Vigenere Cipher

This tool performs classical Vigenere cipher encryption and decryption on alphabetic text using a keyword. The keyword is normalized to uppercase letters and applied as a repeating polyalphabetic shift; non-letter characters pass through unchanged, and the original letter case of the input is preserved in the output.

1. Toggle the Encrypt or Decrypt mode at the top using the Lock / Unlock buttons. 2. Enter the text you want to transform in the Input Text area. 3. Type a keyword in the Keyword field. Only letters are accepted (digits and symbols are stripped automatically). 4. Read the transformed result in the Output area, and use Copy to copy it to your clipboard.

The keyword is cleaned to letters only and uppercased before use. Each letter of the input is shifted by the corresponding keyword letter (A=0, B=1, ... Z=25), cycling through the keyword as the message advances. Lowercase input stays lowercase and uppercase stays uppercase; spaces, digits, punctuation, and newlines do not consume a key index and are emitted verbatim. If the keyword field is empty or contains no letters, the output is left blank and an invalid-key warning appears.