Numbers Stations, The Woodpecker etc.

The so-called “Numbers Stations“ are a favorite topic on the Interwebs. Some obvious open source information is available but wild speculation about them is widespread. I won’t try to duplicate any of that here but below are some of my observations. Although they are not as common as in the past, they are still in operation and can be monitored.

I copied a lot of 5 number/letter Morse Code cypher groups from them over the years. They were excellent code practice as I studied for my various ham license tests.

Below: Putting the CIA’s RS-6 Spy Radio to work listening to various numbers stations. Spying on spies; and keeping your coffee warm while on a late-night Op at the Safe House.

Mission Essential Equipment

Encrypted short wave “numbers” transmission is a favorite cheap, simple and effective method to communicate. It is known to be used for example by the Cuban Dirección de Inteligencia service to communicate with their spies in the U.S.* (Reference 92.)

*During the prosecutions of numerous Cuban spies in the U.S. the FBI was required by court proceedings to reveal seized evidence of the Cuban spies use of Short Wave radio and associated encryption systems.  Sufficient to convince a lay Jury.  Convictions followed, but as a result Castro learned of his own spy communications vulnerabilities.

When a message is encrypted with a One Time Pad (OTP) and used correctly, the cypher is unbreakable: the contents of these “numbers” transmissions are unrecoverable.

This system is presumably used by many other countries and organizations. As an example, the U.S. used the DIANA offline encryption system during combat Ops in Vietnam.

Below are 2 recordings I made late at night of a well known, reportedly Cuban transmitter, dubbed “M08A”. The first 2 using Morse Code cut-numbers of a non-standard (Cuban, or false-flag) variety. 1=A, 2=N, 3=D, 4=U, 5=W, 6=R, 7=I, 8=G, 9=M, 0=T (Reference 93).

The first one sounds like a call up to several “out stations”. Starting with call signs; (note the triple DDD and GGG. Statistically very highly unlikely in an encrypted message, especially in adjacent Groups.) This preceded a transmission of many cypher groups. 5800 kc around 0600Z. Copied with my CIA RS-6 radio set (above). Click the Arrow buttons:
Numbers Station callup

The above MCW (A2A) transmission continued with 5 cut-numbers cypher groups. Someone’s One Time Pad would be getting a workout. The “Cuban 5” spy ring perhaps?

An advantage of using cut-numbers is that the agent does not need to be “Morse Code qualified”. With minimal communications training they just need to know those 10 simple, 2 or 3 element letters. Those are also much faster to send than the actual 5 element numbers in the standard code. That reduces transmitter “on air” and agent exposure time.

A related OTP system transmits letters versus numbers. This system requires higher level Morse Code skills typical of professional communicators; all letters are utilized. As with the “cut-numbers” system the conversion of letters into different letters when mixed with the OTP key is via an UNCLAS “Trigraph” table during encryption/decryption.

There are many posts online demonstrating the construction and use of one-time-pads. Fundamentally, you need simple number-letter-number UNCLAS conversion tables AND a set of truly random 5 digit numbers. Not computer generated as those are deterministic.

A tried, proven, totally random and therefore secure (but slow) technique uses 10-sided dice to generate an OTP key. Make only 2 copies, one for the transmitter, the other for the receiver. Do not store on, or forward via a computer. Destroy keys immediately after use.

Random Number Generation

Although there are some stations transmitting spoken alpha/numeric codes, the classic way is via Morse Code. For a given transmitter power, distance, interference level etc, Morse is much more effective than voice in getting through to the receiving station. Also the transmitter requirements are much simpler. It works. I taught it to our Cub Scouts.

The Soviets taught their children in organizations like ДОСААФ basic radio technology and Morse Code with kits like this: Радио (Radio) Magazine, Vol 4, 1980. “Komplects Radioamateur” (My dice added, but not a stretch.) What kid didn’t like playing with codes and cyphers! Note the blue morse code key.

Soviet Radio Training Kit – Note the Morse Code key

Below is another station I copied on 11462 kc with a voice preamble in Spanish followed by a multi-tone alert and then digital data streams. Someone has gone “high tech” in this one. Can you say Sound Card? Easy to distribute OTP Keys on a thumb drive via a Dead Drop.

Another interesting HF signal that could be called a “numbers station” is the High Frequency Global Communications System, HFGCS. Unvetted observations and wild speculation on this system are available on the Interwebs so nothing further here. But it is interesting to monitor with the trusty R-390A, especially with the echoes.


Do not answer……….. (Shades of the AN/PRT-4 / AN/PRR-9 !)

Then there are the otherwise boring channel markers like “The Buzzer” on 4625 kc and many others. We’ve been using that one for propagation path tests into eastern Europe for many years.

Generating cyphertext for a live transmission by an actual “numbers station”:

The M-209 Converter is an interesting system used by the U.S. during WWII and Korea as an encryption device. This is a low level tactical encryption system that produced cyphertext as letters, to be transmitted either by voice or Morse Code. It used a daily-changing Key detailing the mechanical settings internal to the M-209 versus an OTP-based system.

The M-209 and KeyMat

An exercise by the West Coast Military Radio Collectors Group and the Maritime Historical Radio Society was conducted in 2013. A message encrypted with an M-209 was transmitted on HF via Morse Code at 15 WPM and received by an AN/GRC-109 at the MRCG event site 200 miles away.

The MHRS station KFS is licensed commercially and is authorized to transmit encrypted messages unlike Amateur Radio where that is prohibited. The M-209 Key to this message was published in advance so other M-209 owners could participate in the exercise.

The message sent was an actual message transmitted in 1944 by Merrill’s Marauders during combat operations in Burma during WWII. This is the plaintext:


It was encrypted as a 26 group message and transmitted by KSM on their assigned 8642 kc frequency and simultaneously on their 6 and 12 mc channels. It was received at the MRCG event site, as transcribed below. Note the first and second groups are also the last 2 groups.

26 Group M-209 Message as received

Upon decryption by the above M-209 at the receiver site, we “Rogered” for it on 7050 kc CW with the same GRC-109.

A second message also encrypted by an M-209 was then sent and partially received via RATT using a GRC-46 setup at our receiver site. (NOTE: CW wins the day. Again!)

Further details here: and on M-209 Guru Mark’s site here:

Although not a “numbers station” another HF radio signal one could not avoid was the so-called Russian Woodpecker. This Russian Over the Horizon Radar caused enormous interference to worldwide HF radio communications in the late 1970’s and 1980’s. For more information on that take a look here: