  We discuss aspects of secure quantum communication by proposing and analyzing
a quantum analog of the Vernam cipher (one-time-pad). The quantum Vernam cipher
uses entanglement as the key to encrypt quantum information sent through an
insecure quantum channel. First, in sharp contrast with the classical Vernam
cipher, the quantum key can be recycled securely. We show that key recycling is
intrinsic to the quantum cipher-text, rather than using entanglement as the
key. Second, the scheme detects and corrects for arbitrary transmission errors,
and it does so using only local operations and classical communication (LOCC)
between the sender and the receiver. The application to quantum message
authentication is discussed. Quantum secret sharing schemes with similar
properties are characterized. We also discuss two general issues, the relation
between secret communication and secret sharing, the classification of secure
communication protocols.
