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

From: Marco Costalba <mcostalba@gmail.com>
Date: Apr 21, 2007, 18:28 UTC
Message-ID: <e5bfff550704211128i12035947i7597e920a0eca163@mail.gmail.com>
In-Reply-To: <9e4733910704211059h16c06e11k967d0bdd3e4970fc@mail.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:

Show 5 quoted lines

>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