The PPP DES Encryption Protocol, Version 2 (DESE-bis)
RFC 2419

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Network Working Group                                         K. Sklower
Request for Comments: 2419            University of California, Berkeley
Obsoletes: 1969                                                 G. Meyer
Category: Standards Track                                          Shiva
                                                          September 1998

         The PPP DES Encryption Protocol, Version 2 (DESE-bis)

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.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

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, Philip
   Rakity of Flowpoint, and William Simpson of Daydreamer for helpful
   improvements to the clarity and correctness 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 .............................  4
   4. DESE Configuration Option for ECP ...........................  4
   5. Packet Format for DESE ......................................  5

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RFC 2419                 PPP DES Encryption v2            September 1998

   6. Encryption ..................................................  6
   6.1. Padding Considerations ....................................  7
   6.2. Generation of the Ciphertext ..............................  8
   6.3. Retrieval of the Plaintext ................................  8
   6.4. Recovery after Packet Loss ................................  8
   7. MRU Considerations ..........................................  9
   8. Differences from RFC 1969 ...................................  9
   8.1. When to Pad ...............................................  9
   8.2. Assigned Numbers ..........................................  9
   8.3. Minor Editorial Changes ...................................  9
   9. Security Considerations .....................................  9
   10. References ................................................. 10
   11. Authors' Addresses ......................................... 11
   12. Full Copyright Statement ................................... 12

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 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 RFC 2119 [8].

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

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RFC 2419                 PPP DES Encryption v2            September 1998

   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