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In total we have 1226 312 choices for the key k

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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 letter-by-letter 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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