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Date: Wed, 22 Apr 2015 09:13:21 -0700
From: Bill Cox <>
To: "" <>
Subject: Re: [PHC] Argon2 modulo division

On Tue, Apr 21, 2015 at 10:01 AM, Krisztián Pintér <>

> Bill Cox (at Tuesday, April 21, 2015, 5:48:31 PM):
> >> Generally, small integer division algorithms that are not
> >> constant-time...
> > I do not consider this to be a limitation of Argon2d, though Lyra2
> > (and TwoCats) does protect against related attacks with it's
> > password independent first loop.  However, there are some tin-foil
> > hat attacks
> this is unfortunately not the first time i hear such unscientific
> arguments on the list. it would be a good time to stop that.
> side channel attacks not only bypass the computational hardness. it is
> false that in case of a succesful attack, the hashing scheme reverts
> back to a single hash, or a half-hash if there are two phases. side
> channels are much more sinister. it is possible to steal the password
> in situations when it was not at all possible without them

It turns out that there was not a single entry in the competition that is
power rail analysis resistant, including Gambit.  On line 90 of gambit.cpp,
you have:

            A.block_absorb(pwd, 16, pwd_len);

This code runs in time directly proportional to the password length.
Determining the exact length of the user's password from an oscilloscope
trace of the power rails should be trivial.  While I agree that this side
channel is important, I was not able to get a single person on this list to
consider defending against it.

Why is password length not worth protecting better than we do now?  What
makes it a less significant side-channel weakness than using power rail
analysis to detect mod operation timing?

By the way, looking back at the Gambit code just now was a pleasure.  As
much as we differ on defense philosophy, I have to credit you for writing
clean code.  It's very nice work.


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