Joseph Finnegan (cryptographer)

Joseph Finnegan was a US linguist and cryptanalyst.

Joseph Finnegan is described by Captain Forrest R. "Tex" Biard USN (Ret) in a July 2002 speech to the National Cryptologic Museum Foundation as follows:

Cquote| First, by hindsight, I can say that four miracles blessed us in the Dungeon [the basement offices of the Pearl Harbor cryptanalysts] during that otherwise tragic December. Each of these four miracles was essential and absolutely necessary. Not until much later could we perceive just how very well we had been blessed.

The first miracle was performed on 7 December. The Japanese bombed and badly damaged the battleship TENNESSEE causing it to settle to the bottom of Pearl Harbor. But in doing this they made one fatal mistake. They spared a very special one of her officers, a certain Lieutenant Commander Joseph Finnegan, U.S. Naval Academy Class of 1928, Tokyo-trained Japanese linguist, usually gregarious but sometimes reclusive, intuitive, brilliant, volatile, and a professional U.S. Irishman from Boston. The Japanese paid a high price for that mistake. It cost them the war. http://www.usspennsylvania.com/TheDungeon.htm

Biard also maintains that Finnegan was second only to the master crytoanalyst,Joseph J. Rochefort, head of Hypo and chief naval cryptanalyst in Honolulu. He calls him "the incomparable human dynamo, the top-notch linguist, intuitive code breaker, brilliant thinker, strategic analyst", "inter alia".

Edwin T. Layton, in his book "And I Was There: Pearl Harbor and Midway -- Breaking the Secrets" (1985) recounts a tremendous effort by Finnegan on the Hypo team concerning the exact date on which the attack on Midway would occur. This involved the date-time groups in Japanese naval messages.

Layton refers to the date-time data as being “superenciphered,” meaning that this data was preencoded even before it was added to the JN-25 cipher. When Hypo made their all-out effort to crack this, they started by searching the stacks of printouts and punch cards for five-digit number sequences. Those they found were in low grade codes, a poor starting point, but a starting point.

Layton describes this method as "involving a 12 x 31 (12 rows for months, 31 columns for day) garble check. The 31 kana [Japanese syllabic scripts] of the first row were A, I, U, E, O, KA, KI …………….HA, HI, HU, HE, HO. The second row was I, U, E, O ……………HE, HO, A; the third, U, E, O ……….HO, A, I, and so on, for 12 rows. At the left, representing the 12 months, was a column of 12 kana, different from those in the table – SA, AI, SU, SE, SO, TA, TI, TU, TE, TO, NA, NI (SA for January, NI for December). To encipher, for example 27 May, one picked the 5th line (May=SO), ran across to the twenty-seventh column, HA, and recorded the kana at that intersection, HO. The encipherment, then, was SO, HA, HO, the third kana providing the garble check." (Layton, pp. 427-428)

An intercept of 26 May with orders for two destroyer groups escorting invasion transports was analyzed with this table and “really clinched the pivotal date of the operation” as either 4 or 5 June.