Function encrypt_message_id 

pub fn encrypt_message_id<R: RngCore + CryptoRng>(
    key: &[u8],
    message_id: &[u8],
    rng: &mut R,
) -> Result<Vec<u8>, Error>

Symmetric encryption for message IDs using ChaCha20-Poly1305

This is used in step 7 for encrypting message IDs with a shared secret