full-disclosure - Re: choice-point screw-up and secure hashes

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From: jasonc at science.org (Jason Coombs)
Subject: Re: choice-point screw-up and secure hashes

> reverse hashing

By reverse hashing you mean defeating the protection by forward hashing all possible SSNs, presumably.

-----Original Message-----
From: Valdis.Kletnieks@...edu
Date: Sat, 19 Mar 2005 17:38:09 
To:Atom Smasher <atom@...sher.org>
Cc:Jason Coombs <jasonc@...ence.org>,       Full-Disclosure <full-disclosure@...ts.grok.org.uk>
Subject: Re: [Full-disclosure] Re: choice-point screw-up and secure hashes

On Sat, 19 Mar 2005 13:34:53 EST, Atom Smasher said:

> tell ya what... here's my SSN hashed with a salt:
>  =09e36c98b34d5ba979fb0bf0c64dc7b3a66c9ce841437d6460390e6380810f1440
> 
> as soon as you recover my SSN, just let me know.

Tell you what - give me the salt and the hash algorithm, and it will be
quick work indeed.

Remember that the company probably needs an *invertible* function as they need to
be able to access the original value, so the trick of "hash the SSN and see if
you get the same to compare for equality" isn't usable.  You can use a one-way
function if the only question is "Is 109-00-4368 the SSN for Customer XYZ?". If
you need to be able to answer "What is Customer XYZ's SSN?" you need an invertible
function.  And if you ever do a database JOIN based on SSN, you need the SSN.

And the sad fact is that if the company's servers are compromised sufficiently
to recover the hashed SSN, they're almost certainly compromised sufficiently to
allow the theft of the software that handles the forward and reverse hashing.
So considering the salt and the algorithm secure if the hash has been compromised
is probably a bad idea..