Both the American rotor solution, deemed MAGIC, and Britain's Ultra placed the two countries in difficult positions. Although they knew the enemy's every move ahead of time, they could not show that they knew. This meant that they could not always act on every message if it would make it obvious that the enemy's codes were detected. Additionally, to prevent leaks from the inside, very few even knew about MAGIC or Ultra. In America, generals were only informed that 'reliable sources' had supplied information and that they should act accordingly. Messages intercepted by MAGIC were delivered personally in a small leather pouch only to those of the highest importance. When delivered, the courier would wait while the recipient read the message and then take the message back. No copies were allowed and only small notes could be of the message. Even the President was under these strict regulations.
In Britain, similar safeguards were put in place to protect the discovery of Ultra. The number of persons allowed to receive Ultra messages was restricted (no one in danger of being captured was allowed to know about it), and after a recipient read the message, it was immediately destroyed by its bearer (who was part of a special unit specifically used to deliver the messages). Furthermore, no one could reference Ultra or quote from an Ultra message in case the enemy was reading messages. Furthermore, when an action was being taken because of an Ultra intercept, a cover story had to be made up to supply the source of information - such as a defecting German agent or British spy providing information, or in the case of military action, a spotting plane had to be sent out to 'scout' the area even if it was known the enemy was there.
Probably the most famous success of Purple occurred on December 6th and 7th, 1941. America had intercepted a 14 part message sent to the Japanese diplomat in Washington that was to be delivered to the American government. It was encoded with Purple, and Magic easily decoded the solution. The message's parts had begun transmission around 2 pm on the 6th and by 8:45 that same day the American cryptanalytic bureau had deciphered and typed the 13 parts which had been received (the 14th part hadn't been sent yet). The Magic employees called the President to alert him about the message received. It was the long awaited official response from the Japanese to a previous peace offer by America, and the tone did not sound very good.
Meanwhile, the Japanese embassy had received and deciphered the message but had not typed up a clean copy for presentation. When the 14th part finally arrived in the early morning hours of December 7th, they finally began typing. And while the Japanese Embassy was typing the message to be delivered to the President, he was reading the first thirteen parts lying in his bed after being awoken specially to read the message.
When the Japanese finally delivered their message to the state department at 1:45 officially ending negotiations on December 7th, the Americans already had the message. Furthermore, the attack at Pearl Harbor had begun an hour before hand, making an official declaration slightly unnecessary.
While the information about Axis cryptographic systems is very detailed, that of the Allies is severely lacking. Most likely the Allies employed a similar rotor system though probably improved to augment the weaknesses found in the German and Japanese systems.
