Byte stream selective encryption/decryption device

An apparatus for providing data communications security in point-to-point and multipoint communications networks, wherein enciphered data on a communications line is deciphered on a byte-by-byte basis when received from said communications line, comprising:

• means for receiving said enciphered data from said communications line on a byte-by-byte basis;
• means for generating a plurality of key bits for each byte of said enciphered data received from said communications line;
• means for storing said plurality of key bits;
• means for exclusive ORing each bit of each byte of said enciphered data with one of said plurality of key bits generated for the byte previous to each of said bytes of enciphered data, whereby a byte of plain text data is produced corresponding to each of said bytes of enciphered data;
• and
• means for transmitting each of said bytes of plain text data onto said communications line, whereby each of said bytes of enciphered data is replaced by the corresponding byte of plain text data.

An apparatus for insertion in a bidirectional communications line for providing full duplex data communications security in point-to-point and multipoint communications networks, wherein plain text data being transmitted on said communications line in a first direction is received by said apparatus, enciphered, and transmitted back onto said communications line in said first direction on a byte-by-byte basis, and enciphered data being transmitted on said communications line in a second direction is received by said apparatus, deciphered and transmitted back onto said communications line in said second direction on a byte-by-byte basis, comprising:

• means for receiving said plain text data being transmitted in said first direction from said communications line on a byte-by-byte basis;
• first means for generating a first plurality of key bits for each byte of said plain text data received from said communications line;
• second means for storing said first plurality of key bits;
• third means for exclusive ORing each bit of each received byte of said plain text data with one of said first plurality of key bits generated for the byte of data received previous to each of said bytes of plain text data received, whereby a byte of enciphered data is produced corresponding to each of said received bytes of plain text data;
• means for transmitting each of said produced bytes of enciphered data onto said communications line in said first direction, whereby each of said received bytes of plain text data being transmitted in said first direction is replaced by the corresponding produced byte of enciphered data;
• means for receiving said enciphered data being transmitted in said second direction from said communications line on a byte-by-byte basis;
• fourth means for generating a second plurality of key bits for each byte of said enciphered data received from said communications line;
• fifth means for storing said second plurality of key bits;
• sixth means for exclusive ORing each bit of each received byte of said enciphered data with one of said second plurality of key bits generated for the byte of data received previous to each of said bytes of enciphered data received, whereby a byte of plain text data is produced corresponding to each of said received bytes of enciphered data;
• and
• means for transmitting each of said produced bytes of plain text data onto said communications line in said second direction, whereby each of said received bytes of enciphered data being transmitted in said second direction is replaced by the corresponding produced byte of plain text data.