Function open_detached_precomputed 

pub fn open_detached_precomputed(
    c: &mut [u8],
    mac: &Tag,
    n: &Nonce,
    k: &PrecomputedKey,
) -> Result<(), ()>

open_detached_precomputed() verifies and decrypts a ciphertext c using a precomputed key k and a nonce n. c is decrypted in place, so if this function is successful it will contain the plaintext. If the ciphertext fails verification, open_detached() returns Err(()), and the ciphertext is not modified.