Enhanced Vigenère cipher that extends the key using plaintext itself, eliminating periodic key repetition. Uses initial keyword plus plaintext letters to create non-repeating key sequence. More secure than standard Vigenère. Input domain: every byte is accepted. A-Z and a-z are enciphered in place with their case preserved and are the only bytes that extend the running key, which uses their uppercase form; every other byte - digit, punctuation, whitespace, control or high-bit - is carried through unchanged and takes no part in the key, 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 | Intermediate |
| Inventor | Blaise de Vigenère |
| Year | 1586 |
| Origin | 🇫🇷 France |
| Source | algorithms/classical/autokey.js |
Status: 🎓 Educational Only
| Issue | Description | Mitigation |
|---|---|---|
| Probable Plaintext Attack | If portion of plaintext is known, can recover key and decrypt remainder of message | Avoid predictable beginnings or known phrases |
| Statistical Analysis | While more secure than Vigenère, still vulnerable to advanced statistical attacks | Educational use only |
3 vectors ship with this algorithm and run in the test suite. Byte values are hexadecimal.
Vector 1 — Classic Autokey Example
| Field | Value |
|---|---|
key |
4c454d4f4e |
input |
41545441434b41544441574e |
expected |
4c58464f504b544d4443474e |
Vector 2 — Educational Test Vector
| Field | Value |
|---|---|
key |
4b4559 |
input |
48454c4c4f |
expected |
52494a5353 |
Vector 3 — DCode Reference
| Field | Value |
|---|---|
key |
4155544f4b4559 |
input |
44434f4445 |
expected |
445748524f |