Reciprocal polyalphabetic substitution cipher invented by Sir Francis Beaufort. Uses formula C = (K - P) mod 26 where encryption and decryption are identical operations. Variant of Vigenère cipher with reciprocal property. Input domain: every byte is accepted. A-Z and a-z are enciphered in place with their case preserved and advance the keyword; every other byte - digit, punctuation, whitespace, control or high-bit - is carried through unchanged and leaves the keyword position alone, which is the usual pen-and-paper convention and makes the round trip exact for arbitrary input. Nothing is ever discarded.
| Property | Value |
|---|---|
| Category | Classical Ciphers |
| Sub-category | Classical Cipher |
| Security status | 🎓 Educational Only |
| Complexity | Beginner |
| Inventor | Sir Francis Beaufort |
| Year | 1857 |
| Origin | 🇬🇧 United Kingdom |
| Source | algorithms/classical/beaufort.js |
Status: 🎓 Educational Only
| Issue | Description | Mitigation |
|---|---|---|
| Frequency Analysis | Letter frequencies partially preserved, making frequency analysis effective on longer texts | Use only for educational demonstrations, not for actual security |
| Kasiski Examination | Repeating key patterns can be detected using Kasiski’s method for determining key length | Consider as historical demonstration cipher only |
3 vectors ship with this algorithm and run in the test suite. Byte values are hexadecimal.
Vector 1 — Key shorter than the message. No published source carries this value
| Field | Value |
|---|---|
key |
4c454d4f4e |
input |
41545441434b41544441574e |
expected |
4c4c544f4c4245544c4e5052 |
Vector 2 — Educational Test Vector
| Field | Value |
|---|---|
key |
4b4559 |
input |
48454c4c4f |
expected |
44414e5a51 |
Vector 3 — Reciprocal Property Test
| Field | Value |
|---|---|
key |
434950484552 |
input |
42454155464f5254 |
expected |
4245504e5a444c50 |