Function crypto_box::generate_nonce

pub fn generate_nonce<T>(
    csprng: &mut T
) -> GenericArray<u8, <XSalsa20 as NewCipher>::NonceSize> where
    T: RngCore + CryptoRng, 

Generate a random nonce: every message MUST have a unique nonce!

Do NOT ever reuse the same nonce for two messages!