METHOD AND APPARATUS FOR SYNCHRONIZING ENCRYPTING AND DECRYPTING SYSTEMS

A synchronization method and corresponding apparatus for transmitting or storing encrypted data breaks the data into blocks and appends to each block an error detection code which is calculated from the encrypted data block plus a unique sequence number. The sequence number is generated by a local counter and may be the number of bits, bauds, or characters transmitted and received since a previous resynchronization. The error correcting code is transmitted or stored with the encrypted data block, but although the sequence number is appended to the data block for error code calculation purposes, it is not actually transmitted or stored with the encrypted data and error correcting code. When the encrypted data is retrieved or received, the receiving apparatus appends to the received data blocks a sequence number derived from a local counter which is synchronized to the counter at the transmitting or storing apparatus and a new error detecting code is calculated for comparison to the error detecting code received or retrieved with the encrypted data. A mismatch between the error detecting codes indicates a transmission or synchronization error for that block. In either case the data block can be retransmitted.