Encode and decode Vigenère text with a keyword.
Attack at dawn Lxfopv ef rnhr Because the shift changes with every letter, the same plaintext letter can become different ciphertext letters. The uniform letter patterns that break Caesar ciphers simply do not appear, which is why Vigenère was known as "the indecipherable cipher" for centuries.
Anyone who knows the keyword can decode the message instantly. Short keywords repeat and eventually leak patterns; long random keywords were the foundation of one-time pads, the only provably unbreakable cipher.
It is a polyalphabetic cipher that shifts each letter by a different amount, determined by a keyword. The keyword repeats across the message.
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A Caesar cipher uses one shift for the whole message. Vigenère changes the shift with every letter, so identical plaintext letters can encode differently and frequency analysis stops working.
Enter the same keyword used to encode it. Decoding shifts each letter backward by the keyword position instead of forward.