john-users - Re: Using a pre-computed list of alphanumeric strings. (not rainbow tables)

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Date: Tue, 29 Aug 2006 15:11:17 +0200
From: Simon Marechal <simon@...quise.net>
To: john-users@...ts.openwall.com
Subject: Re: Using a pre-computed list of alphanumeric strings.
 (not rainbow tables)

John wrote:
> Why would you have to generate that for every salt? When you have a word
> list, its just plain text dictionary file, and John uses that.

"if I have a pre-computed hash table with hashes of every
alphanumeric combination up to say, 14 chars long, why couldn't
something like this be used in place of a word list?"

You can only compute the hash WITH the salt. For some ciphers you can
compute a pre-hashed value that would save some cycles. But most of the
time you can't.

> I guess I overlooked something.....when I was cracking NT hashes with
> pre-generated rainbow tables.... I could do it fast and effectively because
> the hash could be broken into two 7 char strings, so really you are only
> cracking 7 chars at a time....a MUCH smaller list then having a table of
> all
> possible 14char alphanumeric combinations....

It's because of that, plus the fact that lm hashes do not care about
case. More importantly, rainbow tables are not lists of all 7 chars
combinations. They are ordered tables of hash-chains. They are a very
effective time-memory trade-off, and do not allow finding 100% of the 7
chars cleartexts.

Here is a more accurate description:

http://en.wikipedia.org/wiki/Rainbow_table

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