phc-discussions - Re: [PHC] Native server relief support for password hashing in browsers

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Date: Sun, 19 Jan 2014 20:42:21 -0600 (CST)
From: Steve Thomas <steve@...tu.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Native server relief support for password hashing in
 browsers

> On January 19, 2014 at 3:21 PM Poul-Henning Kamp <phk@....freebsd.dk> wrote:
>
> a = password
> b = make_integer_from(cryptographic_hash(a)):
> for i N + b:
> a = cryptographic_hash(a)
> [...]
>
> And hand the client a random number X, and have it perform a
> hardly predictable number of rounds, maybe ((X * b) mod (N + b)) or
> similar.
>
> The server would have to store b along with the password for this
> to work. The parameters can probably be tuned so that it only
> leaves a real replay opening, if the server happens to ask for the
> same X again.
>
> But would it be worth the extra complexity ?

A database leak would get you b. Knowing b gives you an early out.
Even if b is 5 bits that speeds up the attacker by 32 times. Also you
might be able to guess what b is by how long it takes the client to
calculate the hash.
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