Lecture 3 - CECS 478: Class Notes September 16, 2008 To...

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CECS 478: Class Notes September 16, 2008 Caesar Cipher C i = E(p i ) = p i + 3 mod 26 Substitution using a key Assuming a key of “suddenly” (remove the double d) A B C D E F G H I J K L M N O P Q R S T U V W X Y Z S U D E N L Y A B C F G H I J K M O P Q R T V W X Z This can be aided with a Vigenere Tableau To decrypt you can use knowledge of the English language to help you. All letters do not occur at the same frequency. E, T, O, and A appear 40% of the time. Other letters are rare (J, Q, K, Z). If the encryption is “fixed” then the letter frequency is preserved. Look for frequent characters in cipher text. These are E, T, O, and A. Cryptographer’s dilemma: Encryption algorithm should be fast and regular but also secure. Perfect secrecy: Unbreakable. There is an algorithm that fits this description, but is not very practical. The one time pad. Assume availability of a random sequence of letters. This sequence is the key. To encrypt: C
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Lecture 3 - CECS 478: Class Notes September 16, 2008 To...

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