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Network Working Group                                            G. PallRequest for Comments: 3078                         Microsoft CorporationCategory: Informational                                          G. ZornUpdates:2118                                              cisco Systems                                                              March 2001Microsoft Point-To-Point Encryption (MPPE) ProtocolStatus 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 (seesection 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 20013.  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 20013.1.  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 20016.  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 Keys7.1.  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)",RFC1962, 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",RFC2246, 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.comPall & Zorn                  Informational                     [Page 11]

RFC 3078                     MPPE Protocol                    March 200113.  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]