North Korea has been blamed for cyber-attacks against South Korea's nuclear reactor operator last December, based on analysis of internet addresses used in the attack, but Pyongyang denied any involvement.
"The malicious codes used for the nuclear operator hacking were the same in composition and working methods as the so-called 'kimsuky' that North Korean hackers use," said a statement from the Seoul central prosecutors' office reported by Reuters.
Between 9 and 12 December 2014 multiple internet protocol addresses based in China but registered in North Korea were used to send 5,986 phishing emails containing malicious codes to 3,571 current and former employees of the nuclear plant operator and its 23 reactors.
South Korean prosecutors made the statement after conducting an investigation of both the publishing of information about the the country's reactors on Twitter last December, and included last week's leak of a blueprint and test data from Korea Hydro and Nuclear Power Co Ltd. The Twitter account is also reported to have demanded money and provided an email address where the user said he could be contacted.
The Wall Street Journal reported that investigators said no critical data was disclosed and the emails were largely unsuccessful in their aim of obtaining remote control access of computers.
There is a history of accusation and counter accusation of cyber-espionage between the two countries, and more recently the US too with the Sony hack attributed to N Korea, and now the South's Ministry of Unification issued a statement saying: "We condemn North Korea's persistent cyber-terror targeting of our country and the international community."
North Korea's domestic intranet, which launched in 2008, is internationally isolated and it is believed that less than 1,000 people have access to the wider internet, making it possible to identify those individuals with clearance to engage in such activity.