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Cryptography

Cryptography. Part 2. Vigenere’s Cipher.

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Cryptography

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  1. Cryptography Part 2

  2. Vigenere’s Cipher • “All nature is merely a cipher and a secret writing. The great name and essence of God and His wonders -- the very deeds, projects, words, actions, and demeanor of mankind -- what are they, for the most part, but a cipher?” Blaise De Vigenère (1523-1596)

  3. Properties of Vigenere’s Cipher • Actually due to Giovani Battista Ballaso (1553) • Polyalphabetic cipher • Can’t be broken with the same frequency analysis we used for a substitution cipher • Used by Confederacy in the Civil War • Involves a secret key that needs to be known to the sender and the receiver (Confederates used "Manchester Bluff", "Complete Victory" and "Come Retribution"). • Lewis Carroll and Scientific American (1917) incorrectly called the code unbreakable.

  4. Exercise Use Vigenere’s Cipher and the method discussed in class to do the followingtasks: • Use the key AGGIES to encrypt the message <spoon is soon> • Use the key LEMON to decipher <LXFOPVEFRNHR> • To review this cipher, see http://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher

  5. Security of Vignere’s Cipher • Remarkable fact about Vigenere’s Cipher: If the key is longer than the plaintext, is used only once and is random, then Vigenere’s Cipher is unbreakable. • This exercise gives a partial reason why: Consider ciphertext <sqrtn>. Find a key that produces plaintext <aggies>. Then find another one that produces <miners>. • How would a codebreaker know the code had been broken? (Think about this!)

  6. One-Time Pad • So Vigenere’s Cipher can be used to make an unbreakable(!) code. • But you need a really long, random key. • Book-length keys, called one-time pads, have been used by spies since at least 1920. • You must never use the key more than once, or good cryptographers can pick up patterns (sort of like very advanced frequency analysis).

  7. Some one-time pads used by KGB.  • US broke Soviet’s one-time pad during Cold War because they used a pad more than once. • Moral: Unbreakable if done right, but very hard to do right!

  8. What’s next? • We’ve seen in just a couple of weeks how to construct an unbreakable code. But it is very, very difficult to implement the code. • We’d settle for a breakable code that is easy to implement but very, very hard to break. • For this, we need to look at things from a mathematical point of view…

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