This very simple web site lets you encrypt and decrypt using One Time Pad (OTP) encryption which is an encryption algorithm that offers perfect secrecy via a single use key.
Perfect secrecy: as long as an attacker doesn't have the key associated with the cyhertext, it can't decode the message in any way but by trying and guess the key. Very simple implementation and usage
You have to exchange the key in a secret and secure way. The key is the same length of the original message, meaning that if you have a very large text, the key will be large too.