Microsoft Point-To-Point Encryption (MPPE) Protocol

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.

   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|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   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.

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.

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.

      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)

6.  Encrypting Data

   Once initialized, data is encrypted using the following function and
   transmitted with the CCP and MPPE headers.

      EncryptedData = rc4(RC4Key, Length_Of_Data, Data)

7.  Changing Keys

7.1.  Stateless Mode Key Changes

   If stateless encryption has been negotiated, the session key changes
   every time the coherency count changes; i.e., on every packet.  In
   stateless mode, the sender MUST change its key before encrypting and
   transmitting each packet and the receiver MUST change its key after
   receiving, but before decrypting, each packet (see "Synchronization",
   below).

7.2.  Stateful Mode Key Changes

   If stateful encryption has been negotiated, the sender MUST change
   its key before encrypting and transmitting any packet in which the
   low order octet of the coherency count equals 0xFF (the "flag"
   packet), and the receiver MUST change its key after receiving, but
   before decrypting, a "flag" packet (see "Synchronization", below).

7.3.  The MPPE Key Change Algorithm

   The following method is used to change keys:

      /*
       * SessionKeyLength is 8 for 40-bit keys, 16 for 128-bit keys.
       *
       * SessionKey is the same as StartKey in the first call for
       * a given session.
       */

      void
      GetNewKeyFromSHA(
      IN  unsigned char *StartKey,
      IN  unsigned char *SessionKey,
      IN  unsigned long SessionKeyLength
      OUT unsigned char *InterimKey )
      {
         unsigned char  Digest[20];

         ZeroMemory(Digest, 20);

         /*
          * SHAInit(), SHAUpdate() and SHAFinal()
          * are an implementation of the Secure
          * Hash Algorithm [7]
          */

         SHAInit(Context);
         SHAUpdate(Context, StartKey, SessionKeyLength);
         SHAUpdate(Context, SHApad1, 40);
         SHAUpdate(Context, SessionKey, SessionKeyLength);
         SHAUpdate(Context, SHApad2, 40);
         SHAFinal(Context, Digest);

         MoveMemory(InterimKey, Digest, SessionKeyLength);
      }

   The RC4 tables are re-initialized using the newly created interim key:

      rc4_key(RC4Key, Length_Of_Key, InterimKey)

   Finally, the interim key is encrypted using the new tables to produce
   a new session key:

      SessionKey = rc4(RC4Key, Length_Of_Key, InterimKey)

   For 40-bit session keys the most significant three octets of the new
   session key are now set to 0xD1, 0x26 and 0x9E respectively; for 56-
   bit keys, the most significant octet is set to 0xD1.

   Finally, the RC4 tables are re-initialized using the new session key:

      rc4_key(RC4Key, Length_Of_Key, SessionKey)

8.  Synchronization

   Packets may be lost during transfer.  The following sections describe
   synchronization for both the stateless and stateful cases.

8.1.  Stateless Synchronization

   If stateless encryption has been negotiated and the coherency count
   in the received packet (C1) is greater than the coherency count in
   the last packet previously received (C2), the receiver MUST perform N
   = C1 - C2 key changes before decrypting the packet, in order to
   ensure that its session key is synchronized with the session key of
   the sender.  Normally, the value of N will be 1; however, if
   intervening packets have been lost, N may be greater than 1.  For
   example, if C1 = 5 and C2 = 02 then N = 3 key changes are required.

   Since the FLUSHED bit is set on every packet if stateless encryption
   was negotiated, the transmission of CCP Reset-Request packets is not
   required for synchronization.

8.2.  Stateful Synchronization

   If stateful encryption has been negotiated, the sender MUST change
   its key before encrypting and transmitting any packet in which the
   low order octet of the coherency count equals 0xFF (the "flag"
   packet), and the receiver MUST change its key after receiving, but
   before decrypting, a "flag" packet.  However, the "flag" packet may
   be lost.  If this happens, the low order octet of the coherency count
   in the received packet will be less than that in the last packet
   previously received.  In this case, the receiver MUST perform a key
   change before decrypting the newly received packet, (since the sender
   will have changed its key before transmitting the packet), then send
   a CCP Reset-Request packet (see below).  It is possible that 256 or
   more consecutive packets could be lost; the receiver SHOULD detect
   this condition and perform the number of key changes necessary to
   resynchronize with the sender.

   If packet loss is detected while using stateful encryption, the
   receiver MUST drop the packet and send a CCP Reset-Request packet
   without data.  After transmitting the CCP Reset-Request packet, the
   receiver SHOULD silently discard all packets until a packet is
   received with the FLUSHED bit set.  On receiving a packet with the
   FLUSHED bit set, the receiver MUST set its coherency count to the one
   received in that packet and re-initialize its RC4 tables using the
   current session key:

      rc4_key(RC4Key, Length_Of_Key, SessionKey)

   When the sender receives a CCP Reset-Request packet, it MUST re-
   initialize its own RC4 tables using the same method and set the
   FLUSHED bit in the next packet sent.  Thus synchronization is
   achieved without a CCP Reset-Ack packet.

9.  Security Considerations

   Because of the way that the RC4 tables are reinitialized during
   stateful synchronization, it is possible that two packets may be
   encrypted using the same key.  For this reason, the stateful mode
   SHOULD NOT be used in lossy network environments (e.g., layer two
   tunnels on the Internet).

   Since the MPPE negotiation is not integrity protected, an active
   attacker could alter the strength of the keys used by modifying the
   Supported Bits field of the CCP Configuration Option packet.  The
   effects of this attack can be minimized through appropriate peer
   configuration, however.

   Peers MUST NOT transmit user data until the MPPE negotiation is
   complete.

   It is possible that an active attacker could modify the coherency
   count of a packet, causing the peers to lose synchronization.

   An active denial-of-service attack could be mounted by methodically
   inverting the value of the 'D' bit in the MPPE packet header.

10.  References

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

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

   [3]  RC4 is a proprietary encryption algorithm available under
        license from RSA Data Security Inc.  For licensing information,
        contact:

                  RSA Data Security, Inc.
                  100 Marine Parkway
                  Redwood City, CA 94065-1031

   [4]  Pall, G., "Microsoft Point-to-Point Compression (MPPC)
        Protocol", RFC 2118, March 1997.

   [5]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

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

   [7]  "Secure Hash Standard", Federal Information Processing Standards
        Publication 180-1, National Institute of Standards and
        Technology, April 1995.

   [8]  Kohl, J. and C. Neuman "The Kerberos Network Authentication
        System (V5)", RFC 1510, September 1993.

   [9]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
        2246, January 1999.

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

11.  Acknowledgements

   Anthony Bell, Richard B. Ward, Terence Spies and Thomas Dimitri, all
   of Microsoft Corporation, significantly contributed to the design and
   development of MPPE.

   Additional thanks to Robert Friend, Joe Davies, Jody Terrill, Archie
   Cobbs, Mark Deuser, and Jeff Haag, for useful feedback.

12.  Authors' Addresses

   Questions about this memo can be directed to:

   Gurdeep Singh Pall
   Microsoft Corporation
   One Microsoft Way
   Redmond, Washington 98052
   USA

   Phone: +1 425 882 8080
   Fax:   +1 425 936 7329
   EMail: gurdeep@microsoft.com


   Glen Zorn
   cisco Systems
   500 108th Avenue N.E.
   Suite 500
   Bellevue, Washington 98004
   USA

   Phone: +1 425 438 8218
   Fax:   +1 425 438 1848
   EMail: gwz@cisco.com

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