Public key encryption has some rather useful properties. Since the decryption key cannot practically be derived from the encryption key, the latter may as well be public knowledge, hence the more common term: ``public key''. If this is done, then anyone can encrypt data with that key. The corresponding decryption key is kept private, hence the term ``private key''; only the possessor(s) of that key can decrypt the encrypted data. Thus: if each person has a public key published in a directory, anyone may send that person a private message. No-one else, not even the author, can decrypt that message because they do not have and cannot derive the corresponding private key. There is no necessity to exchange keys ahead of time as there would be if conventional encryption were used