Function seal_box_under 

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

The box of plaintext under key and nonce: nonce ‖ XChaCha20-Poly1305(key, nonce, label, plaintext).