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Letters-Only LM Hash Database
Peter Clark writes "Disk storage has increased tremendously in the past 5 years and the blatant insecurities in the antiquated LM hashing technique have not gone away; though functionality has been added to disable LM hashes, this is not set by default. With some help from Elcomsoft, simple flat files have been created that hold every combination of LM hash for letters only passwords. Jesko has coded a server application which allows you to access this database. Simply telnet to: beginningtoseethelight.no-ip.org on port 2501 and paste in a LM hash. So how does this differ from Rainbow tables? Well this will return a password 100% of the time, using minimal processor power, in approximately less than 0.2 seconds."
As I understand it LM Hashes can't be reversed. So what someone has done here is pre-hashed an entire dictionary, and created a word=hash lookup system.
You telnet to it, feed it the hash, and it replies with the word.
LM hashes are how a lot of passwords are 'encrypted'
Excuse the fuzzy description... Crypto is not my strong point.
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Windows generates and stores user account passwords by using two different password representations, generally known as "hashes." When you set or change the password for a user account to a password that contains fewer than 15 characters, Windows generates both a LAN Manager hash (LM hash) and a Windows NT hash (NT hash) of the password. These hashes are stored in the local Security Accounts Manager (SAM) database or in Active Directory.
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The LM hash is relatively weak compared to the NT hash, and it is therefore prone to fast brute force attack.
Source: http://support.microsoft.com/default.aspx?scid=KB
http://en.wikipedia.org/wiki/Lm_hash
it's all about a time/space trade-off:
- you have a password hash you want to crack
you can either:
- brute force your way through a long list of possible passwords to find one that matches the hash; everytime you do this, it takes a lot of time and processing power
- or you can go through all possible passwords ONCE and save the resulting hashes; subsequent cracking jobs are very fast, but you wasted a lot of space to store all possible hashes.
thus, if disk space is cheap and you can afford to keep a couple of gigs in pre-computed hashes around, you have a very fast way of cracking weak hashes. a couple of years ago, this was not possible due to hard drives being like 200MB large.
In simple terms, a hash is a sort of "one-way" function. Passwords are often saved as hashes. THe password is fed in one end, and the hash comes out the other. The hash is saved in a file. When one logs in and types in one's password, the system feeds what you typed in through the hash function and checks if the result matches the hash of your password. Since there is no way to reverse the hash process, it's been considered fairly safe to leave these hashes moderately unprotected. This database is, presumably, a sorted collection of the possible hashes from all letters-only passwords up to a certain length. The hash can't be reversed to get the password it came from, but a huge database of all possible hashes and their originating passwords certainly makes that irrelevant.
If a job's not worth doing, it's not worth doing right.
Unless you have machines on your network running 95/98 you should disable LM Hash in Windows. It is there only for backwards compatibility and you can disable it easily:
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http://support.microsoft.com/default.aspx?scid=KB
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You're okay for about 2 hours and 34 minutes: that's how long it takes to traverse every possible alphanumeric input on the author's test rig. Additionally, the article suggests that tables including every possible LM hash for [A-Z0-9] would occupy only 1.2 TB of space, meaning that these lookups could be done in a matter of milliseconds instead.
If you have the LM Hash, and the server accepts LM Authentication, you don't need the password. At all.
Likewise, if you have the NTLM Hash, and the server accepts NTLM, NTLMv2, or LMv2 authentication, then you don't need the password.
The hashes are password equivalent.
I've written it all up in my online book (slashdot review), but...
Basically, the hashes are generated with no salt...nothing to obfuscate them. The algorithm used to log in is challenge/response:
So... The hash is not exposed on the wire. It has to be reversed from the challenge and response. That's possible (and fairly easy with LM Auth), but it's got little to do with the password/LM Hash database.
The only way to use the LM Hash database to reverse the challenge/response is to use it as a hash dictionary.
Chris -)-----