Data is encrypted for transmission over non-secure communication by providing data with a header and a trailer portion containing the full information as to the sender and recipient of the data so as to form a data packet, incrypting the data packet, and providing a further header and trailer portion to form a further data packet. The further header and trailer only contain information identifying the entry and exit nodes at which the further data packet enters and leaves the non-secure network. On arrival at the exit node the further data packet can be decrypted to reconstruct the original packet which is then conveyed to its destination via a local secure network. Padding non-secure network with dummy messages makes it impossible to identify the presence of genuine traffic between particular nodes of the non-secure network. Thus secure data may be sent via commercial non-secure packet switching networks without indicating its presence.