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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Sitaram Chamarty <sitaramc@gmail.com>
Cc: demerphq <demerphq@gmail.com>,
	"brian m. carlson" <sandals@crustytoothpaste.net>,
	Johannes Schindelin <Johannes.Schindelin@gmx.de>,
	Jonathan Nieder <jrnieder@gmail.com>,
	Git Mailing List <git@vger.kernel.org>,
	Adam Langley <agl@google.com>,
	keccak@noekeon.org
Subject: Re: Hash algorithm analysis
Date: Mon, 23 Jul 2018 11:23:34 -0700
Message-ID: <CA+55aFx876XPwdSqRsKDk+Vo4pD-0wVRx=E+SAN6JiZKWUMrHg@mail.gmail.com> (raw)
In-Reply-To: <98111891-a605-1cfd-e92b-a3b5b4186ac2@gmail.com>

On Mon, Jul 23, 2018 at 5:48 AM Sitaram Chamarty <sitaramc@gmail.com> wrote:
>
> I would suggest (a) hash size of 256 bits and (b) choice of any hash
> function that can produce such a hash.  If people feel strongly that 256
> bits may also turn out to be too small (really?) then a choice of 256 or
> 512, but not arbitrary sizes.

Honestly, what's the expected point of 512-bit hashes?

The _only_ point of a 512-bit hash is that it's going to grow objects
in incompressible ways, and use more memory. Just don't do it.

If somebody can break a 256-bit hash, you have two choices:

 (a) the hash function itself was broken, and 512 bits isn't the
solution to it anyway, even if it can certainly hide the problem

 (b) you had some "new math" kind of unexpected breakthrough, which
means that 512 bits might not be much  better either.

Honestly, the number of particles in the observable universe is on the
order of 2**256. It's a really really big number.

Don't make the code base more complex than it needs to be. Make a
informed technical decision, and say "256 bits is a *lot*".

The difference between engineering and theory is that engineering
makes trade-offs. Good software is well *engineered*, not theorized.

Also, I would suggest that git default to "abbrev-commit=40", so that
nobody actually *sees* the new bits by default. So the perl scripts
etc that use "[0-9a-f]{40}" as a hash pattern would just silently
continue to work.

Because backwards compatibility is important (*)

                     Linus

(*) And 2**160 is still a big big number, and hasn't really been a
practical problem, and SHA1DC is likely a good hash for the next
decade or longer.

  parent reply index

Thread overview: 66+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-06-09 20:56 State of NewHash work, future directions, and discussion brian m. carlson
2018-06-09 21:26 ` Ævar Arnfjörð Bjarmason
2018-06-09 22:49 ` Hash algorithm analysis brian m. carlson
2018-06-11 19:29   ` Jonathan Nieder
2018-06-11 20:20     ` Linus Torvalds
2018-06-11 23:27       ` Ævar Arnfjörð Bjarmason
2018-06-12  0:11         ` David Lang
2018-06-12  0:45         ` Linus Torvalds
2018-06-11 22:35     ` brian m. carlson
2018-06-12 16:21       ` Gilles Van Assche
2018-06-13 23:58         ` brian m. carlson
2018-06-15 10:33           ` Gilles Van Assche
2018-07-20 21:52     ` brian m. carlson
2018-07-21  0:31       ` Jonathan Nieder
2018-07-21 19:52       ` Ævar Arnfjörð Bjarmason
2018-07-21 20:25         ` brian m. carlson
2018-07-21 22:38       ` Johannes Schindelin
2018-07-21 23:09         ` Linus Torvalds
2018-07-21 23:59         ` brian m. carlson
2018-07-22  9:34           ` Eric Deplagne
2018-07-22 14:21             ` brian m. carlson
2018-07-22 14:55               ` Eric Deplagne
2018-07-26 10:05                 ` Johannes Schindelin
2018-07-22 15:23           ` Joan Daemen
2018-07-22 18:54             ` Adam Langley
2018-07-26 10:31             ` Johannes Schindelin
2018-07-23 12:40           ` demerphq
2018-07-23 12:48             ` Sitaram Chamarty
2018-07-23 12:55               ` demerphq
2018-07-23 18:23               ` Linus Torvalds [this message]
2018-07-23 17:57             ` Stefan Beller
2018-07-23 18:35             ` Jonathan Nieder
2018-07-24 19:01       ` Edward Thomson
2018-07-24 20:31         ` Linus Torvalds
2018-07-24 20:49           ` Jonathan Nieder
2018-07-24 21:13           ` Junio C Hamano
2018-07-24 22:10             ` brian m. carlson
2018-07-30  9:06               ` Johannes Schindelin
2018-07-30 20:01                 ` Dan Shumow
2018-08-03  2:57                   ` Jonathan Nieder
2018-09-18 15:18                   ` Joan Daemen
2018-09-18 15:32                     ` Jonathan Nieder
2018-09-18 16:50                     ` Linus Torvalds
2018-07-25  8:30             ` [PATCH 0/2] document that NewHash is now SHA-256 Ævar Arnfjörð Bjarmason
2018-07-25  8:30             ` [PATCH 1/2] doc hash-function-transition: note the lack of a changelog Ævar Arnfjörð Bjarmason
2018-07-25  8:30             ` [PATCH 2/2] doc hash-function-transition: pick SHA-256 as NewHash Ævar Arnfjörð Bjarmason
2018-07-25 16:45               ` Junio C Hamano
2018-07-25 17:25                 ` Jonathan Nieder
2018-07-25 21:32                   ` Junio C Hamano
2018-07-26 13:41                     ` [PATCH v2 " Ævar Arnfjörð Bjarmason
2018-08-03  7:20                       ` Jonathan Nieder
2018-08-03 16:40                         ` Junio C Hamano
2018-08-03 17:01                           ` Linus Torvalds
2018-08-03 16:42                         ` Linus Torvalds
2018-08-03 17:43                         ` Ævar Arnfjörð Bjarmason
2018-08-04  8:52                           ` Jonathan Nieder
2018-08-03 17:45                         ` brian m. carlson
2018-07-25 22:56                 ` [PATCH " brian m. carlson
2018-06-11 21:19   ` Hash algorithm analysis Ævar Arnfjörð Bjarmason
2018-06-21  8:20     ` Johannes Schindelin
2018-06-21 22:39     ` brian m. carlson
2018-06-11 18:09 ` State of NewHash work, future directions, and discussion Duy Nguyen
2018-06-12  1:28   ` brian m. carlson
2018-06-11 19:01 ` Jonathan Nieder
2018-06-12  2:28   ` brian m. carlson
2018-06-12  2:42     ` Jonathan Nieder

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