AMERICANS have grown accustomed to North Korean nuclear petulance. Now they are learning to live with its cyber sabre-rattling. Earlier this month the Department of Defence delivered a report to Congress accusing the hermit kingdom's expanding army of “cyber-warriors” of using foreign infrastructure, such as broadband networks, to launch cyber-attacks on American allies, most notably South Korea.

Kim Jong Un, North Korea's fresh-faced dictator, is said to have 4,000 loyal cyber-warriors at his disposal. Brightest sparks at the sharp end of Songbun, the North’s rigid social hierarchy, are plucked from school to train as elite hackers. Following graduation they are often posted in China and Europe to wreak digital havoc, says Sun Chul Kim, a cyber-security expert at Korea University in Seoul.

According to American report, cyber-warfare is a cost-effective way for North Korea to boost its military capabilities, which may explain the keen interest Mr Kim has taken in it. Prominent web security analysts such as Rob Rachwald of FireEye, an American firm, agree that the tools used in a recent cyber-attack on South Korea could have cost just tens of thousands of dollars, compared to the estimated $1.3 billion the North spent on its rocket programme last year.

On March 20th thousands of South Korean banking and broadcasting systems were paralysed by a devastating cyber "time bomb" spread unwittingly via the ViRobot Management System, corporate antivirus software made by Hauri, an internet-security firm. When the bomb exploded, it wiped the master boot record (MBR), which stores the information a computer needs to start the operating system. If the MBR is wiped or damaged, the computer system will not start up.

The attack targeted machines running Windows and Linux operating systems, throwing ATMs offline and crippling national television stations. Derek Manky of Fortinet, another American security firm, who led a team of experts sifting through the virtual debris of the virtual bombing alongside the Korean Internet Security Agency (KISA), says it is hard to identify the perpetrators. But at a time of heightened tension on the Korean Peninsula many fingers in Seoul are pointing at the North.

Similar viruses have been deployed before, most notably against Iran in 2012. Parts of the malicious software, dubbed Trojan.Jokra, were written in English, and on an English-language keyboard, according to FireEye. Mr Manky says such outsourcing is common. But it does suggest that if North Korea is behind the attack, it is plugged into the hacker world.

This worries the South. Since the start of the year it has doubled the number of experts at its "cyber-command" to 1,000. As one of the world’s most connected nations South Korea represents a tempting target for cyber attacks. Whereas its government systems are well protected, South Korean companies and individuals often are not: the attacks on March 20th exploited vulnerabilities that were previously known. Digitally deprived North Korea, meanwhile, is one of the safest places in the world in this respect.