There is https://wiki.gentoo.org/wiki/Handbook:Parts/Installation/Media#Verifying_the_downloaded_files and mention of gpg in https://wiki.gentoo.org/wiki/Handbook:Parts/Installation/Stage#Downloading_the_stage_tarball . But here in https://wiki.gentoo.org/wiki/Handbook:Parts/Installation/Base#Installing_a_Portage_snapshot validation gets lost because stage 3 as of stage3-amd64-20160114.tar.bz2 does not contain the gentoo-keys nor the needed app-crypt/gnupg.
*** This bug has been marked as a duplicate of bug 507774 ***