Infinite one time pads using a pseudorandom hash function
This is called a stream cipher.  It is a practical instantiation of the one-time pad model, where we encrypt a message $m$ with a single-use pad $p$ by using the ciphertext $c = m \oplus p$.

When $p$ is generated from a short key $k$ by $p = F(k)$, obviously $c$ and $m$ are not technically independent—we can't say of the probabilities that $P(m \mid c) = P(m)$, because obviously some pads are impossible—but the advantage the adversary has at breaking the cryptography knowing that we used $F(k)$ is limited by their advantage at distinguishing $F(k)$ from a uniform random bit string.

If $k$ is uniform random and $F$ is, e.g., ChaCha or SHA-256, the adversary has no hope of doing this without a breakthrough in cryptanalysis.