Function open_precomputed 

pub fn open_precomputed(
    c: &[u8],
    n: &Nonce,
    k: &PrecomputedKey,
) -> Result<Vec<u8>, ()>

open_precomputed() verifies and decrypts a ciphertext c using a precomputed key k and a nonce n. It returns a plaintext Ok(m). If the ciphertext fails verification, open_precomputed() returns Err(()).