Messages are decrypted when you read them.<p>It's reasonable to believe that at any point in time Root exploits exist for both iOS and Android.<p>It's viable that the FBI or someone they cooperate with has such exploits from time to time (which doesn't mean they are reliable, or cheap to use).<p>If you root-hack a phone you can easily get all messages the user sees after you hacked it.<p>Even without root hacking you might get some, in some circumstances.<p>EDIT: I should have read the article first, it's more about what content they get <i>without</i> hacking.