Note that signatures do not constitute zero knowledge proofs, however, because I can transmit your signature to a third party and convince them of the same thing you convinced me of, namely the authenticity (or whatever) of a particular document. Of course, larger protocols built around signatures don't suffer, because at worst you just need to make sure you sign a sufficient collection of data (to avoid interpretation as a different message given a different context)... but it's not zero-knowledge.