Re: Salted passwords

[T Biehn <tbiehn () gmail com>]

Thank you for the thoughtful analysis Raid. The hash and salt are both
known to the attacker :)
It looks like I'm going to have to settle with confounding efforts by
the man via increased hash computation cost.

-Travis

On Mon, Aug 10, 2009 at 6:53 PM, <raid () hushmail com> wrote:
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> Travis,
>
> On Mon, 10 Aug 2009 22:50:32 +0200 T Biehn <tbiehn () gmail com> wrote:
> > I don't have control over the set. Sorry I wasn't more explicit
> > about
> > this. Although, it should have been obvious that the solution
> > needed
> > to satisfy the conditions:
> > Data to one way hash.
> > The set has 9,999,999,999 members.
>
> if these are the only two conditions, I wonder why a static salt
> does not satisfy your requirements? If the salt is not publicly
> known, the procedure is secure in respect to the hash-function in
> use...
>
> So, suppose the third condition is the salt may be publicly known.
>
> Suppose, we have plaintext (alphabet E, length of alphabet s = |E|)
> with fixed length, say 'c' chars. So if you insert the salt at a
> random position, there are c+1 possibilities for the position of
> the salt. So the bruteforce attacker has to run c more tests than
> having the salt in a fixed position.
>
> Comparing the two procedures under a theoretically view, there isnt
> a significant difference in terms of runtime complexity:
>
> If the salt is not publicly known and at a fixed position,
> complexity (means: number of possible plaintexts) is at O(s**c).
> Your method only rises complexity by a constant factor: It's at O(
> (c+1) * s**c).
>
> Theoretically this is negligible: If it takes me 2 hours to
> bruteforce procedure 1 (fixed position), why bother about 20 hours
> computing for procedure 2?
>
> Practically it depends on your overall requirements.
>
> Besides, your procedure lowers the latch for DoS... at least
> slightly (same argument as above).
>
> So far, my two cents...
>
> raid
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