25 decrypts were very short messages or partial decrypts of marginal intelligence value. As Whitlock stated, "The reason that not one single JN-25 decrypt made prior to Pearl Harbor has ever been found or declassified is not due to any insidious cover-up... it is due quite simply to the fact that no such decrypt ever existed. It simply was not within the realm of our combined cryptologic capability to produce a usable decrypt at that particular juncture." JN-25 The JN-25 superencrypted code, and its cryptanalysis by the US, is one of the most debated portions of Pearl Harbor lore. JN-25 is the U.S. Navy's last of several names for the cryptosystem of the Imperial Japanese Navy, sometimes referred to as Naval Code D. Other names used for it include five-numeral, 5Num, five-digit, five-figure, AN (JN-25 Able), and AN-1 (JN-25 Baker), and so on. Superenciphered codes of this sort were widely used and were the state of the art in practical cryptography at the time. JN-25 was very similar in principle to the British "Naval Cypher No. 3", known to have been broken by Germany during World War II. Once it was realized what sort of cryptosystem JN-25 was, how to attempt breaking into it was known. Stinnett notes the existence of a USN handbook for attacks on such a system, produced by OP-20-G. Even so, breaking any such code was not easy in actual practice. It took much effort and time, not least in accumulating sufficient 'cryptanalytic depth' in intercepted messages prior to the outbreak of hostilities when IJN radio traffic increased abruptly and substantially; prior to 7 December 1941, IJN radio traffic was limited, since the IJN played only a minor role in the war against China and therefore was only rarely required to send radio messages whatever the highest level crypto system might have been. (As well, interception of IJN traffic off China would have been at best spotty.) Rather oddly however, the official history of GYP-1 shows nearly 45,000 IJN messages intercepted during the period from 1 June 1941 until 4 December 1941. Thus, most Japanese encrypted broadcast military radio traffic was Army traffic associated with the land operations in China, none of which used IJN cryptography. Breaking a superencrypted cipher like JN-25 was a three-step process: (a) determining the "indicator" method to establish the starting point within the additive cipher, (b) stripping away the superencryption to