The report SRH-361 ‘History of the
Signal Security Agency Volume Two The General Cryptanalytic Problems’, p283
says about TOKI/JBA:
JBA, a transposition system of a
degree of security second only to the Purple machine-cipher system (JAA), was
solved by statistical methods within six weeks. This solution is believed to be
the first instance of the recovery of an unknown transposition of an unknown
code by purely statistical means. Beginning groups, and later, code groups
within the body of the text were found by matching stretches of cipher text
from several messages with the same indicator. Frequent digraphs were recorded,
and eventually the transposition patterns and tetragraphic code groups were
recovered despite the presence of occasional trigraphic groups, the use of
blanks in the matrix, and the use of the letters of the signature as nulls
throughout the message.
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