Secure communication system with cross-linked cryptographic codes

A cryptographic communications system includes a method and apparatus for sending and receiving a plurality of encrypted signal packets in a system for metering the use of encrypted information. Plural packet communication between a user terminal containing a remote cryptographic control unit and a cryptographic operations center typically includes uploading usage records and downloading credit. Plural packet messages are protected to detect alterations in the form of packet reordering by cross linking data cryptographic codes and message authentication codes (MAC). In particular, a first cryptographic code key is used to encrypt a first plurality of signal packets. A second cryptographic code key is used to encrypt the encrypted first plurality of signal packets to form an appended MAC. The initial vector for computing the MAC for a given packet is the last encrypted data block of the previous packet encrypted with the first cryptographic code key. In such manner, each MAC is a function of the present packet and all previous packets, thereby providing an increased level of security against signal tampering by reordering of packets.