SEOUL, South Korea — When Pyongyang Radio recently broadcast a mysterious series of seemingly random numbers from inside North Korea, they sounded like a call from the past to one former spy.
Kim Dong Sik, a former intelligence officer for North Korea, said he used to listen for such broadcasts every midnight to check whether his spymasters had a message for him.
“When I arrived in the South, I had five different call signs assigned to me,” said Kim, who now works as a senior analyst at the Institute for National Security Strategy, a think tank run by the South’s National Intelligence Service. “Each night, I listened for my call signs.”
The cryptic numbers, broadcast last month, were the first such coded message in 16 years, leaving South Korean intelligence officials and analysts puzzled by the North’s motives.
An announcer on Pyongyang Radio again broadcast a string of numbers last Friday, reading what she described as “a mathematics review assignment for investigative agent No. 27,” engaged in a “distance learning” program.
“Turn to Page 459, No. 35; Page 913, No. 55; Page 135, No. 86,” she said, citing numbers for 14 minutes.
Decades ago, it was not unusual for late-night radio listeners in South Korea to hear mysterious numbers arriving on static-filled signals from the North — an eerie reminder of the espionage at play across the divided Korean Peninsula. The South would try to block signals like those and barred citizens from listening.
North Korea had stopped sending out such coded messages by shortwave radio after the Koreas held a historic summit meeting in 2000 and agreed to de-escalate their Cold War-era intrigue against each other.
Since then, the North is believed to have adopted more sophisticated methods of communication. When the South’s intelligence service announced the capture of a spy ring in 2011, it said that the officers contacted the North through steganography, a technique for encrypting a message into a text, image or video file delivered online.
Some analysts said that the seeming redeployment by the North of what many had considered a bygone encryption tool was rekindling old fears among South Koreans, a possible escalation of psychological warfare.
In recent weeks, North Korea has been raising tensions over a plan by the United States to deploy an advanced missile defense system in the South. This week, it fired three ballistic missiles, saying that they were used in simulated tests of detonating nuclear warheads over seaports and airfields in the South, where U.S. reinforcements were supposed to arrive should war break out again on the peninsula.
Jeong Joon-hee, a government spokesman for South Korea, has called the resumption of the broadcasts “seriously regrettable” but declined to comment on possible motives. “The North should abandon its old ways,” he said.
In the age of digital media and the internet, South Korea has also resorted to old-school propaganda in recent years, resuming loudspeaker and radio broadcasts into the North and juicing them up with synthesized K-pop music.
Kim said the broadcasts should be taken seriously. He said the North appeared to be bolstering its espionage operations since 2009, when it created the General Bureau of Reconnaissance by merging various party and military agencies in charge of sending spies to the South. (Washington has blacklisted the bureau after North Korean hackers were accused of wreaking havoc on the computer network of Sony’s movie studio in 2014.)
At a time when counterintelligence authorities use sophisticated technology to monitor the digital communication of espionage suspects and closed-circuit television cameras watch over every internet cafe in South Korea, “the old number broadcasts are still a dependable and preferable means of communication for spies,” Kim said.
“We should assume that the North is using the radio broadcasts to communicate with its agents here or is at least using them to train spies,” he added.
He recalled that when he was training in the 1980s, he spent countless hours listening to tape-recorded broadcasts and copying the numbers to master a so-called numbers station technique of encrypted communication.
Kim said he and his handlers in Pyongyang, the North Korean capital, used an agreed-upon book — a popular novel in the South titled “Whale Hunt” — to decipher each other’s codes. As in the broadcast on Friday, a typical five-digit combination started with a three-digit page number from the book. The remaining two digits pointed at two Korean characters in the text of the page, he said.