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Symmetric-key cryptography

Security in Computing (3rd Edition)

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CS 161 Computer Security Fall 2005 Joseph/Tygar/Vazirani/Wagner Notes 8 1 Brief History of Cryptography The word “cryptography” comes from the latin root crypt meaning secret, and graphia, meaning writing. So cryptography is literally the study of how to send secret messages. In the next few lectures we shall study encryption schemes as well as some other fundamental goals of cryptography: authentication and digital signatures. Schemes for sending secret messages go back to antiquity - the Romans used the “Caesar cypher” which consists of permuting the alphabet by simply shifting each letter forward by a Fxed amount. ±or example, Caesar used a shift by 3 so the message “cryptography” would be encoded as ”fubswrjudskb”. With the developement of the telegraph (wireless communication) at the end of the nineteenth century, the need for encryption in military and diplomatic communications became particularly important. The codes that were used during this “pen and ink” period were relatively simple since messages had to be decoded by hand. The codes were not very secure especially with the use of mathematical and statistical techniques. The second phase of cryptography, the “mechanical era” was the result of an intense German project to create a mechanical device for encrypting messages in an unbreakable code. The resulting Enigma machine was a remarkable engineering feat. Even more remarkable was the massive British effort to break the code. The breaking of the Enigma code was an important event that determined the course of World War II, and according to most experts shortened the war by about a year. There were two important factors in the break- ing of the Enigma code: Frst, the British managed to obtain a replica of a working Enigma machine from Poland, which had cracked a simpler version of the code. The second factor was scale and sophistication of the code breaking effort, Frst by the Poles, who employed a large contingent of mathematicians to crack the structure, and then by the British, whose massive effort included Alan Turing, one of the founding fathers of computer science. Modern cryptography is distinguished by its reliance on mathematics and electronic computers. It has its early roots in the work of Claude Shannon following World War II. The analysis of the one-time pad later in this lecture is due to Shannon. The early seventies saw the the introduction of the cryptosystem DES by NIST (the National Institute for Standards in Technology). DES answered the growing need for digital encryption standards in banking and other business. The decade starting in the late seventies saw an explosion of work
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