sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 76F1A20FF987672F

add loic key:
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
source : deb http://download.mono-project.com/repo/debian wheezy main

Linux add key