The 1944 Bletchley Park Cryptographic Dictionary

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TUNING MESSAGE :

A message sent normally by the control station, usually immediately after a change of frequency, for the purpose of ensuring that all the stations in the group are correctly tuned in (to the new frequency).

TUNNY :

1. A German electric letter-subtractor, or virtual letter-subtractor, cipher machine using the teleprinter alphabet, and enciphering each of the five impulses constituting a letter separately on a long key produced (in the simplest form of the machine) from four short period components, two of which are common to the five keys and two independent, i.e. from twelve short-period components in all, the periods being all prime to each other. The exact method of operation can only be probably inferred, but the essential character of it can be represented as a mechanical driving so far as the effect of the motor-wheels (q.v.) on the psi wheels is concerned, and as addition (non-carrying) in the scale of 2 so far as the chi and psi wheel patterns affect the plain language impulses.


Each of the five chi wheels and each of the five psi wheels bears a pattern which can be considered as a succession of ones and noughts (in practice written as crosses and dots), the numbers of ones on any of these wheels being as a rule approximately equal to the number of noughts. In the chi wheels the distribution of ones and noughts is more or less random, i.e. any sign is as likely to be followed by the same sign as by a different sign; but in the psi wheels there are usually many more changes of sign than continuations, apparently in order that the psi-wheel patterns should have an approximately random sequence when extended by the motor. (cf. Turing's method and motor).


Encipherment is effected by the addition (non-carrying) in the scale of two of each of the impulses of a letter to the signs of the appropriate chi and psi wheels which are at the operative position for that letter, the resulting five signs being transmitted as a teleprinter letter. This, on reception, operates, and is operated on by, an identical machine having its twelve wheels set in the same positions as the enciphering machine; and this by the same cycle of operations deciphers the letter and prints the clear equivalent on a tape.

2. Any traffic enciphered on the above machine.

TURING's METHOD :

A method, devised by Mr. Turing, for obtaining the wheel-patterns from a length of Tunny key, utilizing the fact that unextended psi patterns are characterised by many more changes of sign than continuations and the consequent fact that the five extended psi patterns are characterized by either remaining all the same from one position to the next or by all, or nearly all, changing. The key is first differenced at an interval of one, producing at each position, as a result of the above characteristics, either the true chi first differences at that point or the reverse, or almost the reverse, of these. One or two assumptions, usually of the former type (i.e. that the differences are the true differences), are

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