[Assist] Can't upgrade my ParabolaGNUlinux system

[Csányi Pál]

Hi,

when I'm trying to upgrade my system with:

$ yaourt -Syu

command, after download packages, I get error messages, like:

error: linux-libre: signature from "André Silva
<emulatorman at riseup.net>" is unknown trust
:: File /var/cache/pacman/pkg/linux-libre-3.14.5-1-x86_64.pkg.tar.xz
is corrupted (invalid or corrupted package (PGP signature)).
Do you want to delete it? [Y/n]

and so on, on every downloaded packages.

What can I do to solve this problem?

-- 
Regards from Pal