Caesar cipher

Classical Cryptography, Substitution Ciphers

• Substitution cipher

• Symmetric cyptography

How to attack

1. Exhaustive search

2. Statistical analysis

Vigenère Cipher

• Period - length of the key

• polyalphabetic - key has several different letters

How to attack

1. Establish period; call it n

Kasiski examination

Repetitions in the ciphertext occur when characters of the key appear over the same characters in the plaintext



Index of coincidence (IC)

• $IC = [n(n-1)^{-1}]\sum_{0\le i\le25}[F_i(F_i - 1)]$ when $F_i$ is the number of times charater $i$ occurs in ciphertext
• The probability that two randomly chosen letters from ciphertext will be the same for any character
• English IC ~ .066

2. Solve each part, using techniques from breaking Caesar cipher

Decrypting Attack

heuristic and inference