Date: Tue, 13 Nov 2007 09:54:14 +0100 From: Simon Marechal <simon@...quise.net> To: john-users@...ts.openwall.com Subject: advice requested about NTLMv1 implementation Hello, as most people know it's quite easy once you have a valid account on a window computer (not vista) to retrieve an NTLMv1 response to an arbitrary challenge. This makes it quite useful when looking for a valid user password. This response is made of 3 chunks, based on the LM hash (which is made of 2 chunks). The dependency is: C/R LM 1st chunk 7 first bytes of the first LM chunk 2nd chunk last byte of 1st LM chunk and 6 first bytes of 2nd chunk 3rd chunk 2 last bytes of the second LM chunk It should be clear here that there are only 2^16 3rd chunks for a given challenge. Getting the last 2 bytes of the 2nd chunk's LM hash is thus quite fast. In the same way, once you know the 2nd LM chunk, it's quite easy to retrieve the last byte of the 1st. It thus makes sense to only compute the LM hash, and do the full computation on passwords whose last bytes match (with a false positive rate of 1/2^16 and 1/2^8 for the 2nd and 1st chunk). It's straightforward to alter the LM cipher to crack either the last chunk or the first chunk, but I can't see a smart way to attack both in the same cipher, as the split() function would retrieve chunks that are computed differently. Would someone have a good advice on that? -- To unsubscribe, e-mail john-users-unsubscribe@...ts.openwall.com and reply to the automated confirmation request that will be sent to you.
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