  We characterize the complete set of protocols that may be used to securely
encrypt n quantum bits using secret and random classical bits. In addition to
the application of such quantum encryption protocols to quantum data security,
our framework allows for generalizations of many classical cryptographic
protocols to quantum data. We show that the encrypted state gives no
information without the secret classical data, and that 2n random classical
bits are the minimum necessary for informationally secure quantum encryption.
Moreover, the quantum operations are shown to have a surprising structure in a
canonical inner product space. This quantum encryption protocol is a
generalization of the classical one time pad concept. A connection is made
between quantum encryption and quantum teleportation, and this allows for a new
proof of optimality of teleportation.
