The PPP DES Encryption Protocol (DESE)

Network Working Group                                         K. Sklower
Request for Comments: 1969            University of California, Berkeley
Category: Informational                                         G. Meyer
                                                          Spider Systems
                                                               June 1996


                 The PPP DES Encryption Protocol (DESE)

Status of This Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  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.

   The PPP Encryption Control Protocol (ECP) [2] provides a method to
   negotiate and utilize encryption protocols over PPP encapsulated
   links.

   This document provides specific details for the use of the DES
   standard [5, 6] for encrypting PPP encapsulated packets.

Acknowledgements

   The authors extend hearty thanks to Fred Baker of Cisco for helpful
   improvements to the clarity of the document.

Table of Contents

   1. Introduction ................................................    2
   1.1. Motivation ................................................    2
   1.2. Conventions ...............................................    2
   2. General Overview ............................................    2
   3. Structure of This Specification .............................    3
   4. DESE Configuration Option for ECP ...........................    4
   5. Packet Format for DESE ......................................    5
   6. Encryption ..................................................    6
   6.1. Padding Considerations ....................................    6
   6.2. Generation of the Ciphertext ..............................    7
   6.3. Retrieval of the Plaintext ................................    8
   6.4. Recovery after Packet Loss ................................    8
   7. MRU Considerations ..........................................    8
   8. Security Considerations .....................................    9
   9. References ..................................................    9
   10. Authors' Addresses .........................................   10
   11. Expiration Date of this Draft ..............................   10

1.  Introduction

1.1.  Motivation

   The purpose of this memo is two-fold: to show how one specifies the
   necessary details of a "data" or "bearer" protocol given the context
   of the generic PPP Encryption Control Protocol, and also to provide
   at least one commonly-understood means of secure data transmission
   between PPP implementations.

   The DES encryption algorithm is a well studied, understood and widely
   implemented encryption algorithm.  The DES cipher was designed for
   efficient implementation in hardware, and consequently may be
   relatively expensive to implement in software.  However, its
   pervasiveness makes it seem like a reasonable choice for a "model"
   encryption protocol.

   Source code implementing DES in the "Electronic Code Book Mode" can
   be found in [7].  US export laws forbid the inclusion of
   compilation-ready source code in this document.

1.2.  Conventions

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

   o    MUST, SHALL or MANDATORY -- the item is an absolute requirement
        of the specification.

   o    SHOULD or RECOMMENDED -- the item should generally be followed
        for all but exceptional circumstances.

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

2.  General Overview

   The purpose of encrypting packets exchanged between two PPP
   implementations is to attempt to insure the privacy of communication
   conducted via the two implementations.  The encryption process
   depends on the specification of an encryption algorithm and a shared
   secret (usually involving at least a key) between the sender and
   receiver.

   Generally, the encryptor will take a PPP packet including the
   protocol field, apply the chosen encryption algorithm, place the
   resulting cipher text (and in this specification, an explicit
   sequence number) in the information field of another PPP packet.  The
   decryptor will apply the inverse algorithm and interpret the
   resulting plain text as if it were a PPP packet which had arrived
   directly on the interface.

   The means by which the secret becomes known to both communicating
   elements is beyond the scope of this document; usually some form of
   manual configuration is involved.  Implementations might make use of
   PPP authentication, or the EndPoint Identifier Option described in
   PPP Multilink [3], as factors in selecting the shared secret.  If the
   secret can be deduced by analysis of the communication between the
   two parties, then no privacy is guaranteed.

   While the US Data Encryption Standard (DES) algorithm [5, 6] provides
   multiple modes of use, this specification selects the use of only one
   mode in conjunction with the PPP Encryption Control Protol (ECP): the
   Cipher Block Chaining (CBC) mode.  In addition to the US Government
   publications cited above, the CBC mode is also discussed in [7],
   although no C source code is provided for it per se.

   The initialization vector for this mode is deduced from an explicit
   64-bit nonce, which is exchanged in the clear during the negotiation
   phase.  The 56-bit key required by all DES modes is established as a
   shared secret between the implementations.

   One reason for choosing the chaining mode is that it is generally
   thought to require more computation resources to deduce a 64 bit key
   used for DES encryption by analysis of the encrypted communication
   stream when chaining mode is used, compared with the situation where
   each block is encrypted separately with no chaining.  Further, if
   chaining is not used, even if the key is never deduced, the
   communication may be subject to replay attacks.

   However, if chaining is to extend beyond packet boundaries, both the
   sender and receiver must agree on the order the packets were
   encrypted.  Thus, this specification provides for an explicit 16 bit
   sequence number to sequence decryption of the packets.  This mode of
   operation even allows recovery from occasional packet loss; details
   are also given below.

3.  Structure of This Specification

   The PPP Encryption Control Protocol (ECP), provides a framework for
   negotiating parameters associated with encryption, such as choosing
   the algorithm.  It specifies the assigned numbers to be used as PPP
   protocol numbers for the "data packets" to be carried as the
   associated "data protocol", and describes the state machine.

   Thus, a specification for use in that matrix need only describe any
   additional configuration options required to specify a particular
   algorithm, and the process by which one encrypts/decrypts the
   information once the Opened state has been achieved.

4.  DESE Configuration Option for ECP

   Description

        The ECP DESE Configuration Option indicates that the issuing
        implementation is offering to employ this specification for
        decrypting communications on the link, and may be thought of as
        a request for its peer to encrypt packets in this manner.

        The ECP DESE Configuration Option has the following fields,
        which are transmitted from left to right:


                    Figure 1:  ECP DESE Configuration Option


        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     |         Initial Nonce ...
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Type

             1, to indicate the DESE protocol.


        Length

             10


        Initial Nonce

             This field is an 8 byte quantity which is used by the peer
             implementation to encrypt the first packet transmitted
             after the sender reaches the opened state.

             To guard against replay attacks, the implementation SHOULD
             offer a different value during each ECP negotiation.  An
             example might be to use the number of seconds since Jan
             1st, 1970 (GMT/UT) in the upper 32 bits, and the current
             number of nanoseconds relative to the last second mark in
             the lower 32 bits.

             Its formulaic role is described in the Encryption section
             below.

5.  Packet Format for DESE

   Description

        The DESE packets themselves have the following fields:


                Figure 2:  DES Encryption Protocol Packet Format


        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
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |    Address    |    Control    |     0000      |  Protocol ID  |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        | Seq. No. High | Seq. No. Low  |        Ciphertext ...
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


        Address and Control

             These fields MUST be present unless the PPP Address and
             Control Field Compression option (ACFC) has been
             negotiated.

        Protocol ID

             The value of this field is 0x53 or 0x55; the latter
             indicates that ciphertext includes headers for the
             Multilink Protocol, and REQUIRES that the Individual Link
             Encryption Control Protocol has reached the opened state.
             The leading zero MAY be absent if the PPP Protocol Field
             Compression option (PFC) has been negotiated.

        Sequence Number

             These 16-bit numbers are assigned by the encryptor
             sequentially starting with 0 (for the first packet
             transmitted once ECP has reached the opened state.

        Ciphertext

             The generation of this data is described in the next
             section.

6.  Encryption

   Once the ECP has reached the Opened state, the sender MUST NOT apply
   the encryption procedure to LCP packets nor ECP packets.

   If the async control character map option has been negotiated on the
   link, the sender applies mapping after the encryption algorithm has
   been run.

   The encryption algorithm is generally to pad the Protocol and
   Information fields of a PPP packet to some multiple of 8 bytes, and
   apply DES in Chaining Block Cipher mode with a 56-bit key K.

   There are a lot of details concerning what constitutes the Protocol
   and Information fields, in the presence or non-presence of Multilink,
   and whether the ACFC and PFC options have been negotiated, and the
   sort of padding chosen.

   Regardless of whether ACFC has been negotiated on the link, the
   sender applies the encryption procedure to only that portion of the
   packet excluding the address and control field.

   If the Multilink Protocol has been negotiated and encryption is to be
   construed as being applied to each link separately, then the
   encryption procedure is to be applied to the (possibly extended)
   protocol and information fields of the packet in the Multilink
   Protocol.

   If the Multilink Protocol has been negotiated and encryption is to be
   construed as being applied to the bundle, then the multilink
   procedure is to be applied to the resulting DESE packets.

6.1.  Padding Considerations

   Since the DES algorithm operates on blocks of 8 octets, packets which
   are of length not a multiple of 8 octets must be padded.  This can be
   injurious to the interpretation of some protocols which do not
   contain an explicit length field in their protocol headers.
   (Additional padding of the ciphered packet for the purposes of
   transmission by HDLC hardware which requires an even number of bytes
   should not be necessary since the information field will now be of
   length a multiple of 8, and whether or not the packet is of even
   length can be forced by use or absence of a leading zero in the
   protocol field).

   For protocols which do have an explicit length field, such as IP,
   IPX, XNS, and CLNP, then padding may be accomplished by adding random
   trailing garbage.  Even when performing the Multilink protocol, if it
   is only being applied to packets with explicit length fields, and if
   care is taken so that all non-terminating fragments (i.e., those not
   bearing the (E)nd bit) are of lengths divisible by 8; then no ill
   effects will happen if garbage padding is applied only to terminating
   fragments.

   For certain cases, such as the PPP bridging protocol when the
   trailing CRC is forwarded or when any bridging is being applied to
   protocols not having explicit length fields, adding garbage changes
   the interpretation of the packet.  The self-describing padding option
   [4] permits unambiguous removal of padded bytes; although it should
   only be used when absolutely necessary as it may inadvertently
   require adding as many as 8 octets to packets that could otherwise be
   left unaltered.

      Consider a packet, which by unlucky circumstance is already a
      multiple of 8 octets, but terminates in the sequence 0x1, 0x2.
      Self-describing padding would otherwise remove the trailing two
      bytes.  For purposes of coexistence with archaic HDLC chips where
      it is necessary to transmit packets of even length, one would
      normally only have to add an additional two octets (0x1, 0x2),
      which could then be removed.  However, since the packet was
      initially a multiple of 8 bytes, an additional 8 bytes would need
      to be added.

6.2.  Generation of the Ciphertext

   In this discussion, E[k] will denote the basic DES cipher determined
   by a 56-bit key k acting on 64 bit blocks. and D[k] will denote the
   corresponding decryption mechanism.  The padded plaintext described
   in the previous section then becomes a sequence of 64 bit blocks P[i]
   (where i ranges from 1 to n).  The circumflex character (^)
   represents the bit-wise exclusive-or operation applied to 64-bit
   blocks.

   When encrypting the first packet to be transmitted in the opened
   state let C[0] be the result of applying E[k] to the Initial Nonce
   received in the peer's ECP DESE option; otherwise let C[0] be the
   final block of the previously transmitted packet.

   The ciphertext for the packet is generated by the iterative process

                        C[i] = E[k](P[i] ^ C[i-1])

   for i running between 1 and n.

6.3.  Retrieval of the Plaintext

   When decrypting the first packet received in the opened state, let
   C[0] be the result of applying E[k] to the Initial Nonce transmitted
   in the ECP DESE option.  The first packet will have sequence number
   zero.  For subsequent packets, let C[0] be the final block of the
   previous packet in sequence space.  Decryption is then accomplished
   by

                        P[i] = C[i-1] ^ D[k](C[i]),

   for i running between 1 and n.

6.4.  Recovery after Packet Loss

   Packet loss is detected when there is a discontinuity in the sequence
   numbers of consecutive packets.  Suppose packet number N - 1 has an
   unrecoverable error or is otherwise lost, but packets N and N + 1 are
   received correctly.

   Since the algorithm in the previous section requires C[0] for packet
   N to be C[last] for packet N - 1, it will be impossible to decode
   packet N.  However, all packets N + 1 and following can be decoded in
   the usual way, since all that is required is the last block of
   ciphertext of the previous packet (in this case packet N, which WAS
   received).

7.  MRU Considerations

   Because padding can occur, and because there is an additional
   protocol field in effect, implementations should take into account
   the growth of the packets.  As an example, if PFC had been
   negotiated, and if the MRU before had been exactly a multiple of 8,
   then the plaintext resulting combining a full sized data packets with
   a one byte protocol field would require an additional 7 bytes of
   padding, and the sequence number would be an additional 2 bytes so
   that the information field in the DESE protocol is now 10 bytes
   larger than that in the original packet.  Because the convention is
   that PPP options are independent of each other, negotiation of DESE
   does not, by itself, automatically increase the MRU value.

8.  Security Considerations

   Security issues are the primary subject of this memo.  This proposal
   relies on exterior and unspecified methods for authentication and
   retrieval of shared secrets.

   It proposes no new technology for privacy, but merely describes a
   convention for the application of the DES cipher to data transmission
   between PPP implementation.

   Any methodology for the protection and retrieval of shared secrets,
   and any limitations of the DES cipher are relevant to the use
   described here.

9.  References

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

   [2] Meyer, G., "The PPP Encryption Protocol", RFC 1968, Spider
       Systems, June 1996.

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

   [4] Simpson, W., Editor, "PPP LCP Extensions", RFC 1570, Daydreamer,
       January 1994.

   [5] National Bureau of Standards, "Data Encryption Standard", FIPS
       PUB 46 (January 1977).

   [6] National Bureau of Standards, "DES Modes of Operation", FIPS PUB
       81 (December 1980).

   [7] Schneier, B., "Applied Cryptography - Protocols Algorithms, and
       source code in C", John Wiley & Sons, Inc. 1994.  There is an
       errata associated with the book, and people can get a copy by
       sending e-mail to schneier@counterpane.com.

10.  Authors' Addresses

   Keith Sklower
   Computer Science Department
   384 Soda Hall, Mail Stop 1776
   University of California
   Berkeley, CA 94720-1776

   Phone:  (510) 642-9587
   EMail:  sklower@CS.Berkeley.EDU


   Gerry M. 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