The basic ideas of cryptography

Suppose Alice wants to send Bob an 8-bit binary string m (i.e., the string of length 8, consisting of 0s and 1s). They have previously chosen another 8-bit binary string $r$ as a key . What operations can possibly serve as an encryption function? Hint: think when Bob can recover the original message without any doubt.

Hint: we apply the operations to the strings symbolwise (we can also say "bitwise", as every 0 or 1 is called a bit). For single-bit $x$ and $y$, $x \text{ AND } y = 1$ if $x = y = 1$, otherwise it is 0. Also, $x \text{ OR } y = 0$ if $x = y = 0$ and 1 otherwise. Finally, $x \text{ XOR } y = 1$ if $x \neq y$ and 0 otherwise.