  A continuous key distribution scheme is proposed that relies on a pair of
canonically conjugate quantum variables. It allows two remote parties to share
a secret Gaussian key by encoding it into one of the two quadrature components
of a single-mode electromagnetic field. The resulting quantum cryptographic
information vs disturbance tradeoff is investigated for an individual attack
based on the optimal continuous cloning machine. It is shown that the
information gained by the eavesdropper then simply equals the information lost
by the receiver.
