Modern Cryptology Tutorial

For thousands of years, Cryptography has been the art of providing secure communications over insecure channels, and cryptanalysis has been the dual art of hacking those communications. Historically, cryptography (the combined art of encryption and cryptanalysis) was almost exclusively in the hands of the military and diplomats. With the advent of the computer revolution and, more importantly, with the advent of a society in which vast amounts of personal, financial, business and technical information are stored in computer databases and transmitted to across computer networks, the need for civilian encryption has become overwhelming. In Kahn's words, "In the cipher was revealed, the country's best-kept secret".

Who would win the age-old battle between cryptology and cryptanalysis? The great (non-expert) minds of past centuries are different. Voltaire wrote in A Dictionary of Philosophy : "Those who boast of reading ciphers are more liars than those who boast of listening to languages ​​they have not learned" saying: "The houses of cryptanalysis are a bunch of liars , even more so than Champollion t”). Edgar Allan Poe expressed the opposite opinion in his famous story "The Gold-bug" :wit cannot be solved by correct application"

It is now clear that Voltaire was wrong: the most cryptographic systems throughout history have been severely broken, sometimes with disastrous consequences. On the other hand, there are some cryptographic schemes that have been shown to be unbreakable regardless of "skill" or power of the cryptanalyst's computation (such as the one-shot buffers discussed in Section. However, for practical public-key cryptography (which is the subject of Chapter ), this question remains open. The current view is that the increase in computing power in the second half of this century has put cryptographers in an unprecedentedly favorable position, while cryptanalysts have been in that position. Ironically, Colossus was the first electronic computer in history specifically designed to encode German numbers. (Brian Randle is reported to have said, "The ENIAC machine, in my opinion, was not the first computer, but the eleventh." Which may have led to its collapse!

Until recently, a large numbers of qualified cryptanalysts "proved" the supposed reliability of cryptographic systems in unsuccessful attempts to break it. History clearly shows the drawbacks of this approach, as encrypted messages are often decrypted using encryption schemes that users have found unattackable. The destruction of the Enigma by Allied forces during (and even before) World War II is a good example. Readers interested in the historical significance of cryptography are encouraged to read the excellent description by Kahn and other popular books such as.

During this century, mathematicians have worked hard to find objective standards for the security of cryptographic systems, thus transforming this ancient art into an exact science. Shannon developed information theory as a result of his first (originally classified) work on cryptography. For different encryption systems, he was able to estimate the amount of ciphertext needed for cryptanalysis to achieve the desired level of reliability. Eb Melchior, for example, might have saved a trip to Helsingor if he believed he had deciphered the secret message on Shakespeare's tombstone, revealing the existence of the first edition of Hamlet within the last decade.

Computer scientists have worked to base cryptographic security on a new theory of computational complexity rather than Shannon's information theory". The main difference is that Shannon's theory was based on the 'hope that the cryptanalyst would not have enough information to break it.cipher and complexity Arithmetic just waits until the cryptanalyst has time for the reader to do it.

An extensive (but not exhaustive) list of references is provided, although coverage of many topics is necessarily brief. In addition to the history books mentioned above, there are many technical books such as and many popular books and technical books They have surveyed articles.
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