LM hash or LAN Manager hash is one of the formats that Microsoft LAN Manager and Microsoft Windows use to store user passwords that are less than 15 characters long. This type of hash is the only type of encryption used in Microsoft LAN Manager, hence the name, and versions of Windows up to Windows Me. It is also supported in more recent Windows versions for backward compatibility. The LAN Manager was an advanced Network Operating System (NOS) from Microsoft developed in cooperation with 3COM. It is based on the Operating System/2 (OS/2) and NetBEUI protocol similar to its predecessors MS-NET for MS-DOS and Xenix-NET for MS-Xenix. ... “Windows” redirects here. ... A password is a form of secret authentication data that is used to control access to a resource. ... Encrypt redirects here. ... The LAN Manager was an advanced Network Operating System (NOS) from Microsoft developed in cooperation with 3COM. It is based on the Operating System/2 (OS/2) and NetBEUI protocol similar to its predecessors MS-NET for MS-DOS and Xenix-NET for MS-Xenix. ... Windows Millennium Edition, or Windows Me (IPA pronunciation: [miː], [ɛm iː]), is a hybrid 16-bit/32-bit graphical operating system released on September 14, 2000 by Microsoft. ... This article or section does not cite its references or sources. ...

Algorithm

The LM hash is computed as follows.^[1]

1. The user’s password as an OEM string is converted to uppercase.
2. This password is either null-padded or truncated to 14 bytes.
3. The “fixed-length” password is split into two 7-byte halves.
4. These values are used to create two DES keys, one from each 7-byte half, by converting the seven bytes into a bit stream, and inserting a zero bit after every seven bits. This generates the 56 bits needed.
5. Each of these keys is used to DES-encrypt the constant ASCII string “KGS!@#$%”, resulting in two 8-byte ciphertext values.
6. These two ciphertext values are concatenated to form a 16-byte value, which is the LM hash.

Code page is the traditional IBM term used for a specific character encoding table: a mapping in which a sequence of bits, usually a single octet representing integer values 0 through 255, is associated with a specific character. ... Majuscules or capital letters (in the Roman alphabet: A, B, C, ...) are one type of case in a writing system. ... The Data Encryption Standard (DES) is a cipher (a method for encrypting information) selected as an official Federal Information Processing Standard (FIPS) for the United States in 1976, and which has subsequently enjoyed widespread use internationally. ... Image:ASCII fullsvg There are 95 printable ASCII characters, numbered 32 to 126. ...

Security weaknesses

Although it is based on DES, a reasonably secure block cipher, the LM hash can easily be cracked due to two weaknesses in its implementation. First, passwords longer than 7 characters are divided into two pieces and each piece is hashed separately. Second, all lower case letters in the password are changed to upper case before the password is hashed. The first weakness allows each half of the password to be attacked separately. While there are 2⁸⁴ different passwords made of up to 14 mixed case letters and digits, there would be only 2⁴² different 1-7 character password pieces using the same character set. Restricting the character set to upper case letters and digits further reduces the number of possibilities for each half to 2³⁶. By mounting a brute force attack on each half separately, modern desktop machines can crack alphanumeric LM hashes in a few hours. The Data Encryption Standard (DES) is a cipher (a method for encrypting information) selected as an official Federal Information Processing Standard (FIPS) for the United States in 1976, and which has subsequently enjoyed widespread use internationally. ... Encryption Decryption In cryptography, a block cipher is a symmetric key cipher which operates on fixed-length groups of bits, termed blocks, with an unvarying transformation. ... Password cracking is the process of recovering secret passwords from data that has been stored in or transmitted by a computer system. ... The EFFs US$250,000 DES cracking machine contained over 1,800 custom chips and could brute force a DES key in a matter of days — the photograph shows a DES Cracker circuit board fitted with several Deep Crack chips. ... Generally speaking, the term alphanumeric refers to anything that consists of only letters and numbers. ...

Because LM hash does not include salt, a time-memory trade-off cryptanalysis attack, such as rainbow tables, is also feasible. In 2003, Ophcrack, an implementation of the rainbow table technique, was published. It specifically targets the weaknesses of LM encryption, and includes pre-computed data sufficient to crack virtually all alphanumeric LM hashes in a few seconds. Many cracking tools, e.g. RainbowCrack, L0phtCrack and Cain, now incorporate similar attacks and make cracking of LM hashes trivial. In cryptography, a salt consists of random bits used as one of the inputs to a key derivation function. ... A time-memory trade-off is a situation in computer programming where the programmer can reduce memory use at the cost of slower program execution, or can reduce computation time at the cost of increased memory use. ... Cryptanalysis (from the Greek kryptós, hidden, and analýein, to loosen or to untie) is the study of methods for obtaining the meaning of encrypted information, without access to the secret information which is normally required to do so. ... Simplified rainbow table with 3 reduction functions A rainbow table is a lookup table offering a time-memory tradeoff used in recovering the plaintext password from a password hash generated by a hash function, often a cryptographic hash function. ... Year 2003 (MMIII) was a common year starting on Wednesday of the Gregorian calendar. ... Ophcrack is an open source (GPL license) program that cracks Windows LM hashes using rainbow tables. ... RainbowCrack is the name of a computer program which performs password cracking. ... L0phtCrack is a password auditing and recovery application (nowadays called LC5) by L0pht Heavy Industries. ... Cain and Abel is a Windows password recovery tool. ...

To address the security weaknesses inherent in LM encryption, Microsoft introduced the NTLM algorithm with Windows NT 3.1. While LAN Manager is considered obsolete and current Windows operating systems use the stronger NTLM hashing method, all Windows systems still compute and store the LAN Manager hash by default for compatibility with LAN Manager and Windows Me or earlier clients. It is considered good security practice to disable this feature where it isn’t needed.^[2] Microsoft claims that support for LM will be completely eliminated in the new Windows Vista operating system.^[3] However, in practice, the current Vista release does include support for the LM hash, although it is disabled by default. One who wishes to enable it can do so by via the Local Security Policy tool in the Administrative tools. NTLM (New Technology LAN Manager) is a Microsoft authentication protocol used with the SMB protocol. ... Windows NT 3. ... Windows Vista is a line of graphical operating systems used on personal computers, including home and business desktops, notebook computers, Tablet PCs, and media centers. ...

Notes

1. ^ Glass, Eric (2003). The NTLM Authentication Protocol. Retrieved on 2006-06-05.
2. ^ How to prevent Windows from storing a LAN manager hash of your password in Active Directory and local SAM databases. Microsoft Knowledge Base. Retrieved on 2006-06-05.
3. ^ Johansson, Jesper (August 2006). "The Most Misunderstood Windows Security Setting of All Time". TechNet Magazine. Retrieved on 2007-01-08. 

Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 156th day of the year (157th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 156th day of the year (157th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 8th day of the year in the Gregorian calendar. ...

See also

• LAN Manager
• NTLM
• Password cracking
• Dictionary attack

The LAN Manager was an advanced Network Operating System (NOS) from Microsoft developed in cooperation with 3COM. It is based on the Operating System/2 (OS/2) and NetBEUI protocol similar to its predecessors MS-NET for MS-DOS and Xenix-NET for MS-Xenix. ... NTLM (New Technology LAN Manager) is a Microsoft authentication protocol used with the SMB protocol. ... Password cracking is the process of recovering secret passwords from data that has been stored in or transmitted by a computer system. ... In cryptanalysis and computer security, a dictionary attack is a technique for defeating a cipher or authentication mechanism by trying to determine its decryption key or passphrase by searching a large number of possibilities. ...

External links

Wikibooks' [[wikibooks:|]] has more about this subject:

Reverse Engineering/Cracking Windows XP Passwords

• Making a Faster Cryptanalytic Time-Memory Trade-Off, Philippe Oechslin, Advances in Cryptology - CRYPTO 2003
• Ophcrack, the time-memory-trade-off-cracker
• The Shmoo Group offers pre-computed rainbow tables which are downloadable via BitTorrent
• Cain and Abel
• Online LM crack using Rainbow tables