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Date: Thu, 11 Sep 2014 17:36:51 +0200
From: pornin@...et.org
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Makwa parameter generation

> If you'd use a big modulus, you should be able to optimize it with CRT.

Thinking about it, this has serious implications. Suppose that there is
a big modulus N and that a (smallish) prime factor r of N is known
(even if the complete factorization of N is not known). Then, given
a Makwa output using N, one can compute modulo r to see whether a
given password matches or not -- and since r is prime, the "fast path"
is applicable. This yields an efficient, fast dictionary attack.

Thus, unless post-hashing is used, NONE of the factors of N must be
known by outsiders (post-hashing would save the day here, but it is
incompatible with offline work factor increase). An RSA-UFO would not
be appropriate for Makwa without post-hashing. With post-hashing, this
is still open (but this example demonstrates that it requires some more
thinking).


> Why does montgomery multiplication have larger asymptotic cost
> compared to multiplication modulo a power of two?

Montgomery multiplication is the combination of a "normal
multiplication" (which could be optimized with Karatsuba or FFT)
and a special reduction which works word-by-word, on the least
significant bits. That reduction is quadratic.

To get below quadratic cost, one must use other methods which are
more complex and are not worth the effort for "normal"-sized modulus,
but would be appropriate for a large modulus as we are envisioning
here.


        --Thomas Pornin


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