> incidentally, how the actual fuck do we know matey boy used WhatsApp before the attack? a copper went nudge nudge wink wink to a tame journo? they've got his phone and WhatsApp installed?
A far more interesting question, that few have asked so far. I asked myself the same question. From what I have gathered, the arrests in Birmingham happened directly because the attacker sent two whats-app messages to contacts at those addresses before he did his deed.
This leads me to think that they probably had the "metadata" (i.e. they were doing real time scanning of the whatsapp network to see who is messaging who), but are unable to decipher the messages themselves.
So now they want to decrypt the messages to find out if the people they arrested were in on the attack, or just unfortunate people who he texted last (maybe to say good bye or something).
Unless they knew in advance an attack was going to happen, I can only assume they are constantly monitoring who is talking to who on whatsapp, and (for the moment at least) it seems they can't actually read the message contents. Facebook can provide them with access to the network, but the enctyption is still client side "end-to-end".
Perhaps a future version of whatsapp will be crippled by fb, not unlike how MS crippled Skype after they purchased it.