Melbourne, Hong Kong and Batavia. Prior to the attack on Pearl Harbor, the amount of available IJN traffic was low, and little progress had been made on the most important Japanese Navy system, called JN-25 by U.S. analysts. JN-25 was used by the IJN for high level operations: movement and planning commands, for instance. It was a state-of-the-art superencrypted code, eventually a two-book system. Cryptanalytic progress was slow. Most references cite about 10% of messages partially (or sometimes completely) decrypted prior to December 1, 1941, at which time a new version of the system went into effect, requiring the cryptanalysts to start again. Although most references did set the limit of the percent of the messages that were decrypted at 10%, they were not privy to the latest information. Wilford in his Decoding Pearl Harbor: USN Cryptanalysis and the Challenge of JN-25B in 1941, suggests that this view is now untenable and that the JN-25 codes were readable to a great extent and hence, lends "support to the revisionist theories of Toland and Stinnett." After the attack on Pearl Harbor, there was considerably more JN-25 traffic as the Japanese Navy operational tempo increased and geographically expanded, which helped progress against it. Hong Kong's contribution stopped until the crypto station there could be relocated (to Ceylon and eventually Kenya), but and the Dutch at Batavia, in conjunction with and OP-20-G made steady progress. in particular made significant contributions. Its people, including its commander, Joseph Rochefort, thought a forthcoming Japanese attack early in 1942 was intended for the central Pacific, while opinion at OP-20-G, backed by , favored the North Pacific, perhaps in the Aleutians. In early 1942, in response to the Japanese advances in the Philippines (which threatened ), the possibility of an invasion of Hawaii, and the increasing demand for intelligence, another signals intelligence center, known as was formed in Washington, using elements of OP-20-G. In the words of NSA historian Frederick D. Parker: By the middle of March 1942, two viable naval radio intelligence centers existed in the Pacific—one in Melbourne, Australia [], and one, , in Pearl Harbor, Hawaii ... The center on Corregidor () was no longer affiliated with a fleet command, and its collection and processing capabilities were rapidly disintegrating as a result of evacuations of personnel to Australia and destruction of its facilities by bombing and gunfire. Japanese traffic was intercepted regarding a new offensive