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Practical Cryptology Affine Cipher Cryptanalysis The case C = AP (mod 26) (B = 0) is called Decimation Cipher. Example: Decipher the following text and find a key: CFQGE KAZEMF ZMAGVMC NMO VYSV Solution: 1. For each letter, count the number of times it appears in the ciphertext. 2. Choose the letter that appears the most. In our example, M occurs 4 times in the ciphertext – the most. 3. Use the English frequency table below to make a following assumption: Since E is the most common letter, we will make a tentative assumption that ciphertext M (12) corresponds to the plaintext E (4). 4. Solve equation: C = AP (mod 26) to find A using 12 (M) instead of C and 4 (E) instead of P: A*4 ≡ 12 (mod 26) there are 2 solutions in the range 1 to 25, A = 3 and A =16 (not good, not relatively prime with 26) 5. If our assumption was correct, the key A is 3. 6. To find the plain text we need to solve the following equations: 7. To find the plaintext we need to use the following decryption formula: P = 3-1 C (mod 26) , 3-1 = 9 is multiplicative inverse of 3 mod 26. 8. Solving P = 9C (mod 26) for each ciphertext letter you will get the following plaintext: Decrypted message: STOC MARKET REACHES NEW HIGH Frequency Table of English: Letter A B C D E F G H I 0 1 2 3 4 5 6 7 8 frequiency 7 1 3 4 13 3 2 3 8 in % Plaintext N 13 Ciphertext 8 O 14 7 P 15 3 Q 16 <1 R 17 8 S 18 6 T 19 9 U 20 3 V 21 1 J 9 <1 K 10 < 1 W 22 1 X 23 <1 L 11 4 Y 24 1 M 12 3 Z 25 <1