Monoalphabetic Cipher

Trying every possible key on a piece of ciphertext until intelligible translation into plaintext is obtained.

The language of the plaintext is known and easily recognizable.

A monoalphabetic substitution cipher.

To deduce a specific plaintext or to deduce the key being used.

It is easily broken.

A monoalphabetic substitution cipher, such as the Caesar Cipher.

Classical cryptography involves encryption and decryption done by hand, while modern cryptography uses computers to encrypt and decrypt, allowing for more complex ciphers.

The Caesar Cipher involves replacing each letter of the alphabet with the letter standing three places further down the alphabet.

The general Caesar algorithm involves replacing each letter of the alphabet with the letter standing k places further down the alphabet, for k in the range 1 through 25.

It has a key space of 26! or greater than 4x10^26 possible keys, making it virtually impossible to break using brute-force techniques.

Roughly 13% of the letters in a large sample of English text should be E's.

C = E(k, p) = (p + k) mod 26

Frequency analysis

Known plaintext attack

In a ciphertext-only attack, the attacker knows only the ciphertext, whereas in a known plaintext attack, the attacker knows both the plaintext and the corresponding ciphertext.

The frequencies of occurrence of letters in a natural language, such as English, where the most frequent letters are E, T, A, O, N, R, I, S, and H.

To exploit the regularities of the language to decipher the encrypted message.

Ciphertext-only attack.