Joseph Finnegan (cryptographer)

Joseph Finnegan was a US linguist and cryptanalyst with Station Hypo during the Second World War.

War contribution
In 2002, Tex Biard described Finnegan as "intuitive" and "brilliant", and second only to Station Hypo chief Joseph J. Rochefort, saying that Finnegan's survival of the bombing of the USS Tennessee was a "fatal mistake" on the part of the Japanese, and that Finnegan's survival "cost (the Japanese) the war."

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 punched cards for five-digit number sequences. Those they found were in low grade codes, a poor starting point, but a starting point.

Next they had to unravel the cipher itself. It was Lieutenant Joseph Finnegan, a linguist-cryptanalyst, who finally hit upon the method that the Japanese had used to lock up their date-time groups.

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.