A cryptologic veteran's analysis of "day of deceit"--a book review

Cryptologia, Apr 2000 by Jacobsen, Philip H

The book implies more improprieties by the fact that Hypo had no assigned Japanese diplomatic intercept or decrypt authority until RCA President Sarnoff made available RCA cables from Honolulu beginning in early December 1941. Part of the book's overall conspiracy theory includes the allegation that Hypo only decrypted the administrative messages of these low level Japanese diplomatic messages provided by RCA before Pearl Harbor and did not decrypt the "bomb plot" messages until after Pearl Harbor.

Although Stinnett obtained definite information from Captain Whitlock that no significant JN-25B decrypts were made by Station Cast on Corregidor during the period in question, he disputes this fact and misinterprets other documents and sources as proof that Whitlock is wrong. Some navy cryptologic veterans involved in this book have complained Stinnett gained their confidence by agreeing to tell their stories but ignored their version of events in favor of the monstrous conspiracy theory finalized in the book. Admiral Layton terminated his interview with the author, most likely when he learned where the book was going. It should be noted that it took OP-20-G some 14 months to read the much simpler JN-25A system that was used from 1 June 1939 to 1 December 1940. The book misleads its readers by not revealing there were two distinct codes, the earlier JN-25A and its much more complicated successor JN-25B used during the period in question and refers to them collectively as "Code Book D" or "5-Num code." Thus, the final successes of JN-25A are improperly imputed to JN-25B that was not read to any significant extent until early 1942 when the first published decrypt is found. The ever-increasing requirements to provide Japanese diplomatic decrypts and translations during 1941 took most of the time of navy cryptographers so that few people at both Washington and Station Cast were assigned to work on the new version of the Fleet Code, JN-25B. In addition, JN-25B used about eight additive cipher books up through 4 December 1941 further delaying the effort to read any significant amount of this new and far more complicated code and cipher combination.

Stinnett and his sources are apparently not aware that Japanese naval shore broadcast stations transmitted simultaneously on a number of frequencies covering their communications area. It was up to the ships in their communications zone (or U. S. intercept operators) to choose the best frequency on which to copy such broadcast. Thus, the deduction that because an intercept operator copied one message in the 12 MHz. range part of one day and 16 MHz. on part of a different later day means the ship or force has moved further away from the shore station is patently incorrect. These Tokyo broadcast transmitters were active on several of their assigned frequencies simultaneously and the 16 MHz. frequency had long been used by the Tokyo broadcast as a daytime frequency.

Stinnett often claims carriers or fleet units must have transmitted on high frequencies when they are only seen in the headings of messages on fleet broadcasts. He does not tell his readers that many ships are tied up at docks and have landline or cable communications available to them so they do not have to use radio and the original transmissions of such messages will never be heard by foreign intercept operators. In this regard, he maintains that Admiral Yamamoto's messages sent (while tied up at a Kure dock) to the Pearl Harbor attack force and other ships on the Tokyo broadcast violated radio silence when, in fact, the radio silence imposed then only meant that ships (or aircraft) are not permitted to transmit by high frequency radio, not that messages to these units cannot be sent by fleet broadcasts or that fleet units or commands that have land-line, cable or other approved facilities available to them cannot use them.