Re: Which hash function to use, was Re: RFC: Another proposed hash function transition plan
- From
brian m. carlson <sandals@crustytoothpaste.net>
- Date
- Jun 16, 2017, 00:17 UTC
- Message-ID
- <20170616001738.affg4qby7y7yahos@genre.crustytoothpaste.net>
- In-Reply-To
- <CACBZZX5Z3kQHe_5TgOeuJSgzuvpQdaLo6RrgX_EvuZfdz856sA@mail.gmail.com>
On Fri, Jun 16, 2017 at 01:36:13AM +0200, Ævar Arnfjörð Bjarmason wrote:
Show 11 quoted lines
> 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.
Show 21 quoted lines
> 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.
-- brian m. carlson / brian with sandals: Houston, Texas, US https://www.crustytoothpaste.net/~bmc | My opinion only OpenPGP: https://keybase.io/bk2204