ClassicCrypto

# Example alice always starts her messages the same way

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Unformatted text preview: Eve has a copy of the ciphertext and the corresponding plaintext. Example: Alice always starts her messages the same way. – Chosen Plaintext: Eve somehow gains access to the encryption device. She can’t open it up to get the key, but she can input whatever she wants. – Chosen Ciphertext: Eve somehow gains access to the decryption device. She can’t open it up to get the key, but she can input whatever she wants. Most types of information security attacks can be loosely categorized as one of these attacks. Shift Cipher Shift Start with the plaintext alphabet. For example, it may be Z26 if we are using A, B, …, Z. Map these to numbers A =0 B =1 C =2 … Y = 24 Z = 25 Plaintext is mapped to a numerical representation “x”. The key, k , is a letter/number in Z26. Encryption yields the ciphertext: y = x + k (mod 26) Decryption is x = y - k (mod 26) Caesar used k=3. How to Attack the Shift Cipher? How Known Plaintext: You know x and you know y, so you calculate the key easily by: k = y - x...
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## This note was uploaded on 01/07/2013 for the course 332 519 taught by Professor Wadetrappe during the Fall '12 term at Rutgers.

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