LM hash

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LM hash or LAN Manager hash is one of the formats that Microsoft LAN Manager and Microsoft Windows versions previous to Windows Vista use to store user passwords that are fewer 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, although in Windows Vista must be manually enabled for use.

Contents

[edit] 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 64 bits needed for the DES key.
  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.

[edit] Security weaknesses

Although it is based on DES, a well-studied 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 95^{14} \approx 2^{92} different passwords made of up to 14 printable ASCII characters, there would be only 95^{7} \approx 2^{46} different 7 character password pieces using the same character set. Restricting the character set by converting lowercase to uppercase further reduces the number of possibilities for each half to 69^{7} \approx 2^{43}. By mounting a brute force attack on each half separately, modern desktop machines can crack alphanumeric LM hashes in a few hours.

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.

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.

[edit] Passwords especially weak under LM hashing

Because of the splitting of passwords, one with seven or fewer characters is especially vulnerable, as is one that consists of a seven-letter word followed by another dictionary word or a single symbol. However, because LM hashing is not used for passwords of 15 characters or longer, these are relatively strong. Support and use of LM hashes can be turned off if no versions of Windows older than 2000 are in use.[4]

[edit] See also

[edit] References

  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. 
  4. ^ How to prevent Windows from storing a LAN manager hash of your password in Active Directory and local SAM databases

[edit] External links

Wikibooks
Wikibooks' Reverse Engineering/Cracking Windows XP Passwords has more about this subject:
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