How does the Vigenère cipher work?

Each letter of the message is shifted by the corresponding letter of a repeating keyword: A shifts by 0, B by 1, and so on. This makes it a polyalphabetic cipher that is much harder to break than a single Caesar shift.

What happens to spaces and numbers?

Only letters A–Z are enciphered. Digits, spaces and punctuation pass through unchanged, and any non-letter characters you type in the keyword are ignored.

Does the keyword count skip spaces and punctuation?

Yes. The keyword advances only on letters that get enciphered, so spaces and punctuation in your text do not consume a keyword position. The cleaned keyword (letters only) repeats across the letters of the message.

How is it different from a Caesar cipher?

A Caesar cipher shifts every letter by the same fixed amount. Vigenère uses a different shift per position taken from the keyword, so the same plaintext letter can encrypt to different letters — defeating simple frequency analysis.

Is the Vigenère cipher secure today?

No. It withstood casual cryptanalysis for centuries but is broken by the Kasiski examination and the Friedman test, which find the key length, then solve each Caesar slice. Treat it as educational or puzzle-grade, not real encryption.

Does uppercase or lowercase matter?

Case is preserved in the output but does not change the shift. The keyword is treated case-insensitively, so LEMON and lemon give the same result.

Is my text uploaded anywhere?

No. The cipher runs entirely in your browser — nothing is sent to a server.

Vigenère Cipher Encoder & Decoder

The Vigenère cipher is a classic polyalphabetic substitution cipher: instead of shifting every letter by the same amount like a Caesar cipher, it uses a repeating keyword so each position is shifted by a different value. Described by Giovan Battista Bellaso in 1553 and later misattributed to Blaise de Vigenère, it resisted casual codebreaking for 300 years. This tool encrypts and decrypts text with any keyword, entirely in your browser.

How it works

The keyword is first cleaned to letters only and lower-cased, then repeated across the letters of your message (spaces, digits and punctuation pass through and do not advance the keyword). Each plaintext letter is shifted by the alphabet position of the matching keyword letter — A = 0, B = 1, … Z = 25 — wrapping around modulo 26. To decrypt, the tool shifts in the opposite direction (by 26 − shift). Letter case is kept; non-letters are left untouched.

Example

Encrypting ATTACKATDAWN with the keyword LEMON:

Plaintext	A	T	T	A	C	K	A	T	D	A	W	N
Key	L	E	M	O	N	L	E	M	O	N	L	E
Shift	11	4	12	14	13	11	4	12	14	13	11	4
Cipher	L	X	F	O	P	V	E	F	R	N	H	R

The result is LXFOPVEFRNHR. Decrypting it with the same keyword returns the original message. Everything happens in your browser, so your message and keyword never leave your device.