US Codebreaking Activities in Tokyo during the Korean War

I updated my recent article "Korean Cryptography" with additional specimens from John Milmore (2002), #1 Code Break Boy. The book, written like an unedited memorandum of an eyewitness, is interesting in recording US codebreaking activities on North Korean traffic during the Korean War, in which the author was "the principle cryptanalyst" (p.3).

It is more interesting to me because the codebreaking unit was headquartered in Tokyo (Japan was still under occupation after WWII). The photo below is a recent view of the building used as the headquarter (from Wikipedia). 

 

 

How Long Does It Take to Decipher a Despatch?

How long did it take for a secretary to decipher a despatch? Sometimes we see a reply to a ciphered despatch received the night before, in which case we know that the deciphering of the original letter and the enciphering of the reply were done in a day. It is desired to find such an instance for which the original letter in cipher is available. If the letter is a long one, a better upper bound estimate can be made.

An informal experiment I recently read about this (I thought it was in Desenclos and Clinet (2025) (BDN) but apparently I misremembered it) induced me to try it myself. It took me twenty minutes to decipher a page.

But of course, this depends on many factors.

(i) The page is almost entirely in cipher but the number of symbols on the page (about 340) is modest. (One instance of a letter of Mary, Queen of Scots, has more than 4000 symbols on a page.)

(ii) Of these, almost 80% are single letters, with only about 20% being symbols for syllables and names. While the cipher has three or four homophones for each letter of the alphabet, after getting familiar with the cipher, one can work with this much (as well the most frequent syllables and code numbers such as "re", "de", "se", "que", "luy", "dit") from memory. If the cipher used syllables and codes more extensively, it would have taken more time in consulting the cipher table.

(iii) I was familiar with the cipher because I had been working with it for two weeks before the experiment. When one works with a new cipher, much longer time is necessary.

(iv) The deciphering work may be delayed if symbol shapes are hard to identify. In this regard, this particular case posed difficulty because of ink bleed-through. For this experiment, however, its influence was negligible beause I simply skipped the hardest cases for the experiment.

(v) There is interlinear decryption, but I did not refer to it for the experiment. (Actually, I had worked on this particular page before but I'm sure I did not remember the content.)

(vi) I worked on a pdf file, which is not very convenient as far as writing deciphered text is concerned. If I had worked on paper, it would have taken much less time.