PPP ECP: The PPP Encryption Control Protocol (ECP)

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.

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.

      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:

   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:

         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.

        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.

   Data

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

   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
         1               DESE


   All compliant ECP implementations SHOULD implement ECP option 1 - the
   PPP DES Encryption Protocol (DESE) [6].

   Vendors who want to use proprietary encryption MAY use the OUI
   mechanism to negotiate these without recourse to requesting an
   assigned option number from the Internet Assigned Numbers Authority.
   All other encryption options are registered by IANA.  At the time of
   writing only DESE (option 1) is registered.  Other registered options
   may be found by referring to future versions of the Assigned Numbers
   RFC.

4.1 Proprietary Encryption OUI

   Description

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

      Vendor's encryption protocols are distinguished from each other by
      means of an Organisationally Unique Identifier (OUI), namely the
      first three octets of a Vendor's Ethernet address assigned by IEEE
      802.

      Since the first matching encryption will be used, it is
      recommended 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 OUI identifies a proprietary algorithm that the implementation
      can decrypt, and that any vendor 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
      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     |       OUI ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            OUI       |    Subtype    |  Values...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-


   Type

       0

   Length

      >= 6

   IEEE OUI

      The IEEE OUI 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.

   Subtype

      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.

   Values

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

4.2 Publicly Available Encryption Types

   Description

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

      These protocols should be made available to interested parties,
      but may have certain licencing or export restrictions associated
      with them.  For additional information, refer to the encryption
      protocol documents 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...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-


    Type

       1 to 254, indicating the encryption protocol option
       being negotiated.  DESE [6] is option type 1.  Refer to the
       latest Assigned Numbers RFC for other encryption protocols.

    Length

       >= 2

   Values

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

4.3 Negotiating an Encryption Algorithm

   ECP uses LCP option negotiation techniques to negotiate encryption
   algorithms.  In contrast with most other LCP or NCP negotiation of
   multiple options, ECP negotiation is expected to converge on a single
   mutually agreeable option (encryption algorithm) - or none.
   Encryption SHOULD be negotiated in both directions, but the
   algorithms MAY be different.

   An implementation willing to decrypt using a particular encryption
   algorithm (or one of a set of algorithms) offers the algorithm(s) as
   an option (or options) in an ECP Configure-Request - call this end
   the Decrypter; call the peer the Encrypter.

   A Decrypter supporting more than one encryption algorithm may send a
   Configure-Request containing either:

   o  an ordered list of options, with the most-preferred encryption
      algorithm coming first.

   o  Or may just offer its preferred (not already Rejected) option.

   An Encrypter wishing to accept the first option (of several) MAY
   Configure-Ack ALL Options to indicate complete acceptance of the
   first-listed, preferred, algorithm.

   Otherwise, if the Encrypter does not recognise - or is unwilling to
   support - an option it MUST send a Configure-Reject for that option.
   Where more than one option is offered, the Encrypter SHOULD
   Configure-Reject all but a single preferred option.

   If the Encrypter Configure-Rejects all offered ECP options - and the
   Decrypter has no further (non-rejected) options it can offer in a
   Configure-Request - the Encrypter SHOULD take the link down.

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

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 'secret' 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.

References

   [1]  Simpson, W., Editor; "The Point-to-Point Protocol (PPP)", STD
        51, RFC 1661, Daydreamer, July 1994.

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

   [3]  Rand, D., "PPP Reliable Transmission", RFC 1663, Novell, July
        1994.

   [4]  Reynolds, J., and Postel, J.; "ASSIGNED NUMBERS", STD 2,
        RFC 1700, USC/Information Sciences Institute, October 1994.

   [5]  Rand, D., "The PPP Compression Control Protocol (CCP)", RFC
        1962, Novell, June 1996.

   [6]  Sklower, K., and G. Meyer, "The PPP DES Encryption Protocol
        (DESE)", RFC 1969, University of California, Berkeley, June
        1996.

Acknowledgements

   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:

   Karl Fox
   Ascend Communications
   3518 Riverside Drive, Suite 101
   Columbus, Ohio 43221

   EMail: karl@ascend.com

Author's Address

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

   Phone: (UK) 131 554 9424
   Fax:   (UK) 131 554 0649
   EMail: gerry@spider.co.uk