Re: Suggestion on hashing

[Bill Zaumen <bill.zaumen@gmail.com>]

On Fri, 2011-12-02 at 12:54 -0500, Jeff King wrote:
> On Fri, Dec 02, 2011 at 12:08:39AM -0800, Bill Zaumen wrote:

> I think your code is solving the wrong problem (or solving the right
> problem in a half-way manner). The only things that make sense to me
> are:
> 
>   1. Do nothing. SHA-1 is probably not broken yet, even by the NSA, and
>      even if it is, an attack is extremely expensive to mount. This may
>      change in the future, of course, but it will probably stay
>      expensive for a while.
> 
>   2. Decouple the object identifier and digest roles, but insert the
>      digest into newly created objects, so it can be part of the
>      signature chain. I described such a scheme in one of my replies to
>      you. It has some complexities, but has the bonus that we can build
>      directly on older history, preserving its sha1s.
> 
>   3. Replace SHA-1 with a more secure algorithm.

Suppose I make the digest pluggable, something I intended to do
eventually anyway?  Then you just use the existing SHA-1 as an
object identifier and the new digest in a signature chain?  What I
did was essentially to compute the new digest (using a CRC as the
trivial case) whenever an object's SHA-1 hash is computed, plus
using the new digest for low-cost collision checks.

Then you have everything needed to experiment with your second option.
I got the impression that Nguyen had some interest in that, but could
be mistaken.

The use is simple: if you have the SHA-1 hash of an object, you call
a function, currently named "has_sha1_file_crc" and it returns true if
a CRC is available, writing the hash into the buffer supplied as its
second argument.  You can do whatever you like with it.  If you want
a digest of digests, you just traverse a commit's tree, and call
has_sha1_file_crc whenever you want to look up a digest.  So, the API
is actually very simple if you just use the patch to quickly look up
the digest associated with a SHA-1 ID - everything else it does happens
automatically.