Showing papers on "Ciphertext published in 1978"

Past dependent microcomputer cipher apparatus

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. The cipher process is initiated by a control character and starts with predetermined seeds that establish a first origin in the data field for each line of text to be encrypted. This origin is then translated to a second origin of the data field in response to four characters that are randomly generated by a counter. A variable sequence of iterative logical operations transforms cleartext to ciphertext and provides a key stream that is non-linear, random-like and which possesses an encryption period that is substantially longer than the period of any line so that the periodicity of the key stream is never indicated. The cipher keys for each line of text are temporarily stored in a read/write memory to permit editing operations and are erased at the onset of the next line.