The broadcasts come amid concerns about the North, which has raised tension with the United States and its allies by conducting a series of missile tests and has issued bold claims of advances in its quest for a nuclear-tipped long-range missile.

North Korea has reacted strongly to 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 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 any 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. North Korea 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.