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From: "Ævar Arnfjörð Bjarmason" <>
To: Junio C Hamano <>
	"brian m. carlson" <>,
Subject: Re: Is the sha256 object format experimental or not?
Date: Fri, 14 May 2021 10:49:42 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <xmqqo8de9wis.fsf@gitster.g>

On Fri, May 14 2021, Junio C Hamano wrote:

> writes:
>> I think Git should externalize the calculation of object digests just
>> like it externalizes the calcualtion of object digital signatures.
> The hashing algorithms used to generate object names has
> requirements fundamentally different from that of digital
> signatures.  I strongly suspect that that fact would change the
> equation when you rethink what you said above.
> We can "upgrade" digital signature algorithms fairly easily---nobody
> would complain if you suddenly choose different signing algorithm
> over a blob of data, as long as all project participants are aware
> (and self-describing datastream helps here) and are capable of
> grokking the new algorithm we are adopting.  But because object
> names are used by one object to refer to another, and most
> importantly, we do not want a single object to have multiple names,
> we cannot afford to introduce a new hashing algorithm every time we
> feel like it.  In other words, diversity of object naming algorithms
> is to be avoided as much as possible, while diversity of signature
> algorithms is naturally expected.

I agree insofar that I don't see a good reason for us to support some
plethora of hash algorithms, but I wouldn't have objections to adding
more if people find them useful for some reason. See e.g. [1] for an

But I really don't see how anything you've said would present a
technical hurdle once we have SHA-1<->SHA-256 interop in a good enough
state. At that point we'll support re-hashing on arrival of content
hashed with algorithm X into Y, with a local lookup table between X<=>Y.

So if somebody wants to maintain content hashed with algorithm Z locally
we should easily be able to support that. The "diversity of naming"
won't matter past that local repository, any mention of Z will be
translated to X or Y on fetch/push.


  parent reply	other threads:[~2021-05-14  8:56 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-08  2:22 Preserving the ability to have both SHA1 and SHA256 signatures dwh
2021-05-08  6:39 ` Christian Couder
2021-05-08  6:56   ` Junio C Hamano
2021-05-08  8:03     ` Felipe Contreras
2021-05-08 10:11       ` Stefan Moch
2021-05-08 11:12         ` Junio C Hamano
2021-05-09  0:19 ` brian m. carlson
2021-05-10 12:22   ` Is the sha256 object format experimental or not? Ævar Arnfjörð Bjarmason
2021-05-10 22:42     ` brian m. carlson
2021-05-13 20:29       ` dwh
2021-05-13 20:49         ` Konstantin Ryabitsev
2021-05-13 23:47           ` dwh
2021-05-14 13:45             ` Konstantin Ryabitsev
2021-05-14 17:39               ` dwh
2021-05-13 21:03         ` Junio C Hamano
2021-05-13 23:26           ` dwh
2021-05-14  8:49           ` Ævar Arnfjörð Bjarmason [this message]
2021-05-14 18:10             ` dwh
2021-05-18  5:32         ` Jonathan Nieder

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