other preshared credentials, such as a Kerberos Data
 Encryption Standard (DES) key, or an MD4-hashed Microsoft Challenge
 Handshake Authentication Protocol (MSCHAP) password, to provision
 clients, these keys SHOULD still be put through SHA-1 before being
 used.  This serves to protect the credentials from possible
 compromise, and also keeps things uniform.  As an example, consider
 provisioning using an existing Kerberos credential.  The initial key
 computation could be SHA1_128(string2key(password)).  The KDC,
 storing string2key(password), would also be able to compute this
 initial key value.

Appendix B. Implementation Suggestions

 In this section, two implementation strategies are discussed.  The
 first describes how best to implement and deploy EAP-PAX in an
 enterprise network for IEEE 802.11i authentication.  The second
 describes how to use EAP-PAX for device authentication in a 3G-style
 mobile phone network.

B.1. WiFi Enterprise Network

 For the purposes of this section, a wireless enterprise network is
 defined to have the following characteristics:
 o  Users wish to obtain network access through IEEE 802.11 access
    points.
 o  Users can possibly have multiple devices (laptops, PDAs, etc.)
    they wish to authenticate.
 o  A preexisting authentication framework already exists, for
    example, a Microsoft Active Directory domain or a Kerberos realm.
 Two of the biggest challenges in an enterprise WiFi network is key
 provisioning and support for multiple devices.  Consequently, it is
 recommended that the client's Network Access Identifier (NAI) have

Clancy & Arbaugh Informational [Page 27] RFC 4746 EAP-PAX November 2006

 the format username/KID@realm, where KID is a key ID that can be used
 to distinguish between different devices.
 The client's supplicant can use a variety of sources to automatically
 generate the KID.  Two of the better choices would likely be the
 computer's NETBIOS name, or local Ethernet adapter's MAC address.
 The wireless adapter's address may be a suboptimal choice, as the
 user may only have one PCCARD adapter for multiple systems.
 With an authentication system already in place, there is a natural
 choice for the provisioned key.  Clients can authenticate using their
 preexisting password.  When the server is presented with a new KID,
 it can create a new key record on the server and use the user's
 current password as the provisioned key.  For example, for Active
 Directory, the supplicant could use Microsoft's NtPasswordHash
 function to generate a key verifiable by the server.  It is suggested
 that this key then be fed through SHA1_128 before being used in a
 non-Microsoft authentication protocol.
 After a key update, the server should keep track of both the old and
 new authentication keys.  When two keys exist, the server should
 attempt to use both to validate the MACs on transmitted packets.
 Once a client successfully authenticates using the new key, the
 server should discard the old key.  This prevents desynchronization
 attacks.

B.2. Mobile Phone Network

 In a mobile phone system, we no longer need to worry about supporting
 multiple keys per identity.  Presumably, each mobile device has a
 unique identity.  However, if multiple devices per identity are
 desired, a method similar to that presented in Section B.1 could be
 used.
 Provisioning could easily be accomplished by issuing customers a 6-
 digit PIN they could type into their phone's keypad.

Clancy & Arbaugh Informational [Page 28] RFC 4746 EAP-PAX November 2006

Authors' Addresses

 T. Charles Clancy
 DoD Laboratory for Telecommunications Sciences
 8080 Greenmeade Drive
 College Park, MD  20740
 USA
 EMail: clancy@ltsnet.net
 William A. Arbaugh
 University of Maryland
 Department of Computer Science
 College Park, MD  20742
 USA
 EMail: waa@cs.umd.edu

Clancy & Arbaugh Informational [Page 29] RFC 4746 EAP-PAX November 2006

Full Copyright Statement

 Copyright (C) The IETF Trust (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
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 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST,
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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Clancy & Arbaugh Informational [Page 30]

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