Enter your text and a key, and the XOR result is produced instantly. Run the output back through with the same key to decrypt it.
Each byte of the text is XORed with a byte of the repeating key. Applying the same key again reverses the operation.
With a short reused key, no — it is easily broken. Only a truly random key as long as the message (a one-time pad) is secure.
Because a XOR b XOR b equals a, so encrypting and decrypting use the exact same step with the same key.