Abstract: Apparatus for use in a secure data communication system includes a microcomputer that controls a cipher process by which sensitive data is encrypted before transmission. When activated, the apparatus accepts either cleartext or ciphertext and transforms the text to its opposite form according to an algorithm that is past dependent and which is also dependent on a pseudo random cipher key. Ciphertext may then be stored in an information retrieval system or transmitted to a destination terminal where the text is deciphered in real time by a corresponding second apparatus that stores the same key. In either case, security of transmitted and stored data is maintained since the sensitive data is in encrypted form. A set of logical instructions according to the algorithm is stored in a read only memory of the microcomputer which includes an EPROM unit having a data field of predetermined characters.