Re: Which hash function to use, was Re: RFC: Another proposed hash function transition plan

> On Fri, Jun 16, 2017 at 01:36:13AM +0200, Ævar Arnfjörð Bjarmason wrote:
>> On Fri, Jun 16, 2017 at 12:41 AM, brian m. carlson
>> <sandals@crustytoothpaste.net> wrote:
>> > SHA-256 acceleration exists for some existing Intel platforms already.
>> > However, they're not practically present on anything but servers at the
>> > moment, and so I don't think the acceleration of SHA-256 is a
>> > something we should consider.
>>
>> Whatever next-gen hash Git ends up with is going to be in use for
>> decades, so what hardware acceleration exists in consumer products
>> right now is practically irrelevant, but what acceleration is likely
>> to exist for the lifetime of the hash existing *is* relevant.
>
> The life of MD5 was about 23 years (introduction to first document
> collision).  SHA-1 had about 22.  Decades, yes, but just barely.  SHA-2
> was introduced in 2001, and by the same estimate, we're a little over
> halfway through its life.

>> So I don't follow the argument that we shouldn't weigh future HW
>> acceleration highly just because you can't easily buy a laptop today
>> with these features.
>>
>> Aside from that I think you've got this backwards, it's AMD that's
>> adding SHA acceleration to their high-end Ryzen chips[1] but Intel is
>> starting at the lower end this year with Goldmont which'll be in
>> lower-end consumer devices[2]. If you read the github issue I linked
>> to upthread[3] you can see that the cryptopp devs already tested their
>> SHA accelerated code on a consumer Celeron[4] recently.
>>
>> I don't think Intel has announced the SHA extensions for future Xeon
>> releases, but it seems given that they're going to have it there as
>> well. Have there every been x86 extensions that aren't eventually
>> portable across the entire line, or that they've ended up removing
>> from x86 once introduced?
>>
>> In any case, I think by the time we're ready to follow-up the current
>> hash refactoring efforts with actually changing the hash
>> implementation many of us are likely to have laptops with these
>> extensions, making this easy to test.
>
> I think you underestimate the life of hardware and software.  I have
> servers running KVM development instances that have been running since
> at least 2012.  Those machines are not scheduled for replacement anytime
> soon.
>
> Whatever we deploy within the next year is going to run on existing
> hardware for probably a decade, whether we want it to or not.  Most of
> those machines don't have acceleration.

> Furthermore, you need a reasonably modern crypto library to get hardware
> acceleration.  OpenSSL has only recently gained support for it.  RHEL 7
> does not currently support it, and probably never will.  That OS is
> going to be around for the next 6 years.
>
> If we're optimizing for performance, I don't want to optimize for the
> latest, greatest machines.  Those machines are going to outperform
> everything else either way.  I'd rather optimize for something which
> performs well on the whole everywhere.  There are a lot of developers
> who have older machines, for cost reasons or otherwise.

> Here are some stats (cycles/byte for long messages):
>
>                    SHA-256    BLAKE2b
> Ryzen                 1.89       3.06
> Knight's Landing     19.00       5.65
> Cortex-A72            1.99       5.48
> Cortex-A57           11.81       5.47
> Cortex-A7            28.19      15.16
>
> In other words, BLAKE2b performs well uniformly across a wide variety of
> architectures even without acceleration.  I'd rather tell people that
> upgrading to a new hash algorithm is a performance win either way, not
> just if they have the latest hardware.