Date: Sat, 29 Mar 2014 20:22:53 +0400
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Should SkinnyCat have memCost and timeCost?

On Sat, Mar 29, 2014 at 11:54:07AM -0400, Bill Cox wrote:
> It's been a couple of wonderful days since submitting TwoCats, and
> since then, I've only found a couple of typos in the paper.  I'm
> getting real work done again at my day job :-)

I envy you. ;-)

> There is only one minor issue nagging at me: the stripped-down subset
> of TwoCats called SkinnyCat takes only a memCost parameter, not both a
> memCost and a timeCost.  For compute-time hardening in SkinnyCat, I
> would repeat the inner block hash loop 2^timeCost times rather than
> using multiplication chains.  TwoCats, being the full featured
> version, does both.  The idea is that repeatedly hashing the same
> small block of memory that fits in L1 cache over and over before
> moving to the next block is hard for an attacker to speed up, because
> our L1 CPU caches are fast.  It also would allow SkinnyCat to be used
> in low memory situations where long runtimes are still desired.

This specific approach to using timeCost sounds weird to me.  Maybe I
misunderstood, but as described above it sounds like timeCost > 0 would
reduce the whole scheme's usage of memory bandwidth accordingly, which I
find an undesired side-effect.  Set timeCost high enough and attackers
would even be able to use disks instead of RAM.  No?

I actually thought of controlling yescrypt's time cost solely via the
pwxform rounds count (thus, within BlockMix rather than at a higher
level such as in SMix), but decided not to for the reason given above.
The pwxform rounds count should also be tunable (in the full-featured
implementations), but that's for other reasons: it changes the balance
between computation (along with L1 cache lookups) and memory bandwidth
usage, which may be desirable to tune as well.

> On the other hand, many users are likely to get confused with 2 cost
> parameters, and there is already some compute time hardness in
> SkinnyCat due to it's high external memory bandwidth.  It also adds
> complexity to SkinnyCats, which is antithetical to it's design goal.
> 
> I know this isn't terribly important.  I can probably change it in the
> "tweaking" phase anyway.  What do you think?  Should a KISS PHS
> include a time cost parameter?

I have the same dilemma for a scripting-language friendly KISS scheme
that I _might_ submit.  Either choice makes sense to me, although I
think supporting t_cost (via continuing to run for longer after the
desired amount of memory has been filled) is preferable.  In case the
way you can support higher t_cost is only as you described above (and
assuming I understood you correctly), then maybe don't bother. ;-(  What
you described sounds more like a way to tune computation vs. bandwidth
usage than a way to tune the time cost.  I'd even say that the effect it
has on time is a side-effect, and it'd be unfortunate to have people use
this setting for the sole purpose of achieving the side-effect.

I understand that you probably tried to keep the number of tunable
parameters to a minimum, in TwoCats as well.  This may be why you
arrived at only one parameter where there could have been two.

Alexander

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