Authentication completing my OpenBSD 5.2 install.

[Luis Suzuki]

I have installed all of OpenBSD 5.2 that came with the install52.iso and
everything is fine.Now I want to install a GUI(gnome or kde) and other
software that did not come with install52.iso.How can I be certain that the
download is cryptographically secure and the downloaded packages are genuine?
pkg_add does automatically verify signed packages or I do have to use a secure
link : https://ftp.openbsd.org/.../.../... ,or so.