Sixty-eighth edition of the N&O column / Spooks newsletter

(Date: Sun, 04 Jan 2004 22:02:39 +0100)

Crypto

Every now and then I visit my favorite crypto website "Toby's Crypto site" at http://hem.passagen.se/tan01/ because there is a lot of good stuff on his pages. Especially the archives are interesting. Items like the one below are accompanied by pictures of the original papers. So, if you are interested in items like this, be sure to check Toby's website.

The following is an extract from "the Archives".

5-figure codes were the main cryptographic system used by the Soviet Navy in the Baltic Sea during WWII. Swedish signal intelligence intercepted the traffic on a regular basis, and made efforts to break the system. Just when the Swedish code breakers were beginning to get a grasp of the code in early January 1940, the Russians changed it. Despite this, the new code was cracked and the traffic readable by May 1940! The code was superenciphered with a so called additive, which was printed on a sheet - changed daily - having ten columns, and 30 rows with random 5-figure groups. Each row on the sheet was marked with a 3-figure number, and each column with a 2-figure number.

The first few rows of a sheet could look something like this:

	04	09	15	29	33	50	58	64	82	96
017	58201	10035	57184	00472	33727	18483	10622	37310	74512	23295
023	67844	98944	84216	17402	90246	19774	97823	98151	73785	04471
027	44380	53478	05713	75739	48921	60935	99042	63856	02744	43826

The code clerk chose a group on the sheet at random as starting point, and formed a 5-figure indicator group by combining the 3-figure row number with the 2-figure column number. Using the mock-sheet above, and starting on row two, column two, would result in the indicator group 02309. This group was later hidden in the final telegram. Starting at the group chosen on the sheet (=98944 following the example), the clerk then proceeded to write out one group from the sheet under each group he had looked up in the code book when doing the basic encoding of the plaintext. Then the groups were added together, figure by figure, using non-carrying addition ignoring the "tens" (i.e. 8+4 equals 2, not 12), the result of this operation being the final cryptogram. Let's say our first group from the code book is 82697 meaning Warship "Karl Marx". Adding 98944 to this group by non-carrying addition would result in 70531.

The system was further complicated in March 1941 by performing a double superencipherment. The code clerk chose two, different starting points in the series on the sheet, and thus two numbers were added to each code group. This posed a formidable obstacle to cryptanalysis, and the new system was not broken until June 1942, more than a year later.