Function xchacha20poly1305_open 

pub fn xchacha20poly1305_open(
    key: &[u8; 32],
    nonce: &[u8; 24],
    aad: &[u8],
    sealed: &[u8],
) -> Option<Vec<u8>>

XChaCha20-Poly1305 open: verifies the tag (constant-time) and decrypts, or None on tamper/truncation.