Re: Why SHA are 40 bytes? (aka looking for flames)

["Marco Costalba" <mcostalba@gmail.com>]

On 4/21/07, Jon Smirl <jonsmirl@gmail.com> wrote:
>
> You have to store the full SHAs at least once since they are the
> signature against data corruption.
>

This is interesting.

So until know I learnt 3 uses of sha:

- Reference an object
- Check against object data corruption
- Prevent any possible collision among objects


Karl says that we need 160 bits to prevent collisions:

>prevent any possible collision created by a malicious adversary, too,
>so that it's possible to e.g. sign just a commit and be able to trust
>everything it points to. The SHA1 designers felt that 160 bits was a
>good compromise between size and robustness, and we just trust that
>their (and the cryptographic community's) guess is good enough.

Probably we don't need 160bits to reference an object. I really don't
know how many bit we need to be robust against data corruption.

Someone more versed then me in SHA1 could tell the probablity to find
a corrupted object calculating his hash and checking against his
stored 160bits known good signature and *FAIL* to find as corrupt *the
same object* calculating his hash and checking against a truncated sha
to say 20bits.

I would say this probability is veery veery low in random case (not a
malicious attack of course, but I think this is not the case with git
repository as it was with SHA1 designers).

Marco