Importance

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Abstract

In a secure digital transmission system that may utilize the Data Encryption Standard, the message signal is encrypted according to a predetermined algorithm and a key. The key is selected from a library of key material according to a key index signal. The key index signal, which is transmitted to all stations that must decrypt the message signal and therefore subject to interception, is itself encrypted at each station to form the library addresses for key selection. Even though the key selection process is initiated by a transmitted signal, security remains high because in the key selection chain, clear text and associated cypher text are not available for analysis. The key for encrypting the key index signal may itself be selected in a similar manner, thereby cascading the system any number of times for further security. A pseudorandom bit stream generated and encrypted at each station and initialized by the transmitted key index signal can address the key library to provide continually changing key.

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First Claim