Hi @henriques ,
This topic - in french - should helps you :
EDIT :
A priori in your case :
gpg --keyserver pgpkeys.mit.edu5 --recv-key 8B48AD6246925553
gpg -a --export 8B48AD6246925553 | sudo apt-key add -
gpg --keyserver pgpkeys.mit.edu5 --recv-key 7638D0442B90D010
gpg -a --export 7638D0442B90D010| sudo apt-key add -
Then update, but in order to NOT do a messy update to stretch for now, you have to check your /etc/apt/sources.list, you should have jessie and NOT stretch in :
sed -i ‘s|stable|jessie|’ /etc/apt/sources.list
sed -i ‘s|stretch|jessie|’ /etc/apt/sources.list
apt-get update && apt-get dist-upgrade && apt-get autoclean && apt-get autoremove
ppr