Weak key

In cryptography, a weak key is a key, which, used with a specific cipher, makes the cipher behave in some undesirable way. Weak keys usually represent a very small fraction of the overall keyspace, which usually means that, if one generates a random key to encrypt a message, weak keys are very unlikely to give rise to a security problem. Nevertheless, it is considered desirable for a cipher to have no weak keys. A cipher with no weak keys is said to have a flat, or linear, key space.

Virtually all rotor based cipher machines (from 1925 onwards) have implementation flaws that lead to a substantial number of weak keys being created. Some machines have more problems with weak keys than others, as modern block and stream ciphers do.

The German Enigma machine is a family of about a dozen different cipher machine designs, each with its own problems. The military Enigma cipher machine, in its 3 and 4 rotor implementations had the equivalent of weak keys. Certain combinations of rotor order, stepping and initial key were fundamentally weaker than others. The Enigma's reflector (when used) guaranteed that no letter could be enciphered as itself, so an A could never turn back into an A. This helped Polish and, later, British efforts to break the cipher. (See Cryptanalysis of the Enigma and the Enigma rotor details.)

The first stream cipher machines, that were also rotor machines had some of the same problems of weak keys as the more traditional rotor machines. The T52 was one such stream cipher machine that had weak key problems.

The British first detected T52 traffic in Summer and Autumn of 1942. One link was between Sicily and Libya, codenamed "Sturgeon", and another from the Aegean to Sicily, codenamed "Mackerel". Operators of both links were in the habit of enciphering several messages with the same machine settings, producing large numbers of depths.

There were several (mostly incompatible) versions of the T52: the T52a and T52b (which differed only in their electrical noise suppression), T52c, T52d and T52e. While the T52a/b and T52c were cryptologically weak, the last two were more advanced devices; the movement of the wheels was intermittent, the decision on whether or not to advance them being controlled by logic circuits which took as input data from the wheels themselves.

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