munication private. The sender enciphers eachmessage before transmitting it to the receiver. Thereceiver (but no unauthorized person) knows the appropriate deciphering function to apply to the receivedmessage to obtain the original message. An eavesdropper who hears the transmitted message hears only"garbage" (the ciphertext) which makes no sense tohim since he does not know how to decrypt it.The large volume of personal and sensitive information currently held in computerized data banks andtransmitted over telephone lines makes encryptionincreasingly important. In recognition of the fact thatefficient, high-quality encryption techniques are verymuch needed but are in short supply, the NationalBureau of Standards has recently adopted a "DataEncryption Standard" [13, 14], developed at IBM.The new standard does not have property
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