The SZ42: a pre-electronic machine

The Enigma and Hagelin machines provided a much greater degree of security than any earlier systems of encipherment other than the unbreakable one-time pad. The cryptographic principles on which these two machines were based were quite simple. The Enigma provided a large number of substitution alphabets whilst the Hagelin generated a very long stream of pseudo-random key. In theory either machine could be modified in order to make it even more secure. The number of wheels could be increased and in the Hagelin the wheels could be made longer. In practice, modification of an existing cipher machine may present major difficulties of manufacture, distribution and compatibility with the original machine, which may be vital. A four-wheel Enigma was, in fact, introduced in 1942 and compatibility with the original three-wheel version achieved by arranging that with the new components in specified positions the old and new versions were the same cryptographically. Several new models of the Hagelin were produced by that company in the 1950s with different sized wheels and other features, but these were genuinely different machines and no attempt was made to provide compatibility with the original.

It might seem obvious that increasing the number of components in, or increasing the complexity of, a cipher machine will make it more secure, but this is not necessarily so. The more components there are, the more likely it becomes that operators will make errors. The greater the complexity, the greater the chance of a machine malfunction.