Monoalphabetic cipher
A monoalphabetic cipher uses one fixed substitution mapping for the entire message.
Definition
A monoalphabetic cipher assigns each plaintext symbol one fixed ciphertext counterpart for the whole message. If a plaintext letter appears several times, it is normally replaced by the same ciphertext letter each time.
Examples and key space
Caesar, affine, Atbash, and simple substitution are monoalphabetic ciphers. A freely shuffled alphabet has a much larger key space than Caesar, but key-space size alone does not remove the statistical structure of language.
Why it is vulnerable
The fixed mapping preserves frequency ranks, repeated-letter patterns, and word shapes. Frequency analysis combined with common words and digrams can recover long ordinary messages, although short or unusual texts may remain ambiguous.
Yes. Its single shifted alphabet is used unchanged throughout the message.
It has many possible keys, but long natural-language messages still leak patterns that enable cryptanalysis.