Network Working Group                                         G.M. Meyer
Internet Draft                                            Spider Systems
Expires Jun 17, 1995                                            Dec 1994

               The PPP Encryption Control Protocol (ECP)

Status of this Memo

   This document is a submission to the Point-to-Point Protocol Working
   Group of the Internet Engineering Task Force (IETF).  Comments should
   be submitted to the mailing list.

   Distribution of this memo is unlimited.

   This document is an Internet-Draft.  Internet Drafts are working
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   The Point-to-Point Protocol (PPP) [1] provides a standard method for
   transporting multi-protocol datagrams over point-to-point links.  PPP
   also defines an extensible Link Control Protocol.

   This document defines a method for negotiating data encryption over
   PPP links.


   The following language conventions are used in the items of
   specification in this document:

   o  MUST -- the item is an absolute requirement of the specification.

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      MUST is only used where it is actually required for interopera-
      tion, not to try to impose a particular method on implementors
      where not required for interoperability.

   o  SHOULD -- the item should be followed for all but exceptional cir-

   o  MAY or optional -- the item is truly optional and may be followed
      or ignored according to the needs of the implementor.

      The words "should" and "may" are also used, in lower case, in
      their more ordinary senses.

                          Table of Contents

      1. Introduction ...........................................  2

      2. Encryption Control Protocol (ECP) ......................  3
          2.1 Sending Encrypted Datagrams .......................  4

      3. Additional Packets .....................................  5
          3.1 Reset-Request and Reset-Ack .......................  5

      4. ECP Configuration Options ..............................  6
          4.1 Proprietary Encryption OUI ........................  7
          4.2 Other Encryption Types ............................  9

      5. Security Considerations ................................  9

1. Introduction

   In order to establish communications over a PPP link, each end of the
   link must first send LCP packets to configure and test the data link
   during Link Establishment phase.  After the link has been esta-
   blished, optional facilities may be negotiated as needed.

   One such facility is data encryption.  A wide variety of encryption
   methods may be negotiated, although typically only one method is used
   in each direction of the link.

   A different encryption algorithm may be negotiated in each direction,
   for speed, cost, memory or other considerations.

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2. Encryption Control Protocol (ECP)

   The Encryption Control Protocol (ECP) is responsible for configuring
   and enabling data encryption algorithms on both ends of the point-
   to-point link.

   ECP uses the same packet exchange mechanism as the Link Control Pro-
   tocol (LCP).  ECP packets may not be exchanged until PPP has reached
   the Network-Layer Protocol phase.  ECP packets received before this
   phase is reached should be silently discarded.

   The Encryption Control Protocol is exactly the same as the Link Con-
   trol Protocol [1] with the following exceptions:

      Frame Modifications

         The packet may utilise any modifications to the basic frame
         format which have been negotiated during the Link Establishment

      Data Link Layer Protocol Field

         Exactly one ECP packet is encapsulated in the PPP Information
         field, where the PPP Protocol field indicates type hex 8053
         (Encryption Control Protocol).

         When individual link data encryption is used in a multiple link
         connection to a single destination [2], the PPP Protocol field
         indicates type hex 8055 (Individual link Encryption Control

      Code field

         ECP uses codes 1 through 7 (Configure-Request, Configure-Ack,
         Configure-Nak, Configure-Reject, Terminate-Request, Terminate-
         Ack and Code-Reject), code 14 (Reset-Request) and code 15
         (Reset-Ack).  Other Codes should be treated as unrecognised and
         should result in Code-Rejects.


         ECP packets may not be exchanged until PPP has reached the
         Network-Layer Protocol phase.  An implementation should be
         prepared to wait for Authentication and Link Quality Determina-
         tion to finish before timing out waiting for a Configure-Ack or
         other response.

         An implementation MUST NOT transmit data until ECP negotiation

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         has completed successfully.  And if ECP negotiation is not suc-
         cessful the link MUST be brought down.

       Configuration Option Types

         ECP has a distinct set of Configuration Options.

2.1 Sending Encrypted Datagrams

   Before any encrypted packets may be communicated, PPP must reach the
   Network-Layer Protocol phase, and the Encryption Control Protocol
   must reach the Opened state.

   An encrypted packet is encapsulated in the PPP Information field,
   where the PPP Protocol field indicates type hex 0053 (Encrypted

   When using multiple PPP links to a single destination [2], there are
   two methods of employing data encryption.  The first method is to
   encrypt the data prior to sending it out through the multiple links.
   The second is to treat each link as a separate connection, that may
   or may not have encryption enabled.  In the second case, the PPP Pro-
   tocol field MUST be type hex 0055 (Individual link encrypted

   Only one primary algorithm in each direction is in use at a time, and
   that is negotiated prior to sending the first encrypted frame.  The
   PPP Protocol field of the encrypted datagram indicates that the frame
   is encrypted, but not the algorithm with which it was encrypted.

   The maximum length of an encrypted packet transmitted over a PPP link
   is the same as the maximum length of the Information field of a PPP
   encapsulated packet.  If the encryption algorithm is likely to
   increase the size of the message beyond that, multilink should also
   be negotiated to allow fragmentation of the frames (even if only
   using a single link).

   If the encryption algorithm carries history between frames, the
   encryption algorithm must supply a way of determining if it is pass-
   ing data reliably, or it must require the use of a reliable transport
   such as LAPB [3].

   If both compression and encryption have been negotiated, compression
   MUST be performed on the data prior to encryption.  It is explicitly
   stated here to aid interoperability.  Performing them in this order
   should maximise the effect of compression.  Truly encrypted data is
   unlikely to be compressible.

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3. Additional Packets

   The Packet format and basic facilities are already defined for LCP

   Up-to-date values of the ECP Code field are specified in the most
   recent "Assigned Numbers" RFC [4].  This specification concerns the
   following values:

         14      Reset-Request
         15      Reset-Ack

3.1 Reset-Request and Reset-Ack


      ECP includes Reset-Request and Reset-Ack Codes in order to provide
      a mechanism for indicating a decryption failure in one direction
      of a decrypted link without affecting traffic in the other direc-
      tion.  Individual algorithms need to specify a mechanism for
      determining how to detect a decryption failure.  Some algorithms
      may not require this facility.

      On initial detection of a decryption failure, an ECP implementa-
      tion SHOULD transmit an ECP packet with the Code field set to 14
      (Reset-Request).  The Data field may be filled with any desired

      Once a Reset-Request has been sent, any decrypted packets received
      are discarded.  Further Reset-Requests MAY be sent with the same
      Identifier, until a valid Reset-Ack is received.

      When the link is busy, one decryption error is usually followed by
      several more before the Reset-Ack can be received.  It is undesir-
      able to transmit Reset-Requests more frequently than the round-
      trip-time of the link, since this will result in redundant Reset-
      Requests and Reset-Acks being transmitted and processed.  The
      receiver MAY elect to limit transmission of Reset-Requests (to say
      one per second) while a Reset-Ack is outstanding.

      Upon reception of a Reset-Request, the transmitting encrypter is
      reset to an initial state.  An ECP packet MUST be transmitted with
      the Code field set to 15 (Reset-Ack), the Identifier field copied
      from the Reset-Request packet, and the Data field filled with any
      desired data.

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      On receipt of a Reset-Ack, the receiving decrypter is reset to an
      initial state.  Since there may be several Reset-Acks in the pipe,
      the decrypter MUST be reset for each Reset-Ack which matches the
      currently expected identifier.

      A summary of the Reset-Request and Reset-Ack packet formats is
      shown below.  The fields are transmitted from left to right.

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


      14 for Reset-Request;

      15 for Reset-Ack.


      On transmission, the Identifier field MUST be changed whenever the
      content of the Data field changes, and whenever a valid reply has
      been received for a previous request.  For retransmissions, the
      Identifier MAY remain unchanged.

      On reception, the Identifier field of the Reset-Request is copied
      into the Identifier field of the Reset-Ack packet.


      The Data field is zero or more octets and contains uninterpreted
      data for use by the sender.  The data may consist of any binary
      value and may be of any length from zero to the peer's established
      MRU minus four.

4. ECP Configuration Options

   ECP Configuration Options allow negotiation of encryption algorithms
   and their parameters.  ECP uses the same Configuration Option format
   defined for LCP [1], with a separate set of Options.

   Configuration Options, in this protocol, indicate algorithms that the

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   receiver is willing or able to use to decrypt data sent by the
   sender.  Systems may offer to accept several algorithms, and nego-
   tiate a single one that will be used.

   There is the possibility of not being able to agree on an encryption
   algorithm.  In that case the link MUST be brought down.

   We expect that many vendors will want to use proprietary encryption
   algorithms, and have made a mechanism available to negotiate these
   without encumbering the Internet Assigned Number Authority with
   proprietary number requests.

   The LCP option negotiation techniques are used.  If an option is
   unrecognised, a Configure-Reject MUST be sent.  If all protocols the
   sender implements are Configure-Rejected by the receiver the link
   MUST be brought down.

   If an option is recognised, but not acceptable due to values in the
   request (or optional parameters not in the request), a Configure-Nak
   MUST be sent with the option modified appropriately.  The Configure-
   Nak MUST contain only those options that will be acceptable.  A new
   Configure-Request SHOULD be sent with only the single preferred
   option, adjusted as specified in the Configure-Nak.

   Up-to-date values of the ECP Option Type field are specified in the
   most recent "Assigned Numbers" RFC [4].  Current values are assigned
   as follows:

         ECP Option      Encryption type

         0               OUI

4.1 Proprietary Encryption OUI


      This Configuration Option provides a way to negotiate the use of a
      proprietary encryption protocol.

      Since the first matching encryption will be used, it is recom-
      mended that any known OUI encryption options be transmitted first,
      before the common options are used.

      Before accepting this option, the implementation must verify that
      the Organisation Unique Identifier identifies a proprietary algo-
      rithm that the implementation can decrypt, and that any vendor

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      specific negotiation values are fully understood.

      A summary of the Proprietary Encryption OUI Configuration Option
      format is shown below.  The fields are transmitted from left to

          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
         |     Type      |    Length     |       OUI ...
               OUI       |    Subtype    |  Values...




      >= 6


      The vendor's IEEE Organisation Unique Identifier (OUI), which is
      the most significant three octets of an Ethernet Physical Address,
      assigned to the vendor by IEEE 802.  This identifies the option as
      being proprietary to the indicated vendor.  The bits within the
      octet are in canonical order, and the most significant octet is
      transmitted first.


      This field is specific to each OUI, and indicates an encryption
      type for that OUI.  There is no standardisation for this field.
      Each OUI implements its own values.

      This field is zero or more octets, and contains additional data as
      determined by the vendor's encryption protocol.

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4.2 Other Encryption Types


      These Configuration Options provide a way to negotiate the use of
      a publicly defined encryption algorithm.

      These protocols will be made available to all interested parties,
      but may have certain licencing restrictions associated with them.
      For additional information, refer to the encryption protocol docu-
      ments that define each of the encryption types.

      A summary of the Encryption Type Configuration Option format is
      shown below.  The fields are transmitted from left to right.

          0                   1                   2                   3
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
         |     Type      |    Length     |  Values...


       1 to 254


       >= 2


      This field is zero or more octets, and contains additional data as
      determined by the encryption protocol.

5. Security Considerations

   Negotiation of encryption using PPP is designed to provide protection
   against eavesdropping on that link.  The strength of the protection
   is dependent on the encryption algorithm used and the care with which
   any 'secrets' used by the encryption algorithm is protected.

   It must be recognised that complete security can only be obtained
   through end-to-end security between hosts.

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   [1]  Simpson, W., Editor; "The Point-to-Point Protocol (PPP)", RFC
        1548, Computer Systems Consulting Services, December 1993.

   [2]  Sklower, K., Lloyd, B., McGregor, G. and Carr, D., "The PPP Mul-
        tilink Protocol (MP)", RFC 1717, University of California,
        Berkeley, November 1994.

   [3]  Rand, D., "PPP Reliable Transmission", RFC 1663, Novell, July

   [4]  Reynolds, J., and Postel, J.; "ASSIGNED NUMBERS", RFC 1700,
        USC/Information Sciences Institute, October 1994.

   [5]  Rand, D., "The PPP Compression Control Protocol (CCP)", work in
        progress, Novell.


   The style and approach of this proposal owes much to the work on the
   Compression CP [5].

Chair's Address

   The working group can be contacted via the current chair:

   Fred Baker
   Cisco Systems
   519 Lado Drive
   Santa Barbara
   California 93111

Author's Address:

   Gerry Meyer
   Spider Systems
   Stanwell Street
   Edinburgh EH6 5NG
   Scotland, UK

   Phone: (UK) 31 554 9424
   Fax:   (UK) 31 554 0649

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