The practice was halted in 2000 after the first inter-Korea summit between North Korean President Kim Jong Il and South Korean President Kim Dae Jun.
The 14-minute broadcast of two sets of numbers by a female voice appears to have been the work of the Voice of Korea (formerly Radio Pyongyang), a North Korean radio propaganda station that broadcasts accolades of the Kim family.
A retired US National Security Agency source said the fact it was a 10-11 meter frequency band in the middle of the night, considering that North Korea does not have relay stations like many other shortwave stations, would make the target local to South Korea, Japan or northern China.
"Sun Spot cycle is low to zero right now so would not expect it to be a DX [long distance] transmission," according to the NSA source.
The station is using old Soviet transmitters that give it a distinctive humming sound when broadcasting, said Keith Perron, an expert on spy number stations who runs the international shortwave and FM station, PCJ Radio, which broadcasts news, entertainment and serves as a relay for other content.
The hum is created due to the poor quality of transmitters and the fact that North Korea does not use microwaves to relay the message to the antennas but rather old telephone wires.
Perron said that Voice of Korea has sometimes broadcast gibberish between news stories that are also designed as messages to spies. This was a common tactic by the BBC during World War II to alert the French Resistance.
Number stations can be traced back to World War I and were made famous during the Cold War in Europe. At the end of the Cold War, number stations began shutting down and now are occasionally broadcast by Communist remnants, such as Cuba, China and North Korea, though Israel and Taiwan still use them.
The method is simple: The broadcasts contain a set of four or five numbers that correspond to letters or words that are decipherable using a one-time pad by a deep cover spy listening to a shortwave radio. Messages are broadcast at schedules and frequencies assigned to the spy. Perron claims North Korean one-time pads have never been broken by counter-intelligence.
There have been media reports out of South Korea that the North Koreans are using a more sophisticated method of sending secrets via steganography, a method of concealing a message within another file, image, or video, which makes the recent number broadcast by North Korea odd.