Monday, August 08, 2005

Number stations

Those one-time pads remind me of strange and spooky radio broadcastings I used to hear when sweeping through shortwave frequencies back in the 80s. Unknown stations transmitted groups of four or five numbers or letters, sometimes male, female or child voices, sometimes in Morse code. The voices often in German, Russian or Spanish, but also in English, appeared for a few minutes and disappeared again.

The letters or numbers are messages, send to secret agents or spies in the field, encrypted with the one-time pads. The reasons are obvious. Shortwave stations are hard to locate, because of the many reflections they create in the atmosphere. Figuring out who is receiving the message is impossible. Why one-time pads? Because they are unbreakable and easy to carry around, without being noticed. They are ideal for field encryption purposes. This is still the case today, where security and easy use without special encryption devices is important.

Most of these number stations were active during the Cold War, in Germany, former Sovjet countries, South-America and Cuba, but also on different locations in the US and England. Although the end of the Cold War brought a significant decrease in transmissions and broadcast locations, they still appear, mostly in digital modes or Morse. You can listen to some recordings and find more details on how these stations operate on my website Some enthusiastic radio amateurs still monitor and record these transmissions. If you want to taste a bit of this spooky secrecy, or want to search HF numbers stations yourself, you should visit Numbers & Oddities or Enigma2000.


College Boy said...

There is a technique for detecting crystal recievers tuned to a specific frequency. You send a very powerful burst at the frequency they are listening to and listen for the receiver to emit another, lower frequency after the crystal steps it down.


Dirk said...

The tuner's HF oscilator circuits also emit a bit of unwanted signal, but receivers must follow legal emission rules, to make sure they cause no interference. These emissions can only get picked up with very sensitive devices on very close distance. (those old illegal and interferring scanners could get picked up a block away) Tracking down someone in your city,listning with a small world-receiver to a Russian HF station is something else, that's a mission impossible.

Chris said...

The British broke the russian one time pads! Each one time pad is, as you say unbreakable, but if you use a machine to print out a sequence of one time pads then there is a flaw.
The machine uses an algorithm and that algorithm can be broken with enough data. If you know the order in which the pads are used you get a lot more material for decrypting the codes than you for a single one time pad. Then you can throw a powerful computer at it and you get to read unbreakable codes.

Dirk said...

Hi Chris, I believe you're mixing two stories here. Russian One-Time Pad were broken during the Verona project, the reason was that they used the same OTP's more than once.

On the other hand you talk about machines creating OTP's. There are two kinds of machines: those who generate real random mostly noise based, or older version mechanical like lottry stuff) and algorithm based (computer program). The latter is NO One Time Pad, but a PRNG, a PSEUDO random generator!!!

If we talk about OTP's, we talk about real random, and this is ALWAYS unbreakable (see shannon).

Unfortunally, a PRNG is often called OTP, and that's pure snake-oil talk!