SEOUL (Reuters) - This classroom in Korea University is called the "war room".

It's where students like Noh train to defend his country against North Korea's cyber attacks. He is a 21-year-old student at Korea University's Department of Cyber Defence, and has requested to be known by his surname.

The atmosphere's secretive, courses are known only by number, and students keep their identities hidden.

Noh has to complete a seven-year course in hacking and cracking codes.

"It's not a burden on my time, I think it's part of the process to build my career. I am absolutely committed. Becoming a cyber warrior means devoting myself to serve my country," he said.

North Korea has a 6,000-strong cyber army.

That's more than 10 times the size of South Korea's, one of the world's most technologically-advanced countries.

That means it's also a bigger target, as everything, from power grids to the banking system, is vulnerable to attacks.