Signing pictures -- how hard, how long?

[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. Errors on these tapes happen rarely,
but most video manufacturers aren't really going to bother
worrying about occasional bit errors because they're usually
invisible to the eye. Why waste all that extra effort on
error correction if it's not worth the trouble. So signed
photographs will also need to contain all of the error 
correction necessary and that will make them more expensive.
This isn't any real cost on a general purpose machine, but
it matters in some places.