Re: Hash algorithm choice

From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Aug 9, 2009, 17:46 UTC
Message-ID: <alpine.LFD.2.01.0908091038300.3288@localhost.localdomain>
In-Reply-To: <f448a46a0908090917s102b4c83pbad6f298a8e127cc@mail.gmail.com>

On Sun, 9 Aug 2009, Jerome Baum wrote:

Show 5 quoted lines

> 
> How difficult would it be to allow users to choose a hash function
> during git-init which is then globally used in the repo? Are there
> many changes needed or are changes in git-hash-object and git-init
> sufficient?

If youlimit the hash size to 20 bytes, there are almost no changes necessary.

You'd need to hijack the 'SHA1_Init/SHA1_Update/SHA1_Final' functions, of course, and you'd likely want to rename them (and eventually a lot of other functions too), but that renaming is mechanical and isn't even needed for proper working.

Now, if you would ever want to extend the _size_ of the hash, that's a much much bigger problem, but if you're ok with just changing the hash and then truncating the result to 20 bytes (ie kind of like sha-512-160), or you're ok with limiting yourself to 20-byte hashes like REIPMD-160, the size of the changes should be minimal.

			Linus