Back in the days of the Cold War, spies found ingenious and unusual ways to communicate with each other, hiding their communications in plain sight, whether encrypted or not. For instance, an intelligence agent (spy) might contact his case officer (handler) by placing an ad in the personals section of the local newspaper. This practice, it seems, continues today.

On or around May 8, the following personal ad appeared on the Internet classified ad site Craigslist. (It has since been removed.)

For mein fraulein Mein Fraulein, I haven’t heard from you in a while. Won’t you

call me? 212 //// 796 //// 0735

If you actually called the number, up until a couple of days ago you would have heard this prerecorded message (MP3). It’s a head scratcher to keep you National Security Agency analysts occupied in your spare time. Each block of numbers is repeated twice; but below I have transcribed them only once for clarity.

Group 415

01305 60510 12079 04606 50100

93000 08203 90130 94069 01207

81080 17028 01706 90220 73038

01401 70150 15073 00402 00680

12013 12510 00540 04091 01401

30150 86022 09608 10660 02082

05507 00020 00000 02208 30290

08022 01200 40710 13065 02709

40190 29014 02200 80020 11083

07300 30260 19000 00700 00000

86

You used to hear broadcasts not unlike this on shortwave radio all the time, stations which broadcast nothing but numbers all day. Nobody has yet come forward with an answer as to what exactly these stations were broadcasting, or to whom, or what was the point of the messages. If anyone does know, they likely can’t say because it’s classified information.

On or around May 12, Craigslist pulled the ad, but the number remained up and running, and someone tipped off our friends at 2600 Magazine. Its editor Emmanuel Goldstein mentioned the number on the May 24 episode (MP3) of the WBAI radio show Off The Hook. After that, it was flooded with calls from aspiring phone phreaks and other curiosity seekers, and that’s when some more of the details behind the number began coming out, through brute force, as it were.

The number is a VoIP setup from a small provider yet to be identified, but the wholesaler is a company called RNK Telecom. The account was prepaid, and the flood of calls after the radio show quickly depleted the account balance; callers over Memorial Day weekend would have heard a message (MP3) saying your account balance was too low. Now the number gives nothing but a busy signal.

Many questions remain, of course, now that the number seems to be no longer functioning. Who put the ad on Craigslist? What is the encrypted message? Is this all some sort of elaborate prank? Or could it be a viral marketing scheme of some type?

The question which most interests me, though, is what does the message mean? The Craigslist posting is typical of how intelligence agents might communicate in hostile countries where they are actively trying to evade the country’s intelligence services. The message at that phone number, though, seems like an easier avenue of attack for this particular problem, at least right now.

The first thing I notice is that it contains an unusually high number of zeros. And that’s just about where my cryptanalytic skills end. Anybody else want to take a crack at it?

Update: Ryan Singel (of Wired ) thinks it’s just two young cryptanalysts in love, “sending love notes and taunting Mossad, the NSA and the phone phreakers at the same time.’ He also points to some links indicating those shortwave numbers stations are still around. Go take a listen.

Update: There is now a second phone numbers station message for you to puzzle over, too.

Update: And now there’s a third phone numbers station as well.