At the start of WWII and for most of the conflict the standard crypto system used by the British for high level messages was the codebook enciphered with subtractor tables. Both the Foreign Office and the military services relied on these Cyphers for their most important traffic.
The War Office Cypher was the Army’s universal high-grade codebook (4-figure) and carried traffic between Whitehall, Commands, Armies, Corps and, later, divisions. There were different sets of enciphering tables for each geographic area (Home Forces, Middle East, etc)
The Germans captured two copies of the WOC in 1940. One during the Norway campaign and the other near Dunkirk. The compromise of the code allowed them to focus only on stripping the cipher sequence. This was achieved by taking advantage of ‘depths’ (messages enciphered with the same numeric sequence).
From early 1941 the German Army’s signal intelligence service was able to read messages from the Middle East theatre. A considerable number of messages were read during the Cyrenaica offensive of General Wavell. In the summer of ’41 Tobruk was encircled by Rommel’s forces and had to rely strictly on radio communications. Since the WOC was used many of the messages could be read. In late ’41 a message from Tobruk indicated that their Typex machine was in need of repairs.
During 1941 the WOC decodes provided intelligence on the enemy’s strength, OOB and movement of units in the M.E. Theatre. For example an Enigma message decoded by Bletchley Park in October ’41 gave a summary of the increase in British ground strength in Egypt and the tank strength estimate was so accurate that the War Office was very concerned.
The British knew that the WOC was in enemy hands and could be exploited but they had no alternative than to keep using it. Security was upgraded in late ’41 and from early ’42 the German success was hindered but not eliminated.
The Germans continued to read some WOC traffic till summer ’42, when the intercept company NFAK 621 was captured in N.Africa. The files of that unit revealed to the Brits that many of their systems were compromised and they immediately took action to change them. From then on the WOC-ME would be secure.
This was not the end of the German solution. According to Herzfeld, an army cryptanalyst, the WOC used by Home Forces in Britain was solved in 1943. Some traffic of January and February ’43 was read but success ended in March. It was at this time that the subtractor tables were replaced by the new stencil cipher which proved to be unbreakable.
Overall the solution of the War Office Cypher in the M.E. Theatre in 1941-42 was an important achievement for the German side and a serious defeat for the British.
Sources: ‘Intelligence and strategy: selected essays’, ‘British intelligence in the Second World War’ vol2, TICOM reports I-51, I-113, IF-107, CSDIC SIR 1704-‘The organization and history of the Cryptologic service within the German Army’, CSDIC/CMF/Y 40-'First Detailed Interrogation Report on Barthel Thomas’, ‘European Axis Signal Intelligence in World War II’ vol1 and 4, , Cryptologia article: ‘Brigadier John Tiltman: One of Britain’s finest cryptologists’