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 Mr. 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.