[mstolpm]

Why do you think terrorists use (algorithmic) encryption in common communication channels at all? There are so many ways to transport a hidden message in plain sight that using encryption itself might give intelligence a hint that terrorists would avoid. Embed the crucial information in videos, photos or a text about your mothers cheese macaroni recipe. Or use a side channel for communication, not mail or some messenger.

The idea of finding clever criminals by decryption their communications is stupid by itself. And stupid criminals leave so many trails bragging about their plans that you don't need decryption and back doors. Most european terrorist attacks were done by criminals already known by intelligence, but not monitored strongly (see Berlin). Monitoring all communications would lead to an explosition of the number of suspects. It would not lead to more security.