Beaufort Cipher

Explore a reciprocal polyalphabetic cipher

The Beaufort cipher combines each alphabetic input character with a repeating key. Letters are represented from zero through twenty-five, and the plaintext value is subtracted from the matching key value modulo twenty-six. Nonletters are preserved and do not advance the key position. Unlike many familiar keyed ciphers, Beaufort is reciprocal: running ciphertext through the same transformation with the same key reconstructs the original letters.

With plaintext HELLO and key KEY, the repeating key sequence produces ciphertext DANZQ. Entering DANZQ with the same key returns HELLO. The page normalizes letters to uppercase so the numerical alphabet is unambiguous. Spaces and punctuation remain in place, making phrase-level experiments readable while the repeating key aligns only with alphabetic characters.

Keep historical ciphers out of security work

The transformation happens in your browser and does not send the message or key anywhere. Beaufort is appropriate for classical-cryptography study, hand-worked examples, escape rooms, and recreational puzzles. It does not provide authentication or modern confidentiality, and repeated-key statistical patterns are vulnerable to analysis. Use vetted contemporary encryption software for private messages, credentials, files, or any information that requires actual protection.