Vigenere Cipher
Encrypt and decrypt text using the classic Vigenere polyalphabetic cipher.
Plaintext
Only letters are used; key repeats cyclically
Ciphertext
Key Visualization (first 40 letters)
PlaintextH e l l o W o r l d
KeyS E C R E T S E C R
Each letter shifted by key letter position (A=0, B=1 ... Z=25)