The principal series of operational ciphers of the Imperial Japanese Navy [IJN] from 1939 to 1945, collectively called JN25, used five-digit code groups which were all multiples of three. This is shown to have been a quite unnecessary major flaw with very considerable consequences.

A detailed discussion of the JN-25 and related ciphers.

FLAW IN THE JN25 SERIES OF CIPHERS, THE Cryptologia - Find Articles