One Word, Enciphered

A monoalphabetic substitution runs by one fixed mapping — and enciphering a short word by hand, then deciphering it, proves both the mechanism and its reversibility in under a minute.

Fix a mapping: every letter moves three forward in the alphabet — A becomes D, B becomes E, and so on, one rule throughout, which is what monoalphabetic means. Take the plaintext word BUS. Encipher: B moves to E; U moves to X; S moves to V. The ciphertext reads EXV — scrambled by the encryption key, which here is the number three and the direction forward. Now the receiver's side: the decryption key is the same mapping reversed — three backward. E returns to B; X returns to U; V returns to S. The plaintext BUS is restored, letter for letter, and the whole cast from video 0632 appeared on a page: plaintext, algorithm, key, ciphertext, decryption. Now the weakness, in the same small case: the mapping never varied — every B in a longer message would become E, every pattern in the plaintext surviving as a pattern in the ciphertext. That regularity is what a solver attacks, and why the polyalphabetic refinement shifts the mapping as it goes.