monoalfabetiskt

Monoalfabetiskt refers to a type of substitution cipher in which every letter of the plaintext is replaced by a fixed corresponding letter according to a single substitution alphabet. The same mapping is used for the entire message. This is distinct from polyalphabetic ciphers, which use multiple alphabets or shifting keys depending on position.

Encryption is performed by applying the key’s mapping: for a standard Latin alphabet, each letter is replaced by the letter it maps to. The key can be a simple permutation of the alphabet or a keyword-based arrangement, examples being Atbash (reversing the alphabet) or a Caesar shift (fixed shift).

Common monoalfabetiska ciphers include the Caesar cipher and various substitution ciphers where letters are permuted according to a secret key. In typical implementations, spaces and punctuation are either preserved or removed according to the rules of the system.

Strengths and weaknesses: monoalfabetiska ciphers are easy to implement and understand, but they are highly vulnerable to frequency analysis, since each plaintext letter corresponds to a single ciphertext letter regardless of context. With enough text, the substitution mapping can often be deduced. The number of possible keys for a 26-letter alphabet (26!) makes brute-force search theoretically possible but impractical for long texts without computational aid.

Historisk och modern användning: dessa ciphers dominate classical cryptography and are studied for educational purposes and in puzzles. De används sällan för verklig säkerhet i modern kommunikation, där mer avancerade tekniker krävs.

Se även: monoalfabetisk substitution, polyalphabetisk cipher, Caesar cipher, frekvensanalys, kryptologi.