Security of continuous-variable quantum cryptography

We discuss a quantum key distribution scheme in which small phase and amplitude modulations of cw light beams carry the key information The presence of EPR-type correlations provides the quantum protection. We identify universal constraints on the level of shared information between the intended receiver (Bob) and any eavesdropper (Eve) and use this to make a general evaluation of security. We identify teleportation as an optimum eavesdropping technique.