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INFORMATIONAL

Internet Engineering Task Force (IETF)                        Y. Sheffer
Request for Comments: 6124                                   Independent
Category: Informational                                          G. Zorn
ISSN: 2070-1721                                              Network Zen
                                                           H. Tschofenig
                                                  Nokia Siemens Networks
                                                              S. Fluhrer
                                                                   Cisco
                                                           February 2011


                 An EAP Authentication Method Based on
               the Encrypted Key Exchange (EKE) Protocol

Abstract

   The Extensible Authentication Protocol (EAP) describes a framework
   that allows the use of multiple authentication mechanisms.  This
   document defines an authentication mechanism for EAP called EAP-EKE,
   based on the Encrypted Key Exchange (EKE) protocol.  This method
   provides mutual authentication through the use of a short, easy to
   remember password.  Compared with other common authentication
   methods, EAP-EKE is not susceptible to dictionary attacks.  Neither
   does it require the availability of public-key certificates.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6124.











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Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
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   Without obtaining an adequate license from the person(s) controlling
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   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Message Flows  . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Message Formats  . . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  EAP-EKE Header . . . . . . . . . . . . . . . . . . . . . .  7
     4.2.  EAP-EKE Payloads . . . . . . . . . . . . . . . . . . . . .  8
       4.2.1.  The EAP-EKE-ID Payload . . . . . . . . . . . . . . . .  8
       4.2.2.  The EAP-EKE-Commit Payload . . . . . . . . . . . . . . 10
       4.2.3.  The EAP-EKE-Confirm Payload  . . . . . . . . . . . . . 11
       4.2.4.  The EAP-EKE-Failure Payload  . . . . . . . . . . . . . 12
     4.3.  Protected Fields . . . . . . . . . . . . . . . . . . . . . 13
     4.4.  Encrypted Fields . . . . . . . . . . . . . . . . . . . . . 14
     4.5.  Channel Binding Values . . . . . . . . . . . . . . . . . . 14
   5.  Protocol Sequence  . . . . . . . . . . . . . . . . . . . . . . 15
     5.1.  EAP-EKE-Commit/Request . . . . . . . . . . . . . . . . . . 15
     5.2.  EAP-EKE-Commit/Response  . . . . . . . . . . . . . . . . . 17
     5.3.  EAP-EKE-Confirm/Request  . . . . . . . . . . . . . . . . . 18
     5.4.  EAP-EKE-Confirm/Response . . . . . . . . . . . . . . . . . 18
     5.5.  MSK and EMSK . . . . . . . . . . . . . . . . . . . . . . . 19
   6.  Cryptographic Details  . . . . . . . . . . . . . . . . . . . . 19
     6.1.  Generating Keying Material . . . . . . . . . . . . . . . . 19
     6.2.  Diffie-Hellman Groups  . . . . . . . . . . . . . . . . . . 20
     6.3.  Mandatory Algorithms . . . . . . . . . . . . . . . . . . . 20
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 24
     8.1.  Cryptographic Analysis . . . . . . . . . . . . . . . . . . 27
     8.2.  Diffie-Hellman Group Considerations  . . . . . . . . . . . 28
     8.3.  Resistance to Active Attacks . . . . . . . . . . . . . . . 28
     8.4.  Identity Protection, Anonymity, and Pseudonymity . . . . . 28
     8.5.  Password Processing and Long-Term Storage  . . . . . . . . 29
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 29
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 29
     10.2. Informative References . . . . . . . . . . . . . . . . . . 31

1.  Introduction

   The predominant access method for the Internet today is that of a
   human using a username and password to authenticate to a computer
   enforcing access control.  Proof of knowledge of the password
   authenticates the human to the computer.

   Typically, these passwords are not stored on a user's computer for
   security reasons and must be entered each time the human desires
   network access.  Therefore, the passwords must be ones that can be



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   repeatedly entered by a human with a low probability of error.  They
   will likely not possess high entropy and it may be assumed that an
   adversary with access to a dictionary will have the ability to guess
   a user's password.  It is therefore desirable to have a robust
   authentication method that is secure even when used with a weak
   password in the presence of a strong adversary.

   EAP-EKE is an EAP method [RFC3748] that addresses the problem of
   password-based authenticated key exchange, using a possibly weak
   password for authentication and to derive an authenticated and
   cryptographically strong shared secret.  This problem was first
   described by Bellovin and Merritt in [BM92] and [BM93].
   Subsequently, a number of other solution approaches have been
   proposed, for example [JAB96], [LUC97], [BMP00], and others.

   This proposal is based on the original Encrypted Key Exchange (EKE)
   proposal, as described in [BM92].  Some of the variants of the
   original EKE have been attacked, see e.g., [PA97], and improvements
   have been proposed.  None of these subsequent improvements have been
   incorporated into the current protocol.  However, we have used only
   the subset of [BM92] (namely the variant described in Section 3.1 of
   that paper) that has withstood the test of time and is believed
   secure as of this writing.

2.  Terminology

   This document uses Encr(Ke, ...) to denote encrypted information, and
   Prot(Ke, Ki, ...) to denote encrypted and integrity protected
   information.

   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].

3.  Protocol

   EAP is a two-party protocol spoken between an EAP peer and an EAP
   server (also known as "authenticator").  An EAP method defines the
   specific authentication protocol being used by EAP.  This memo
   defines a particular method and therefore defines the messages sent
   between the EAP server and the EAP peer for the purpose of
   authentication and key derivation.

3.1.  Message Flows

   A successful run of EAP-EKE consists of three message exchanges: an
   Identity exchange, a Commit exchange, and a Confirm exchange.  This
   is shown in Figure 1.



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   The peer and server use the EAP-EKE Identity exchange to learn each
   other's identities and to agree upon a ciphersuite to use in the
   subsequent exchanges.  In the Commit exchange, the peer and server
   exchange information to generate a shared key and also to bind each
   other to a particular guess of the password.  In the Confirm
   exchange, the peer and server prove liveness and knowledge of the
   password by generating and verifying verification data (note that the
   second message of the Commit exchange already plays an essential part
   in this liveness proof).

         +--------+                                     +--------+
         |        |                  EAP-EKE-ID/Request |        |
         |  EAP   |<------------------------------------|  EAP   |
         |  peer  |                                     | server |
         |  (P)   | EAP-EKE-ID/Response                 |   (S)  |
         |        |------------------------------------>|        |
         |        |                                     |        |
         |        |              EAP-EKE-Commit/Request |        |
         |        |<------------------------------------|        |
         |        |                                     |        |
         |        | EAP-EKE-Commit/Response             |        |
         |        |------------------------------------>|        |
         |        |                                     |        |
         |        |             EAP-EKE-Confirm/Request |        |
         |        |<------------------------------------|        |
         |        |                                     |        |
         |        | EAP-EKE-Confirm/Response            |        |
         |        |------------------------------------>|        |
         |        |                                     |        |
         |        |          EAP-Success                |        |
         |        |<------------------------------------|        |
         +--------+                                     +--------+

                Figure 1: A Successful EAP-EKE Exchange

   Schematically, the original exchange as described in [BM92] (and with
   the roles reversed) is:

  Server                              Peer
  ------                              ----

  Encr(Password, y_s) ->

                     <- Encr(Password, y_p), Encr(SharedSecret, Nonce_P)

  Encr(SharedSecret, Nonce_S | Nonce_P) ->

                                          <- Encr(SharedSecret, Nonce_S)



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   Where:

   o  Password is a typically short string, shared between the server
      and the peer.  In other words, the same password is used to
      authenticate the server to the peer, and vice versa.

   o  y_s and y_p are the server's and the peer's, respectively,
      ephemeral public key, i.e., y_s = g ^ x_s (mod p) and
      y_p = g ^ x_p (mod p).

   o  Nonce_S, Nonce_P are random strings generated by the server and
      the peer as cryptographic challenges.

   o  SharedSecret is the secret created by the Diffie-Hellman
      algorithm, namely SharedSecret = g^(x_s * x_p) (mod p).  This
      value is calculated by the server as: SharedSecret = y_p ^ x_s
      (mod p), and by the peer as: SharedSecret = y_s ^ x_p (mod p).

   The current protocol extends the basic cryptographic protocol, and
   the regular successful exchange becomes:

      Message                   Server                       Peer
     ---------                 --------                     ------
   ID/Request         ID_S, CryptoProposals ->

   ID/Response                                 <- ID_P, CryptoSelection

   Commit/Request     Encr(Password, y_s) ->

   Commit/Response        <- Encr(Password, y_p), Prot(Ke, Ki, Nonce_P)

   Confirm/Request    Prot(Ke, Ki, Nonce_S | Nonce_P), Auth_S ->

   Confirm/Response                    <- Prot(Ke, Ki, Nonce_S), Auth_P


   Where, in addition to the above terminology:

   o  Encr means encryption only, and Prot is encryption with integrity
      protection.

   o  Ke is an encryption key, and Ki is an integrity-protection key.

   Section 5 explains the various cryptographic values and how they are
   derived.






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   As shown in the exchange above, the following information elements
   have been added to the original protocol: identity values for both
   protocol parties (ID_S, ID_P), negotiation of cryptographic
   protocols, and signature fields to protect the integrity of the
   negotiated parameters (Auth_S, Auth_P).  In addition, the shared
   secret is not used directly.  In this initial exposition, a few
   details were omitted for clarity.  Section 5 should be considered as
   authoritative regarding message and field details.

4.  Message Formats

   EAP-EKE defines a small number of message types, each message
   consisting of a header followed by a payload.  This section defines
   the header, several payload formats, as well as the format of
   specific fields within the payloads.

   As usual, all multi-octet strings MUST be laid out in network byte
   order.

4.1.  EAP-EKE Header

   The EAP-EKE header consists of the standard EAP header (see Section 4
   of [RFC3748]), followed by an EAP-EKE exchange type.  The header has
   the following structure:

    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      |   EKE-Exch    |              Data            ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 2: EAP-EKE Header

   The Code, Identifier, Length, and Type fields are all part of the EAP
   header as defined in [RFC3748].  The Type field in the EAP header is
   53 for EAP-EKE Version 1.

   The EKE-Exch (EKE Exchange) field identifies the type of EAP-EKE
   payload encapsulated in the Data field.  This document defines the
   following values for the EKE-Exch field:

   o  0x00: Reserved

   o  0x01: EAP-EKE-ID exchange

   o  0x02: EAP-EKE-Commit exchange



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   o  0x03: EAP-EKE-Confirm exchange

   o  0x04: EAP-EKE-Failure message

   Further values of this EKE-Exch field are available via IANA
   registration (Section 7.7).

4.2.  EAP-EKE Payloads

   EAP-EKE messages all contain the EAP-EKE header and information
   encoded in a single payload, which differs for the different
   exchanges.

4.2.1.  The EAP-EKE-ID Payload

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | NumProposals  |   Reserved    |           Proposal           ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ...    Proposal                  |    IDType     |  Identity    ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 3: EAP-EKE-ID Payload

   The EAP-EKE-ID payload contains the following fields:

   NumProposals:

      The NumProposals field contains the number of Proposal fields
      subsequently contained in the payload.  In the EAP-EKE-ID/Request
      message, the NumProposals field MUST NOT be set to zero (0), and
      in the EAP-EKE-ID/Response message, the NumProposals field MUST be
      set to one (1).  The offered proposals in the Request are listed
      contiguously in priority order, most preferable first.  The
      selected proposal in the Response MUST be fully identical with one
      of the offered proposals.

   Reserved:

      This field MUST be sent as zero, and MUST be ignored by the
      recipient.









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   Proposal:

      Each proposal consists of four one-octet fields, in this order:

      Group Description:

         This field's value is taken from the IANA registry for Diffie-
         Hellman groups defined in Section 7.1.

      Encryption:

         This field's value is taken from the IANA registry for
         encryption algorithms defined in Section 7.2.

      PRF:

         This field's value is taken from the IANA registry for pseudo-
         random functions defined in Section 7.3.

      MAC:

         This field's value is taken from the IANA registry for keyed
         message digest algorithms defined in Section 7.4.

   IDType:

      Denotes the Identity Type.  This is taken from the IANA registry
      defined in Section 7.5.  The server and the peer MAY use different
      identity types.  All implementations MUST be able to receive two
      identity types: ID_NAI and ID_FQDN.

   Identity:

      The meaning of the Identity field depends on the values of the
      Code and IDType fields.

      *  EAP-EKE-ID/Request: server ID

      *  EAP-EKE-ID/Response: peer ID

      The length of the Identity field is computed from the Length field
      in the EAP header.  Specifically, its length is

         eap_header_length - 9 - 4 * number_of_proposals.

      This field, like all other fields in this specification, MUST be
      octet-aligned.




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4.2.2.  The EAP-EKE-Commit Payload

   This payload allows both parties to send their encrypted ephemeral
   public key, with the peer also including a Challenge.

   In addition, a small amount of data can be included by the server
   and/or the peer, and used for channel binding.  This data is sent
   here unprotected, but is verified later, when it is signed by the
   Auth_S/Auth_P payloads of the EAP-EKE-Confirm exchange.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         DHComponent_S/DHComponent_P                           ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                                                               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          PNonce_P                                             ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                                                               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          CBValue (zero or more occurrences)                   ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                                                               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 4: EAP-EKE-Commit Payload

   DHComponent_S/DHComponent_P:

      This field contains the password-encrypted Diffie-Hellman public
      key, which is generated as described in Section 5.1.  Its size is
      determined by the group and the encryption algorithm.

   PNonce_P:

      This field only appears in the response, and contains the
      encrypted and integrity-protected challenge value sent by the
      peer.  The field's size is determined by the encryption and MAC
      algorithms being used, since this protocol mandates a fixed nonce
      size for a given choice of algorithms.  See Section 5.2.










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   CBValue:

      This structure MAY be included both in the request and in the
      response, and MAY be repeated multiple times in a single payload.
      See Section 4.5.  Each structure contains its own length.  The
      field is neither encrypted nor integrity protected, instead it is
      protected by the AUTH payloads in the Confirm exchange.

4.2.3.  The EAP-EKE-Confirm Payload

   Using this payload, both parties complete the authentication by
   generating a shared temporary key, authenticating the entire
   protocol, and generating key material for the EAP consumer protocol.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          PNonce_PS/PNonce_S                                   ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                                                               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Auth_S/Auth_P                                        ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                                                               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 5: EAP-EKE-Confirm Payload

   PNonce_PS/PNonce_S:

      This field ("protected nonce") contains the encrypted and
      integrity-protected response to the other party's challenge; see
      Sections 5.3 and 5.4.  Similarly to the PNonce_P field, this
      field's size is determined by the encryption and MAC algorithms.

   Auth_S/Auth_P:

      This field signs the preceding messages, including the Identity
      and the negotiated fields.  This prevents various possible
      attacks, such as algorithm downgrade attacks.  See Section 5.3 and
      Section 5.4.  The size is determined by the pseudo-random function
      negotiated.









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4.2.4.  The EAP-EKE-Failure Payload

   The EAP-EKE-Failure payload format is defined as follows:
    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Failure-Code                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 6: EAP-EKE-Failure Payload

   The payload's size is always exactly 4 octets.

   The following Failure-Code values are defined:

   +------------+----------------+-------------------------------------+
   | Value      | Name           | Meaning                             |
   +------------+----------------+-------------------------------------+
   | 0x00000000 | Reserved       |                                     |
   | 0x00000001 | No Error       | This code is used for failure       |
   |            |                | acknowledgement, see below.         |
   | 0x00000002 | Protocol Error | A failure to parse or understand a  |
   |            |                | protocol message or one of its      |
   |            |                | payloads.                           |
   | 0x00000003 | Password Not   | A password could not be located for |
   |            | Found          | the identity presented by the other |
   |            |                | protocol party, making              |
   |            |                | authentication impossible.          |
   | 0x00000004 | Authentication | Failure in the cryptographic        |
   |            | Failure        | computation, most likely caused by  |
   |            |                | an incorrect password or an         |
   |            |                | inappropriate identity type.        |
   | 0x00000005 | Authorization  | While the password being used is    |
   |            | Failure        | correct, the user is not authorized |
   |            |                | to connect.                         |
   | 0x00000006 | No Proposal    | The peer is unwilling to select any |
   |            | Chosen         | of the cryptographic proposals      |
   |            |                | offered by the server.              |
   +------------+----------------+-------------------------------------+

   Additional values of this field are available via IANA registration,
   see Section 7.8.

   When the peer encounters an error situation, it MUST respond with
   EAP-EKE-Failure.  The server MUST reply with an EAP-Failure message
   to end the exchange.





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   When the server encounters an error situation, it MUST respond with
   EAP-EKE-Failure.  The peer MUST send back an EAP-EKE-Failure message
   containing a "No Error" failure code.  Then the server MUST send an
   EAP-Failure message to end the exchange.

   Implementation of the "Password Not Found" code is not mandatory.
   For security reasons, implementations MAY choose to return
   "Authentication Failure" also in cases where the password cannot be
   located.

4.3.  Protected Fields

   Several fields are encrypted and integrity-protected.  They are
   denoted Prot(...).  Their general structure is as follows:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Initialization Vector (IV) (optional)                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Encrypted Data                         ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                                                               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~               |            Random Padding                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Integrity Check Value (ICV)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 7: Protected Field Structure

   The protected field is a concatenation of three octet strings:

   o  An optional IV, required when the encryption algorithm/mode
      necessitates it, e.g., for CBC encryption.  The content and size
      of this field are determined by the selected encryption algorithm.
      In the case of CBC encryption, this field is a random octet string
      having the same size as the algorithm's block size.

   o  The original data, followed if necessary by random padding.  This
      padding has the minimal length (possibly zero) required to
      complete the length of the encrypted data to the encryption
      algorithm's block size.  The original data and the padding are
      encrypted together.







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   o  ICV, a Message Authentication Code (MAC) cryptographic checksum of
      the encrypted data, including the padding.  The checksum is
      computed over the encrypted, rather than the plaintext, data.  Its
      length is determined by the MAC algorithm negotiated.

   We note that because of the requirement for an explicit ICV, this
   specification does not support authenticated encryption algorithms.
   Such algorithms may be added by a future extension.

4.4.  Encrypted Fields

   Two fields are encrypted but are not integrity protected.  They are
   denoted Encr(...).  Their format is identical to a protected field
   (Section 4.3), except that the Integrity Check Value is omitted.

4.5.  Channel Binding Values

   This protocol allows higher-level protocols to transmit limited
   opaque information between the peer and the server.  This information
   is integrity protected but not encrypted, and may be used to ensure
   that protocol participants are identical at different protocol
   layers.  See Section 7.15 of [RFC3748] for more information on the
   rationale behind this facility.

   EAP-EKE neither validates nor makes any use of the transmitted
   information.  The information MUST NOT be used by the consumer
   protocol until it is verified in the EAP-EKE-Confirm exchange
   (specifically, until it is integrity protected by the Auth_S, Auth_P
   payloads).  Consequently, it MUST NOT be relied upon in case an error
   occurs at the EAP-EKE level.

   An unknown Channel Binding Value SHOULD be ignored by the recipient.

   Some implementations may require certain values to be present, and
   will abort the protocol if they are not.  Such policy is out of scope
   of the current protocol.















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   Each Channel Binding Value is encoded using a TLV structure:

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          CBType               |           Length              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          Value                                               ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 8: Channel Binding Value

   CBType:

      This is the Channel Binding Value's type.  This document defines
      the value 0x0000 as reserved.  Other values are available for IANA
      allocation, see Section 7.6.

   Length:

      This field is the total length in octets of the structure,
      including the CBType and Length fields.

   This facility should be used with care, since EAP-EKE does not
   provide for message fragmentation.  EAP-EKE is not a tunneled method
   and should not be used as a generic transport; specifically,
   implementors should refrain from using the Channel Binding facility
   to transmit posture information, in the sense of [RFC5209].

5.  Protocol Sequence

   This section describes the sequence of messages for the Commit and
   Confirm exchanges, and lists the cryptographic operations performed
   by the server and the peer.

5.1.  EAP-EKE-Commit/Request

   The server computes:

      y_s = g ^ x_s (mod p),

   where x_s is a randomly chosen number in the range 2 .. p-1.  The
   randomly chosen number is the ephemeral private key, and the
   calculated value is the corresponding ephemeral public key.  The
   server and the peer MUST both use a fresh, random value for x_s and
   the corresponding x_p on each run of the protocol.





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   The server computes and transmits the encrypted field (Section 4.4)

      temp = prf(0+, password)

      key = prf+(temp, ID_S | ID_P)

      DHComponent_S = Encr(key, y_s).

   See Section 6.1 for the prf+ notation.  The first argument to "prf"
   is a string of zero octets whose length is the output size of the
   base hash algorithm, e.g., 20 octets for HMAC-SHA1; the result is of
   the same length.  The first output octets of prf+ are used as the
   encryption key for the negotiated encryption algorithm, according to
   that algorithm's key length.

   Since the PRF function is required to be an application of the HMAC
   operator to a hash function, the above construction implements HKDF
   as defined in [RFC5869].

   When using block ciphers, it may be necessary to pad y_s on the
   right, to fit the encryption algorithm's block size.  In such cases,
   random padding MUST be used, and this randomness is critical to the
   security of the protocol.  Randomness recommendations can be found in
   [RFC4086]; also see [NIST.800-90.2007] for additional recommendations
   on cryptographic-level randomness.  When decrypting this field, the
   real length of y_s is determined according to the negotiated Diffie-
   Hellman group.

   If the password needs to be stored on the server, it is RECOMMENDED
   to store a randomized password value as a password-equivalent, rather
   than the cleartext password.  We note that implementations may choose
   the output of either of the two steps of the password derivation.
   Using the output of the second step, where the password is salted by
   the identity values, is more secure; however, it may create an
   operational issue if identities are likely to change.  See also
   Section 8.5.

   This protocol supports internationalized, non-ASCII passwords.  The
   input password string SHOULD be processed according to the rules of
   the [RFC4013] profile of [RFC3454].  A password SHOULD be considered
   a "stored string" per [RFC3454], and unassigned code points are
   therefore prohibited.  The output is the binary representation of the
   processed UTF-8 [RFC3629] character string.  Prohibited output and
   unassigned code points encountered in SASLprep preprocessing SHOULD
   cause a preprocessing failure and the output SHOULD NOT be used.






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5.2.  EAP-EKE-Commit/Response

   The peer computes:

      y_p = g ^ x_p (mod p)

   Then computes:

      temp = prf(0+, password)

      key = prf+(temp, ID_S | ID_P)

      DHComponent_P = Encr(key, y_p)

   formatted as an encrypted field (Section 4.4).

   Both sides calculate

      SharedSecret = prf(0+, g ^ (x_s * x_p) (mod p))

   The first argument to "prf" is a string of zero octets whose length
   is the output size of the base hash algorithm, e.g., 20 octets for
   HMAC-SHA1; the result is of the same length.  This extra application
   of the pseudo-random function is the "extraction step" of [RFC5869].
   Note that the peer needs to compute the SharedSecret value before
   sending out its response.

   The encryption and integrity protection keys are computed:

      Ke | Ki = prf+(SharedSecret, "EAP-EKE Keys" | ID_S | ID_P)

   And the peer generates the Protected Nonce:

      PNonce_P = Prot(Ke, Ki, Nonce_P),

   where Nonce_P is a randomly generated binary string.  The length of
   Nonce_P MUST be the maximum of 16 octets, and half the key size of
   the negotiated prf (rounded up to the next octet if necessary).  The
   peer constructs this value as a protected field (Section 4.3),
   encrypted using Ke and integrity protected using Ki with the
   negotiated encryption and MAC algorithm.

   The peer now sends a message that contains the two generated fields.

   The server MUST verify the correct integrity protection of the
   received nonce, and MUST abort the protocol if it is incorrect, with
   an "Authentication Failure" code.




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5.3.  EAP-EKE-Confirm/Request

   The server constructs:

      PNonce_PS = Prot(Ke, Ki, Nonce_P | Nonce_S),

   as a protected field, where Nonce_S is a randomly generated string,
   of the same size as Nonce_P.

   It computes:

      Ka = prf+(SharedSecret, "EAP-EKE Ka" | ID_S | ID_P | Nonce_P |
      Nonce_S)

   whose length is the preferred key length of the negotiated prf (see
   Section 5.2).  It then constructs:

      Auth_S = prf(Ka, "EAP-EKE server" | EAP-EKE-ID/Request | EAP-EKE-
      ID/Response | EAP-EKE-Commit/Request | EAP-EKE-Commit/Response).

   The messages are included in full, starting with the EAP header, and
   including any possible future extensions.

   This construction of the Auth_S (and Auth_P) value implies that any
   future extensions MUST NOT be added to the EAP-EKE-Confirm/Request or
   EAP-EKE-Confirm/Response messages themselves, unless these extensions
   are integrity-protected in some other manner.

   The server now sends a message that contains the two fields.

   The peer MUST verify the correct integrity protection of the received
   nonces and the correctness of the Auth_S value, and MUST abort the
   protocol if either is incorrect, with an "Authentication Failure"
   code.

5.4.  EAP-EKE-Confirm/Response

   The peer computes Ka, and generates:

      PNonce_S = Prot(Ke, Ki, Nonce_S)

   as a protected field.  It then computes:

      Auth_P = prf(Ka, "EAP-EKE peer" | EAP-EKE-ID/Request | EAP-EKE-ID/
      Response | EAP-EKE-Commit/Request | EAP-EKE-Commit/Response)

   The peer sends a message that contains the two fields.




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   The server MUST verify the correct integrity protection of the
   received nonce and the correctness of the Auth_P value, and MUST
   abort the protocol if either is incorrect, with an "Authentication
   Failure" code.

5.5.  MSK and EMSK

   Following the last message of the protocol, both sides compute and
   export the shared keys, each 64 bytes in length:

      MSK | EMSK = prf+(SharedSecret, "EAP-EKE Exported Keys" | ID_S |
      ID_P | Nonce_P | Nonce_S)

   When the RADIUS attributes specified in [RFC2548] are used to
   transport keying material, then the first 32 bytes of the MSK
   correspond to MS-MPPE-RECV-KEY and the second 32 bytes to MS-MPPE-
   SEND-KEY.  In this case, only 64 bytes of keying material (the MSK)
   are used.

   At this point, both protocol participants MUST discard all
   intermediate cryptographic values, including x_p, x_s, y_p, y_s, Ke,
   Ki, Ka, and SharedSecret.  Similarly, both parties MUST immediately
   discard these values whenever the protocol terminates with a failure
   code or as a result of timeout.

6.  Cryptographic Details

6.1.  Generating Keying Material

   Keying material is derived as the output of the negotiated pseudo-
   random function (prf) algorithm.  Since the amount of keying material
   needed may be greater than the size of the output of the prf
   algorithm, we will use the prf iteratively.  We denote by "prf+" the
   function that outputs a pseudo-random stream based on the inputs to a
   prf as follows (where "|" indicates concatenation):

      prf+ (K, S) = T1 | T2 | T3 | T4 | ...

   where:

      T1 = prf(K, S | 0x01)

      T2 = prf(K, T1 | S | 0x02)

      T3 = prf(K, T2 | S | 0x03)

      T4 = prf(K, T3 | S | 0x04)




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   continuing as needed to compute all required keys.  The keys are
   taken from the output string without regard to boundaries (e.g., if
   the required keys are a 256-bit Advanced Encryption Standard (AES)
   key and a 160-bit HMAC key, and the prf function generates 160 bits,
   the AES key will come from T1 and the beginning of T2, while the HMAC
   key will come from the rest of T2 and the beginning of T3).

   The constant concatenated to the end of each string feeding the prf
   is a single octet.  In this document, prf+ is not defined beyond 255
   times the size of the prf output.

6.2.  Diffie-Hellman Groups

   Many of the commonly used Diffie-Hellman groups are inappropriate for
   use in EKE.  Most of these groups use a generator that is not a
   primitive element of the group.  As a result, an attacker running a
   dictionary attack would be able to learn at least 1 bit of
   information for each decrypted password guess.

   Any MODP Diffie-Hellman group defined for use in this protocol MUST
   have the following properties to ensure that it does not leak a
   usable amount of information about the password:

   1.  The generator is a primitive element of the group.

   2.  The most significant 64 bits of the prime number are 1.

   3.  The group's order p is a "safe prime", i.e., (p-1)/2 is also
       prime.

   The last requirement is related to the strength of the Diffie-Hellman
   algorithm, rather than the password encryption.  It also makes it
   easy to verify that the generator is primitive.

   Suitable groups are defined in Section 7.1.

6.3.  Mandatory Algorithms

   To facilitate interoperability, the following algorithms are
   mandatory to implement:

   o  ENCR_AES128_CBC (encryption algorithm)

   o  PRF_HMAC_SHA1 (pseudo-random function)

   o  MAC_HMAC_SHA1 (keyed message digest)

   o  DHGROUP_EKE_14 (DH-group)



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7.  IANA Considerations

   IANA has allocated the EAP method type 53 from the range 1-191, for
   "EAP-EKE Version 1".

   Per this document, IANA created the registries described in the
   following sub-sections.  Values (other than private-use ones) can be
   added to these registries per Specification Required [RFC5226], with
   two exceptions: the Exchange and Failure Code registries can only be
   extended per RFC Required [RFC5226].

7.1.  Diffie-Hellman Group Registry

   This section defines an IANA registry for Diffie-Hellman groups.

   This table defines the initial contents of this registry.  The Value
   column is used when negotiating the group.  Additional groups may be
   defined through IANA allocation.  Any future specification that
   defines a non-MODP group MUST specify its use within EAP-EKE and MUST
   demonstrate the group's security in this context.

   +-----------------+---------+---------------------------------------+
   | Name            | Value   | Description                           |
   +-----------------+---------+---------------------------------------+
   | Reserved        | 0       |                                       |
   | DHGROUP_EKE_2   | 1       | The prime number of the 1024-bit      |
   |                 |         | Group 2 [RFC5996], with the generator |
   |                 |         | 5 (decimal)                           |
   | DHGROUP_EKE_5   | 2       | The prime number of the 1536-bit      |
   |                 |         | Group 5 [RFC3526], g=31               |
   | DHGROUP_EKE_14  | 3       | The prime number of the 2048-bit      |
   |                 |         | Group 14 [RFC3526], g=11              |
   | DHGROUP_EKE_15  | 4       | The prime number of the 3072-bit      |
   |                 |         | Group 15 [RFC3526], g=5               |
   | DHGROUP_EKE_16  | 5       | The prime number of the 4096-bit      |
   |                 |         | Group 16 [RFC3526], g=5               |
   | Available for   | 6-127   |                                       |
   | allocation via  |         |                                       |
   | IANA            |         |                                       |
   | Reserved for    | 128-255 |                                       |
   | Private Use     |         |                                       |
   +-----------------+---------+---------------------------------------+









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7.2.  Encryption Algorithm Registry

   This section defines an IANA registry for encryption algorithms:

     +-----------------+---------+-----------------------------------+
     | Name            | Value   | Definition                        |
     +-----------------+---------+-----------------------------------+
     | Reserved        | 0       |                                   |
     | ENCR_AES128_CBC | 1       | AES with a 128-bit key, CBC mode  |
     |                 | 2-127   | Available for allocation via IANA |
     |                 | 128-255 | Reserved for Private Use          |
     +-----------------+---------+-----------------------------------+

7.3.  Pseudo-Random Function Registry

   This section defines an IANA registry for pseudo-random function
   algorithms:

   +-------------------+---------+-------------------------------------+
   | Name              | Value   | Definition                          |
   +-------------------+---------+-------------------------------------+
   | Reserved          | 0       |                                     |
   | PRF_HMAC_SHA1     | 1       | HMAC SHA-1, as defined in [RFC2104] |
   | PRF_HMAC_SHA2_256 | 2       | HMAC SHA-2-256 [SHA]                |
   |                   | 3-127   | Available for allocation via IANA   |
   |                   | 128-255 | Reserved for Private Use            |
   +-------------------+---------+-------------------------------------+

   A pseudo-random function takes two parameters K and S (the key and
   input string respectively), and, to be usable in this protocol, must
   be defined for all lengths of K between 0 and 65,535 bits
   (inclusive).

   Any future pseudo-random function MUST be based on the HMAC
   construct, since the security of HKDF is only known for such
   functions.















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7.4.  Keyed Message Digest (MAC) Registry

   This section defines an IANA registry for keyed message digest
   algorithms:

   +-------------------+---------+--------------+----------------------+
   | Name              | Value   | Key Length   | Definition           |
   |                   |         | (Octets)     |                      |
   +-------------------+---------+--------------+----------------------+
   | Reserved          | 0       |              |                      |
   | MAC_HMAC_SHA1     | 1       | 20           | HMAC SHA-1, as       |
   |                   |         |              | defined in [RFC2104] |
   | MAC_HMAC_SHA2_256 | 2       | 32           | HMAC SHA-2-256       |
   | Reserved          | 3-127   |              | Available for        |
   |                   |         |              | allocation via IANA  |
   | Reserved          | 128-255 |              | Reserved for Private |
   |                   |         |              | Use                  |
   +-------------------+---------+--------------+----------------------+

7.5.  Identity Type Registry

   This section defines an IANA registry for identity types:

   +-----------+---------+---------------------------------------------+
   | Name      | Value   | Definition                                  |
   +-----------+---------+---------------------------------------------+
   | Reserved  | 0       |                                             |
   | ID_OPAQUE | 1       | An opaque octet string                      |
   | ID_NAI    | 2       | A Network Access Identifier, as defined in  |
   |           |         | [RFC4282]                                   |
   | ID_IPv4   | 3       | An IPv4 address, in binary format           |
   | ID_IPv6   | 4       | An IPv6 address, in binary format           |
   | ID_FQDN   | 5       | A fully qualified domain name, see note     |
   |           |         | below                                       |
   | ID_DN     | 6       | An LDAP Distinguished Name formatted as a   |
   |           |         | string, as defined in [RFC4514]             |
   |           | 7-127   | Available for allocation via IANA           |
   |           | 128-255 | Reserved for Private Use                    |
   +-----------+---------+---------------------------------------------+

   An example of an ID_FQDN is "example.com".  The string MUST NOT
   contain any terminators (e.g., NULL, CR, etc.).  All characters in
   the ID_FQDN are ASCII; for an internationalized domain name, the
   syntax is as defined in [RFC5891], for example
   "xn--tmonesimerkki-bfbb.example.net".






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7.6.  EAP-EKE Channel Binding Type Registry

   This section defines an IANA registry for the Channel Binding Type
   registry, a 16-bit long code.  The value 0x0000 has been defined as
   Reserved.  All other values up to and including 0xfeff are available
   for allocation via IANA.  The remaining values up to and including
   0xffff are available for Private Use.

7.7.  Exchange Registry

   This section defines an IANA registry for the EAP-EKE Exchange
   registry, an 8-bit long code.  Initial values are defined in
   Section 4.1.  All values up to and including 0x7f are available for
   allocation via IANA.  The remaining values up to and including 0xff
   are available for private use.

7.8.  Failure-Code Registry

   This section defines an IANA registry for the Failure-Code registry,
   a 32-bit long code.  Initial values are defined in Section 4.2.4.
   All values up to and including 0xfeffffff are available for
   allocation via IANA.  The remaining values up to and including
   0xffffffff are available for private use.

8.  Security Considerations

   Any protocol that claims to solve the problem of password-
   authenticated key exchange must be resistant to active, passive, and
   dictionary attack and have the quality of forward secrecy.  These
   characteristics are discussed further in the following paragraphs.

   Resistance to Passive Attack:  A passive attacker is one that merely
      relays messages back and forth between the peer and server,
      faithfully, and without modification.  The contents of the
      messages are available for inspection, but that is all.  To
      achieve resistance to passive attack, such an attacker must not be
      able to obtain any information about the password or anything
      about the resulting shared secret from watching repeated runs of
      the protocol.  Even if a passive attacker is able to learn the
      password, she will not be able to determine any information about
      the resulting secret shared by the peer and server.

   Resistance to Active Attack:  An active attacker is able to modify,
      add, delete, and replay messages sent between protocol
      participants.  For this protocol to be resistant to active attack,
      the attacker must not be able to obtain any information about the
      password or the shared secret by using any of its capabilities.
      In addition, the attacker must not be able to fool a protocol



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      participant into thinking that the protocol completed
      successfully.  It is always possible for an active attacker to
      deny delivery of a message critical in completing the exchange.
      This is no different than dropping all messages and is not an
      attack against the protocol.

   Resistance to Dictionary Attack:  For this protocol to be resistant
      to dictionary attack, any advantage an adversary can gain must be
      directly related to the number of interactions she makes with an
      honest protocol participant and not through computation.  The
      adversary will not be able to obtain any information about the
      password except whether a single guess from a single protocol run
      is correct or incorrect.

   Forward Secrecy:  Compromise of the password must not provide any
      information about the secrets generated by earlier runs of the
      protocol.

   [RFC3748] requires that documents describing new EAP methods clearly
   articulate the security properties of the method.  In addition, for
   use with wireless LANs, [RFC4017] mandates and recommends several of
   these.  The claims are:

   1.  Mechanism: password.

   2.  Claims:

       *  Mutual authentication: the peer and server both authenticate
          each other by proving possession of a shared password.  This
          is REQUIRED by [RFC4017].

       *  Forward secrecy: compromise of the password does not reveal
          the secret keys (MSK and EMSK) from earlier runs of the
          protocol.

       *  Replay protection: an attacker is unable to replay messages
          from a previous exchange either to learn the password or a key
          derived by the exchange.  Similarly, the attacker is unable to
          induce either the peer or server to believe the exchange has
          successfully completed when it hasn't.

       *  Key derivation: a shared secret is derived by performing a
          group operation in a finite cyclic group (e.g.,
          exponentiation) using secret data contributed by both the peer
          and server.  An MSK and EMSK are derived from that shared
          secret.  This is REQUIRED by [RFC4017].





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       *  Dictionary attack resistance: an attacker can only make one
          password guess per active attack, and the protocol is designed
          so that the attacker does not gain any confirmation of her
          guess by observing the decrypted y_s or y_p value (see below).
          The advantage she can gain is through interaction not through
          computation.  This is REQUIRED by [RFC4017].

       *  Session independence: this protocol is resistant to active and
          passive attacks and does not enable compromise of subsequent
          or prior MSKs or EMSKs from either passive or active attacks.

       *  Denial-of-service resistance: it is possible for an attacker
          to cause a server to allocate state and consume CPU.  Such an
          attack is gated, though, by the requirement that the attacker
          first obtain connectivity through a lower-layer protocol
          (e.g., 802.11 authentication followed by 802.11 association,
          or 802.3 "link-up") and respond to two EAP messages: the
          EAP-ID/Request and the EAP-EKE-ID/Request.

       *  Man-in-the-Middle Attack resistance: this exchange is
          resistant to active attack, which is a requirement for
          launching a man-in-the-middle attack.  This is REQUIRED by
          [RFC4017].

       *  Shared state equivalence: upon completion of EAP-EKE, the peer
          and server both agree on the MSK and EMSK values.  The peer
          has authenticated the server based on the Server_ID and the
          server has authenticated the peer based on the Peer_ID.  This
          is due to the fact that Peer_ID, Server_ID, and the generated
          shared secret are all combined to make the authentication
          element that must be shared between the peer and server for
          the exchange to complete.  This is REQUIRED by [RFC4017].

       *  Fragmentation: this protocol does not define a technique for
          fragmentation and reassembly.

       *  Resistance to "Denning-Sacco" attack: learning keys
          distributed from an earlier run of the protocol, such as the
          MSK or EMSK, will not help an adversary learn the password.

   3.  Key strength: the strength of the resulting key depends on the
       finite cyclic group chosen.  Sufficient key strength is REQUIRED
       by [RFC4017].  Clearly, "sufficient" strength varies over time,
       depending on computation power assumed to be available to
       potential attackers.






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   4.  Key hierarchy: MSKs and EMSKs are derived from the secret values
       generated during the protocol run, using a negotiated pseudo-
       random function.

   5.  Vulnerabilities (note that none of these are REQUIRED by
       [RFC4017]):

       *  Protected ciphersuite negotiation: the ciphersuite proposal
          made by the server is not protected from tampering by an
          active attacker.  However, if a proposal was modified by an
          active attacker, it would result in a failure to confirm the
          message sent by the other party, since the proposal is bound
          by each side into its Confirm message, and the protocol would
          fail as a result.  Note that this assumes that none of the
          proposed ciphersuites enables an attacker to perform real-time
          cryptanalysis.

       *  Confidentiality: none of the messages sent in this protocol
          are encrypted, though many of the protocol fields are.

       *  Integrity protection: protocol messages are not directly
          integrity protected; however, the ID and Commit exchanges are
          integrity protected through the Auth payloads exchanged in the
          Confirm exchange.

       *  Channel binding: this protocol enables the exchange of
          integrity-protected channel information that can be compared
          with values communicated via out-of-band mechanisms.

       *  Fast reconnect: this protocol does not provide a fast
          reconnect capability.

       *  Cryptographic binding: this protocol is not a tunneled EAP
          method and therefore has no cryptographic information to bind.

       *  Identity protection: the EAP-EKE-ID exchange is not protected.
          An attacker will see the server's identity in the EAP-EKE-ID/
          Request and see the peer's identity in EAP-EKE-ID/Response.
          See also Section 8.4.

8.1.  Cryptographic Analysis

   When analyzing the Commit exchange, it should be noted that the base
   security assumptions are different from "normal" cryptology.
   Normally, we assume that the key has strong security properties, and
   that the data may have few or none.  Here, we assume that the key has
   weak security properties (the attacker may have a list of possible
   keys), and hence we need to ensure that the data has strong



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   properties (indistinguishable from random).  This difference may mean
   that conventional wisdom in cryptology might not apply in this case.
   This also imposes severe constraints on the protocol, e.g., the
   mandatory use of random padding and the need to define specific
   finite groups.

8.2.  Diffie-Hellman Group Considerations

   It is fundamental to the dictionary attack resistance that the
   Diffie-Hellman public values y_s and y_p are indistinguishable from a
   random string.  If this condition is not met, then a passive attacker
   can do trial-decryption of the encrypted DHComponent_P or
   DHComponent_S values based on a password guess, and if they decrypt
   to a value that is not a valid public value, they know that the
   password guess was incorrect.

   For MODP groups, Section 6.2 gives conditions on the group to make
   sure that this criterion is met.  For other groups (for example,
   Elliptic Curve groups), some other means of ensuring this must be
   employed.  The standard way of expressing Elliptic Curve public
   values does not meet this criterion, as a valid Elliptic Curve X
   coordinate can be distinguished from a random string with probability
   of approximately 0.5.

   A future document might introduce a group representation, and/or a
   slight modification of the password encryption scheme, so that
   Elliptic Curve groups can be accommodated.  [BR02] presents several
   alternative solutions for this problem.

8.3.  Resistance to Active Attacks

   An attacker, impersonating either the peer or the server, can always
   try to enumerate all possible passwords, for example by using a
   dictionary.  To counter this likely attack vector, both peer and
   server MUST implement rate-limiting mechanisms.  We note that locking
   out the other party after a small number of tries would create a
   trivial denial-of-service opportunity.

8.4.  Identity Protection, Anonymity, and Pseudonymity

   By default, the EAP-EKE-ID exchange is unprotected, and an
   eavesdropper can observe both parties' identities.  A future
   extension of this protocol may support anonymity, e.g., by allowing
   the server to send a temporary identity to the peer at the end of the
   exchange, so that the peer can use that identity in subsequent
   exchanges.





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   EAP-EKE differs in this respect from tunneled methods, which
   typically provide unconditional identity protection to the peer by
   encrypting the identity exchange, but reveal information in the
   server certificate.  It is possible to use EAP-EKE as the inner
   method in a tunneled EAP method in order to achieve this level of
   identity protection.

8.5.  Password Processing and Long-Term Storage

   This document recommends that a password-equivalent (a hash of the
   password) be stored instead of the cleartext password.  While this
   solution provides a measure of security, there are also tradeoffs
   related to algorithm agility:

   o  Each stored password must identify the hash function that was used
      to compute the stored value.

   o  Complex deployments and migration scenarios might necessitate
      multiple stored passwords, one per each algorithm.

   o  Changing the algorithm can require, in some cases, that the users
      manually change their passwords.

   The reader is referred to Section 10 of [RFC3629] for security
   considerations related to the parsing and processing of UTF-8
   strings.

9.  Acknowledgements

   Much of this document was unashamedly picked from [RFC5931] and
   [EAP-SRP], and we would like to acknowledge the authors of these
   documents: Dan Harkins, Glen Zorn, James Carlson, Bernard Aboba, and
   Henry Haverinen.  We would like to thank David Jacobson, Steve
   Bellovin, Russ Housley, Brian Weis, Dan Harkins, and Alexey Melnikov
   for their useful comments.  Lidar Herooty and Idan Ofrat implemented
   this protocol and helped us improve it by asking the right questions,
   and we would like to thank them both.

10.  References

10.1.  Normative References

   [RFC2104]           Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
                       Keyed-Hashing for Message Authentication",
                       RFC 2104, February 1997.






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

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

   [RFC3454]           Hoffman, P. and M. Blanchet, "Preparation of
                       Internationalized Strings ("stringprep")",
                       RFC 3454, December 2002.

   [RFC3526]           Kivinen, T. and M. Kojo, "More Modular
                       Exponential (MODP) Diffie-Hellman groups for
                       Internet Key Exchange (IKE)", RFC 3526, May 2003.

   [RFC3629]           Yergeau, F., "UTF-8, a transformation format of
                       ISO 10646", STD 63, RFC 3629, November 2003.

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

   [RFC4013]           Zeilenga, K., "SASLprep: Stringprep Profile for
                       User Names and Passwords", RFC 4013,
                       February 2005.

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

   [RFC4514]           Zeilenga, K., "Lightweight Directory Access
                       Protocol (LDAP): String Representation of
                       Distinguished Names", RFC 4514, June 2006.

   [RFC5891]           Klensin, J., "Internationalized Domain Names in
                       Applications (IDNA): Protocol", RFC 5891,
                       August 2010.

   [RFC5996]           Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
                       "Internet Key Exchange Protocol Version 2
                       (IKEv2)", RFC 5996, September 2010.

   [SHA]               National Institute of Standards and Technology,
                       U.S. Department of Commerce, "Secure Hash
                       Standard", NIST FIPS 180-3, October 2008.






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10.2.  Informative References

   [BM92]              Bellovin, S. and M. Merritt, "Encrypted Key
                       Exchange: Password-Based Protocols Secure Against
                       Dictionary Attacks", Proc. IEEE Symp. on Research
                       in Security and Privacy , May 1992.

   [BM93]              Bellovin, S. and M. Merritt, "Augmented Encrypted
                       Key Exchange: A Password-Based Protocol Secure
                       against Dictionary Attacks and Password File
                       Compromise", Proc. 1st ACM Conference on Computer
                       and Communication Security , 1993.

   [BMP00]             Boyko, V., MacKenzie, P., and S. Patel, "Provably
                       Secure Password Authenticated Key Exchange Using
                       Diffie-Hellman", Advances in Cryptology,
                       EUROCRYPT 2000 , 2000.

   [BR02]              Black, J. and P. Rogaway, "Ciphers with Arbitrary
                       Finite Domains", Proc. of the RSA Cryptographer's
                       Track (RSA CT '02), LNCS 2271 , 2002.

   [EAP-SRP]           Carlson, J., Aboba, B., and H. Haverinen, "EAP
                       SRP-SHA1 Authentication Protocol", Work
                       in Progress, July 2001.

   [JAB96]             Jablon, D., "Strong Password-Only Authenticated
                       Key Exchange", ACM Computer Communications
                       Review Volume 1, Issue 5, October 1996.

   [LUC97]             Lucks, S., "Open Key Exchange: How to Defeat
                       Dictionary Attacks Without Encrypting Public
                       Keys", Proc. of the Security Protocols
                       Workshop LNCS 1361, 1997.

   [NIST.800-90.2007]  National Institute of Standards and Technology,
                       "Recommendation for Random Number Generation
                       Using Deterministic Random Bit Generators
                       (Revised)", NIST SP 800-90, March 2007.

   [PA97]              Patel, S., "Number Theoretic Attacks On Secure
                       Password Schemes", Proceedings of the 1997 IEEE
                       Symposium on Security and Privacy , 1997.

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



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   [RFC4086]           Eastlake, D., Schiller, J., and S. Crocker,
                       "Randomness Requirements for Security", BCP 106,
                       RFC 4086, June 2005.

   [RFC5209]           Sangster, P., Khosravi, H., Mani, M., Narayan,
                       K., and J. Tardo, "Network Endpoint Assessment
                       (NEA): Overview and Requirements", RFC 5209,
                       June 2008.

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

   [RFC5869]           Krawczyk, H. and P. Eronen, "HMAC-based Extract-
                       and-Expand Key Derivation Function (HKDF)",
                       RFC 5869, May 2010.

   [RFC5931]           Harkins, D. and G. Zorn, "Extensible
                       Authentication Protocol (EAP) Authentication
                       Using Only a Password", RFC 5931, August 2010.































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Authors' Addresses

   Yaron Sheffer
   Independent

   EMail: yaronf.ietf@gmail.com


   Glen Zorn
   Network Zen
   227/358 Thanon Sanphawut
   Bang Na, Bangkok  10260
   Thailand

   Phone: +66 (0) 87-040-4617
   EMail: gwz@net-zen.net


   Hannes Tschofenig
   Nokia Siemens Networks
   Linnoitustie 6
   Espoo  02600
   Finland

   Phone: +358 (50) 4871445
   EMail: Hannes.Tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at


   Scott Fluhrer
   Cisco Systems.
   1414 Massachusetts Ave.
   Boxborough, MA  01719
   USA

   EMail: sfluhrer@cisco.com















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