phc-discussions - Re: [PHC] How important is salting really?

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Date: Fri, 12 Dec 2014 13:09:57 -0800
From: epixoip <epixoip@...dshell.nl>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] How important is salting really?

On 12/12/2014 1:05 PM, Samuel Neves wrote:
> On 12-12-2014 18:57, Steve Thomas wrote:
>> And now for the other "salt table". For those that don't see the need for this,
>> it's because you probably haven't ran into a scheme that has lots of salt
>> collisions: crypt(3) (12 bit salt
>> http://en.wikipedia.org/wiki/Crypt_(C)#Traditional_DES-based_scheme) or
>> vBulletin (3 character salt). These cause massive amounts of salt collisions and
>> as such you have a table of unique salts "salt table". You run through the salt
>> table and remove them when they are no longer needed. If salts are large enough
>> there is little difference between a salt table and a list of all the hashes
>> with their salts.
> Thanks for clearing this up. Since most good password hash functions use salts with at least 128 bits, it is easy to
> wonder why indexing by salt would be a good idea.

It's not just about indexing by salt, though. You still have to maintain
a list of salts to hash each plaintext candidate with, and remove salts
from said list when a salt is eliminated. Regardless of how you do it,
it is the number of salts, not the number of hashes, that slows down a
cracking job. Unless you are working with very large lists on AMD GPUs,
but that's a whole nother can of worms.