A method of communicating a classical message M between a first party A and a second distant party B over a public channel F, comprises the steps of sharing a key between the parties, the shared key K comprising a short-term-secure key KS and/or a long-term-secure key KL; at A, encoding M as a quantum codeword, A using K to encode M into a first encrypted codeword belonging to a publicly known quantum code; communicating the first encrypted codeword from A to B over F whose output is a second codeword; unitarily transforming the second codeword into a third codeword by using a N-mode interferometer controlled by B, placed at the output of F and keyed by K; determining an estimate of M, at B, by performing a measurement on the third codeword and by processing the measurement using K.