ChaCha20 is a stream cipher. See https://tools.ietf.org/html/rfc7539.
CRYPTO_chacha_20 encrypts in_len bytes from in with the given key and nonce and writes the result to out. If in and out alias, they must be equal. The initial block counter is specified by counter.
OPENSSL_EXPORT void CRYPTO_chacha_20(uint8_t *out, const uint8_t *in, size_t in_len, const uint8_t key, const uint8_t nonce, uint32_t counter);