phc-discussions - Re: [PHC] Scripting memory (not so) high vs Catena in PHP (with optimizations)

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Date: Mon, 13 Jan 2014 04:05:14 -0600 (CST)
From: Steve Thomas <steve@...tu.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Scripting memory (not so) high vs Catena in PHP (with
 optimizations)

Oops I just noticed a bug on line 21 (this shouldn't change the speed):
        for ($i = 0; $i < $c; $i++)
should be:
        for ($i = 1; $i < $c; $i++)

> On January 13, 2014 at 3:31 AM Solar Designer <solar@...nwall.com> wrote:
>
> On Mon, Jan 13, 2014 at 02:50:22AM -0600, Steve Thomas wrote:
> > I averaged 7 calls and took the average of 3 runs. So your i7-4770K is
> > 4.45x faster.
>
> Ouch. I guess you're using a 32-bit build of PHP. Right?

Nope, it's 64 bit. I think it might be slow because I'm using WAMP and It's
PHP 5.3.4.


> > > > $v = array();
> > >
> > > BTW, if we do that, we can also discourage TMTO by random writes in the
> > > last loop:
> > >
> > > http://www.openwall.com/lists/crypt-dev/2013/11/20/1
> > >
> > > I think the same is not easy to do on a string, where substr() can only
> > > be used on the right hand side, right? We're lacking an equivalent of
> > > Perl's vec() in PHP, right?
> >
> > I believe so, but you could do this with reading and writing characters.
> > Only
> > problem is this is slow.
>
> Do you mean treating the string as array of chars?

Yes.


> > > I think PHP arrays are always associative (they use hash tables), which
> > > means they're somewhat inefficient for our needs here. Right?
> >
> > Correct, I don't know what they use to store arrays but it's order
> > preserving.
> > If I had to guess it would be a b-tree with two extra pointers for next and
> > previous.
> > I was actually impressed with the speed of Catena in PHP. I thought it would
> > be much worse because of the use of arrays.
>
> Yes, the only ~3.5x slowdown for Catena vs. a PHP-tuned algorithm is not
> that bad.
>
> Speeds aside, I like the secret-dependent indexing better, despite of
> the side-channels risk, because it introduces a dependency on memory
> latency for the attacker. With pre-determined indices, the attacker may
> prefetch blocks way in advance.
>
> I think the side-channels issue may be mitigated by combining both
> approaches: switching to the risky secret-dependent indexing at some
> point during hash computation, so that the impact of a potential
> early-reject (possible after side-channel monitoring) is fairly low.

This is OK until you upgrade your hashes then it will do the secret-dependent
indexing throwout.
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