turn a 20-byte SHA1 hash into a blob of data as large as the key's N,
        using PKCS1's \"emsa-pkcs1-v1_5\" encoding.  totally bizarre.

def _pkcs1imify(self, data):
        """
        turn a 20-byte SHA1 hash into a blob of data as large as the key's N,
        using PKCS1's \"emsa-pkcs1-v1_5\" encoding.  totally bizarre.
        """
        size = len(util.deflate_long(self.n, 0))
        filler = max_byte * (size - len(SHA1_DIGESTINFO) - len(data) - 3)
        return zero_byte + one_byte + filler + zero_byte + SHA1_DIGESTINFO + data