TheCodeBreakers
then placing this group at the head of the cryptogram. If he wished to
change this number during a message, he simply repeated the new
encipherment group length five times, inserted it into the body of the
cryptogram, and used that length from then on.
Cryptograms in the Russian Army cipher thus consisted of groups of
monoalphabetically enciphered letters, with the length of the groups
clearly indicated by the unmistakable appearance of, say a 99999 (the
maximum length) or a 66666. Aside from being vulnerable to the usual
techniques of frequency analysis, the cipher would often mirror the
telltale repeated-letter pattern of an underlying plaintext word, such as
attack or division, that had fallen entirely within a single encipherment
group and so had been monoalphabetically enciphered. Such a system
does not interpose insuperable difficulties to the cryptanalyst, especially
when, as with the Russians, it was poorly used, often with intermixture
of plaintext
annaabi.ee 
