๐ Vigenรจre Cipher
Lxfopv ef rnhr
Try out the Vigenรจre cipher, which encrypts and decrypts text by shifting each letter by an amount determined by a repeating keyword. Unlike the Caesar cipher, the shift amount varies letter by letter, making it a polyalphabetic cipher that resists simple frequency analysis. Useful for learning about the history of cryptography or just for fun with text.
How to use
- Choose "Encode" or "Decode" mode.
- Enter the text to convert and a keyword (letters only) to use for the encryption.
- The converted result appears automatically โ use "Copy" to copy it.
How the calculation works
The Vigenรจre cipher, popularised in the 16th century, uses a keyword to vary the shift from letter to letter โ in effect, several Caesar ciphers combined. Encrypt: cipher letter = (plain letter + key letter) mod 26 Decrypt: plain letter = (cipher letter โ key letter) mod 26 Key letters count as A = 0, B = 1 โฆ Z = 25, and the key repeats to match the length of the text. Here, non-letters (spaces, symbols) are left unchanged and do not advance the key. Non-letters in the key are ignored.
Worked example
Plaintext: ATTACKATDAWN Key: LEMON (repeated as LEMONLEMONLE) Ciphertext: LXFOPVEFRNHR First letter: A (0) + L (11) = L Second letter: T (19) + E (4) = X (23) The default "Attack at dawn" keeps its spaces and becomes "Lxfopv ef rnhr".
Things to be aware of
- It was once called "the indecipherable cipher", but in the 19th century Kasiski and others found ways to determine the key length and break it.
- Shorter keys repeat the same shifts more often and are easier to break.
- It cannot protect real secrets. Use it to learn about the history and workings of ciphers.
FAQ
How is this different from the Caesar cipher?
The Caesar cipher shifts every letter in the text by the same fixed amount, while the Vigenรจre cipher varies the shift for each letter based on the keyword โ making it harder to break.
Can the keyword include numbers or symbols?
Non-alphabetic characters in the keyword are automatically stripped out โ only the letters are used to calculate the shift amounts.
Is this a practical cipher for modern use?
No, it was already shown to be breakable in the 19th century using techniques like the Kasiski examination, and it isn't secure by modern standards. It's purely for learning about historical cryptography.