def reg_signature(client_data_json)
      payload = [
        "\x00",
        ::U2F::DIGEST.digest(app_id),
        ::U2F::DIGEST.digest(client_data_json),
        key_handle_raw,
        origin_public_key_raw
      ].join
      cert_key.sign(::U2F::DIGEST.new, payload)
    end
def reg_signature(client_data_json)
      payload = [
        "\x00",
        ::U2F::DIGEST.digest(app_id),
        ::U2F::DIGEST.digest(client_data_json),
        key_handle_raw,
        origin_public_key_raw
      ].join
      cert_key.sign(::U2F::DIGEST.new, payload)
    end
The signature field of a registrationData field of a RegisterResponse.

 client_data_json - The JSON encoded clientData String.

 Returns an ECDSA signature String.