  By analogy to classical cryptography, we develop a "quantum public key" based
cryptographic scheme in which the two public and private keys consist in each
of two entangled beams of squeezed light. An analog message is encrypted by
modulating the phase of the beam sent in public. The knowledge of the degree of
non classical correlation between the beam quadratures measured in private and
in public allows only the receiver to decrypt the message. Finally, in a view
towards absolute security, we formally prove that any external intervention of
an eavesdropper makes him vulnerable to any subsequent detection.
