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Microsoft PPP CHAP Extensions.
G. Zorn, S. Cobb. October 1998.

 
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Network Working Group G. Zorn Request for Comments: 2433 S. Cobb Category: Informational Microsoft Corporation October 1998 Microsoft PPP CHAP Extensions 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 (1998). All Rights Reserved. IESG Note The protocol described here has significant vulnerabilities. People planning on implementing or using this protocol should read section 12, "Security Considerations". 1. 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 1994 [2], which the reader should have at hand. The algorithms used in the generation of various MS-CHAP protocol fields are described in an appendix. 2. Introduction Microsoft created MS-CHAP to authenticate remote Windows workstations, providing the functionality to which LAN-based users are accustomed while integrating the encryption and hashing algorithms used on Windows networks. Zorn & Cobb Informational [Page 1]
RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998 Where possible, MS-CHAP is consistent with standard CHAP. Briefly, the 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's Windows NT 3.5, 3.51 and 4.0, and Windows95 networking products. The MS-CHAP format does not require the authenticator to store a clear-text or reversibly encrypted password. * MS-CHAP provides authenticator-controlled authentication retry and password changing mechanisms. * MS-CHAP defines a set of reason-for-failure codes returned in the Failure packet Message field. 3. 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 [2]. 4. 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. PPP implementations which do not support MS-CHAP, but correctly implement LCP Config-Rej, should have no problem dealing with this non-standard option. 5. 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. Peers need not duplicate Microsoft's algorithm for selecting the 8-octet value, but the standard guidelines on randomness [1,2,7] SHOULD be observed. Microsoft authenticators do not currently provide information in the Name field. This may change in the future. Zorn & Cobb Informational [Page 2]
RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998 6. 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 routine LmChallengeResponse() (see section A.1, below). LAN Manager passwords are limited to 14 case-insensitive OEM characters. Note that use of the LAN Manager compatible challenge response has been deprecated; peers SHOULD NOT generate it, and the sub-field SHOULD be zero-filled. The algorithm used in the generation of the LAN Manager compatible challenge response is described here for informational purposes only. The Windows NT compatible challenge response is an encoded function of the password and the received challenge as output by the routine NTChallengeResponse() (see section A.5, below). The Windows NT password is a string of 0 to (theoretically) 256 case-sensitive Unicode [8] characters. Current versions of Windows NT limit passwords to 14 characters, mainly for compatibility reasons; this may change in the future. 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. If the flag is 0, the Windows NT response is ignored and the LAN Manager response is used. Since the use of LAN Manager authentication has been deprecated, this flag SHOULD always be set (1) and the LAN Manager compatible challenge response field SHOULD be zero-filled. The Name field identifies the peer's user account name. The Windows NT domain name may prefix the user's account name (e.g. "BIGCO\johndoe" where "BIGCO" is a Windows NT domain containing the user account "john-doe"). If a domain is not provided, the backslash should also be omitted, (e.g. "johndoe"). Zorn & Cobb Informational [Page 3]
RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998 7. Success Packet The Success packet is identical in format to the standard CHAP Success packet. 8. 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, affects 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 the authenticator sets this flag to "1" it disables short timeouts, expecting the peer to prompt the user for new credentials and resubmit the response. The "cccccccccccccccc" is 16 hexadecimal digits representing an ASCII representation of a new challenge value. This field is optional. If it is not sent, the 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. Windows 95 authenticators may send this field. Windows NT authenticators do not, but may in the future. Both systems implement peer support of this field. 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 to be 1; since use of the version 1 Zorn & Cobb Informational [Page 4]
RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998 Change Password packet has been deprecated, this field SHOULD always contain a value greater than or equal to 2. Implementations should accept but ignore additional text they do not recognize. 9. Change Password Packet (version 1) The version 1 Change Password packet does not appear in standard CHAP. It allows the peer 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) and V is either missing or equal to one in the Message field of the Failure packet. The use of the Change Password Packet (version 1) has been deprecated; the format of the packet is described here for informational purposes, but peers SHOULD NOT transmit it. 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 5 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 72 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 the last received challenge value, as output by the routine LmEncryptedPasswordHash() (see section A.8, below). Zorn & Cobb Informational [Page 5]
RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998 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 the last received challenge value, as output by the pseudo-code routine NtEncryptedPasswordHash() (see section A.10, below). Password Length The length in octets of the LAN Manager compatible form of the new password. If this value is greater than or equal to zero and 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 This field is two octets in length. It is a bit field of option flags where 0 is the least significant bit of the 16-bit quantity: Bit 0 If this bit is set (1), it indicates that the encrypted Windows NT hashed passwords are valid and should be used. If this bit is cleared (0), the Windows NT fields are not used and the LAN Manager fields must be provided. Bits 1-15 Reserved, always clear (0). 10. Change Password Packet (version 2) The version 2 Change Password packet does not appear in standard CHAP. It allows the peer to change the password on the account specified in the preceding 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 greater in the Message field of the Failure packet. This packet type is supported by Windows NT 3.51, 4.0 and recent versions of Windows 95. It is not supported by Windows NT 3.5 or early versions of Windows 95. The format of this packet is as follows: 1 octet : Code 1 octet : Identifier 2 octets : Length 516 octets : Password Encrypted with Old NT Hash Zorn & Cobb Informational [Page 6]
RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998 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 6 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 (see section A.11, below). 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 (see section A.14, below). 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 described in section A.15, below. Note, however, that the use of this field has been deprecated: peers SHOULD NOT generate it, and this field SHOULD be zero-filled. 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 (see section A.16, below). Note, however, that the use of this field has been deprecated: peers SHOULD NOT generate it, and this field SHOULD be zero-filled. Zorn & Cobb Informational [Page 7]
RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998 LAN Manager compatible challenge response Windows NT compatible challenge response The challenge response field (as described in the Response packet description), but calculated on the new password and the same challenge used in the last response. Note that use of the LAN Manager compatible challenge response has been deprecated; peers SHOULD NOT generate it, and the field SHOULD be zero- filled. Flags This field is two octets in length. It is a bit field of option flags where 0 is the least significant bit of the 16-bit quantity. The format of this field is illustrated in the following diagram: 1 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Bit 0 The "use Windows NT compatible challenge response" flag as described in the Response packet. Bit 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. Clear (0) indicates these fields are not valid. This bit SHOULD always be clear (0). Bits 2-15 Reserved, always clear (0). 11. Security Considerations As an implementation detail, the authenticator SHOULD limit the number of password retries allowed to make brute-force password guessing attacks more difficult. Because the challenge value is encrypted using the password hash to form the response and the challenge is transmitted in clear-text form, both passive known-plaintext and active chosen-plaintext attacks against the password hash are possible. Suitable precautions (i.e., frequent password changes) SHOULD be taken in environments where eavesdropping is likely. Zorn & Cobb Informational [Page 8]
RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998 The Change Password (version 1) packet is vulnerable to a passive eavesdropping attack which can easily reveal the new password hash. For this reason, it MUST NOT be sent if eavesdropping is possible. 12. References [1] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994. [2] Simpson, W., "PPP Challenge Handshake Authentication Protocol (CHAP)", RFC 1994, August 1996. [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [4] "Data Encryption Standard (DES)", Federal Information Processing Standard Publication 46-2, National Institute of Standards and Technology, December 1993. [5] Rivest, R., "MD4 Message Digest Algorithm", RFC 1320, April 1992. [6] 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 [7] Eastlake, D., Crocker, S., and J. Schiller, "Randomness Recomnendations for Security", RFC 1750, December 1994. [8] "The Unicode Standard, Version 2.0", The Unicode Consortium, Addison-Wesley, 1996. ISBN 0-201-48345-9. [9] "DES Modes of Operation", Federal Information Processing Standards Publication 81, National Institute of Standards and Technology, December 1980 13. Acknowledgements Thanks (in no particular order) to Jeff Haag (Jeff_Haag@3com.com), Bill Palter (palter@network-alchemy.com), Bruce Johnson (bjohnson@microsoft.com), Tony Bell (tonybe@microsoft.com), Benoit Martin (ehlija@vircom.com), and Joe Davies (josephd@microsoft.com) for useful suggestions and feedback. Zorn & Cobb Informational [Page 9]
RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998 14. Chair's Address The PPP Extensions Working Group can be contacted via the current chair: Karl Fox Ascend Communications 3518 Riverside Drive Suite 101 Columbus, OH 43221 Phone: +1 614 326 6841 EMail: karl@ascend.com 15. Authors' Addresses Questions about this memo can also be directed to: Glen Zorn Microsoft Corporation One Microsoft Way Redmond, Washington 98052 Phone: +1 425 703 1559 Fax: +1 425 936 7329 EMail: glennz@microsoft.com Steve Cobb Microsoft Corporation One Microsoft Way Redmond, Washington 98052 EMail: stevec@microsoft.com Zorn & Cobb Informational [Page 10]
RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998 Appendix A - Pseudocode The routines mentioned in the text are described in pseudocode below. A.1 LmChallengeResponse() 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 ) } A.2 LmPasswordHash() 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 ) } A.3 DesHash() DesHash( IN 7-octet Clear, OUT 8-octet Cypher ) { /* * Make Cypher an irreversibly encrypted form of Clear by * encrypting known text using Clear as the secret key. * The known text consists of the string * * KGS!@#$% */ Set StdText to "KGS!@#$%" Zorn & Cobb Informational [Page 11]
RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998 DesEncrypt( StdText, Clear, giving Cypher ) } A.4 DesEncrypt() DesEncrypt( IN 8-octet Clear, IN 7-octet Key, OUT 8-octet Cypher ) { /* * Use the DES encryption algorithm [4] in ECB mode [9] * 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. */ } A.5 NtChallengeResponse() 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 ) } A.6 NtPasswordHash() NtPasswordHash( IN 0-to-256-unicode-char Password, OUT 16-octet PasswordHash ) { /* * Use the MD4 algorithm [5] to irreversibly hash Password * into PasswordHash. Only the password is hashed without * including any terminating 0. */ Zorn & Cobb Informational [Page 12]
RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998 } A.7 ChallengeResponse() 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 ) } A.8 LmEncryptedPasswordHash() 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 ) } A.9 PasswordHashEncryptedWithBlock() PasswordHashEncryptedWithBlock( IN 16-octet PasswordHash, IN 8-octet Block, OUT 16-octet Cypher ) { Zorn & Cobb Informational [Page 13]
RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998 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 ) } A.10 NtEncryptedPasswordHash() 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 ) } A.11 NewPasswordEncryptedWithOldNtPasswordHash() 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 ) } A.12 EncryptPwBlockWithPasswordHash() EncryptPwBlockWithPasswordHash( IN 0-to-256-unicode-char Password, IN 16-octet PasswordHash, Zorn & Cobb Informational [Page 14]
RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998 OUT datatype-PWBLOCK PwBlock ) { Fill ClearPwBlock with random octet values PwSize = lstrlenW( Password ) * sizeof( unicode-char ) PwOffset = sizeof( ClearPwBlock.Password ) - PwSize Move PwSize octets to (ClearPwBlock.Password + PwOffset ) from Password ClearPwBlock.PasswordLength = PwSize Rc4Encrypt( ClearPwBlock, sizeof( ClearPwBlock ), PasswordHash, sizeof( PasswordHash ), giving PwBlock ) } A.13 Rc4Encrypt() Rc4Encrypt( IN x-octet Clear, IN integer ClearLength, IN y-octet Key, IN integer KeyLength, OUT x-octet Cypher ) { /* * Use the RC4 encryption algorithm [6] 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. */ } A.14 OldNtPasswordHashEncryptedWithNewNtPasswordHash() 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 ) NtPasswordHashEncryptedWithBlock( OldPasswordHash, NewPasswordHash, giving EncryptedPasswordHash ) } Zorn & Cobb Informational [Page 15]
RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998 A.15 NewPasswordEncryptedWithOldLmPasswordHash() 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 ) } A.16 OldLmPasswordHashEncryptedWithNewNtPasswordHash() 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 ) NtPasswordHashEncryptedWithBlock( OldPasswordHash, NewPasswordHash, giving EncrytptedPasswordHash ) } A.17 NtPasswordHashEncryptedWithBlock() NtPasswordHashEncryptedWithBlock( IN 16-octet PasswordHash, IN 16-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, 2nd 7-octets Block, giving 2nd 8-octets Cypher ) } Zorn & Cobb Informational [Page 16]
RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998 Appendix B - Examples B.1 Negotiation Examples Here are some examples of typical negotiations. The peer 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 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. B.1.1 Successful authentication <- Challenge Response -> <- Success B.1.2 Failed authentication with no retry allowed <- Challenge Response -> <- Failure (E=691 R=0) B.1.3 Successful authentication after retry <- Challenge Response -> <- Failure (E=691 R=1), disable short timeout Response (++ID) to first challenge+23 -> <- Success B.1.4 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 -> Zorn & Cobb Informational [Page 17]
RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998 <- Failure (E=691 R=0) B.1.5 Successful authentication with password change <- Challenge Response -> <- Failure (E=648 R=0 V=2), disable short timeout ChangePassword (++ID) to first challenge -> <- Success B.1.6 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 V=2), disable short timeout ChangePassword (++ID) to first challenge+23 -> <- Success B.2 Hash Example Intermediate values for password "MyPw". 8-octet Challenge: 10 2D B5 DF 08 5D 30 41 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 B.3 Example of DES Key Generation DES uses 56-bit keys, expanded to 64 bits by the insertion of parity bits. After the parity of the key has been fixed, every eighth bit is a parity bit and the number of bits that are set (1) in each octet is odd; i.e., odd parity. Note that many DES engines do not check parity, however, simply stripping the parity bits. The following example illustrates the values resulting from the use of the 16-octet NTPasswordHash shown in Appendix B.2 to generate a pair of DES keys Zorn & Cobb Informational [Page 18]
RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998 (e.g., for use in the NtPasswordHashEncryptedWithBlock() described in Appendix A.17). 16-octet NtPasswordHash: FC 15 6A F7 ED CD 6C 0E DD E3 33 7D 42 7F 4E AC First "raw" DES key (initial 7 octets of password hash): FC 15 6A F7 ED CD 6C First parity-corrected DES key (eight octets): FD 0B 5B 5E 7F 6E 34 D9 Second "raw" DES key (second 7 octets of password hash) 0E DD E3 33 7D 42 7F Second parity-corrected DES key (eight octets): 0E 6E 79 67 37 EA 08 FE Zorn & Cobb Informational [Page 19]
RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998 Full Copyright Statement Copyright (C) The Internet Society (1998). 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. Zorn & Cobb Informational [Page 20]

   

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