passlib.hash.lmhash - LanManager Hash

New in version 1.6.

Warning

This scheme has been deprecated since Windows NT, and is notoriously weak. It should be used for compatibility with existing systems; do not use in new code.

This class implements the LanManager Hash (aka LanMan or LM hash). It was used by early versions of Microsoft Windows to store user passwords, until it was supplanted (though not entirely replaced) by the nthash algorithm in Windows NT. It continues to crop up in production due to it’s integral role in the legacy NTLM authentication protocol. This class can be used directly as follows:

>>> from passlib.hash import lmhash

>>> # encrypt password
>>> h = lmhash.encrypt("password")
>>> h
'e52cac67419a9a224a3b108f3fa6cb6d'

>>> # verify correct password
>>> lmhash.verify("password", h)
True
>>> # verify incorrect password
>>> lmhash.verify("secret", h)
False

See also

the generic PasswordHash usage examples

Interface

class passlib.hash.lmhash

This class implements the Lan Manager Password hash, and follows the Password Hash Interface.

It has no salt and a single fixed round.

The encrypt() and verify() methods accept a single optional keyword:

Parameters:encoding (str) – This specifies what character encoding LMHASH should use when calculating digest. It defaults to cp437, the most common encoding encountered.

Note that while this class outputs digests in lower-case hexidecimal, it will accept upper-case as well.

Issues with Non-ASCII Characters

Passwords containing only ascii characters should hash and compare correctly across all LMhash implementations. However, due to historical issues, no two LMhash implementations handle non-ascii characters in quite the same way. While Passlib makes every attempt to behave as close to correct as possible, the meaning of “correct” is dependant on the software you are interoperating with. If you think you will have passwords containing non-ascii characters, please read the Deviations section (below) for details about the known interoperability issues. It’s a mess of codepages.

Format & Algorithm

A LM hash consists of 32 hexidecimal digits, which encode the 16 byte digest. An example hash (of password) is e52cac67419a9a224a3b108f3fa6cb6d.

The digest is calculated as follows:

1. First, the password should be converted to uppercase, and encoded using the “OEM Codepage” of the Windows release that the host / target server is running [2].

   For pure-ASCII passwords, this step can be performed using the us-ascii encoding (as most OEM Codepages are ASCII-compatible). However, for passwords with non-ASCII characters, this step is fraught with compatibility issues and border cases (see Deviations for details).

2. The password is then truncated to 14 bytes, or the end NULL padded to 14 bytes; as appropriate.

3. The first 7 bytes of the truncated password from step 2 are used as a key to DES encrypt the constant KGS!@#$%, resulting in the first 8 bytes of the final digest.

4. Step 3 is repeated using the second 7 bytes of the password from step 2, resulting in the second 8 bytes of the final digest.

5. The combined digests from 3 and 4 are then encoded to hexidecimal.

Security Issues

Due to a myriad of flaws, and the existence high-speed password cracking software dedicated to LMHASH, this algorithm should be considered broken. The major flaws include:

• It has no salt, making hashes easily pre-computable.
• It limits the password to 14 characters, and converts the password to uppercase before hashing, greatly reducing the keyspace.
• By breaking the password into two independant chunks, they can be attacked independantly and simultaneously.
• The independance of the chunks reveals significant information about the original password: The second 8 bytes of the digest are the same for all passwords < 8 bytes; and for passwords of 8-9 characters, the second chunk can be broken much faster, revealing part of the password, and reducing the likely keyspace for the first chunk.

Deviations

Passlib’s implementation differs from others in a few ways, all related to the handling of non-ASCII characters.

• Unicode Policy:

  Officially, unicode passwords should be encoded using the “OEM Codepage” used [2] by the specific release of Windows that the host or target server is running. Common encodings include cp437 (used by the English edition of Windows XP), cp580 (used by many Western European editions of XP), and cp866 (used by many Eastern European editions of XP). Complicating matters further, some third-party implementations are known to use encodings such as latin-1 and utf-8, which cause non-ASCII characters to hash in a manner incompatible with the canonical MS Windows implementation.

  Thus if an application wishes to provide support for non-ASCII passwords, it must decide which encoding to use. Passlib uses cp437 as a default, but this may need to be overridden via lmhash.encrypt(secret, encoding="some-other-codec"). All known encodings are us-ascii-compatible, so for ASCII passwords, the default should be sufficient.

• Upper Case Conversion:

  Note

  Future releases of Passlib may change this behavior as new information and code is integrated.

  Once critical step in the LMHASH algorithm is converting the password to upper case. While ASCII characters are uppercased as normal, non-ASCII characters are converted in implementation-dependant ways:

  Windows systems encode the password first, and then convert it to uppercase using an codepage-specific table. For the most part these tables seem to agree with the Unicode specification, but there are some codepoints where they deviate (for example, Unicode uppercases U+00B5 -> U+039C, but cp437 leaves it unchanged [3]).

  In contrast, most third-party implementations (Passlib included) perform the uppercase conversion first using the Unicode specification, and then encode the password second; despite the non-ASCII border cases where the resulting hash would not match the official Windows hash.

Footnotes

[1]	Article used as reference for algorithm - http://www.linuxjournal.com/article/2717.

[2]	(1, 2) The OEM codepage used by specific Window XP (and earlier) releases can be found at http://msdn.microsoft.com/nl-nl/goglobal/cc563921%28en-us%29.aspx.

[3]	Online discussion dealing with upper-case encoding issues - http://www.openwall.com/lists/john-dev/2011/08/01/2.