Friday, December 20, 2013

US Chungking embassy message

I’ve written a lot about how the M-138-A strip cipher was used (and misused) by the US State Department during WWII.

Time for a challenge!
The following message was sent from the US embassy in Chungking, China to Washington in March 1943. It has the signature Vincent, which probably refers to Counselor John Carter Vincent.

Can YOU decipher it?

I know the following things about the indicator system. According to the cipher instructions of May 1944 all messages had to use the channel elimination table. The cipher clerk had to select 5 different letters at random and then use the channel elimination table to find out which of the 30 positions in the panel would be empty.

Although the cipher instructions did not include a sample channel elimination table I think that the version used at the time must have been similar to that used by the US Armed forces in the early parts of WWII:

The first code groups of the message must be the date (taken from either the Gray or Brown codebook) and then the channel elimination indicator that was repeated at the end. Following these instructions it would mean that XSUEF is a date group and TONZS the channel elimination indicator. Note that TONZS is not repeated at the end but that could be due to the fact that not all the codegroups were intercepted.

Since I don’t have the previous version of ‘Instructions for cipher device M-138A’ (issued in January 1942) i can’t be sure that channel elimination was used in 1943 by the State Department. Instead they could have been using only the split generatrix system (15 cipher letters from one column and 15 from another).

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