monoalphabetikus

Monoalphabetikus, or monoalphabetic, refers to a class of substitution ciphers in cryptography that uses a fixed substitution alphabet to map every plaintext symbol to a ciphertext symbol. In practice, each letter of the plaintext alphabet is assigned a unique counterpart in a second alphabet, and this mapping remains constant for the duration of the message. The method is defined by a key, which is the chosen permutation of the alphabet.

Encryption replaces each plaintext letter with its fixed ciphertext equivalent according to the substitution alphabet, while decryption uses the inverse mapping. Non-letter characters are typically preserved or treated separately. Common examples include the Caesar cipher, which shifts the alphabet by a fixed number, and the Atbash cipher, which reverses the alphabet. More generally, any simple substitution cipher applies a fixed permutation to the alphabet, and some variants can be described by simple mathematical functions modulo the alphabet size.

Security and limitations: monoalphabetic ciphers are vulnerable to frequency analysis because each plaintext letter maps to a consistent ciphertext letter, preserving letter and digram frequencies. This makes it possible to infer the key from longer ciphertexts, especially for languages with characteristic letter distributions. As a result, monoalphabetic ciphers are considered insecure for modern communications, though they remain useful for educational purposes and as puzzles.

History and context: these ciphers are among the oldest known encryption methods and laid groundwork for later techniques. They are typically contrasted with polyalphabetic ciphers, which employ multiple alphabets to counter frequency analysis.