Unformatted text preview: pg 15 for what can happen when you don’t have this!)
There are 12 choices for α since there are 12 numbers with
gcd(α,26) = 1. (Check this!)
We also need to choose β , and there are 26 possibilities.
In total, we have 12*26 = 312 choices for the key k=(α,β) Attacks on Affine Cipher
Attacks
Ciphertext Only: Just try all 312 possible keys.
Known Plaintext: If you have two pieces of plaintext, you may
set up a system of equations
y l = αx1 + β (mod 26) y 2 = αx 2 + β (mod 26 ) Solving is just algebra!
Chosen Plaintext: Choose ‘ab’ as the plaintext. The first
character of the ciphertext will be β, while the second will be
α+β.
Chosen Ciphertext: Similar to Chosen Plaintext. Vigenere Cipher
Vigenere
The Vigenere cipher is an extension of the shift cipher.
Basically, your key is a secret word of unknown length.
Encryption proceeds as:
– Line your key up with your plaintext, repeating it as necessary.
– Perform a letterbyletter shift using the letter of the key beneath
it. Example:
H e 21 4 r e i s h o 2 19 14 17 21 4 w i t 2 19 14 Shift CI T XWJ CSY BH How to Break the Vigenere
Vigenere
There are two tasks to breaking Vigenere:
– Find the length of the key
– Find the key itself. If we know the length of the key (L), finding the key itself is not
that hard:
– All we need is to do to find the first letter...
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Full Document
 Fall '12
 WadeTrappe
 Cryptography, Encryption, Plaintext, Key Length, Vigenere

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