checksum_seed

[jw schultz]

On Tue, Feb 10, 2004 at 09:56:00PM -0800, Craig Barratt wrote:
> On Mon, Feb 09, 2004 at 09:14:06AM -0500, Jason M. Felice wrote:
> 
> > I got the go-ahead from the client on my --link-by-hash proposal, and
> > the seed is making the hash unstable.  I can't figure out why the seed
> > is there so I don't know whether to cirumvent it in my particular case
> > or calculate a separate, stable hash.
> 
> I believe the checksum seed is meant to reduce the chance that different
> data could repeatedly produce the same md4 digest over multiple runs.
> If a collision happens the hope is that a different checksum seed will
> break the collision.
> 
> However, my guess is that it doesn't make any difference.  Certainly
> adding the seed at the end of the block won't change a collision even
> if the seed changes over multiple runs.  File MD4 checksums add the
> seed at the beginning, which might help breaking collisions, although
> I'm not sure.
> 
> Wayne Davison writes:
> 
> > There was some talk last year about adding a --fixed-checksum-seed
> > option, but no consensus was reached.  It shouldn't hurt to make the
> > seed value constant for certain applications, though, so you can feel
> > free to proceed in that direction for what you're doing for your client.
> > 
> > FYI, I just checked in some changes to the checksum_seed code that will
> > make it easier to have other options (besides the batch ones) specify
> > that a constant seed value is needed.
> 
> I would really like a --fixed-csumseed option become a standard
> feature in rsync.  Just using the batch value (32761) is fine.
> Can I contribute a patch?  The reason I want this is the next
> release of BackupPC will support rsync checksum caching, so that
> backups don't need to recompute block or file checksums.  This
> requires a fixed checksum seed on the remote rsync, hence the
> need for --fixed-csumseed.  I've included this feature in a
> pre-built rsync for cygwin that I include on the SourceForge
> BackupPC downloads.

1.  Yes, you may contribute a patch.  I favor the idea of
being able to supply a checksum seed.

2.  Lets get the option name down to a more reasonable
length.  --checksum-seed should be sufficient.

-- 
________________________________________________________________
	J.W. Schultz            Pegasystems Technologies
	email address:		jw at pegasys.ws

		Remember Cernan and Schmitt