How to use the asu.common.verify_usign function in asu

def setup_ib():
        """Setup ImageBuilder based on `request`

        This function downloads and verifies the ImageBuilder archive. Existing
        setups are automatically updated if newer version are available
        upstream.
        """
        log.debug("Setting up ImageBuilder")
        if (cache / subtarget).is_dir():
            rmtree(cache / subtarget)

        download_file("sha256sums.sig", sig_file)
        download_file("sha256sums", sums_file)

        assert verify_usign(
            sig_file, sums_file, request["version_data"]["pubkey"]
        ), "Bad signature for cheksums"

        # openwrt-imagebuilder-ath79-generic.Linux-x86_64.tar.xz
        ib_search = re.search(
            r"^(.{64}) \*(openwrt-imagebuilder-.+?\.Linux-x86_64\.tar\.xz)$",
            sums_file.read_text(),
            re.MULTILINE,
        )

        assert ib_search, "No ImageBuilder in checksums found"

        ib_hash, ib_archive = ib_search.groups()

        download_file(ib_archive)