Type your text and a keyword, and the tool encrypts it with the classic Vigenère polyalphabetic cipher, shifting each letter by the keyword's repeating letters.
Each plaintext letter is shifted by the corresponding letter of a repeating keyword. Encryption is C = (P + K) mod 26 and decryption is P = (C − K) mod 26, where K is the keyword letter's position (A=0).
Because the shift changes with every keyword letter, the same plaintext letter can map to different ciphertext letters. This defeats the simple frequency analysis that breaks single-shift Caesar ciphers.
Only A–Z letters are enciphered and output is uppercased. Spaces, digits, and punctuation pass through unchanged, and the keyword only advances on letters.