Network Working Group G. Zorn
Request for Comments: 2759 Microsoft Corporation
Category: Informational January 2000
Microsoft PPP CHAP Extensions, Version 2
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 (2000). All Rights Reserved.
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 version two of Microsoft's PPP CHAP dialect
(MS-CHAP-V2). MS-CHAP-V2 is similar to, but incompatible with, MS-
CHAP version one (MS-CHAP-V1, described in [9]). In particular,
certain protocol fields have been deleted or reused but with
different semantics. In addition, MS-CHAP-V2 features mutual
authentication.
The algorithms used in the generation of various MS-CHAP-V2 protocol
fields are described in section 8. Negotiation and hash generation
examples are provided in section 9.
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 [3].
Zorn Informational [Page 1]
RFC 2759 Microsoft MS-CHAP-V2 January 2000 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. LCP Configuration . . . . . . . . . . . . . . . . . . . . . . . 3 3. Challenge Packet . . . . . . . . . . . . . . . . . . . . . . . 3 4. Response Packet . . . . . . . . . . . . . . . . . . . . . . . . 4 5. Success Packet . . . . . . . . . . . . . . . . . . . . . . . . 4 6. Failure Packet . . . . . . . . . . . . . . . . . . . . . . . . 5 7. Change-Password Packet . . . . . . . . . . . . . . . . . . . . 6 8. Pseudocode . . . . . . . . . . . . . . . . . . . . . . . . . . 7 8.1. GenerateNTResponse() . . . . . . . . . . . . . . . . . . . . 7 8.2. ChallengeHash() . . . . . . . . . . . . . . . . . . . . . . . 8 8.3. NtPasswordHash() . . . . . . . . . . . . . . . . . . . . . . 9 8.4. HashNtPasswordHash() . . . . . . . . . . . . . . . . . . . . 9 8.5. ChallengeResponse() . . . . . . . . . . . . . . . . . . . . . 9 8.6. DesEncrypt() . . . . . . . . . . . . . . . . . . . . . . . . 10 8.7. GenerateAuthenticatorResponse() . . . . . . . . . . . . . . . 10 8.8. CheckAuthenticatorResponse() . . . . . . . . . . . . . . . . 12 8.9. NewPasswordEncryptedWithOldNtPasswordHash() . . . . . . . . . 12 8.10. EncryptPwBlockWithPasswordHash() . . . . . . . . . . . . . . 13 8.11. Rc4Encrypt() . . . . . . . . . . . . . . . . . . . . . . . . 13 8.12. OldNtPasswordHashEncryptedWithNewNtPasswordHash() . . . . . 14 8.13. NtPasswordHashEncryptedWithBlock() . . . . . . . . . . . . . 14 9. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 9.1. Negotiation Examples . . . . . . . . . . . . . . . . . . . . 14 9.1.1. Successful authentication . . . . . . . . . . . . . . . . . 15 9.1.2. Authenticator authentication failure . . . . . . . . . . . 15 9.1.3. Failed authentication with no retry allowed . . . . . . . . 15 9.1.4. Successful authentication after retry . . . . . . . . . . . 15 9.1.5. Failed hack attack with 3 attempts allowed . . . . . . . . 15 9.1.6. Successful authentication with password change . . . . . . 16 9.1.7. Successful authentication with retry and password change. . 16 9.2. Hash Example . . . . . . . . . . . . . . . . . . . . . . . . 16 9.3. Example of DES Key Generation . . . . . . . . . . . . . . . . 17 10. Security Considerations . . . . . . . . . . . . . . . . . . . 17 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19 13. Author's Address . . . . . . . . . . . . . . . . . . . . . . . 19 14. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 20 Zorn Informational [Page 2]
RFC 2759 Microsoft MS-CHAP-V2 January 2000 1. Introduction Where possible, MS-CHAP-V2 is consistent with both MS-CHAP-V1 and standard CHAP. Briefly, the differences between MS-CHAP-V2 and MS- CHAP-V1 are: * MS-CHAP-V2 is enabled by negotiating CHAP Algorithm 0x81 in LCP option 3, Authentication Protocol. * MS-CHAP-V2 provides mutual authentication between peers by piggybacking a peer challenge on the Response packet and an authenticator response on the Success packet. * The calculation of the "Windows NT compatible challenge response" sub-field in the Response packet has been changed to include the peer challenge and the user name. * In MS-CHAP-V1, the "LAN Manager compatible challenge response" sub-field was always sent in the Response packet. This field has been replaced in MS-CHAP-V2 by the Peer-Challenge field. * The format of the Message field in the Failure packet has been changed. * The Change Password (version 1) and Change Password (version 2) packets are no longer supported. They have been replaced with a single Change-Password packet. 2. LCP Configuration The LCP configuration for MS-CHAP-V2 is identical to that for standard CHAP, except that the Algorithm field has value 0x81, rather than the MD5 value 0x05. PPP implementations which do not support MS-CHAP-V2, but correctly implement LCP Config-Rej, should have no problem dealing with this non-standard option. 3. Challenge Packet The MS-CHAP-V2 Challenge packet is identical in format to the standard CHAP Challenge packet. MS-CHAP-V2 authenticators send an 16-octet challenge Value field. Peers need not duplicate Microsoft's algorithm for selecting the 16- 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 Informational [Page 3]
RFC 2759 Microsoft MS-CHAP-V2 January 2000 4. Response Packet The MS-CHAP-V2 Response packet is identical in format to the standard CHAP Response packet. However, the Value field is sub-formatted differently as follows: 16 octets: Peer-Challenge 8 octets: Reserved, must be zero 24 octets: NT-Response 1 octet : Flags The Peer-Challenge field is a 16-octet random number. As the name implies, it is generated by the peer and is used in the calculation of the NT-Response field, below. Peers need not duplicate Microsoft's algorithm for selecting the 16-octet value, but the standard guidelines on randomness [1,2,7] SHOULD be observed. The NT-Response field is an encoded function of the password, the user name, the contents of the Peer-Challenge field and the received challenge as output by the routine GenerateNTResponse() (see section 8.1, 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. When computing the NT-Response field contents, only the user name is used, without any associated Windows NT domain name. This is true regardless of whether a Windows NT domain name is present in the Name field (see below). The Flag field is reserved for future use and MUST be zero. The Name field is a string of 0 to (theoretically) 256 case-sensitive ASCII characters which 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 "johndoe"). If a domain is not provided, the backslash should also be omitted, (e.g. "johndoe"). 5. Success Packet The Success packet is identical in format to the standard CHAP Success packet. However, the Message field contains a 42-octet authenticator response string and a printable message. The format of the message field is illustrated below. "S=<auth_string> M=<message>" Zorn Informational [Page 4]
RFC 2759 Microsoft MS-CHAP-V2 January 2000 The <auth_string> quantity is a 20 octet number encoded in ASCII as 40 hexadecimal digits. The hexadecimal digits A-F (if present) MUST be uppercase. This number is derived from the challenge from the Challenge packet, the Peer-Challenge and NT-Response fields from the Response packet, and the peer password as output by the routine GenerateAuthenticatorResponse() (see section 8.7, below). The authenticating peer MUST verify the authenticator response when a Success packet is received. The method for verifying the authenticator is described in section 8.8, below. If the authenticator response is either missing or incorrect, the peer MUST end the session. The <message> quantity is human-readable text in the appropriate charset and language [12]. 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 operation of the protocol. The Message field format is: "E=eeeeeeeeee R=r C=cccccccccccccccccccccccccccccccc V=vvvvvvvvvv M=<msg>" where The "eeeeeeeeee" is the ASCII representation of a 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 an ASCII 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 "cccccccccccccccccccccccccccccccc" is the ASCII representation of a hexadecimal challenge value. This field MUST be exactly 32 octets long and MUST be present. Zorn Informational [Page 5]
RFC 2759 Microsoft MS-CHAP-V2 January 2000 The "vvvvvvvvvv" is the ASCII representation of a decimal version code (need not be 10 digits) indicating the password changing protocol version supported on the server. For MS-CHAP-V2, this value SHOULD always be 3. <msg> is human-readable text in the appropriate charset and language [12]. 7. Change-Password Packet The Change-Password packet does not appear in either standard CHAP or MS-CHAP-V1. It allows the peer to change the password on the account specified in the preceding Response packet. The 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 recent versions of Windows NT 4.0, Windows 95 and Windows 98. It is not supported by Windows NT 3.5, Windows NT 3.51, or early versions of Windows NT 4.0, Windows 95 and Windows 98. The format of this packet is as follows: 1 octet : Code 1 octet : Identifier 2 octets : Length 516 octets : Encrypted-Password 16 octets : Encrypted-Hash 16 octets : Peer-Challenge 8 octets : Reserved 24 octets : NT-Response 2-octet : Flags Code 7 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 586 Zorn Informational [Page 6]
RFC 2759 Microsoft MS-CHAP-V2 January 2000 Encrypted-Password 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 8.9, below). Encrypted-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 8.12, below). Peer-Challenge A 16-octet random quantity, as described in the Response packet description. Reserved 8 octets, must be zero. NT-Response The NT-Response field (as described in the Response packet description), but calculated on the new password and the challenge received in the Failure packet. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Bits 0-15 Reserved, always clear (0). 8. Pseudocode The routines mentioned in the text above are described in pseudocode in the following sections. 8.1. GenerateNTResponse() GenerateNTResponse( IN 16-octet AuthenticatorChallenge, IN 16-octet PeerChallenge, Zorn Informational [Page 7]
RFC 2759 Microsoft MS-CHAP-V2 January 2000 IN 0-to-256-char UserName, IN 0-to-256-unicode-char Password, OUT 24-octet Response ) { 8-octet Challenge 16-octet PasswordHash ChallengeHash( PeerChallenge, AuthenticatorChallenge, UserName, giving Challenge) NtPasswordHash( Password, giving PasswordHash ) ChallengeResponse( Challenge, PasswordHash, giving Response ) } 8.2. ChallengeHash() ChallengeHash( IN 16-octet PeerChallenge, IN 16-octet AuthenticatorChallenge, IN 0-to-256-char UserName, OUT 8-octet Challenge { /* * SHAInit(), SHAUpdate() and SHAFinal() functions are an * implementation of Secure Hash Algorithm (SHA-1) [11]. These are * available in public domain or can be licensed from * RSA Data Security, Inc. */ SHAInit(Context) SHAUpdate(Context, PeerChallenge, 16) SHAUpdate(Context, AuthenticatorChallenge, 16) /* * Only the user name (as presented by the peer and * excluding any prepended domain name) * is used as input to SHAUpdate(). */ SHAUpdate(Context, UserName, strlen(Username)) SHAFinal(Context, Digest) memcpy(Challenge, Digest, 8) } Zorn Informational [Page 8]
RFC 2759 Microsoft MS-CHAP-V2 January 2000 8.3. 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. */ } 8.4. HashNtPasswordHash() HashNtPasswordHash( IN 16-octet PasswordHash, OUT 16-octet PasswordHashHash ) { /* * Use the MD4 algorithm [5] to irreversibly hash * PasswordHash into PasswordHashHash. */ } 8.5. 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 ) } Zorn Informational [Page 9]
RFC 2759 Microsoft MS-CHAP-V2 January 2000 8.6. DesEncrypt() DesEncrypt( IN 8-octet Clear, IN 7-octet Key, OUT 8-octet Cypher ) { /* * Use the DES encryption algorithm [4] in ECB mode [10] * 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. */ } 8.7. GenerateAuthenticatorResponse() GenerateAuthenticatorResponse( IN 0-to-256-unicode-char Password, IN 24-octet NT-Response, IN 16-octet PeerChallenge, IN 16-octet AuthenticatorChallenge, IN 0-to-256-char UserName, OUT 42-octet AuthenticatorResponse ) { 16-octet PasswordHash 16-octet PasswordHashHash 8-octet Challenge /* * "Magic" constants used in response generation */ Magic1[39] = {0x4D, 0x61, 0x67, 0x69, 0x63, 0x20, 0x73, 0x65, 0x72, 0x76, 0x65, 0x72, 0x20, 0x74, 0x6F, 0x20, 0x63, 0x6C, 0x69, 0x65, 0x6E, 0x74, 0x20, 0x73, 0x69, 0x67, 0x6E, 0x69, 0x6E, 0x67, 0x20, 0x63, 0x6F, 0x6E, 0x73, 0x74, 0x61, 0x6E, 0x74}; Zorn Informational [Page 10]
RFC 2759 Microsoft MS-CHAP-V2 January 2000 Magic2[41] = {0x50, 0x61, 0x64, 0x20, 0x74, 0x6F, 0x20, 0x6D, 0x61, 0x6B, 0x65, 0x20, 0x69, 0x74, 0x20, 0x64, 0x6F, 0x20, 0x6D, 0x6F, 0x72, 0x65, 0x20, 0x74, 0x68, 0x61, 0x6E, 0x20, 0x6F, 0x6E, 0x65, 0x20, 0x69, 0x74, 0x65, 0x72, 0x61, 0x74, 0x69, 0x6F, 0x6E}; /* * Hash the password with MD4 */ NtPasswordHash( Password, giving PasswordHash ) /* * Now hash the hash */ HashNtPasswordHash( PasswordHash, giving PasswordHashHash) SHAInit(Context) SHAUpdate(Context, PasswordHashHash, 16) SHAUpdate(Context, NTResponse, 24) SHAUpdate(Context, Magic1, 39) SHAFinal(Context, Digest) ChallengeHash( PeerChallenge, AuthenticatorChallenge, UserName, giving Challenge) SHAInit(Context) SHAUpdate(Context, Digest, 20) SHAUpdate(Context, Challenge, 8) SHAUpdate(Context, Magic2, 41) SHAFinal(Context, Digest) /* * Encode the value of 'Digest' as "S=" followed by * 40 ASCII hexadecimal digits and return it in * AuthenticatorResponse. * For example, * "S=0123456789ABCDEF0123456789ABCDEF01234567" */ } Zorn Informational [Page 11]
RFC 2759 Microsoft MS-CHAP-V2 January 2000 8.8. CheckAuthenticatorResponse() CheckAuthenticatorResponse( IN 0-to-256-unicode-char Password, IN 24-octet NtResponse, IN 16-octet PeerChallenge, IN 16-octet AuthenticatorChallenge, IN 0-to-256-char UserName, IN 42-octet ReceivedResponse, OUT Boolean ResponseOK ) { 20-octet MyResponse set ResponseOK = FALSE GenerateAuthenticatorResponse( Password, NtResponse, PeerChallenge, AuthenticatorChallenge, UserName, giving MyResponse) if (MyResponse = ReceivedResponse) then set ResponseOK = TRUE return ResponseOK } 8.9. 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 ) } Zorn Informational [Page 12]
RFC 2759 Microsoft MS-CHAP-V2 January 2000 8.10. EncryptPwBlockWithPasswordHash() EncryptPwBlockWithPasswordHash( IN 0-to-256-unicode-char Password, IN 16-octet PasswordHash, 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 ) } 8.11. 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. */ } Zorn Informational [Page 13]
RFC 2759 Microsoft MS-CHAP-V2 January 2000 8.12. 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 ) } 8.13. 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 ) } 9. Examples The following sections include protocol negotiation and hash generation examples. 9.1. 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. Zorn Informational [Page 14]
RFC 2759 Microsoft MS-CHAP-V2 January 2000 9.1.1. Successful authentication <- Authenticator Challenge Peer Response/Challenge -> <- Success/Authenticator Response (Authenticator Response verification succeeds, call continues) 9.1.2. Authenticator authentication failure <- Authenticator Challenge Peer Response/Challenge -> <- Success/Authenticator Response (Authenticator Response verification fails, peer disconnects) 9.1.3. Failed authentication with no retry allowed <- Authenticator Challenge Peer Response/Challenge -> <- Failure (E=691 R=0) (Authenticator disconnects) 9.1.4. Successful authentication after retry <- Authenticator Challenge Peer Response/Challenge -> <- Failure (E=691 R=1), disable short timeout Response (++ID) to challenge in failure message -> <- Success/Authenticator Response (Authenticator Response verification succeeds, call continues) 9.1.5. Failed hack attack with 3 attempts allowed <- Authenticator Challenge Peer Response/Challenge -> <- Failure (E=691 R=1), disable short timeout Response (++ID) to challenge in Failure message -> <- Failure (E=691 R=1), disable short timeout Response (++ID) to challenge in Failure message -> <- Failure (E=691 R=0) Zorn Informational [Page 15]
RFC 2759 Microsoft MS-CHAP-V2 January 2000 9.1.6. Successful authentication with password change <- Authenticator Challenge Peer Response/Challenge -> <- Failure (E=648 R=0 V=3), disable short timeout ChangePassword (++ID) to challenge in Failure message -> <- Success/Authenticator Response (Authenticator Response verification succeeds, call continues) 9.1.7. Successful authentication with retry and password change <- Authenticator Challenge Peer Response/Challenge -> <- 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/Authenticator Response (Authenticator Response verification succeeds, call continues) 9.2. Hash Example Intermediate values for user name "User" and password "clientPass". All numeric values are hexadecimal. 0-to-256-char UserName: 55 73 65 72 0-to-256-unicode-char Password: 63 00 6C 00 69 00 65 00 6E 00 74 00 50 00 61 00 73 00 73 00 16-octet AuthenticatorChallenge: 5B 5D 7C 7D 7B 3F 2F 3E 3C 2C 60 21 32 26 26 28 16-octet PeerChallenge: 21 40 23 24 25 5E 26 2A 28 29 5F 2B 3A 33 7C 7E 8-octet Challenge: D0 2E 43 86 BC E9 12 26 16-octet PasswordHash: 44 EB BA 8D 53 12 B8 D6 11 47 44 11 F5 69 89 AE Zorn Informational [Page 16]
RFC 2759 Microsoft MS-CHAP-V2 January 2000 24 octet NT-Response: 82 30 9E CD 8D 70 8B 5E A0 8F AA 39 81 CD 83 54 42 33 11 4A 3D 85 D6 DF 16-octet PasswordHashHash: 41 C0 0C 58 4B D2 D9 1C 40 17 A2 A1 2F A5 9F 3F 42-octet AuthenticatorResponse: "S=407A5589115FD0D6209F510FE9C04566932CDA56" 9.3. 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 password "MyPw" to generate a pair of DES keys (e.g., for use in the NtPasswordHashEncryptedWithBlock() described in section 8.13). 0-to-256-unicode-char Password: 4D 79 50 77 16-octet PasswordHash: 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 10. 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. Zorn Informational [Page 17]
RFC 2759 Microsoft MS-CHAP-V2 January 2000 11. 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 Recommendations 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] Zorn, G. and Cobb, S., "Microsoft PPP CHAP Extensions", RFC 2433, October 1998. [10] "DES Modes of Operation", Federal Information Processing Standards Publication 81, National Institute of Standards and Technology, December 1980. [11] "Secure Hash Standard", Federal Information Processing Standards Publication 180-1, National Institute of Standards and Technology, April 1995. [12] Zorn, G., "PPP LCP Internationalization Configuration Option", RFC 2484, January 1999. Zorn Informational [Page 18]
RFC 2759 Microsoft MS-CHAP-V2 January 2000 12. Acknowledgements Thanks (in no particular order) to Bruce Johnson, Tony Bell, Paul Leach, Terence Spies, Dan Simon, Narendra Gidwani, Gurdeep Singh Pall, Jody Terrill, Brad Robel-Forrest, and Joe Davies for useful suggestions and feedback. 13. Author's Address 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: gwz@acm.org Zorn Informational [Page 19]
RFC 2759 Microsoft MS-CHAP-V2 January 2000 14. Full Copyright Statement Copyright (C) The Internet Society (2000). 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. Zorn Informational [Page 20]
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