Zimmermann Telegram - illegal UK interception of US cable in 1917 to aid war effort

Zimmermann Telegram - Wikipedia, the free encyclopedia: "Hall passed the telegram to the Foreign Office on 5 February, but still warned against releasing it. Meanwhile, the British discussed possible cover stories: to explain to the Americans how they got the ciphertext of the telegram without admitting to the cable snooping; and to explain how they got the cleartext of the telegram without letting the Germans know their codes were broken. Furthermore, the British needed to find a way to convince the Americans the message was not a forgery.

For the first story, the British obtained the ciphertext of the telegram from the Mexican commercial telegraph office. The British knew that the German Embassy in Washington would relay the message by commercial telegraph, so the Mexican telegraph office would have the ciphertext. "Mr. H", a British agent in Mexico, bribed an employee of the commercial telegraph company for a copy of the message. (Sir Thomas Hohler, then British ambassador in Mexico, claimed to have been "Mr. H", or at least involved with the interception, in his autobiography.) This ciphertext could be shown to the Americans without embarrassment. Moreover, the retransmission was enciphered using cipher 13040, so by mid-February the British not only had the complete text, but also the ability to release the telegram without revealing the extent to which the latest German codes had been broken—at worst, the Germans might have realized that the 13040 code had been compromised, but weighed against the possibility of United States entry into the war that was a risk worth taking.

Finally, since copies of the 13040 ciphertext would also have been deposited in the records of the American commercial telegraph, the British had the ability to prove the authenticity of the message to the United States government.
As a cover story, the British could publicly claim that their agents had stolen the telegram's deciphered text in Mexico. Privately, the British needed to give the Americans the 13040 cipher so that the United States government could verify the authenticity of the message independently with their own commercial telegraphic records, however the Americans agreed to back the official cover story. The German Foreign Office refused to consider a possible code break, and instead sent von Eckardt on a witch-hunt for a traitor in the embassy in Mexico." 'via Blog this'