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Date: Sat, 19 Apr 2014 15:30:26 -0400
From: Bill Cox <waywardgeek@...il.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Any other authores ready for benchmarking? (was [PHC] Lyra2
 initial review)

On Sat, Apr 19, 2014 at 11:14 AM, Justin Cappos <jcappos@....edu> wrote:

> Is better performance strictly better?   Is this meant to quantify
> algorithm quality in some way or just to figure out which algorithms are in
> the same equivalence class for performance?
>

Knowing nothing else about a memory-hard algorithm, I would say hashing
more memory faster is better.  That's because the cost per guess to an
attacker is somewhat proportional to how much memory is used, and how long
it is used for.  For any given runtime, using more memory will cost an
attacker more.  However, that's just one metric, though an important one.

More important than how fast you can fill memory with hashed data is how
much memory your algorithm uses on average while doing it.  For example, an
attacker with virtual memory will care about the total of several runs at
once, which will depend more on the average per run than the peak.

Once you know how much memory is used on average, you should get a feel for
how long an attacker will have to do the same to run the algorithm.  What's
important here is that there be some sort of runtime hardness to the
algorithm, forcing an attacker to take a similar amount of time.  The most
basic limit to the runtime of a memory-hard algorithm is it's memory
bandwidth.  Per dollar, we can roughly buy 10X the bandwidth capability
using GPUs rather than Intel boxes, which isn't all that bad.  So, if an
algorithm maxes out bandwidth while hashing data (like Yescript, Lyra2, and
TwoCats), most attackers will only be able to get around a 10X cost benefit
vs defenders.  That rocks compared to doing PBKDF2-SHA256!

For cache-bound memory-hard algorithms, again bandwidth is the primary
limiting factor in many cases.  My Intel CPU does about 100GiB/s to L1
cache, though you can also limit an attacker with less than that to L3
cache.

After these simple measurements, it's time to look at the algorithm and the
author's claims to see what other defenses they have.  For example,
Yescript and TwoCats do multi-threading with TMTO defense between threads,
and multiplication chains for compute-time hardening.  Lyra2 has some
serious TMTO defense, and may just win outright in that category.  Yescript
has a laundry list of defenses  greater than any of the others, IMO.

Finally, and likely most importantly will be cryptanalysis.  High
performance and multiple lines of defense are all great, but if there are
security weaknesses in the algorithm, it shouldn't be used.

Bill

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