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Date: Tue, 24 Jun 2014 04:55:01 +0400
From: Solar Designer <>
Subject: Re: [PHC] auto-tuning parallelism

On Tue, May 27, 2014 at 07:44:04AM -0700, Andy Lutomirski wrote:
> On Mon, May 26, 2014 at 5:36 PM, Solar Designer <> wrote:
> > When editing the wiki page at
> > yesterday, I added
> > this (which has been in my head for months) to "Drawbacks":
> >
> > "While pwxform's default 4x 128-bit SIMD parallelism fits SIMD-enabled
> > builds running on modern CPUs well, it is excessive for older CPUs and
> > non-SIMD builds, making such slower builds of yescrypt weaker than
> > similar builds of bcrypt in terms of GPU attack resistance (when
> > yescrypt's memory usage is low)
> >
> > Using much lower parallelism settings when appropriate should address
> > this, but the risk of people (inadvertently?) using a high SIMD- and
> > instruction-level parallelism mode in a build not suitable for that will
> > remain"
> >
> > Here's an idea on how to deal with this: besides possibly supporting
> > explicit specification of the desired SIMD- and instruction-level
> > parallelism as input parameters to yescrypt, support a magic input value
> > or a flag that would request the current build's optimal settings.  When
> > invoked like this, e.g. yescrypt-opt.c (scalar) would use settings for
> > one 64-bit lane, whereas yescrypt-simd.c would use 4x 128-bit SIMD.
> > Indeed, this will affect the computed hash value, so it's only usable
> > when setting a new password or initially encrypting some data, and the
> > actual parallelism settings would need to be output by yescrypt and
> > encoded along with the hash or derived key.
> >
> > This fits nicely in the currently defined yescrypt_gensalt*() API -
> > they'd generate salt strings optimal for the current build of yescrypt,
> > when requested to do that by a flag.  Those salt strings will have the
> > actual parallelism settings encoded in them.
> >
> > Unfortunately, it doesn't fit nearly as well into other APIs - those
> > that don't use ASCII strings for salts+hashes.  Should this feature be
> > limited to the crypt(3)-like strings-based APIs, or should it somehow be
> > added to (binary) key derivation APIs as well?  I'd do both, but I'm
> > concerned it might not be intuitive for non-strings-based APIs.
> For the binary case, it might be simpler to have a second function to
> call to ask for good parameters.  Then the caller can, for example,
> call it once, hash a bunch of passwords, and know that they have the
> same parameters (and therefore not store a copy for each password).

I will likely implement that.  Thank you!


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