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Date: Sat, 18 Jan 2014 11:27:32 -0500
From: Bill Cox <waywardgeek@...il.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 9:38 AM, Solar Designer <solar@...nwall.com> wrote:
> So I guess I need to read up on Catena.  Maybe it does these upgrades
> better than I had thought, avoiding or improving upon the tradeoffs I
> mentioned in the previous message in this thread.

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.  If the user ever does log in again, maybe the server can
recompute a hash for a higher m_cost, and set garlic back to 0.

I've shamelessly stolen other good ideas, too, such as Blakerypt's use
of a session key when available, and a few of your ideas.  Maybe if,
in the very unlikely even that NoelKDF is chosen, I can add those who
contributed ideas as official-ish authors if they allow?  This sharing
of ideas in a competition is super cool, but also very unusual.  My
13-year-old daughter says we must all be terrible competitors.

By the way, I'm quite interested in access to your high-end machines,
but I have enough on my plate now that I'm back to my real job that I
wont need access for some time.  I'll email you when I need it, and
thank you for the offer!

Bill

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