So let me get this straight: the police arrested them on suspicion of conspiracy to commit (generic, unspecified) "terrorism," but can't accuse them of plotting any specific terrorist act, and they excuse this by saying that the details of the alleged plots must all be in the encrypted communications that they can't read?
So the French police can just make up crimes that haven't happened, and then claim that the total lack of evidence is because the defendant has hidden the evidence (which "must" exist) so well? Damn.