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Date: Thu, 9 Jan 2014 08:10:53 -0500
From: Bill Cox <>
Subject: Re: [PHC] What's your favorite entry so far, and why?

However, on my laptop, for TrueCrypt or my private id_rsa key, I want 2GB,
not 1.
On Thu, Jan 9, 2014 at 4:56 AM, Christian Forler <> wrote:

> On 09.01.2014 04:07, Bill Cox wrote:
> > From a theoretical point of view, Catena wins so far.  Avoiding cache
> > timing attacks is desirable, and Catena shows how to do it.  I am
> > concerned that the authors recommend only 10-ish MB of memory, when
> > escrypt can hash 1GB in a second.  Protection in memory-hard KDFs is
> > proportional to memory used!  While I'm a fan of the algorithm, if
> > Catena were more speed competitive, I would consider it a stronger
> > candidate.
> Do you really want to allocate 1GB of RAM on a tablet or smartphone to
> perform the login process? A lot of those devices have less then
> 1GB of RAM. In general I'm not convinced that a significant amount of
> useres/admins are not pleased with a login process that
> allocates a huge amount of the systems RAM. On many host, such a
> solution will virtually enforce swapping. Just saying. :-)

That's why Catena's Sever Relief idea makes sense.  The key stretching
should be matched to the machine it's running on.  For my laptop, I would
prefer 2GB.  For my phone, maybe 256MB.  On a loaded server, maybe 10MB is
all it can afford.

We designed Catena to run smoothly on (almost) any modern computer
> without causing extra pain for regular users/admins.

That's awesome, but I think we should not develop for the lowest common
denominator machine.  Scrypt measures the available RAM and CPU speed and
picks parameters automatically.  For client-side KDF, I think this is a
good way to go.

For KDF that runs on my machine, I want a good fraction of a second of KDF
with maximum memory usage.  I think server admins will appreciate a
password protection framework that's easy to use, flexible, and stays out
of their way if needed.  I think the Catena framework sounds like the right
approach.  My only  gripe is that if I'm going to spend 1 second on a KDF,
I'm going to want to hash a lot of memory.  Script's speed (about 1/4
GB/second on my development machine) should be considered the lower bound
on acceptable efficiency, IMO.

There is no reason Catena has to run slowly, and I haven't looked at the
code, so I don't know what efficiency changes have been made.  For example,
you could run with two rows small enough to fit in L1 cache, and then do a
ton of rows, followed by a final round that hashes all memory from all rows
4KB at a time.  You could replace memory locations in Catena's rows with
blocks of memory of significant size, and have randomized edges between
blocks instead of a single edge between nodes.  You also could also use an
ultra-fast non-cryptographic hash in the inner loop.  You also could modify
the graph a bit so that the inner loop could process a user-selectable
number of hashes in parallel.  That alone makes a 3X speed difference on my
development machine.  Adding multiple threads is as simple as running N
copies at the same time, and you could hash memory between them as a
post-process to force cheaters to keep it all.

Catena seems like a great framework, with an outstanding theoretical basis.
 However, to be the best KDF, someone has to get dirty in the code and make
fast.  That person will need to be able to change the algorithm.
 Optimization can't be done purely as a post-process to the algorithm
definition.  I haven't looked to see if you guys are doing these things.
 The only reason I suspect not is the low memory numbers I've read.  10MB
is just to low for many applications.  1GB/s is easily achieved on modern
laptops if you do some tuning.


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