SEOUL, South Korea — In an era of sophisticated spycraft, North Korea appears to be returning to the days of shortwave radio.
The North broadcast a series of seemingly random numbers on Pyongyang Radio twice recently, an eerie reminder of the days when the North encrypted messages to its spies in South Korea.
In the latest episode last Friday, an announcer read 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, continuing to cite numbers for 14 minutes.
Decades ago, it was not unusual for late-night radio listeners in the South to hear mysterious numbers arriving on static-filled signals from the North. The South Korean government in Seoul tried to block the signals and barred its citizens from listening.
Kim Dong-sik, a former intelligence officer for North Korea, said he used to listen for such broadcasts at midnight each night to check whether his spymasters had a message for him. Mr. Kim was caught by the South in 1995 after a gun battle with South Korean agents and police officers.
“When I arrived in the South, I had five different call signs assigned to me,” said Mr. Kim, who now works as a senior analyst at the Institute for National Security Strategy, a think tank run by South Korea’s National Intelligence Service. “Each night, I listened for my call signs.”
The June 24 and July 15 broadcasts, confirmed by the South Korean government on Wednesday, were the first such coded messages in 16 years, leaving intelligence officials and analysts puzzled by the North’s motives.
The broadcasts come amid concerns about the behavior of the North, which has raised tension with the United States and its allies by conducting a series of missile tests. It has also issued bold claims of advances in its quest for a nuclear-tipped long-range missile, even though Washington’s attention has been focused elsewhere.
North Korea has reacted strongly to a plan by the United States to deploy anadvanced missile defense system in the South. This week, it fired three ballistic missiles, saying that they were used in simulated tests to detonate nuclear warheads over seaports and airfields in the South, where American reinforcements are supposed to arrive in the event of a war.
The tests defied a new round of sanctions that the United Nations Security Council imposed against the North after a nuclear test in January and a long-range rocket launch in February.
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.
South Korea itself has resorted to old-school propaganda in recent years,resuming loudspeaker and radio broadcasts into the North and juicing them up with synthesized Korean music known as K-pop.
Some analysts said the North’s use of a bygone encryption tool was rekindling old fear among South Koreans of an escalation in psychological warfare on the peninsula.
North Korea had stopped sending out such coded messages by shortwave radio after the Koreas held a summit meeting in 2000, agreeing to de-escalate the Cold War-era intrigue on the divided peninsula.
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.
Mr. Kim, the analyst and former spy, 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 blacklisted the bureau after North Korean hackers were accused ofwreaking havoc on the computer networkof Sony’s movie studio in 2014.
At a time when the counterintelligence authorities use sophisticated technology to monitor the digital communication of espionage suspects, “the old number broadcasts are still a dependable and preferable means of communication for spies,” Mr. 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.
Mr. Kim said he and his handlers in the North used an agreed-upon book — “Whale Hunt,” a popular novel in the South — to decipher one another’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.
Sometimes, four digits of each five-digit set yielded one Korean character.
Long after the end of the Cold War, the two Koreas still accuse each other of spying. The North is holding at least four South Koreans, some of them sentenced to a labor camp for life, on charges of espionage.
In recent years, the South’s intelligence service has arrested people it deemed spies as they entered the country disguised as refugees. Last week, prosecutors said they arrested two South Korean men in their 50s on charges of spying for the North. They released closed-circuit video of counterespionage officers overpowering a suspect at an internet cafe.
The men used encrypted emails to contact their handlers in the North, the prosecutors said.
Mr. Kim said that during his days as a spy, the radio was a main tool of communication.
“If there was a certain song broadcast by Pyongyang Radio at an agreed-upon hour, that meant that there was something wrong and I should immediately abort my mission,” he said. “If not, it was all clear.”