Secure communications between microcomputer systems

A data security communications interface unit has been developed to allow data transfer between Apple terminals in either plain or encrypted format under user control. The unit employs the Data Encryption Standard algorithm and has a degree of sophistication sufficient to meet most user needs. The unit uses the 6502 microprocessor to control encryption, decryption and communications. In addition to the transfer of encrypted data, the interface also provides a facility for storing encrypted program and data files locally in the Apple disc system. Further, the encryption system has been designed to allow storage and retrieval of completely encrypted or partly encrypted frames of information on the Prestel database. The interface has been tested extensively using several DES modes of operation.