rfc1968.txt

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Network Working Group                                           G. Meyer
Request for Comments: 1968                                Spider Systems
Category: Standards Track                                      June 1996


               The PPP Encryption Control Protocol (ECP)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   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.

Conventions

   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.
      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-
      cumstances.

   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.









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RFC 1968                     PPP Encryption                    June 1996


Table of Contents

      1. Introduction ...........................................  2
      2. Encryption Control Protocol (ECP) ......................  2
          2.1 Sending Encrypted Datagrams .......................  3
      3. Additional Packets .....................................  4
          3.1 Reset-Request and Reset-Ack .......................  5
      4. ECP Configuration Options ..............................  6
          4.1 Proprietary Encryption OUI ........................  7
          4.2 Publicly Available Encryption Types ...............  8
          4.3 Negotiating an Encryption Algorithm ...............  9
      5. Security Considerations ................................ 10

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
   established, 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.

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
   Protocol (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 LCP [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
         phase.





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RFC 1968                     PPP Encryption                    June 1996


      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
         Protocol).

      Code field

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

      Negotiation

         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
         Determination to finish before timing out waiting for a
         Configure-Ack or other response.

         An implementation MUST NOT transmit data until ECP negotiation
         has completed successfully.  If ECP negotiation is not
         successful the link SHOULD 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
   datagram).

   When using multiple PPP links to a single destination [2], there are
   two methods of employing data encryption:




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RFC 1968                     PPP Encryption                    June 1996


   o  The first method is to encrypt the data prior to sending it out
      through the multiple links.

      The PPP Protocol field MUST indicate type hex 0053.

   o  The second is to treat each link as a separate connection, that
      may or may not have encryption enabled.

      On links which have negotiated encryption, the PPP Protocol field
      MUST be type hex 0055 (Individual link encrypted datagram).

   Only one encryption 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
   passing data reliably, or it must require the use of a reliable
   transport such as LAPB [3].

   Compression may also be negotiated using the Compression Control
   Protocol [5].  To ensure interoperability, plain text MUST be:

   o  First compressed.

   o  Then encrypted.

   This order has been chosen since it should result in smaller output
   and more secure encryption.

3. Additional Packets

   The Packet format and basic facilities are already defined for LCP
   [1].

   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:




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RFC 1968                     PPP Encryption                    June 1996


         14      Reset-Request
         15      Reset-Ack

3.1 Reset-Request and Reset-Ack

   Description

      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
      direction.  Some encryption algorithms may not require this
      mechanism.

      Individual algorithms need to specify a mechanism for determining
      how to detect a decryption failure.  On initial detection of a
      decryption failure, an ECP implementation SHOULD transmit an ECP
      packet with the Code field set to 14 (Reset-Request).  The Data
      field may be filled with any desired data.

      Once a Reset-Request has been sent, any encrypted 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
      undesirable 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.

      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.







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RFC 1968                     PPP Encryption                    June 1996


        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 ...
       +-+-+-+-+


   Code

      14 for Reset-Request;

      15 for Reset-Ack.

   Identifier

      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 SHOULD remain unchanged.

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