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
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
Chosen Ciphertext: Similar to Chosen Plaintext. Vigenere Cipher
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
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
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
– All we need is to do to find the first letter...
View Full Document
- Fall '12
- Cryptography, Encryption, Plaintext, Key Length, Vigenere