- 90 -
TURING's METHOD, (Cont'd)
made and repeated at the respective chi periods throughout the
differenced key. From the agreement or disagreement of these
assumptions with the differences at these periods inferences are
made as to whether the true difference or its reverse occurs at that
point, and by correlating the results of these inferences the chi
first differences (and from these the chi patterns and from these
the psi and motor wheel patterns) can be obtained. (in practice it
does not matter much whether the original assumption is correct or
not as the wrong assumption gives results for the chi first
differences which only required to be reversed.)
TURN-OVER :
A usually periodic forward movement of a wheel or other cyclic
component of a cipher machine; especially of any wheel other than
the fast wheel.
TUTTE's METHOD :
A method, devised by Mr. Tutte, for obtaining the chi wheel-patterns
(and hence the psi and motor wheel-patterns) of a tunny cipher message,
or for setting chi wheels, of which the patterns are already known (and
hence the psi and motor wheels and so deciphering the message), utilizing
the psi wheel characteristics described under Turing's method (q.v.)
and previously ascertained useful measures of agreement or disagreement
between pairs of impulses of the teleprinter letters and other signs
constituting the clear text of similar messages. Certain variations
in the method can be introduced according to the capacity of the
machinery used to apply it , but essentially it consists in adding the
two selected impulses of the cipher letters together and differencing
the result at an interval of one, (thus largely eliminating the psi
wheel contribution, as well as the clear language contribution in so
far as the two impulses concerned agree or disagree). The resulting
difference, being, in consequence of these eliminations, in appreciably
more than fifty per cent of its signs the sum of the first differences
of the two chi wheel patterns concerned, is written diagonally into a
rectangle combining the two chi periods and this rectangle is resolved
into the two chi first difference patterns. Other suitable pairs of
impulses are similarly treated until the five chi first differences
are all determined. From these the five chi wheel patterns (and their
settings for the message) are found, and the chi wheel contributions
are subtracted from the cipher text. The resulting de-chi (q.v.) is
then examined especially for repeated letters, which often correspond
to repeated psi patterns combined with repeated plain letters or
other symbols, or for consecutive letters having otherwise significant
differences, and the characteristics of extended psi patterns are
again utilized to confirm and then to extend 'cribs' located by this
means until the complete psi patterns and finally the motor-wheel
pattern are obtained.
TWIDDLE :
(in Enigma). To turn round the wheels of an Enigma machine in hand-
testing for a particular constatation, esp. in the procedure for
setting duds. Cf.Clonk.
| previous page
| next page | index page |