EMU Working Group                                              T. Clancy
Internet-Draft                                                       LTS
Intended status: Standards Track                           H. Tschofenig
Expires: April 18, May 4, 2007                                             Siemens
                                                        October 15, 31, 2006

               EAP Generalized Pre-Shared Key (EAP-GPSK)
                     draft-ietf-emu-eap-gpsk-00.txt
                     draft-ietf-emu-eap-gpsk-01.txt

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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This Internet Draft defines an Extensible Authentication Protocol
   method called EAP Generalized Pre-Shared Key (EAP-GPSK).  This method
   is a lightweight shared-key authentication protocol supporting mutual
   authentication and key derivation.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4

   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5

   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  7

   4.  Key Derivation . . . . . . . . . . . . . . . . . . . . . . . .  9

   5.  Ciphersuites . . . . . . . . . . . . . . . . . . . . . . . . . 11

   6.  Ciphersuites Processing Rules  . . . . . . . . . . . . . . . . 13
     6.1.  Ciphersuite #1 . . . . . . . . . . . . . . . . . . . . . . 13
       6.1.1.  Encryption . . . . . . . . . . . . . . . . . . . . . . 13
       6.1.2.  Integrity  . . . . . . . . . . . . . . . . . . . . . . 13
       6.1.3.  Key Derivation . . . . . . . . . . . . . . . . . . . . 14
     6.2.  Ciphersuite #2 . . . . . . . . . . . . . . . . . . . . . . 14
       6.2.1.  Encryption . . . . . . . . . . . . . . . . . . . . . . 14
       6.2.2.  Integrity  . . . . . . . . . . . . . . . . . . . . . . 14
       6.2.3.  Key Derivation . . . . . . . . . . . . . . . . . . . . 15

   7.  Packet Formats . . . . . . . . . . . . . . . . . . . . . . . . 15
     7.1.  Header Format  . . . . . . . . . . . . . . . . . . . . . . 15
     7.2.  Ciphersuite Formatting . . . . . . . . . . . . . . . . . . 16
     7.3.  Payload Formatting . . . . . . . . . . . . . . . . . . . . 16
     7.4.  Protected Data . . . . . . . . . . . . . . . . . . . . . . 20

   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
     8.1.  Mutual Authentication  . . . . . . . . . . . . . . . . . . 21
     8.2.  Protected Result Indications . . . . . . . . . . . . . . . 22
     8.3.  Integrity Protection . . . . . . . . . . . . . . . . . . . 22
     8.4.  Replay Protection  . . . . . . . . . . . . . . . . . . . . 22
     8.5.  Reflection attacks . . . . . . . . . . . . . . . . . . . . 22
     8.6.  Dictionary Attacks . . . . . . . . . . . . . . . . . . . . 22
     8.7.  Key Derivation . . . . . . . . . . . . . . . . . . . . . . 22
     8.8.  Denial of Service Resistance . . . . . . . . . . . . . . . 22
     8.9.  Session Independence . . . . . . . . . . . . . . . . . . . 23
     8.10. Exposition of the PSK  . . . . . . . . . . . . . . . . . . 23
     8.11. Fragmentation  . . . . . . . . . . . . . . . . . . . . . . 24
     8.12. Channel Binding  . . . . . . . . . . . . . . . . . . . . . 24
     8.13. Fast Reconnect . . . . . . . . . . . . . . . . . . . . . . 24
     8.14. Identity Protection  . . . . . . . . . . . . . . . . . . . 24
     8.15. Protected Ciphersuite Negotiation  . . . . . . . . . . . . 24
     8.16. Confidentiality  . . . . . . . . . . . . . . . . . . . . . 24
     8.17. Cryptographic Binding  . . . . . . . . . . . . . . . . . . 24

   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 24
   10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 25

   11. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . 25

   12. Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . . 26

   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 26
     13.2. Informative References . . . . . . . . . . . . . . . . . . 26

   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27
   Intellectual Property and Copyright Statements . . . . . . . . . . 29

1.  Introduction

   EAP Generalized Pre-Shared Key (EAP-GPSK) is an EAP method defining a
   generalized pre-shared key authentication technique.  Mutual
   authentication is achieved through a nonce-based exchange that is
   secured by a pre-shared key.

   At present, several pre-shared key EAP methods are specified, most
   notably

   o  EAP-PAX [I-D.clancy-eap-pax]
   o  EAP-PSK [I-D.bersani-eap-psk]
   o  EAP-TLS-PSK [I-D.otto-emu-eap-tls-psk] and
   o  EAP-SAKE [I-D.vanderveen-eap-sake].

   Each proposal has its particular benefits but also its particular
   deficiencies.  EAP-GPSK is a new EAP method that tries to combine the
   most valuable characteristics of each of these methods and therefore
   attempts to address a broad range of usage scenarios.

   The main design goals of EAP-GPSK are

   Simplicity:

      EAP-GPSK should be easy to implement and therefore quickly
      available.

   Wide applicability:

      EAP-GPSK has been designed in a threat model where the attacker
      has full control over the communication channel.  This is the EAP
      threat model that is presented in Section 7.1 of [RFC3748].  Thus,
      it is particularly suited for wireless or battery powered devices.

   Efficiency:

      EAP-GPSK does not make use of public key cryptography and fully
      relies of symmetric cryptography.  The restriction on symmetric
      cryptographic computations allows for low computational overhead.
      Hence, EAP-GPSK is lightweight and well suited for any type of
      device, especially those with little processing power and memory.

   Flexibility:

      EAP-GPSK offers cryptographic flexibility.  At the beginning, the
      EAP server selects a set of cryptographic algorithms and key
      sizes, a so called ciphersuite.  The current version of EAP-GPSK
      comprises two ciphersuites, but additional ones can be easily
      added.

   Extensibility:

      The design of EAP-GPSK allows to securely exchange information
      between the EAP peer and the EAP server using protected data
      fields.  These fields might, for example, be used to exchange
      channel binding information or to provide support for identity
      confidentiality.

2.  Terminology

   In this document, several words are used to signify the requirements
   of the specification.  These words are often capitalized.  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].

   This section describes the various variables and functions used in
   the EAP-GPSK method.

   Variables:

   PD_Payload_X:

      Data carried within the X-th protected data payload
   CSuite_List:

      An octet array listing available ciphersuites (variable length)

   CSuite_Sel:

      Ciphersuite selected by the client (1 octet or 7 octets)

   ID_Client:

      Client NAI [RFC2486bis]

   ID_Server:

      Server identity as an opaque blob.

   KS:

      Integer representing the key size in octets of the selected
      ciphersuite CSuite_Sel

   RAND_Client:

      Random integer generated by the client (256 bits)

   RAND_Server:

      Random integer generated by the server (256 bits)

   Operations:

   A || B:

      Concatenation of octet strings A and B

   ENC_X(Y):

      Encryption of message Y with a symmetric key X, using a defined
      block cipher

   KDF_X(Y):

      Key Derivation Function that generates an arbitrary number of
      octets of output using secret X and seed Y

   length(X):

      Function that returns the length of input X in octets, encoded as
      a 16-bit integer in network byte order

   MAC_X(Y):

      Keyed message authentication code computed over Y with symmetric
      key X

   SEC_X(Y):

      SEC is a function that provides integrity protection based on the
      chosen ciphersuite.  The function SEC uses the algorithm defined
      by the selected ciphersuite and applies it to the message content
      Y with key X. As an output the message returns Y concatenated with
      MAC_X(Y).

   X[A..B]:

      Notation representing octets A through B of octet array X

   The following abbreviations are used for the keying material:

   PK:

      Session key generated from the MK and used during protocol
      exchange to encrypt protected data (size defined by ciphersuite)

   SK:

      Session key generated from the MK and used during protocol
      exchange to prove knowledge of PSK (size defined by ciphersuite)

   EMSK:

      Extended Master Session Key is exported by the EAP method (512
      bits)

   MK:

      Master Key between the client and EAP server from which all other
      EAP method session keys are derived (KS octets)

   MSK:

      Master Session Key exported by to the EAP method (512 bits)

   MID:

      Method ID exported by the EAP method according to the EAP keying
      framework [I-D.ietf-eap-keying] (128 bits)

   PSK:

      Long-term key shared between the client and the server (PL octets)

3.  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
   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 |        |
   |  EAP   |<------------------------------------|  EAP   |
   |  peer  |                                     | server |
   |        | EAP-Response/Identity               |        |
   |        |------------------------------------>|        |
   |        |                                     |        |
   |        |                  EAP-Request/GPSK-1 |        |
   |        |<------------------------------------|        |
   |        |                                     |        |
   |        | EAP-Response/GPSK-2                 |        |
   |        |------------------------------------>|        |
   |        |                                     |        |
   |        |                  EAP-Request/GPSK-3 |        |
   |        |<------------------------------------|        |
   |        |                                     |        |
   |        | EAP-Response/GPSK-4                 |        |
   |        |------------------------------------>|        |
   |        |                                     |        |
   |        |          EAP-Success or EAP-Failure |        |
   |        |<------------------------------------|        |
   +--------+                                     +--------+

   EAP-GPSK performs mutual authentication between EAP peer ("Client")
   and EAP server ("Server") based on a pre-shared key (PSK).  The
   protocol consists of two EAP message exchanges, in which both sides

   o  exchange nonces and their identities and

   o  compute and exchange a Message Authentication Code (MAC) over the
      previously exchanged values, keyed with the pre-shared key.  This
      MAC is considered as proof of possession of the pre-shared key.

   The full EAP-GPSK protocol is as follows:
   GPSK-1:

      ID_Server, RAND_Server, CSuite_List

   GPSK-2:

      SEC_SK(ID_Client, ID_Server, RAND_Client, RAND_Server,
      CSuite_List, CSuite_Sel [, ENC_PK(PD_Payload_1), ... ] )
   GPSK-3

      SEC_SK(RAND_Client, RAND_Server, CSuite_Sel [,
      ENC_PK(PD_Payload_2) ] )

   GPSK-4:

      [ SEC_SK(ENC_PK(PD_Payload_3)) ]

   The EAP server begins EAP-GPSK creating a random number RAND_Server
   and by encoding the supported ciphersuites into CSuite_List.  A
   ciphersuite consists of an encryption algorithm, a key derivation
   function and a message authentication code.

   In GPSK-1, the EAP server sends its identity ID_Server, a random
   number RAND_Server and the identifier of the chosen ciphersuite.  The
   decision which ciphersuite to use is policy-dependent and therefore
   outside the scope of this document.

   In GPSK-2, the peer sends its identity ID_Client, a random number
   RAND_Client.  Furthermore, it repeats the received parameters of the
   GPSK-1 message and computes a Message Authentication Code over all
   these parameters.

   The EAP server verifies the received Message Authentication Code.  In
   case of successful verification, the EAP server computes a Message
   Authentication Code over the session parameter and returns it to the
   client (within GPSK-3).  Within GPSK-2 and GPSK-3, peer and EAP
   server have the possibility to exchange encrypted protected data
   parameters.

   The peer verifies the received Message Authentication Code.  If the
   verification is successful, GPSK-4 is prepared.  This message can
   optionally contain the client's protected data parameters.

   Upon receipt of GPSK-4, the server assures that the peer has derived
   session keys SK and PK properly.  Then, the EAP server sends an EAP
   Success message to indicate the successful outcome of the
   authentication.

4.  Key Derivation

   EAP-GPSK provides key derivation in compliance to the requirements of
   [RFC3748] and [I-D.ietf-eap-keying].

   The long-term credential shared between EAP peer and EAP server
   SHOULD be a strong pre-shared key PSK of at least 16 bytes, though
   its length and entropy is variable.  While it is possible to use a
   password or passphrase, doing so is NOT RECOMMENDED as it would make
   EAP-GPSK vulnerable to dictionary attacks.

   During an EAP-GPSK authentication, a Master Key MK, a Session Key SK
   and a Protected Data Encryption Key PK are derived using the
   ciphersuite-specified KDF and data exchanged during the execution of
   the protocol, namely 'RAND_Client || ID_Client || RAND_Server ||
   ID_Server' referred as inputString as its short-hand form.

   In case of successful completion, EAP-GPSK derives and exports an MSK
   and EMSK both in length of 64 bytes.  This keying material is derived
   using the ciphersuite-specified KDF as follows:

   o  inputString = RAND_Client || ID_Client || RAND_Server || ID_Server
   o  MK = KDF_Zero-String (PL || PSK || CSuite_Sel ||
      inputString)[0..KS-1]
   o  SK = KDF_MK (inputString)[128..127+KS]
   o  PK = KDF_MK (inputString)[128+KS..127+2*KS]
   o  MSK = KDF_MK (inputString)[0..63]
   o  EMSK = KDF_MK (inputString)[64..127]
   o  MID = KDF_Zero-String ("Method ID" || EAP_Method_Type ||
      CSuite_Sel || inputString)[0..15]

   Note that the term 'Zero-String' refers to a sequence of 0x00 values,
   KS octets in length.  EAP_Method_Type refers to the integer value of
   the IANA allocated EAP Type code.

   Figure 2 depicts the key derivation procedure of EAP-GPSK.

   +-------------+     +-------------------------------+
   |   PL-octet  |     | RAND_Client || ID_Client ||   |
   |     PSK     |     | RAND_Server || ID_Server      |
   +-------------+     +-------------------------------+
          |                            |            |
          v                            v            |
   +--------------------------------------------+   |
   |                    KDF                     |   |
   +--------------------------------------------+   |
                             |                      |
                             v                      |
                      +-------------+               |
                      |   KS-octet  |               |
                      |     MK      |               |
                      +-------------+               |
                             |                      |
                             v                      v
   +---------------------------------------------------+
   |                      KDF                          |
   +---------------------------------------------------+
        |             |             |            |
        v             v             v            v
   +---------+   +---------+  +----------+  +----------+
   | 512-bit |   | 512-bit |  | KS-octet |  | KS-octet |
   |   MSK   |   |  EMSK   |  |    SK    |  |   PK     |
   +---------+   +---------+  +----------+  +----------+

                     Figure 2: EAP-GPSK Key Derivation

5.  Ciphersuites

   The design of EAP-GPSK allows cryptographic algorithms and key sizes,
   called ciphersuites, to be negotiated during the protocol run.  The
   ability to specify block-based and hash-based ciphersuites is
   offered.  Extensibility is provided with the introduction of new
   ciphersuites; this document specifies an initial set.  The CSuite/
   Specifier column in Figure 3 uniquely identifies a ciphersuite.

   For a vendor-specific ciphersuite the first three octets are the
   vendor-specific OID, and the last three octets are vendor assigned
   for the specific ciphersuite.

   The following ciphersuites are specified in this document:

   +-----------+----+-------------+---------------+--------------------+
   | CSuite/   | KS | Encryption  | Integrity     | Key Derivation     |
   | Specifier |    |             |               | Function           |
   +-----------+----+-------------+---------------+--------------------+
   | 0x000001  | 16 | AES-CBC-128 | AES_CMAC_128  | GKDF-128           |
   +-----------+----+-------------+---------------+--------------------+
   | 0x000002  | 32 | NULL        | HMAC-SHA256   | GKDF-256           |
   +-----------+----+-------------+---------------+--------------------+

                          Figure 3: Ciphersuites

   Ciphersuite 1, which is based on AES as a cryptographic primitive, is
   mandatory to implement.  This document specifies also a second
   ciphersuite, but its support is only optional.

   Each ciphersuite needs to specify a key derivation function.  The
   ciphersuites defined in this document make use of the Generalized Key
   Distribution Function (GKDF).  Future ciphersuites can use any other
   formally specified KDF that takes as arguments a key and a seed
   value, and produces at least 1024+2*KS bits of output.

   If GKDF is invoked by a MAC-based ciphersuite, then the variable
   "size" contains the MAC output size in octets.  In case of a block
   cipher-based ciphersuite, "size" contains the block size in octets.

   GKDF has the following structure:

   GKDF-X(Y, Z)

   X  length, in octets, of the desired output
   Y  secret key used to protect the computation
   Z  data specific for the protocol run

   GKDF-X (Y, Z)
   {
       n = int( X / size - 1 ) + 1; /* determine number of
                                       output blocks */
       M_0 = "";
       result = "";

       for i=1 to n {
           M_i = MAC_Y (M_{i-1} || Z || i || X);
           result = result || M_i;
       }

       return truncate (result; X)
   }
   Note that the variables 'i' and 'X' in M_i are represented as 16-bit
   values in network byte order.

6.  Ciphersuites Processing Rules

6.1.  Ciphersuite #1

6.1.1.  Encryption

   With this ciphersuite all cryptography is built around a single
   cryptographic primitive, AES-128.  Within the protected data frames,
   AES-128 is used in CBC mode of operation (see [CBC]).  The CBC mode
   is well-defined and well-understood.  This mode requires an
   Initialization Vector (IV) that has the same size as the block size.
   For security reasons, the IV should be randomly generated.

   In a nutshell, the CBC mode proceeds as follows.  The IV is XORed
   with the first plaintext block before it is encrypted.  Then for
   successive blocks, the previous ciphertext block is XORed with the
   current plaintext, before it is encrypted.

   Note that in order to provide integrity protection, the CBC-encrypted
   ciphertext MUST be accompanied by a MAC.

   [Editor's Note: Description about the computation of the IV is
   missing]

6.1.2.  Integrity

   Ciphersuite 1 uses CMAC as Message Authentication Code.  CMAC is
   recommended by NIST.  Among its advantages, CMAC is capable to work
   with messages of arbitrary length.  A detailed description of CMAC
   can be found in [CMAC].

   The following instantiation is used: AES-128-CMAC(SK, Input) denotes
   the MAC of Input under the key SK.

   where Input refers to the following content:

   o  Value of SEC_SK in message GPSK-2
   o  Value of SEC_SK in message GPSK-3
   o  Value of SEC_SK in message GPSK-4

6.1.3.  Key Derivation

   This ciphersuite instantiates the KDF in the following way:

   MK

   inputString = GKDF-16 (PSK[0..15], RAND_Client || ID_Client || RAND_Server ||
   ID_Server)

   KDF_out ID_Server

   MK = GKDF-160 (MK, RAND_Client GKDF-16 (Zero-String, PL || ID_Client PSK || RAND_Server CSuite_SEL ||
   ID_Server) inputString)

   KDF_out = GKDF-160 (MK, inputString)

   MSK = KDF_out[0..63]

   EMSK = KDF_out[64..127]

   SK = KDF_out[128..143]

   PK = KDF_out[144..159]

   MID = GKDF-16 (Zero-String, "Method ID" || EAP_Method_Type ||
   RAND_Client || ID_Client || RAND_Server
   CSuite_Sel || ID_Server) inputString)

6.2.  Ciphersuite #2

6.2.1.  Encryption

   Ciphersuite 2 does not include an algorithm for encryption.  With a
   NULL encryption algorithm, encryption is defined as:

   E_X(Y) = Y

   When using this ciphersuite, the data exchanged inside the protected
   data blocks is not encrypted.  Therefore this mode MUST NOT be used
   if confidential information appears inside the protected data blocks.

6.2.2.  Integrity

   Ciphersuite 2 uses the keyed MAC function HMAC, with the SHA256 hash
   algorithm.

   For integrity protection the following instantiation is used:

   HMAC-SHA256(SK, Input) denotes the MAC of Input under the key SK
   where Input refers to the following content:

   o  Value of SEC_SK in message GPSK-2
   o  Value of SEC_SK in message GPSK-3
   o  Value of SEC_SK in message GPSK-4

6.2.3.  Key Derivation

   This ciphersuite instantiates the KDF in the following way:

   MK

   inputString = GKDF-32 (PSK, RAND_Client || ID_Client || RAND_Server ||
   ID_Server)

   KDF_out ID_Server

   MK = GKDF-192 (MK, RAND_Client GKDF-32 (Zero-String, PL || ID_Client PSK || RAND_Server CSuite_SEL ||
   ID_Server) inputString)

   KDF_out = GKDF-192 (MK, inputString)

   MSK = KDF_out[0..63]

   EMSK = KDF_out[64..127]

   SK = KDF_out[128..159]

   PK = KDF_out[160..191]

   MID = GKDF-16 (Zero-String, "Method ID" || EAP_Method_Type ||
   RAND_Client || ID_Client || RAND_Server
   CSuite_Sel || ID_Server) inputString)

7.  Packet Formats

   This section defines the packet format of the EAP-GPSK messages.

7.1.  Header Format

   The EAP-GPSK header has the following structure:

   --- 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    |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |                                                               |
   ...                         Payload                           ...
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 5

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

   The OP-Code field is one of four values:

   o  0x01 : GPSK-1
   o  0x02 : GPSK-2
   o  0x03 : GPSK-3
   o  0x04 : GPSK-4

7.2.  Ciphersuite Formatting

   Ciphersuites are encoded as 6-octet arrays.  The first three octets
   indicate the CSuite/Vendor field.  For vendor-specific ciphersuites,
   this represents the vendor OID.  The last three octets indicate the
   CSuite/Specifier field, which identifies the particular ciphersuite.
   The 3-byte CSuite/Vendor value 0x000000 indicates ciphersuites
   allocated by the IETF.

   Graphically, they are represented as

   --- 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        CSuite/Vendor = 0x000000 or OID        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          CSuite / Specifier       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 6

   CSuite_Sel is encoded as a 6-octet ciphersuite CSuite/Vendor and
   CSuite/Specifier pair.

   CSuite_List is a variable-length octet array of ciphersuites.  It is
   encoded by concatenating encoded ciphersuite values.  Its length in
   octets MUST be a multiple of 6.

7.3.  Payload Formatting

   Payload formatting is based on the protocol exchange description in
   Section 3.

   The GPSK-1 payload format is defined as follows:

   --- 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       length(ID_Server)       |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |                                                               |
   ...                         ID_Server                         ...
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ...                   32-octet RAND_Server                    ...
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      length(CSuite_List)      |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |                                                               |
   ...                        CSuite_List                        ...
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 7: GPSK-1 Payload

   The GPSK-2 payload format is defined as follows:

   --- 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       length(ID_Client)       |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |                                                               |
   ...                         ID_Client                         ...
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       length(ID_Server)       |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |                                                               |
   ...                         ID_Server                         ...
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ...                    32-octet RAND_Client                   ...
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ...                    32-octet RAND_Server                   ...
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      length(CSuite_List)      |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |                                                               |
   ...                        CSuite_List                        ...
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           CSuite_Sel                          |
   +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |     length(PD_Payload_1)      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ...                   optional PD_Payload_1                   ...
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ...                   KS-octet payload MAC                    ...
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 8: GPSK-2 Payload

   If the optional protected data payload is not included, then
   length(PD_Payload)=0 and the PD payload is excluded.

   The GPSK-3 payload is defined as follows:

   --- 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ...                    32-octet RAND_Client                   ...
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ...                    32-octet RAND_Server                   ...
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           CSuite_Sel                          |
   +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |     length(PD_Payload_2)      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ...                   optional PD_Payload_2                   ...
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ...                   KS-octet payload MAC                    ...
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 9: GPSK-3 Payload

   If the optional protected data payload is not included, then
   length(PD_Payload)=0 and the PD payload is excluded.

   The GPSK-4 payload format is defined as follows:

   --- 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     length(PD_Payload_3)      |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |                                                               |
   ...                   optional PD_Payload_3                   ...
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ...                   KS-octet payload MAC                    ...
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 10: GPSK-4 Payload

   If the optional protected data payload is not included, then
   length(PD_Payload)=0 and the PD payload is excluded.  The MAC MUST
   always be included, regardless of the presence of PD_Payload_3.

7.4.  Protected Data

   The protected data blocks are a generic mechanism for the client and
   server to securely exchange data.  If the specified ciphersuite has a
   NULL encryption primitive, then this channel only offers
   authenticity, and not confidentiality.

   These payloads are encoded as the concatenation of type-length-value
   (TLV) tripples.

   Type values are encoded as a 6-octet string and represented by a
   3-octet vendor and 3-octet specifier field.  The vendor field
   indicates the type as either standards-specified or vendor-specific.
   If these three octets are 0x000000, then the value is standards-
   specified, and any other value represents a vendor-specific OID.

   The specifier field indicates the actual type.  For vendor field
   0x000000, the specifier field is maintained by IANA.  For any other
   vendor field, the specifier field is maintained by the vendor.

   Length fields are specified as 2-octet integers in network byte
   order, and reflect only the length of the value, and do not include
   the length of the type and length fields.

   Graphically, this can be depicted as follows:

   --- 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   PData/Vendor                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            PData/Specifier        |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ...                      PD_Payload Value                     ...
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   For PData/Vendor field 0x000000, the following PData/Specifier fields
   are defined:
   o  0x000000 : Reserved
   o  0x000001 : Protected Results Indication
   o  0x000002 through 0xFFFFFF : Unallocated

   [Editor's Note: Text for protected results indication needs to be
   added here.]

8.  Security Considerations

   [RFC3748] highlights several attacks that are possible against EAP
   since EAP itself does not provide any security.

   This section discusses the claimed security properties of EAP-GPSK as
   well as vulnerabilities and security recommendations in the threat
   model of [RFC3748].

8.1.  Mutual Authentication

   EAP-GPSK provides mutual authentication.

   The server believes that the peer is authentic because it can
   calculate a valid MAC and the peer believes that the server is
   authentic because it can calculate another valid MAC.

   The key used for mutual authentication is computed again based on the
   long-term secret PSK that has to provide sufficient entropy and
   therefore sufficient strength.  In this way EAP-GPSK is no different
   than other authentication protocols based on pre-shared keys.

8.2.  Protected Result Indications

   EAP-GPSK offers the capability to exchange protected result
   indications using the protected data payloads.

8.3.  Integrity Protection

   EAP-GPSK provides integrity protection based on the ciphersuites
   suggested in this document.

8.4.  Replay Protection

   EAP-GPSK provides replay protection of its mutual authentication part
   thanks to the use of random numbers RAND_Server and RAND_P. Since
   RAND_Server is 128 bit long, one expects to have to record 2**64
   (i.e., approximately 1.84*10**19) EAP-GPSK successful authentication
   before an protocol run can be replayed.  Hence, EAP-GPSK provides
   replay protection of its mutual authentication part as long as
   RAND_Server and RAND_Client are chosen at random, randomness is
   critical for security.

8.5.  Reflection attacks

   EAP-GPSK provides protection against reflection attacks in case of an
   extended authentication because of the messages are constructed in a
   different fashion.

8.6.  Dictionary Attacks

   EAP-GPSK relies on a long-term shared secret (PSK) that MUST be based
   on at least 128 bits of entropy to guarantee security against
   dictionary attacks.  Users who use passwords or weak keys are not
   guaranteed security against dictionary attacks.  Derivation of the
   long-term shared secret from a password is highly discouraged.

8.7.  Key Derivation

   EAP-GPSK supports key derivation as shown in Section 4.

8.8.  Denial of Service Resistance

   Denial of Service resistance (DoS) has not been a design goal for
   EAP-GPSK.

   It is however believed that EAP-GPSK does not provide any obvious and
   avoidable venue for such attacks.

   It is worth noting that the server has to maintain some state when it
   engages in an EAP-GPSK conversation, namely to generate and to
   remember the 16-byte RAND_S. This should however not lead to resource
   exhaustion as this state and the associated computation are fairly
   lightweight.

   It is recommended that EAP-GPSK does not allow EAP notifications to
   be interleaved in its dialog to prevent potential DoS attacks.
   Indeed, since EAP Notifications are not integrity protected, they can
   easily be spoofed by an attacker.  Such an attacker could force a
   peer that allows EAP Notifications to engage in a discussion which
   would delay his authentication or result in the peer taking
   unexpected actions (e.g., in case a notification is used to prompt
   the peer to do some "bad" action).

   It is up to the implementation of EAP-GPSK or to the peer and the
   server to specify the maximum number of failed cryptographic checks
   that are allowed.

8.9.  Session Independence

   Thanks to its key derivation mechanisms, EAP-GPSK provides session
   independence: passive attacks (such as capture of the EAP
   conversation) or active attacks (including compromise of the MSK or
   EMSK) do not enable compromise of subsequent or prior MSKs or EMSKs.
   The assumption that RAND_Client and RAND_Server are random is central
   for the security of EAP-GPSK in general and session independance in
   particular.

8.10.  Exposition of the PSK

   EAP-GPSK does not provide perfect forward secrecy.  Compromise of the
   PSK leads to compromise of recorded past sessions.

   Compromise of the PSK enables the attacker to impersonate the peer
   and the server and it allows the adversary to compromise future
   sessions.

   EAP-GPSK provides no protection against a legitimate peer sharing its
   PSK with a third party.  Such protection may be provided by
   appropriate repositories for the PSK, which choice is outside the
   scope of this document.  The PSK used by EAP-GPSK must only be shared
   between two parties: the peer and the server.  In particular, this
   PSK must not be shared by a group of peers communicating with the
   same server.

   The PSK used by EAP-GPSK must be cryptographically separated from
   keys used by other protocols, otherwise the security of EAP-GPSK may
   be compromised.

8.11.  Fragmentation

   EAP-GPSK does not support fragmentation and reassembly since the
   message size is kept small.

8.12.  Channel Binding

   This document enables the ability to exchange channel binding
   information.  It does not, however, define the encoding of channel
   binding information in the document.

8.13.  Fast Reconnect

   EAP-GPSK does not provide the fast reconnect capability since this
   method is already at the lower limit of the number of roundtrips and
   the cryptographic operations.

8.14.  Identity Protection

   Identity protection is not specified in this document.  Extensions
   can be defined that enhanced this protocol to provide this feature.

8.15.  Protected Ciphersuite Negotiation

   EAP-GPSK provides protected ciphersuite negotiation via the
   indication of available ciphersuites by the server in the first
   message and a confirmation by the client in the subsequent message.

8.16.  Confidentiality

   Although EAP-GPSK provides confidentiality in its protected data
   payloads, it cannot claim to do so as per Section 7.2.1 of [RFC3748].

8.17.  Cryptographic Binding

   Since EAP-GPSK does not tunnel another EAP method, it does not
   implement cryptographic binding.

9.  IANA Considerations

   This document requires IANA to allocate a new EAP Type for EAP-GPSK.

   This document requires IANA to create a new registry for ciphersuites
   and protected data types.  IANA is furthermore instructed to add the
   specified ciphersuites and protected data types to this registry as
   defined in this document.  Values can be added or modified with
   informational RFCs defining either block-based or hash-based
   ciphersuites.  Each ciphersuite needs to provide processing rules and
   needs to specify how the following algorithms are instantiated:
   Encryption, Integrity and Key Derivation.  Additionally, the
   preferred key size needs to be specified.

   The following layout represents the initial ciphersuite CSuite/
   Specifier registry setup:

   o  0x000000 : Reserved
   o  0x000001 : AES-CBC-128, AES-CMAC-128, GKDF-128
   o  0x000002 : NULL, HMAC-SHA256, GKDF-256
   o  0x000003 through 0xFFFFFF : Unallocated

   The following is the initial protected data PData/Specifier registry
   setup:

   o  0x000000 : Reserved
   o  0x000001 : Protected Results Indication
   o  0x000002 through 0xFFFFFF : Unallocated

10.  Contributors

   This work is a joint effort of the EAP Method Update (EMU) design
   team of the EMU Working Group that was created to develop a mechanism
   based on strong shared secrets that meets RFC 3748 [RFC3748] and RFC
   4017 [RFC4017] requirements.  The contributors (in alphabetical
   order) include:

   o  Jari Arkko
   o  Mohamad Badra
   o  Uri Blumenthal
   o  Charles Clancy
   o  Lakshminath Dondeti
   o  David McGrew
   o  Joe Salowey
   o  Sharma Suman
   o  Hannes Tschofenig
   o  Jesse Walker

   Finally, we would like to thank Thomas Otto for his draft reviews,
   feedback and text contributions.

11.  Acknowledgment

   We would like to thank
   o  Jouni Malinen and Bernard Aboba for their early draft comments in
      June 2006.
   o  Lakshminath Dondeti, David McGrew, Bernard Aboba, Michaela
      Vanderveen and Ray Bell for their input to the ciphersuite
      discussions between July and August 2006.
   o  Lakshminath for his detailed draft review (sent to the EMU ML on
      the 12th July 2006).

12.  Open Issues

   The list of open issues can be found at:
   http://www.tschofenig.com:8080/eap-gpsk/

   A first prototype implementation by Jouni Malinen can be found at:
   http://hostap.epitest.fi/releases/snapshots/

13.  References

13.1.  Normative References

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

   [RFC2486bis]
              Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
              Network Access Identifier",
              draft-ietf-radext-rfc2486bis-06 (work in progress),
              July 2005.

   [RFC3174]  Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1
              (SHA1)", RFC 3174, September 2001.

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

13.2.  Informative References

   [I-D.clancy-eap-pax]
              Clancy, C. and W. Arbaugh, "EAP Password Authenticated
              Exchange", draft-clancy-eap-pax-11 (work in progress),
              September 2006.

   [I-D.bersani-eap-psk]
              Tschofenig, H. and F. Bersani, "The EAP-PSK Protocol: a
              Pre-Shared Key EAP Method", draft-bersani-eap-psk-11 (work
              in progress), June 2006.

   [I-D.otto-emu-eap-tls-psk]
              Otto, T. and H. Tschofenig, "The EAP-TLS-PSK
              Authentication Protocol", draft-otto-emu-eap-tls-psk-00 draft-otto-emu-eap-tls-psk-01
              (work in progress), April October 2006.

   [I-D.vanderveen-eap-sake]
              Vanderveen, M. and H. Soliman, "Extensible Authentication
              Protocol Method for Shared-secret Authentication  and Key
              Establishment (EAP-SAKE)", draft-vanderveen-eap-sake-02
              (work in progress), May 2006.

   [I-D.ietf-eap-keying]
              Aboba, B., "Extensible Authentication Protocol (EAP) Key
              Management Framework", draft-ietf-eap-keying-14 draft-ietf-eap-keying-15 (work in
              progress), June October 2006.

   [RFC4017]  Stanley, D., Walker, J., and B. Aboba, "Extensible
              Authentication Protocol (EAP) Method Requirements for
              Wireless LANs", RFC 4017, March 2005.

   [CMAC]     National Institute of Standards and Technology,
              "Recommendation for Block Cipher Modes of Operation: The
              CMAC Mode for Authentication", Special Publication
              (SP) 800-38B, May 2005.

   [CBC]      National Institute of Standards and Technology,
              "Recommendation for Block Cipher Modes of Encryption.
              Methods and Techniques.", Special Publication (SP) 800-
              38A, December 2001.

Authors' Addresses

   T. Charles Clancy
   DoD Laboratory for Telecommunication Sciences
   8080 Greenmeade Drive
   College Park, MD  20740
   USA

   Email: clancy@ltsnet.net
   Hannes Tschofenig
   Siemens
   Otto-Hahn-Ring 6
   Munich, Bavaria  81739
   Germany

   Email: Hannes.Tschofenig@siemens.com
   URI:   http://www.tschofenig.com

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