Purpose of encryption: To stop people - other than those intended - being able to see, or infer, the contents of a message.
Sorry, but it's working exactly as designed.
"Weakening" it or "backdooring" it makes it stop being encryption.
And unless you can stop everyone, everywhere, across the planet having access to ... gosh... mathematics... you're not going to stop it.
Sure, you can ask Skype for a backdoor but you could do that legally anyway without any need to break encryption whatsoever as Skype are part of the "intended recipients" for most things. But if people are using an OTR plugin or similar to communicate USING Skype, there's nothing you can do.
And guess what the terrorists are doing, as compared to Granny who just wants to talk to Fred in Australia?
It's like saying "Oh, yes, we'd really like a way that no bullets in the world would ever fire in any gun, anywhere, except our own". Although "true", it makes you sound just as stupid.
You had your chance when PKE was declared a weapon and that all got invalidated, and only ever really took effect in one country. You can't hide "maths" any more than you can uninvent "chemistry" to get rid of bombs.
Rather than chase encrypted messages, put a few more people on the ground, in airports, and do a few more checks on people at the borders.
P.S. Though I would never suggest anyone - even a mathematician - does so, it's possible to encrypt using nothing more than pen and paper, and it's possible to extend existing source code to use unbelievable complex keys that take 20+ minutes to sign a single message. Even when "weakened" by using all the known flaws in the algorithm, that means it's still not going to be cracked this side of armageddon.
Rather than chase the dream of an encryption scheme you can crack every time, acknowledge that you are no more likely to crack the encryption than infiltrate the groups in question, or get a bug onto their PC directly, or work it out by other means. Just like foreign militaries.