Re: Hash algorithm choice

From: Junio C Hamano <gitster@pobox.com>
Date: Aug 9, 2009, 18:03 UTC
Message-ID: <7vljls986b.fsf@alter.siamese.dyndns.org>
In-Reply-To: <alpine.LFD.2.01.0908091038300.3288@localhost.localdomain>

Linus Torvalds <torvalds@linux-foundation.org> writes:

Show 13 quoted lines

> 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.

Just in case Jerome really wants to go further, "almost no changes" and "minimal" refers to the fact that we have a few hard-coded hash values known to the code, such as the object name for an empty blob and an empty tree.