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I have Ubuntu 10.10 on a USB bootstick. Chntpw is installed as well. Are there any programs available which will crack the password?

EDIT: I understand now that a SAM file contains hashes, which can't be cracked as how encryption can be. Instead the data that produces the hash must be brute-forced.

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You need to retrieve the password hashes from the SAM (which there are many tools for, ophcrack can retrieve the hashes) and then using something called a rainbow table to attempt to crack the hash. If you can retrieve LM hashes, this may be fairly easy. Problem is, newer versions of Windows usually don't retain the LM hash (because it's easy to crack), so you may only be able to get the NT hash. Look in to rcracki and large-size rainbow tables. Realize that this could require a significant dedication of time and effort and is almost certainly not worth it.

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According to Wikipedia, the MD4 hash (which Windows NT uses) has serious vulnerabilities. – grawity Jan 26 '11 at 10:21
The NT hash absolutely is vulnerable, but it is significantly less vulnerable than the LM hash. This means that cracking it will require much more time, such that it may not be practical to a casual user. – jcrawfordor Jan 27 '11 at 23:13

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