$ ftp https://alpha.de.repo.voidlinux.org/live/current/void-live-x86_64-20210218.iso
    $ ftp https://raw.githubusercontent.com/void-linux/void-packages/master/srcpkgs/void-release-keys/files/void-release-20210218.pub
    $ ftp https://alpha.de.repo.voidlinux.org/live/current/sha256sum.sig
    $ signify -C -p void-release-20210218.pub -x sha256sum.sig void-live-x86_64-20210218.iso