MS-CHAP

MS-CHAP Protocol

Network Working Group S. Cobb

Informational Memo Microsoft

Revision 1.3 March 1997

Microsoft PPP CHAP Extensions

Status of this Memo

This document has no official Internet Engineering Task Force

status. It is submitted as an example of one vendor's working

solution to several authentication issues not yet standardized by

the Point-to-Point Working Group.

The protocol described is implemented in Microsoft Windows NT 3.5

and 3.51 and in Microsoft Windows95. Differences between the

platforms are noted in the text. This information, plus that in

the references, is believed sufficient to implement an

interoperating peer.

Distribution of this memo is unlimited.

Abstract

The Point-to-Point Protocol (PPP) [1] provides a standard method

for transporting multi-protocol datagrams over point-to-point

links. PPP defines an extensible Link Control Protocol and a

family of Network Control Protocols (NCPs) for establishing and

configuring different network-layer protocols.

This document describes Microsoft's PPP CHAP dialect (MS-CHAP),

which extends the user authentication functionality provided on

Windows networks to remote workstations. MS-CHAP is closely

derived from the PPP Challenge/Handshake Authentication Protocol

described in RFC 1334 [2], which the reader should have at hand.

History

Rev 1.21: (Sect 6) Fix error in implicit challenge description

Rev 1.22: (Sect 7) Fix error in sub-field table ordering

Rev 1.3: (Sect 10) Added hash example section

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Table Of Contents

1. Introduction................................................3

2. LCP Configuration...........................................4

3. Challenge Packet............................................4

4. Response Packet.............................................4

5. Success Packet..............................................8

6. Failure Packet..............................................8

7. Change Password Packet (version 1)..........................9

8. Change Password Packet (version 2).........................12

9. Negotiation Examples.......................................16

10. Hash Example...............................................16

REFERENCES.....................................................18

CHAIR'S ADDRESS................................................19

AUTHOR'S ADDRESS...............................................19

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1. Introduction

Microsoft created MS-CHAP to authenticate remote Windows

workstations, providing the functionality to which LAN-based users

are accustomed.

The closest fit available in standard PPP is CHAP which is

primarily used for mutual secure authentication between WAN-aware

routers. Unfortunately, CHAP is not widely used in support of

remote workstations where providers commonly require an insecure

text login session in place of PPP authentication protocols. To

date, several remote workstation issues have not been adequately

addressed in CHAP. MS-CHAP addresses these issues and also

integrates the encryption and hashing algorithms used on Windows

networks.

Where possible, MS-CHAP is consistent with standard CHAP, and the

differences are easily modularized. Microsoft implements MS-CHAP

as extensions to it's standard CHAP code base. Briefly,

differences between MS-CHAP and standard CHAP are:

* MS-CHAP is enabled by negotiating CHAP Algorithm 0x80 in LCP

option 3, Authentication Protocol.

* The MS-CHAP Response packet is in a format designed for

compatibility with Microsoft Windows NT 3.5 and 3.51,

Microsoft Windows95, and Microsoft LAN Manager 2.x networking

products. The MS-CHAP format does not require the

authenticator to store a clear or reversibly encrypted

password.

* MS-CHAP provides an authenticator controlled authentication

retry mechanism.

* MS-CHAP provides an authenticator controlled change password

mechanism.

* MS-CHAP defines a set of reason-for-failure codes returned in

the Failure packet Message field.

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2. LCP Configuration

The LCP configuration for MS-CHAP is identical to that for

standard CHAP, except that the Algorithm field has value 0x80,

rather than the MD5 value 0x05. Non-MS-CHAP-aware implementations

that correctly implement LCP Config-Rej have no problem dealing

with this non-standard option.

Microsoft currently negotiates authentication only on the

server->workstation configuration. Mutual authentication may be

supported in the future.

3. Challenge Packet

The MS-CHAP Challenge packet is identical in format to the

standard CHAP Challenge packet.

MS-CHAP authenticators send an 8-octet challenge Value field. It

is not necessary for peers to duplicate Microsoft's algorithm for

selecting the 8-octet value, but the CHAP guidelines on randomness

should be observed.

Microsoft authenticators do not currently provide information in

the Name field. This may change in the future.

4. Response Packet

The MS-CHAP Response packet is identical in format to the standard

CHAP Response packet. However, the Value field is sub-formatted

differently as follows:

24 octets: LAN Manager compatible challenge response

24 octets: Windows NT compatible challenge response

1 octet : "Use Windows NT compatible challenge response" flag

The LAN Manager compatible challenge response is an encoded

function of the password and the received challenge as output by

the pseudo-code routine LmChallengeResponse below. LAN Manager

passwords are limited to 14 case-insensitive OEM characters.

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LmChallengeResponse(

IN 8-octet Challenge,

IN 0-to-14-oem-char Password,

OUT 24-octet Response )

{

LmPasswordHash(

Password,

giving PasswordHash )

ChallengeResponse(

Challenge,

PasswordHash,

giving Response )

}

LmPasswordHash(

IN 0-to-14-oem-char Password,

OUT 16-octet PasswordHash )

{

Set UcasePassword to the uppercased Password

Zero pad UcasePassword to 14 characters

DesHash(

1st 7-octets of UcasePassword,

giving 1st 8-octets of PasswordHash )

DesHash(

2nd 7-octets of UcasePassword,

giving 2nd 8-octets of PasswordHash )

}

DesHash(

IN 7-octet Clear,

OUT 8-octet Cypher )

{

Make Cypher an irreversibly encrypted form of Clear by

encrypting known text [6] using Clear as the secret key,

that is...

DesEncrypt(

StdText,

Clear,

giving Cypher )

}

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DesEncrypt(

IN 8-octet Clear,

IN 7-octet Key,

OUT 8-octet Cypher )

{

Use the DES encryption algorithm [3] to encrypt Clear into

Cypher such that Cypher can only be decrypted back to

Clear by providing Key. Note that the DES algorithm takes

as input a 64-bit stream where the 8th, 16th, 24th, etc

bits are parity bits ignored by the encrypting algorithm.

Unless you write your own DES to accept 56-bit input

without parity, you will need to insert the parity bits

yourself.

}

ChallengeResponse(

IN 8-octet Challenge,

IN 16-octet PasswordHash,

OUT 24-octet Response )

{

Set ZPasswordHash to PasswordHash zero padded to 21 octets

DesEncrypt(

Challenge,

1st 7-octets of ZPasswordHash,

giving 1st 8-octets of Response )

DesEncrypt(

Challenge,

2nd 7-octets of ZPasswordHash,

giving 2nd 8-octets of Response )

DesEncrypt(

Challenge,

3rd 7-octets of ZPasswordHash,

giving 3rd 8-octets of Response )

}

The Windows NT compatible challenge response is an encoded

function of the password and the received challenge as output by

the NtChallengeResponse routine below. The Windows NT password is

a string of 0 to (theoretically) 256 case-sensitive Unicode

characters. Current versions of Windows NT limit passwords to 14

characters, mainly for compatibility reasons, though this may

change in the future.

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NtChallengeResponse(

IN 8-octet Challenge,

IN 0-to-256-unicode-char Password,

OUT 24-octet Response )

{

NtPasswordHash(

Password,

giving PasswordHash )

ChallengeResponse(

Challenge,

PasswordHash,

giving Response )

}

NtPasswordHash(

IN 0-to-256-unicode-char Password,

OUT 16-octet PasswordHash )

{

Use the MD4 algorithm [4] to irreversibly hash Password

into PasswordHash. Only the password is hashed without

including any terminating 0.

}

The "use Windows NT compatible challenge response" flag, if 1,

indicates that the Windows NT response is provided and should be

used in preference to the LAN Manager response. The LAN Manager

response will still be used if the account does not have a Windows

NT password hash, e.g. if the password has not been changed since

the account was uploaded from a LAN Manager 2.x account database.

The LAN Manager response need not be provided (set to 0's) if the

implementation expects all user accounts to be stored only in

fresh Windows NT account databases or ones where all uploaded

passwords have been changed. However, doing so may sacrifice

downward compatibility with non-Windows-NT servers.

If the flag is 0, the Windows NT response is ignored and the LAN

Manager response is used. If the password is LAN Manager

compatible, interoperability may be achieved without providing the

Windows NT challenge response (set to 0's), and providing only the

LAN Manager response. This is what Microsoft Windows95 does,

though this may change in the future. Doing so may sacrifice

interoperability with OEM-specific versions of Windows NT designed

for maximum security in Windows-NT-only networks.

Implementors seeking the broadest possible interoperability are

advised to supply both responses when the password is LAN Manager

compatible. This is what Microsoft Windows NT does.

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The Name field identifies the authenticatee's user account name.

The Windows NT domain name may prefix the user's account name in

the typical Windows NT format, e.g. "redmond\stevec" where

"redmond" is a Windows NT domain containing the user account

"stevec". If a domain is not provided, the backslash should also

be omitted, e.g. "stevec".

5. Success Packet

The Success packet is identical in format to the standard CHAP

Success packet.

6. Failure Packet

The Failure packet is identical in format to the standard CHAP

Failure packet. There is, however, formatted text stored in the

Message field which, contrary to the standard CHAP rules, does

affect the protocol. The Message field format is:

"E=eeeeeeeeee R=r C=cccccccccccccccc V=vvvvvvvvvv"

where

The "eeeeeeeeee" is the decimal error code (need not be 10

digits) corresponding to one of those listed below, though

implementations should deal with codes not on this list

gracefully.

646 ERROR_RESTRICTED_LOGON_HOURS

647 ERROR_ACCT_DISABLED

648 ERROR_PASSWD_EXPIRED

649 ERROR_NO_DIALIN_PERMISSION

691 ERROR_AUTHENTICATION_FAILURE

709 ERROR_CHANGING_PASSWORD

The "r" is a flag set to "1" if a retry is allowed, and "0" if

not. When authenticator sets this flag to "1" it disables

short timeouts, expecting the authenticatee to prompt the user

for new credentials and resubmit the response.

The "cccccccccccccccc" is 16 hex digits representing an ASCII

representation of a new challenge value. This field is

optional. If it is not sent, authenticator expects the

resubmitted response to be calculated based on the previous

challenge value plus decimal 23 in the first octet, i.e. the

one immediately following the Value Size field. Windows95

authenticators may send this field. Windows NT authenticators

do not, but may in the future. Both systems implement

authenticatee support of this field.

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The "vvvvvvvvvv" is the decimal version code (need not be 10

digits) indicating the MS-CHAP protocol version supported on

the server. Currently, this is interesting only in selecting

a Change Password packet type. If the field is not present

the version should be assumed 1.

Implementations should accept but ignore additional text they do

not recognize.

7. Change Password Packet (version 1)

The version 1 Change Password packet does not appear in standard

CHAP. It allows the authenticatee to change the password on the

account specified in the previous Response packet. The version 1

Change Password packet should be sent only if the authenticator

reports ERROR_PASSWD_EXPIRED (E=648) in the Message field of the

Failure packet.

This packet type is supported by Windows NT 3.5 and 3.51. It is

not supported by Windows95, though this may change in the future.

See also Change Password Packet (version 2).

The format of this packet is as follows:

1 octet : Code (=5)

1 octet : Identifier

2 octets: Length (=72)

16 octets: Encrypted LAN Manager Old password Hash

16 octets: Encrypted LAN Manager New Password Hash

16 octets: Encrypted Windows NT Old Password Hash

16 octets: Encrypted Windows NT New Password Hash

2 octets: Password Length

2 octets: Flags

Code

Identifier

The Identifier field is one octet and aids in matching

requests and replies. The value is the Identifier of the

received Failure packet to which this packet responds plus 1.

Length

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Encrypted LAN Manager New Password Hash

Encrypted LAN Manager Old Password Hash

These fields contain the LAN Manager password hash of the new

and old passwords encrypted with an 8-octet key value [6], as

output by the pseudo-code routine LmEncryptedPasswordHash

below.

LmEncryptedPasswordHash(

IN 0-to-14-oem-char Password,

IN 8-octet KeyValue,

OUT 16-octet Cypher )

{

LmPasswordHash(

Password,

giving PasswordHash )

PasswordHashEncryptedWithBlock(

PasswordHash,

KeyValue,

giving Cypher )

}

PasswordHashEncryptedWithBlock(

IN 16-octet PasswordHash,

IN 7-octet Block,

OUT 16-octet Cypher )

{

DesEncrypt(

1st 8-octets PasswordHash,

1st 7-octets Block,

giving 1st 8-octets Cypher )

DesEncrypt(

2nd 8-octets PasswordHash,

1st 7-octets Block,

giving 2nd 8-octets Cypher )

}

Encrypted Windows NT New Password Hash

Encrypted Windows NT Old Password Hash

These fields contain the Windows NT password hash of the new

and old passwords encrypted with an 8-octet key value [6], as

output by the pseudo-code routine NtEncryptedPasswordHash

below.

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NtEncryptedPasswordHash(

IN 0-to-14-oem-char Password

IN 8-octet Challenge

OUT 16-octet Cypher )

{

NtPasswordHash(

Password,

giving PasswordHash )

PasswordHashEncryptedWithBlock(

PasswordHash,

Challenge,

giving Cypher )

}

Password Length

The length in octets of the LAN Manager compatible form of the

new password. If this value is less than or equal to 14 it is

assumed that the encrypted LAN Manager password hash fields

are valid. Otherwise, it is assumed these fields are not

valid, in which case the Windows NT compatible passwords must

be provided.

Flags

Bit field of option flags where 0 is the least significant bit

of the 16-bit quantity:

0 : Set 1 indicates that the encrypted Windows NT

hashed passwords are valid and should be used. If

0, the Windows NT fields are not used and the LAN

Manager fields must be provided.

For the broadest possible interoperability,

implementations are encouraged to provide both the

Windows NT and LAN Manager fields when the password

is LAN Manager compatible. This is what Windows NT

does.

1-15 : Reserved, always set 0.

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8. Change Password Packet (version 2)

The version 2 Change Password packet does not appear in standard

CHAP. It allows the authenticatee to change the password on the

account specified in the previous Response packet. The version 2

Change Password packet should be sent only if the authenticator

reports ERROR_PASSWD_EXPIRED (E=648) and a version of 2 or more in

the Message field of the Failure packet.

This packet type is supported by Windows NT 3.51. It is not

supported by Windows NT 3.5 or Windows95, though the latter may

change in the future. The version 2 change password packet type

is preferable to the version 1 type and should be offered and

accepted where possible.

The format of this packet is as follows:

1 octet : Code (=6)

1 octet : Identifier

2 octet : Length (=1070)

516 octets : Password Encrypted with Old NT Hash

16 octets : Old NT Hash Encrypted with New NT Hash

516 octets : Password Encrypted with Old LM Hash

16 octets : Old LM Hash Encrypted With New NT Hash

24 octets : LAN Manager compatible challenge response

24 octets : Windows NT compatible challenge response

2-octet : Flags

Code

Identifier

The Identifier field is one octet and aids in matching

requests and replies. The value is the Identifier of the

received Failure packet to which this packet responds plus 1.

Length

1118

Password Encrypted with Old NT Hash

This field contains the PWBLOCK form of the new Windows NT

password encrypted with the old Windows NT password hash, as

output by the NewPasswordEncryptedWithOldNtPasswordHash

routine below:

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datatype-PWBLOCK

{

256-unicode-char Password

4-octets PasswordLength

}

NewPasswordEncryptedWithOldNtPasswordHash(

IN 0-to-256-unicode-char NewPassword,

IN 0-to-256-unicode-char OldPassword,

OUT datatype-PWBLOCK EncryptedPwBlock )

{

NtPasswordHash(

OldPassword,

giving PasswordHash )

EncryptPwBlockWithPasswordHash(

NewPassword,

PasswordHash,

giving EncryptedPwBlock )

}

EncryptPwBlockWithPasswordHash(

IN 0-to-256-unicode-char Password,

IN 16-octet PasswordHash,

OUT datatype-PWBLOCK PwBlock )

{

Fill ClearPwBlock with random octet values

lstrcpyW( to ClearPwBlock.Password, from Password )

ClearPwBlock.PasswordLength = lstrlenW( Password )

Rc4Encrypt(

ClearPwBlock,

sizeof( ClearPwBlock ),

PasswordHash,

sizeof( PasswordHash ),

giving PwBlock )

}

Rc4Encrypt(

IN x-octet Clear,

IN integer ClearLength,

IN y-octet Key,

IN integer KeyLength,

OUT x-octet Cypher )

{

Use the RC4 encryption algorithm [5] to encrypt Clear of

length ClearLength octets into a Cypher of the same length

such that the Cypher can only be decrypted back to Clear

by providing a Key of length KeyLength octets.

}

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Old NT Hash Encrypted with New NT Hash

This field contains the old Windows NT password hash encrypted

with the new Windows NT password hash, as output by the

OldNtPasswordHashEncryptedWithNewNtPasswordHash routine below:

OldNtPasswordHashEncryptedWithNewNtPasswordHash(

IN 0-to-256-unicode-char NewPassword,

IN 0-to-256-unicode-char OldPassword,

OUT 16-octet EncryptedPasswordHash )

{

NtPasswordHash(

OldPassword,

giving OldPasswordHash )

NtPasswordHash(

NewPassword,

giving NewPasswordHash )

PasswordHashEncryptedWithBlock(

OldPasswordHash,

NewPasswordHash,

giving EncrytptedPasswordHash )

}

Password Encrypted with Old LM Hash

This field contains the PWBLOCK form of the new Windows NT

password encrypted with the old LAN Manager password hash, as

output by the NewPasswordEncryptedWithOldLmPasswordHash

routine below:

NewPasswordEncryptedWithOldLmPasswordHash(

IN 0-to-256-unicode-char NewPassword,

IN 0-to-256-unicode-char OldPassword,

OUT datatype-PWBLOCK EncryptedPwBlock )

{

LmPasswordHash(

OldPassword,

giving PasswordHash )

EncryptPwBlockWithPasswordHash(

NewPassword,

PasswordHash,

giving EncryptedPwBlock )

}

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Old LM Hash Encrypted with New NT Hash

This field contains the old LAN Manager password hash encrypted

with the new Windows NT password hash, as output by the

OldLmPasswordHashEncryptedWithNewNtPasswordHash routine below:

OldLmPasswordHashEncryptedWithNewNtPasswordHash(

IN 0-to-256-unicode-char NewPassword,

IN 0-to-256-unicode-char OldPassword,

OUT 16-octet EncryptedPasswordHash )

{

LmPasswordHash(

OldPassword,

giving OldPasswordHash )

NtPasswordHash(

NewPassword,

giving NewPasswordHash )

PasswordHashEncryptedWithBlock(

OldPasswordHash,

NewPasswordHash,

giving EncrytptedPasswordHash )

}

LAN Manager compatible challenge response

Windows NT compatible challenge response

The challenge response fields as described in the Response

packet description, but calculated on the new password and the

same challenge used in the last response.

Flags

Bit field of option flags:

0 : The "use Windows NT compatible challenge response"

flag as described in the Response packet.

1 : Set 1 indicates that the "Password Encrypted with

Old LM Hash" and "Old LM Hash Encrypted With New NT

Hash" fields are valid and should be used. Set 0

indicates these fields are not valid.

For the broadest possible interoperability,

implementations are encouraged to provide both the

Windows NT and LAN Manager fields when the password

is LAN Manager compatible. This is what Windows NT

does.

2-15 : Reserved, always set 0.

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9. Negotiation Examples

Here are some examples of typical negotiations. The authenticatee

is on the left and the authenticator is on the right.

The packet sequence ID is incremented on each authentication retry

Response and on the change password response. All cases where the

packet sequence ID is updated are noted below.

Response retry is never allowed after either Change Password.

Change Password may occur after Response retry. The implied

challenge form is shown in the examples, though all cases of

"first challenge+23" should be replaced by the

"C=cccccccccccccccc" challenge if authenticator supplies it in the

Failure packet.

Successful authentication

<- Challenge

Response ->

<- Success

Failed authentication with no retry allowed

<- Challenge

Response ->

<- Failure (E=691 R=0)

Successful authentication after retry

<- Challenge

Response ->

<- Failure (E=691 R=1), disable short timeout

Response (++ID) to first challenge+23 ->

<- Success

Failed hack attack with 3 attempts allowed

<- Challenge

Response ->

<- Failure (E=691 R=1), disable short timeout

Response (++ID) to first challenge+23 ->

<- Failure (E=691 R=1), disable short timeout

Response (++ID) to first challenge+23+23 ->

<- Failure (E=691 R=0)

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Successful authentication with password change

<- Challenge

Response ->

<- Failure (E=648 R=0), disable short timeout

ChangePassword (++ID) to first challenge ->

<- Success

Successful authentication with retry and password change

<- Challenge

Response ->

<- Failure (E=691 R=1), disable short timeout

Response (++ID) to first challenge+23 ->

<- Failure (E=648 R=0), disable short timeout

ChangePassword (++ID) to first challenge+23 ->

<- Success

10. Hash Example

Intermediate values for password "MyPw".

8-octet Challenge:

10 2D B5 DF 08 5D 30 41

0-to-14-oem-char LmPassword:

4D 59 50 57

16-octet LmPasswordHash:

75 BA 30 19 8E 6D 19 75 AA D3 B4 35 B5 14 04 EE

24-octet LmChallengeResponse:

91 88 1D 01 52 AB 0C 33 C5 24 13 5E C2 4A 95 EE

64 E2 3C DC 2D 33 34 7D

0-to-256-unicode-char NtPassword:

4D 00 79 00 50 00 77 00

16-octet NtPasswordHash:

FC 15 6A F7 ED CD 6C 0E DD E3 33 7D 42 7F 4E AC

24-octet NtChallengeResponse:

4E 9D 3C 8F 9C FD 38 5D 5B F4 D3 24 67 91 95 6C

A4 C3 51 AB 40 9A 3D 61

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REFERENCES

[1] Simpson, W., "The Point-to-Point Protocol (PPP)", RFC 1331,

Daydreamer, May 1992

[2] LLoyd, B and Simpson, W., "PPP Authentication Protocols",

RFC 1334, L&A and Daydreamer respectively, Octobet 1992

[3] "Data Encryption Standard (DES)" is Federal Information

Processing Standard publication 46, National Institute of

Standard and Techology.

[4] Rivest, R., "MD4 Message Digest Algorithm", RFC 1320, MIT

Laboratory for Computer Science and RSA Data Security, Inc.,

April 1992.

[5] RC4 is an encryption standard available from RSA Data Security

Inc.

[6] The 8-octet StdText string used in the LAN Manager compatible

password hashing and the 8-octet KeyValue used in the Change

Password (version 1) packet are not available for public

distribution at this time. Contact the Microsoft Developer

Relations group (at time of writing dbeaver@microsoft.com) for

details on obtaining these values. On this particular point

the author can't help you.

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CHAIR'S ADDRESS

The working group can be contacted via the current chair:

Fred Baker

Email: fred@cisco.com

AUTHOR'S ADDRESS

The author is a developer in Microsoft's Windows NT

Internetworking group, which monitors the ietf-ppp@merit.edu

discussions. Questions can also be directed as below, where email

is preferred.

Steve Cobb

Microsoft Corporation

One Microsoft Way

Redmond, WA 98052-6399