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

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Message-ID: <20140119144906.GA20749@openwall.com>
Date: Sun, 19 Jan 2014 18:49:06 +0400
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] cost upgrades (Re: [PHC] Scripting memory (not so) high vs Catena in PHP (with optimizations))

On Sat, Jan 18, 2014 at 09:21:26PM +0400, Solar Designer wrote:
> On Sat, Jan 18, 2014 at 11:27:32AM -0500, Bill Cox wrote:
> > I totally stole Catena's update scheme.  All we have to do is add an
> > outer loop that iterates "garlic" number of times, computing the hash
> > with the user's password.  Inside the loop, at the end, we double the
> > memory.  This makes the scheme take 2X longer for the same memory
> > usage (for garlic == 3, it does 8*m_cost + 4*m_cost + 2*m_cost +
> > m_cost), so it's not ideal, but it is trivial to take the hash for
> > garlic = g, and compute the hash for garlic = g+1.  The input to each
> > loop iteration is just the output from the previous.
> > 
> > So, for brand spanking new hashes, set garlic = 0, so the user has no
> > penalty at all.  As the password hash ages, incrementing the garlic
> > can help.
> 
> Thank you for explaining this so concisely and so clearly!  In my terms,
> this corresponds to 2x granularity and good design (so never-upgraded
> hashes have their full AT cost).

I was wrong.  It's not exactly what I had meant by "2x granularity".
My "2x granularity":

Upgrade #	t_cost	AT cost
0		1	1 (100%)
1		2	2 (50%)
2		4	6 (37.5%)
3		8	22 (34.375%)
4		16	86 (33.594%)

Catena (as described by Bill above):

Upgrade #	t_cost	AT cost
0		1	1 (100%)
1		3	5 (55.556%)
2		7	21 (42.857%)
3		15	85 (37.778%)
4		31	341 (35.484%)

While both converge to 1/3 of AT cost of non-upgradable hashes, Catena's
upgrades are steeper, which results in slightly higher efficiency, but
slightly lower flexibility (especially for the very first upgrade: 2x
vs. 3x difference in t_cost).  I'm not sure which is better.

Alexander