British Codebooks Bergen 1940

The basic book was captured from the British consulate in Bergen in May 1940, allowing the Germans to solve ‘depths’ during the period 1940-43



By August 1941 the Germans were regularly reading the War Office high-grade hand cypher which carried a good deal of Eighth Army's W/T traffic down to division level, and they continued to do so until January 1942.





The German exploitation of the ID Cypher ended on 15 June 1943 when the codebook was changed.



Had the Brits introduced new versions in late 1940 or early 1941 the German success would have been nipped in the bud.


http://chris-intel-corner.blogspot.com/2014/02/british-cryptologic-security-failures.html

Depth[edit]

Sending two or more messages with the same key is an insecure process. To a cryptanalyst the messages are then said to be "in depth".[21] This may be detected by the messages having the same indicator by which the sending operator informs the receiving operator about the key generator initial settings for the message.[22]


Admiral Luigi Donini, one of the top Italian codebreakers of WWII, some of the main British mistakes were:
1). Reuse of the same key sequence over and over, leading to ‘depths’.