Two years ago, Thae Yong Ho, then North Korea's deputy ambassador in London, received an unexpected phone call from the ruling Workers' Party Central Committee in Pyongyang telling him to get ready to receive a very important e-mail.

"Please go to the Albert Hall and buy four tickets," said the cryptic message from a disposable e-mail address designed to throw off Western intelligence agencies.

The message wasn't code — Thae found out later he was being asked to take leader Kim Jong Un's brother to an Eric Clapton concert.