This thesis investigates conventional and public key cryptographic techniques for secure data communication. Block and stream cipher methods to provide secure communication over an insecure channel are discussed with particular reference to the Data Encryption Standard (DES) algorithm. A microprocessor based data encryption interface unit has been designed and constructed using the DES to provide both communication and file security. Several chaining techniques using the system have also been investigated enabling a study of their error characteristics, speed of operation, level of security and their ability to overcome difficulties due to data redundancy and structure. A statistical analysis of the randomness of the output sequences in each of these techniques has been made. Furthermore, the developed system can be used on the Prestel public network allowing storage and retrieval of completely and partly encrypted frames of information on the Prestel database.

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