Mr. Squirrel

[Eric Hughes]

A signature on a message is dependent on the contents of the message;
it is not a free floating bit of information.  You can't copy a
signature, therefore, without copying the message or find another
message that hashes to the same value.  This is the design criterion
behind one-way functions--that you can't (feasibly) find a message
that hashes to a given value.

Eric