[2] The Soviet Unions highest grade ciphers that were used in the East, were the 5-figure codebook enciphered with the Blocknot book, and were generally considered unbreakable.
[9] The Chi-number was the serial numbering of all 5-figure messages passing through the hands of the Cipher Officer, starting on the first of January and ending on thirty-first December of the current year.
After only a relatively short time, the individual curves separated sharply and the type of formation could be recognized by the height of the Chi-number alone.
[3] Blocknots were tracked in a card index, that was maintained by the Signal Intelligence Evaluation Centre (NAAS).
A careful recording and study of blocks provided the positive clues in the identification and tracking of formations using 5-figure ciphers.
The index was subdivided into two files:[10] Inspector Berger, who was the chief cryptanalyst of NAAS 1 stated that the two files formed: The Blocknots were also used in the Stationary Intercept Company (Feste), the military unit that were designed to work at a lower level to the NAAS, at the Army level and were semi-motorized, and closer to the front.
[10] Johannes Marquart was a mathematician and cryptanalyst who initially worked for Inspectorate 7/VI and later led Referat Ia of Group IV of the General der Nachrichtenaufklärung.
[12] All the counts which they made, however, failed to reveal any non-random characteristics in the design of the tables, and while they thought the Blocknots must have been generated by machine, they were never able to draw any concrete deductions as a result of their research.