Disable check package sign

$ vim /etc/pacman.conf
...
#SigLevel = Required DatabaseOptional
SigLevel = Never
...

Or import key

$ sudo pacman-key --init
$ sudo pacman-key -r 7F2D434B9741E8AC

https://wiki.archlinux.org/index.php/Pacman/Package_signing

使用 yaourt 安装软件提醒“所需的密钥从密钥环中丢失”