RFC3078 - 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.

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

Length

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 MSLD 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 ABCD 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, RFC1661, 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", RFC2118, March 1997.

[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement

Levels", BCP 14, RFC2119, March 1997.

[6] Rand, D., "PPP Reliable Transmission", RFC1663, 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)", RFC1510, 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", RFC1570, 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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