This is just so deliciously geeky -

Shortwave radio bands, ignored by commercial broadcasters because of their low fidelity, have long been home to government activity -- whether for national broadcasts such as the BBC World Service, Voice of America and Radio France International, or propaganda broadcasts from the likes of Radio Havana or the U.S.-backed Radio Free Iraq.

Meanwhile, for the last 30 years an altogether more curious kind of international station has been noted on the airwaves.

Across the world, high-powered transmitters with global reach are broadcasting seemingly meaningless strings of numbers or letters, along with a lot of buzzing and beeping noises.

Some have speculated that the signals from these "numbers stations" are operated by drug cartels. However, it's more likely they're run by intelligence agencies, as tacitly acknowledged by the British government, and accidentally by the Cubans...

Having done a sliver of crypto work back in the day, one of my first reactions to this was "ah-hah!   I betcha it's a one-time cipher pad".  One time pads are important in crypto because they do actually produce theoretically unbreakable codes.   Keep in mind we aren't talking about "difficult to break" codes such as most symmetric & assymetric systems like Public Key.   One time pads are truely - in the mathematical sense - unbreakable.  

Large chunks of cryptanalysis (the science of breaking codes) rely upon finding cyclical regularities within the encrypted text.   Trivial examples based on the original unencyrpted text include knowledge of letter frequencies (for ex., 'e' being the most commonly used letter in English) and word frequencies ("the" being one of the most commonly used 3 letter words, etc.).   These insights coupled with some back-breakingly tedious analysis formed the basis of much important code breaking throughout history (Bletchly Park & the Enigma, etc.).   Of course, the folks who create ciphers recognize this & put a lot of effort in their algorithms to create a uniform distribution from decidedly non-uniform input text.  

There's a secondary part of frequency analysis due to the encryption key typically being far smaller than the text you're encrypting - think of your password vs. the size of your encrypted word doc.  The process of "locking" a document is sorta like an algorithm that distributes the key's "signature" across the block of text.  All's well but now the key itself is the piece of data that's repeated & thus there's some room to attack here.  In fact, part of the reason large prime numbers figure so significantly in many crypto schemes is because they create an internal "beat" within a block of encrypted text that's, well, very long.  (Although nowadays, the primary reason they're important has to do with the difficulty of factoring the product of primes... a long story for a different day.)

A onetime pad is interesting because it starts with a password to encrypt & decrypt our text that's as long or longer than the text itself.  In old school spy lore, this used to be things like taking your message and "adding" it to the contents on page 253 of War and Peace.  Through  math & number theory, it's now provably possible to construct an unbreakable crypto system.  The catch is that the sender & recipient need a a high confidence, secure way of distributing a long, constantly changing, one-time-use pad to the agent in the field.   Enter the numbers station?