North Korea defector on cyber threat
For 20 years Prof Kim Heung-Kwang taught computer science at Hamheung Computer Technology University, before escaping the country in 2004.
While Prof Kim did not teach hacking techniques himself, his former students have gone on to form North Korea's notorious hacking unit Bureau 121.
The bureau, widely believed to be operating out of China, has been credited for numerous hacks.
In an exclusive interview with the BBC's Dave Lee, Prof Kim gives his view on the potential for future cyber attacks from the isolated nation.
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29 May 2015