I don't think he's figured that part out yet.  And even if he did, what's to stop terrorists making their plans outside of the UK, using whatever encrypted services they like, then coming into the UK to carry out the plan?  He's just a gibbering idiot who needs a slap across his giant shiny face.  Let's see how long the list of reasons why he's wrong is now...

1. It's not actually possible to ban an entire branch of mathematics
2. Even if it were, nefarious people could still use encrypted messages outside of the country
3. It's duplicitous when they're trying to cripple FOI requests and deleting their own documents so the public can't find out what they're getting up to
4. The UK economy relies on encryption and the Tories are desperately trying (but failing) to paint themselves as a party we can trust with the economy (but we don't)
5. The business sectors who tend to rely most on encryption, which he would leave alone, are the ones who pay the most towards his election campaigns, so more double standards there
6. He's undermining his own security and intelligence agency because they want to educate the public about encryption and recognise the benefits to the UK economy
7. It's a blatant underhand attempt to sneak towards increased authoritarianism and no one in their right mind is going to allow that
8. He's a moronic moonfaced cockwomble who needs to be fired out of a cannon into the sun

Anyone got anything else?