Pablo Waberski crossed the border from Mexico at Nogales, Arizona on February 1, 1918. His passport indicated he was a Russian citizen. Mexico was a neutral country during the First World War (1916-1919) and, like Portugal during the Second World War, it was a favorite hangout for spies. It was particularly convenient for German spies intent on entering the United States undetected. For that reason, the U.S. had a large number of undercover agents operating in Mexico, trying to identify foreign agents. A warning had been relayed to U.S. border officials to be on the lookout for Pablo Waberski. American undercover agents in Mexico suspected he was a German spy whose real name was Lather Witcke! Because of the warning, Waberski was arrested at Nogales and taken to a nearby military base and searched. Military Intelligence officers found nothing of interest except a sheet of paper with a series of 10-letter codegroups written on it. Without being able to read the code, or even prove that it was a code, they would not be able to hold Waberski for more than 24 hours.
Here is what the first few lines of the actual code that Waberski was carrying looked like:
A copy of the paper with the 10-letter groups was sent to Washington and turned over to Herbert O. Yardley. Yardley was the head of a small but effective group of cryptanalysts in a secret department called simply MI-8. Among the secret agencies of the world it was also called “The American Black Chamber.” Yardley was given the task of “cracking” the apparent code and producing the proof that Pablo Waberski was not a tourist from Russian visiting the United States, but a German spy named Lather Witcke. Yardley and his team of cryptanalysts had less than 24 hours in which to do it.
Page 33 from “SECRET CODE BREAKER – A Cryptanalyst’s Handbook