Function verify 

pub(crate) fn verify<D>(
    pub_key: &RsaPublicKey,
    hashed: &[u8],
    sig: &BoxedUint,
    sig_len: usize,
    digest: &mut D,
    salt_len: Option<usize>,
) -> Result<()>
where
    D: Digest + FixedOutputReset,