Signing pictures -- how hard, how long?

[Nick Szabo]

Peter Wayner:
> I think signing photographs and movie images is a difficult
> problem. Why? Because one flipped bit will completely screw
> up the hash function.

Is a one-way hash function or digital signature possible with the
following property: signature verification doesn't just determine
the boolean altered vs. pristine, but also shows the picture
distance between the altered and original?  Forgery may often
require an editing distance much greater than the error rate
from media noise or lossy compression.

Nick Szabo				szabo at netcom.com