Vigenère Cipher Explained
The Vigenère cipher applies a different Caesar shift to each letter based on a repeating keyword. Encrypt and decrypt on Cipher Portal or read the Vigenère decoder page for decrypt-focused steps.
What is the Vigenère cipher?
Invented in the 16th century and once called “le chiffre indéchiffrable,” Vigenère uses a keyword such as KEY or SECRET. Each plaintext letter aligns with one keyword letter; that letter’s alphabet index becomes the shift. The keyword repeats until the message ends.
Mathematically it is a series of Caesar ciphers keyed by the keyword stream. Identical plaintext letters can encrypt to different symbols when keyword letters differ—flattening simple frequency spikes compared with monoalphabetic ciphers.
How Vigenère encryption works
Write the keyword repeatedly under the plaintext (spaces often ignored). Add letter indices modulo 26 for encryption; subtract for decryption. Cipher Portal accepts contiguous A–Z keys in the key field when Vigenère is selected.
Security against pencil-and-paper opponents improved historically, but Kasiski examination and Friedman tests estimate keyword length; once length is known, each column behaves like Caesar. Compare with Caesar and monoalphabetic substitution.
When students use Vigenère
Courses introduce Vigenère immediately after Caesar to show how repeating keys change statistics. Hobby cryptograms and CTF puzzles still use short keywords.
Without the keyword, start with the Vigenère cipher cracker guide and decrypt without a key workflow.
See Vigenère examples for HELLO/KEY → RIJVS and verification on the live tool.
Frequently asked questions
What is a Vigenère cipher?
A polyalphabetic cipher that applies rotating Caesar shifts derived from a repeating keyword.
How is Vigenère different from Caesar?
Caesar uses one shift for the whole message; Vigenère changes the shift per letter based on the keyword.
Can Vigenère be broken?
Yes—estimate keyword length, split columns, then attack each as Caesar.
How do I use Vigenère online?
Select Vigenère on Cipher Portal, enter plaintext/ciphertext and keyword, then encrypt or decrypt.