Re: Starting to think about sha-256?

[David Lang <david.lang@digitalinsight.com>]

--On Sunday, August 27, 2006 03:35:20 PM -0700 Linus Torvalds 
<torvalds@osdl.org> wrote:
>
> On Mon, 28 Aug 2006, Johannes Schindelin wrote:
>> Even if the breakthrough really comes to full SHA-1, you still have to
>> add  _at least_ 20 bytes of gibberish. Which would be harder to spot,
>> but it  would be spotted.
>
> Yeah, I don't think this is at all critical, especially since git really
> on a security level doesn't _depend_ on the hashes being
> cryptographically  secure. As I explained early on (ie over a year ago,
> back when the whole  design of git was being discussed), the _security_
> of git actually depends  on not cryptographic hashes, but simply on
> everybody being able to secure  their own _private_ repository.
>
> So the only thing git really _requires_ is a hash that is _unique_ for
> the  developer (and there we are talking not of an _attacker_, but a
> benign  participant).
>
> That said, the cryptographic security of SHA-1 is obviously a real bonus.
> So I'd be disappointed if SHA-1 can be broken more easily (and I
> obviously  already argued against using MD5, exactly because generating
> duplicates of  that is fairly easy). But it's not "fundamentally
> required" in git per se.


>> This made me think about the use of hashes in git. Why do we need a hash
>> here (in no particular order):
>>
>> 1) integrity checking,
>> 2) fast lookup,
>> 3) identifying objects (related to (2)),
>> 4) trust.
>>
>> Except for (4), I do not see why SHA-1 -- even if broken -- should not
>> be  adequate. It is not like somebody found out that all JPGs tend to
>> have  similar hashes so that collisions are more likely.
>
> Correct. I'm pretty sure we had exactly this discussion around May 2005,
> but I'm too lazy to search ;)

just to double check.

if you already have a file A in git with hash X is there any condition 
where a remote file with hash X (but different contents) would overwrite 
the local version?

what would happen if you ended up with two packs that both contained a file 
with hash X but with different contents and then did a repack on them? 
(either packs from different sources, or packs downloaded through some 
mechanism other then the git protocol are two ways this could happen that I 
can think of)

David Lang