rfc3078.txt

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Network Working Group                                            G. Pall
Request for Comments: 3078                         Microsoft Corporation
Category: Informational                                          G. Zorn
Updates: 2118                                              cisco Systems
                                                              March 2001


          Microsoft Point-To-Point Encryption (MPPE) Protocol

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

   The Point-to-Point Protocol (PPP) provides a standard method for
   transporting multi-protocol datagrams over point-to-point links.

   The PPP Compression Control Protocol provides a method to negotiate
   and utilize compression protocols over PPP encapsulated links.

   This document describes the use of the Microsoft Point to Point
   Encryption (MPPE) to enhance the confidentiality of PPP-encapsulated
   packets.

Specification of Requirements

   In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
   "recommended", "SHOULD", and "SHOULD NOT" are to be interpreted as
   described in [5].

1.  Introduction

   The Microsoft Point to Point Encryption scheme is a means of
   representing Point to Point Protocol (PPP) packets in an encrypted
   form.

   MPPE uses the RSA RC4 [3] algorithm to provide data confidentiality.
   The length of the session key to be used for initializing encryption
   tables can be negotiated.  MPPE currently supports 40-bit and 128-bit
   session keys.




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RFC 3078                     MPPE Protocol                    March 2001


   MPPE session keys are changed frequently; the exact frequency depends
   upon the options negotiated, but may be every packet.

   MPPE is negotiated within option 18 [4] in the Compression Control
   Protocol.

2.  Configuration Option Format


   Description

      The CCP Configuration Option negotiates the use of MPPE on the
      link.  By default (i.e., if the negotiation of MPPE is not
      attempted), no encryption is used.  If, however, MPPE negotiation
      is attempted and fails, the link SHOULD be terminated.

   A summary of the CCP 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     |        Supported Bits         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        Supported Bits         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      18

   Length

      6

   Supported Bits

      This field is 4 octets, most significant octet first.

         3                   2                   1
       1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             |H|                               |M|S|L|D|     |C|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+







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RFC 3078                     MPPE Protocol                    March 2001


   The 'C' bit is used by MPPC [4] and is not discussed further in this
   memo.  The 'D' bit is obsolete; although some older peers may attempt
   to negotiate this option, it SHOULD NOT be accepted.  If the 'L' bit
   is set (corresponding to a value of 0x20 in the least significant
   octet), this indicates the desire of the sender to negotiate the use
   of 40-bit session keys.  If the 'S' bit is set (corresponding to a
   value of 0x40 in the least significant octet), this indicates the
   desire of the sender to negotiate the use of 128-bit session keys.
   If the 'M' bit is set (corresponding to a value of 0x80 in the least
   significant octet), this indicates the desire of the sender to
   negotiate the use of 56-bit session keys.  If the 'H' bit is set
   (corresponding to a value of 0x01 in the most significant octet),
   this indicates that the sender wishes to negotiate the use of
   stateless mode, in which the session key is changed after the
   transmission of each packet (see section 10, below).  In the
   following discussion, the 'S', 'M' and 'L' bits are sometimes
   referred to collectively as "encryption options".

   All other bits are reserved and MUST be set to 0.

2.1.  Option Negotiation

   MPPE options are negotiated as described in [2].  In particular, the
   negotiation initiator SHOULD request all of the options it supports.
   The responder SHOULD NAK with a single encryption option (note that
   stateless mode may always be negotiated, independent of and in
   addition to an encryption option).  If the responder supports more
   than one encryption option in the set requested by the initiator, the
   option selected SHOULD be the "strongest" option offered.
   Informally, the strength of the MPPE encryption options may be
   characterized as follows:

      STRONGEST
         128-bit encryption ('S' bit set)
         56-bit  encryption ('M' bit set)
         40-bit  encryption ('L' bit set)
      WEAKEST

   This characterization takes into account the generally accepted
   strength of the cipher.

   The initiator SHOULD then either send another request containing the
   same option(s) as the responder's NAK or cancel the negotiation,
   dropping the connection.







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RFC 3078                     MPPE Protocol                    March 2001


3.  MPPE Packets

   Before any MPPE packets are transmitted, PPP MUST reach the Network-
   Layer Protocol phase and the CCP Control Protocol MUST reach the
   Opened state.

   Exactly one MPPE datagram is encapsulated in the PPP Information
   field.  The PPP Protocol field indicates type 0x00FD for all
   encrypted datagrams.

   The maximum length of the MPPE datagram transmitted over a PPP link
   is the same as the maximum length of the Information field of a PPP
   encapsulated packet.

   Only packets with PPP Protocol numbers in the range 0x0021 to 0x00FA
   are encrypted.  Other packets are not passed thru the MPPE processor
   and are sent with their original PPP Protocol numbers.

      Padding

         It is recommended that padding not be used with MPPE.  If the
         sender uses padding it MUST negotiate the Self-Describing-
         Padding Configuration option [10] during LCP phase and use
         self-describing pads.

      Reliability and Sequencing

         The MPPE scheme does not require a reliable link.  Instead, it
         relies on a 12-bit coherency count in each packet to keep the
         encryption tables synchronized.  If stateless mode has not been
         negotiated and the coherency count in the received packet does
         not match the expected count, the receiver MUST send a CCP
         Reset-Request packet to cause the resynchronization of the RC4
         tables.

         MPPE expects packets to be delivered in sequence.

         MPPE MAY be used over a reliable link, as described in "PPP
         Reliable Transmission" [6], but this typically just adds
         unnecessary overhead since only the coherency count is
         required.

      Data Expansion

         The MPPE scheme does not expand or compress data.  The number
         of octets input to and output from the MPPE processor are the
         same.




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RFC 3078                     MPPE Protocol                    March 2001


3.1.  Packet Format

   A summary of the MPPE packet 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          PPP Protocol         |A|B|C|D|    Coherency Count    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Encrypted Data...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      PPP Protocol

         The PPP Protocol field is described in the Point-to-Point
         Protocol Encapsulation [1].

         When MPPE is successfully negotiated by the PPP Compression
         Control Protocol, the value of this field is 0x00FD.  This
         value MAY be compressed when Protocol-Field-Compression is
         negotiated.

      Bit A

         This bit indicates that the encryption tables were initialized
         before this packet was generated.  The receiver MUST re-
         initialize its tables with the current session key before
         decrypting this packet.  This bit is referred to as the FLUSHED
         bit in this document.  If the stateless option has been
         negotiated, this bit MUST be set on every encrypted packet.
         Note that MPPC and MPPE both recognize the FLUSHED bit;
         therefore, if the stateless option is negotiated, it applies to
         both MPPC and MPPE.

      Bit B

         This bit does not have any significance in MPPE.

      Bit C

         This bit does not have any significance in MPPE.

      Bit D

         This bit set to 1 indicates that the packet is encrypted.  This
         bit set to 0 means that this packet is not encrypted.




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RFC 3078                     MPPE Protocol                    March 2001


      Coherency Count

         The coherency count is used to assure that the packets are sent
         in proper order and that no packet has been dropped.  It is a
         monotonically increasing counter which incremented by 1 for
         each packet sent.  When the counter reaches 4095 (0x0FFF), it
         is reset to 0.

      Encrypted Data

         The encrypted data begins with the protocol field.  For
         example, in case of an IP packet (0x0021 followed by an IP
         header), the MPPE processor will first encrypt the protocol
         field and then encrypt the IP header.

         If the packet contains header compression, the MPPE processor
         is applied AFTER header compression is performed and MUST be
         applied to the compressed header as well.  For example, if a
         packet contained the protocol type 0x002D (for a compressed
         TCP/IP header), the MPPE processor would first encrypt 0x002D
         and then it would encrypt the compressed Van-Jacobsen TCP/IP
         header.

      Implementation Note

         If both MPPE and MPPC are negotiated on the same link, the MPPE
         processor MUST be invoked after the MPPC processor by the
         sender and the MPPE processor MUST be invoked before the MPPC
         processor by the receiver.

4.  Initial Session Keys

   In the current implementation, initial session keys are derived from
   peer credentials; however, other derivation methods are possible.
   For example, some authentication methods (such as Kerberos [8] and
   TLS [9]) produce session keys as side effects of authentication;
   these keys may be used by MPPE in the future.  For this reason, the
   techniques used to derive initial MPPE session keys are described in
   separate documents.

5.  Initializing RC4 Using a Session Key

   Once an initial session key has been derived, the RC4 context is
   initialized as follows:

      rc4_key(RC4Key, Length_Of_Key, Initial_Session_Key)





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