The following report is taken from the Special Operations Executive Signals history. It details the use of 'finger printing' Morse W/T operators in order to accurately identify the individual by their signals signature using statistical methods.
SIGNAL SECURITY (FINGER PRINTING) SECTION
Miss Cameron's Section deals with "Finger Printing", a method by which the characteristics of an operator, after being suitably recorded on a tape can be classified and actually identified by trained personnel, so that it is possible to ensure that on any radio circuit the identity of the operator at the away Station is known.
Although it has been recognised for sometime that a good operator could recognise the sending characteristics of another, the development of this "Tina" system (taena - tape worm) has enabled recognition to be mathematically precise. Early in 1943 it was felt that "Finger Printing" should be developed by S.O.2. and accordingly Tina recordings were started of all students at the Training Schools and the classification system set up. By the Autumn of 1943 the Main Stations were equipped to take tape records of agents in the Field and the system for checking these agents was commenced.
The tendency on the part of operators in the Field to recruit pupils was soon noticed, the first example found by Tina was WOODCHUCK. We were able to check that a pupil had been put on the circuit and changed with WOODCHUCK who had omitted to notify us of the operator change.
In the beginning of 1944 the system was working as a matter of routine and S.I.S. decided to investigate our methods. They were sceptical of the possibility of actually identifying any one operator and arranged a trial by which S.O.E. were to attempt to recognise one special operator who began the test transmission and was subsequently replaced at regular intervals by other operators. The test proved the complete success of the Tina method, since S.O.E. were not only able to identify the special operator but were able in addition to state how many other operators had been employed and also to give a timetable for each of the operators used on the circuit. Since this test S.I.S. have themselves adopted our methods in their entirety.
During the past year the Tina service has proved invaluable in the following ways:-
1. It has proved that all our plans are working either with their original operators or has shown us when enemy operators take over the circuits.
2. It has enabled us to recognise and given us a good basis for comment on the pupils recruited in the Field.
3. It has kept alive several of our operators who have been taken by the Gestapo, since the agent has no obligation to keep secret the fact that our operators are checked by this method.
A recent example of the utility of the Section was shown in current operations when messages received raised doubts as to the identity of the operator working the Jedburgh circuit GEORGE. It was possible by the "Finger Print" records to prove that the original operator was still working the plan.
During the past year over 500 operators have been "Finger Printed" and the service has made regular comparisons on the basis of once per fortnight, and whilst the more spectacular results have been when the Held Operator has been replaced, the steady routine work has proved of inestimable value to the SIGNALS DIRECTORATE and the COUNTRY SECTIONS by confirming the other security checks.
[Source: TNA HS 7/34, transcribed by www.arcre.com]