TheCodeBreakers
strategic system; border patrol and N.K.V.D. units had their own
systems, usually 4-digit. In addition, the Soviets got some Hagelin M-
209s in Lend Lease, which they apparently used as models for their own
constructions, though it is not known where these were used.
With enough traffic, enciphered code can of course be read. One of the
first to do so in the case of the Russian military was the Swedish expert
Arne Beurling. During the bitter struggle of Finland against Russian
aggressors in the Winter War of 1939-1940, Sweden fed intelligence
based on cryptanalysis to her neighbor. Beurling attacked the top
system, the 5-digit strategic, which was actually a 4-digit code with an
extra digit added as some form of check. In several of the codes, the page
digit—the second— was repeated, so that the groups would look like
52217, 88824, and so on. In others, the fifth digit gave the unit total of
the preceding four digits, so that 6432 would have a check digit of 5,
annaabi.ee 
