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rfc:rfc3078

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.

Pall & Zorn Informational [Page 1] 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|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Pall & Zorn Informational [Page 2] 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.

Pall & Zorn Informational [Page 3] 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.

Pall & Zorn Informational [Page 4] 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.

Pall & Zorn Informational [Page 5] 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)

Pall & Zorn Informational [Page 6] 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);

Pall & Zorn Informational [Page 7] 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.

Pall & Zorn Informational [Page 8] 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).

Pall & Zorn Informational [Page 9] RFC 3078 MPPE Protocol March 2001

 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.

Pall & Zorn Informational [Page 10] RFC 3078 MPPE Protocol March 2001

 [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

Pall & Zorn Informational [Page 11] RFC 3078 MPPE Protocol March 2001

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.

Pall & Zorn Informational [Page 12]

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