L+f: the riddle of the missing 9s

L+f: the riddle of the missing 9s

The American crypto expert Matt Blaze tells in his blog a highly interesting and probably true spy story. According to this story, spies were provided with instructions via regular radio transmissions. In it, an automaton voice read out numbers in Spanish. The radio station was located in Cuba, the receiver or receivers could be anywhere within the reception range of the shortwave transmitter – in other words, just about anywhere.

The messages were encoded, as usual in the spy field, with the provably unbreakable one-time-pad encryption. To decipher them, one needed the corresponding codebook with the corresponding real random numbers. But Blaze (and apparently others) noticed something odd about the messages: Some of them did not contain 9s. So in the complete announcement there was no "nueve" in front of.

Treacherous dummies

It turned out that these messages were just dummies that were sent when there was nothing to say. They were meant to prevent any patterns from being deduced from the transmission times. And the Cubans apparently created these dummy messages with a broken random number generator that didn’t spit out 9s.

The result was that the FBI was able to match the times of dummy transmissions without 9s with the inactivity periods of a couple suspected of espionage. This observation helped to raise this suspicion and finally to overtake the couple. At least that’s what it says in the recently published book of an ex-FBI agent.

Blaze enriches the story in his blog post A Cryptologic Mystery with a lot of nice details. Especially impressive: the original recording of a shortwave transmission. That voice *creepy…

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