CRYPTOGRAPHIC TRANSMISSION SYSTEM

In a cryptographic communication system, a first encryptor transmits to a first decryptor and a second encryptor, co-located with the first decryptor, transmits to a second decryptor, which is co-located with the first encryptor. Each encryptor/decryptor pair is adapted to communicate using a selected non-self-synchronizing cryptographic mode. Whenever it is determined that synchronization between, say, the first encryptor and the first decryptor has been lost, both the first decryptor and the co-located, second encryptor are switched from the non-self-synchronizing mode to the self-synchronizing mode. This causes a loss of synchronization between the second encryptor and second decryptor since the latter is still operating in the non-self-synchronizing mode. Upon detection of this loss of synchronization, the second decryptor and the co-located, first encryptor are also switched from the non-self-synchronizing mode to the self-synchronizing mode. In due course, then, synchronization in both transmission directions automatically restores. Return of each encryptor/decryptor pair to operation using the non-self-synchronizing mode is thereafter initiated by having each encryptor transmit a string of "1"s followed by a "0". The encryptor thereupon returns to the non-self-synchronizing mode. The string of "1"s is long enough to ensure that synchronization with the encryptor is re-established and when the "1"-to-"0" transition is detected at the decryptor, it, too, is switched back to the non-self-synchronizing mode.