HTTP 'HASH' Method

I think a 'HASH' method should be implemented into HTTP, whereby the server
responds with a hash (md5/sha) of the requested resource. This would be a
godsend for large networks which tend to use caching extensively, because
caching-proxy servers could verify the source hasn't changed while creating
a minimal amount of traffic, thereby allow the amount of time the proxy
stores cache to be be greatly increased only at the cost of hard drive
space.
Not only would this benefit caching applications, but as I'm sure you could
imagine, it could be used in may other fields, especially security.