Re: [PATCH 2/2] doc hash-function-transition: pick SHA-256 as NewHash

[Junio C Hamano <gitster@pobox.com>]

Ævar Arnfjörð Bjarmason  <avarab@gmail.com> writes:

> @@ -125,19 +122,19 @@ Detailed Design
>  ---------------
>  Repository format extension
>  ~~~~~~~~~~~~~~~~~~~~~~~~~~~
> -A NewHash repository uses repository format version `1` (see
> +A SHA-256 repository uses repository format version `1` (see
>  Documentation/technical/repository-version.txt) with extensions
>  `objectFormat` and `compatObjectFormat`:
>  
>  	[core]
>  		repositoryFormatVersion = 1
>  	[extensions]
> -		objectFormat = newhash
> +		objectFormat = sha256
>  		compatObjectFormat = sha1

Whenever we said SHA1, somebody came and told us that the name of
the hash is SHA-1 (with dash).  Would we be nitpicker-prone in the
same way with "sha256" here?

> @@ -155,36 +152,36 @@ repository extensions.
>  Object names
>  ~~~~~~~~~~~~
>  Objects can be named by their 40 hexadecimal digit sha1-name or 64
> -hexadecimal digit newhash-name, plus names derived from those (see
> +hexadecimal digit sha256-name, plus names derived from those (see
>  gitrevisions(7)).

Seeing this hunk makes me respond to the above question with another
question: "having to write sha-256-name, sha-1-name, gpgsig-sha-256,
and sha-256-content is sort of ugly, no?"

I guess names with two dashes are not _too_ bad, so I dunno.

>  Selection of a New Hash
>  -----------------------
> @@ -611,6 +608,10 @@ collisions in 2^69 operations. In August they published details.
>  Luckily, no practical demonstrations of a collision in full SHA-1 were
>  published until 10 years later, in 2017.
>  
> +It was decided that Git needed to transition to a new hash
> +function. Initially no decision was made as to what function this was,
> +the "NewHash" placeholder name was picked to describe it.
> +
>  The hash function NewHash to replace SHA-1 should be stronger than
>  SHA-1 was: we would like it to be trustworthy and useful in practice
>  for at least 10 years.

This sentence needs a bit of updating to match the new paragraph
inserted above.  "should be stronger" is something said by those
who are still looking for one and/or trying to decide.  Perhaps
something like this?

	...
	the "NewHash" placeholder name was used to describe it.

	We wanted to choose a hash function to replace SHA-1 that is
	stronger than SHA-1 was, and would like it to be trustworthy
	and useful in practice for at least 10 years.

	Some other relevant properties we wanted in NewHash are:

> @@ -630,14 +631,19 @@ Some other relevant properties:
>  4. As a tiebreaker, the hash should be fast to compute (fortunately
>     many contenders are faster than SHA-1).
>  
> -Some hashes under consideration are SHA-256, SHA-512/256, SHA-256x16,
> +Some hashes under consideration were SHA-256, SHA-512/256, SHA-256x16,
>  K12, and BLAKE2bp-256.
>  
> +Eventually in July 2018 SHA-256 was chosen to be the NewHash. See the
> +thread starting at <20180609224913.GC38834@genre.crustytoothpaste.net>
> +for the discussion
> +(https://public-inbox.org/git/20180609224913.GC38834@genre.crustytoothpaste.net/)
> +