The present invention is concerned with a method to encrypt OFDM modulation, by multiplying its underlying QAM symbols by a complex-valued generalized key stream sequence. Performing encryption on the physical layer ensures that all services and applications running over the OFDM modem will be protected against eavesdropping. It is also proposed to include in the encryption any training symbols used for synchronization and channel estimation. Only the legitimate receiver knowing the encryption key is hence able to synchronize and correctly demodulate the received signal. Attackers will not even be able to acquire the encrypted signal.