The most secure form of cryptography, indeed, the only provably secure form is the One Time Pad.
Pedantically the one time pad is not a single algorithm, but in general it is applying pre-agreed randomness to each letter.
The fundamental problem with the One Time Pad is the Key Distribution Problem, how do two people on opposite sides of the world pre agree the same randomness in a secure fashion?
It is for this reason that Embassies use the Diplomatic Pouch and Courier, and illegal agents carried physical pads into the field with them.
The next problem, as far as the agents are concerned, is how messages are sent without compromising the agent, without making the counter espionage people wonder ‘why is THAT person receiving apparently random messages?’
Where messages were to go one way only (e.g. control to agent) this problem could be solved by the so called ‘number stations’. At pre agreed times, a mechanical voice would read out a string of apparently random numbers. It didn’t matter who heard this as to decode the message one needed to subtract the randomness, and only the agent (listening in private) would have the correct sequence of randomness.
Numbers stations still exist today, you can listen to some recordings from the 80s and 90s online (link spotted via Dirk)
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Excellent! Just when I was starting to wonder if there were any recordings online.
And a question. Why do they (number stations) transmit numbers? Shouldn’t they transmit letters since the ciphertext is composed of letters?
(edit: Numbers can represent letters, this is what happens in ASCII. In Numbers can represent any concept, from ‘Commence secret operation’ to ‘Abort, come home’ to ‘Egg fried Rice’. They’re also rather easy to manipulate! - Murk)
Very good article. Doing some hobby programming for a poly-alphabetic substitution, have been creating “key” documents which I’m calling the one-time pad (OTP) files… since it is intended for single usage.
The dilemma presented by distribution becomes majour in that posting to world-wide viewable areas such as newsgroups or even specific message boards presents it’s own sets of problems in regards to getting the data to the user(s); having huge fears of being compromised.