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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.
 Fall '12
 WadeTrappe

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