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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  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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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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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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     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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RFC 3078                     MPPE Protocol                    March 2001


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);




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RFC 3078                     MPPE Protocol                    March 2001


        /*
         * 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.



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RFC 3078                     MPPE Protocol                    March 2001


  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).







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  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.



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  [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: [email protected]


  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: [email protected]














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13.  Full Copyright Statement

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

  This document and translations of it may be copied and furnished to
  others, and derivative works that comment on or otherwise explain it
  or assist in its implementation may be prepared, copied, published
  and distributed, in whole or in part, without restriction of any
  kind, provided that the above copyright notice and this paragraph are
  included on all such copies and derivative works.  However, this
  document itself may not be modified in any way, such as by removing
  the copyright notice or references to the Internet Society or other
  Internet organizations, except as needed for the purpose of
  developing Internet standards in which case the procedures for
  copyrights defined in the Internet Standards process must be
  followed, or as required to translate it into languages other than
  English.

  The limited permissions granted above are perpetual and will not be
  revoked by the Internet Society or its successors or assigns.

  This document and the information contained herein is provided on an
  "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
  TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
  BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
  HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
  MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

  Funding for the RFC Editor function is currently provided by the
  Internet Society.



















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