Magic (cryptography) - History

History

When PURPLE was broken by the U.S. Army's Signals Intelligence Service (SIS), several problems arose for the Americans: who would get the decrypts, which decrypts, how often, under what circumstances, and crucially (given interservice rivalries) who would do the delivering. Both the U.S. Navy and Army were insistent they alone handle all decrypted traffic delivery, especially to highly placed policy makers in the U.S. Eventually, after much toing and froing, a compromise was reached: the Army would be responsible for the decrypts on one day, and the Navy the next.

The distribution list eventually included some — but not all — military intelligence leaders in Washington and elsewhere, some — but, again, not all — civilian policy leaders in Washington. The eventual routine for distribution included the following steps:

• the duty officer (Army or Navy, depending on the day) would decide which decrypts were significant or interesting enough to distribute
• they would be collected, locked into a briefcase, and turned over to a relatively junior officer (not always cleared to read the decrypts) who would 'make the rounds' to the appropriate offices.
• no copies of any decrypts were left with anyone on the list. The recipient would be allowed to read the translated decrypt, in the presence of the distributing officer, and was required to return it immediately upon finishing. Before the beginning of the second week in December 1941, that was the last time anyone on the list saw that particular decrypt.