phc-discussions - Re: [PHC] "Why I Don't Recommend Scrypt"

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Date: Sat, 22 Mar 2014 23:10:15 -0400
From: Bill Cox <waywardgeek@...il.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] "Why I Don't Recommend Scrypt"

On Sat, Mar 22, 2014 at 9:57 PM, Solar Designer <solar@...nwall.com> wrote:
> This sounds cool in theory, and I think Jean-Philippe wanted this sort
> of submissions to PHC - not only self-contained schemes like most we've
> been discussing so far, but also generic schemes aimed to replace
> PBKDF2.  (Maybe Catena is a valid example of this.)
>
> However, how can you keep "stretching" at this higher level while also
> providing a "memory-hard" underlying function?  Wouldn't the stretching
> need to be inside that function, then?  Or would you move the
> memory-hardness to the higher level, too?  (If we treat Catena as such
> higher-level scheme, then it has its memory-hardness at this higher
> level.)  Efficiency (and thus security) would probably suffer, like it
> does in PBKDF2 (and I think in Catena too), but maybe for some use cases
> (what would they be?) that's an acceptable price for having a more
> generic scheme.

The complexity of both of our projects is my main concern about them.
For example, I would certainly consider Escrypt for any new secure
remote storage app I might right, but I'd just use the code you wrote.
 I would not want to write a version in Java, for example, if I avoid
it.  Same thing with TwoCats.  Also, if there is going to be a winner
in this level of complex password hashing scheme, I can't think why it
should not be Escript.  Faster hashing per thread is great, but upward
compatibility at the application level is critical at this level of
complexity.  We just don't have the bandwidth to critically review
multiple schemes of this size.  TwoCats might gain a couple of bits of
resistance vs Escript against some attacks in the 1 or 2 thread case,
but I have trouble justifying a whole new scheme of this complexity
for that.

Catena is much simpler, and has a higher likelihood of being ported to
various languages.  I was thinking today of how I might include a very
fast block hash function in Catena, with fewer changes than I made in
my waywardgeek branch.  If I could just change H_LEN to something
reasonable, like >= 1KiB, it could be very fast, with minimal changes.

Still, Catena is more complex than something like HKDF, but no so
complex that it feels more like an application than a password hashing
function.  It loses security in half a dozen ways, but addressing
those security issues is what makes our code more complex.  Thus the
desire for a SkinnyCat - not so much as a reaction to Catena as a
reaction to complexity that bothers me.  I would drop the cache-timing
resistant loop first, I think, reverting to something closer to my
original KISS password hashing scheme.

Bill