Function xchacha20poly1305_seal 

pub fn xchacha20poly1305_seal(
    key: &[u8; 32],
    nonce: &[u8; 24],
    aad: &[u8],
    plaintext: &[u8],
) -> Vec<u8> 

XChaCha20-Poly1305 seal: ciphertext ‖ tag[16]. Its 192-bit nonce is wide enough to draw at random for every message under one key, which a 96-bit nonce is not.