Function sign_digest 

pub(crate) fn sign_digest<T: CryptoRngCore + ?Sized, D: Digest + FixedOutputReset>(
    rng: &mut T,
    blind: bool,
    priv_key: &RsaPrivateKey,
    hashed: &[u8],
    salt_len: usize,
) -> Result<Vec<u8>>