The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography

by Simon Singh

Human error at more senior levels also compromised the security of the Enigma cipher. Those responsible for compiling the codebooks had to decide which scramblers would be used each day, and in which positions. They tried to ensure that the scrambler settings were unpredictable by not allowing any scrambler to remain in the same position for two days in a row. So, if we label the scramblers 1, 2, 3, 4 and 5, then on the first day it would be possible to have the arrangement 134, and on the second day it would be possible to have 215, but not 214, because scrambler number 4 is not allowed to remain in the same position for two days in a row. This might seem a sensible strategy because the scramblers are constantly changing position, but enforcing such a rule actually makes life easier for the cryptanalyst. Excluding certain arrangements to avoid a scrambler remaining in the same position meant that the codebook compilers reduced by half the number of possible scrambler arrangements. The Bletchley cryptanalysts realized what was happening and made the most of it. Once they identified the scrambler arrangement for one day, they could immediately rule out half the scrambler arrangements for the next day. Hence, their workload was reduced by half.