No key was as keen on Stichworts as Shark which had a Stichwort permanently in force as a routine security measure. At first the Stichwort was changed comparatively rarely but by the end of 1943 it rarely remained unchanged for more than a fortnight. The series of Stichwort with code name Andromeda followed by a number, which never exceeded 24, made use of column 412 of the K book where the trigram indicated by the number was used as the additive - first letter for wheels, second for clips, third for Stecker. The Bellatrix series of Stichworts had a four figure number and could be solved by looking up the number's log-tangent in 5 figure tables and taking the last 3 digits as the additive. In early 1944 there was great nervousness about compromise and a series of messages were sent out to individual boats which could only be decyphered by the boats themselves as message settings were derived in a complicated fashion from the initials of relatives of members of the crew and the number of the house they lived in. These "Captain's wife" messages were broken and merely gave instructions for reversing the Stichwort procedure in certain respects and re-christening it Bellatrix Alpha.
It was a constant illusion of the German Security Service that the security of the Stichwort was increased by increasing the difficulty of working out the additive. The final Narwhal Stichwort was the supreme example of this and I reprint the description of it from Communications Intelligence No. 683:
"Among the cyphers handed over at Flensburg were copies of the 'Stichwortbefehl Castor zum Schluessel M Niobe' and of the 'Ausfuehrungsbestimmungen' describing the procedure. A pencilled note on one of the envelopes stated that the 'Stichwortbefehl' came into force on 3.4.45.
"The calculation of the daily settings as altered by this 'Stichwortbefehl' is very complicated. The characteristic number ('Bezeichnungszahl'), varied with each announcement, is 6-figure; the first figure, subtracted from the second and fifth, gives a 5-figure group read as the day of the year and the hour of the day to be looked up in the tables of the moon's ascension in the valid 'Nautisches Jahrbuch', from which a further 6-figure number ('Aenderungswert') is obtained. For example, 'Stichwortbefehl Castor 578555' gives the 285th day of the year and the 5th hour of the day, for which the 'Aenderungswert' is 173237.
"The last 4-figures of the 'Aenderungswert' are then applied to the wheel-order and 'Stecker' values of the printed settings to give an intermediate set of values ('Zwischenwert'), which are used in conjunction with the 'Aenderungswert' to build an irregularly shaped cage of 34 squares ('Kammwuerfel'). Into this cage are entered horizontally the intermediate values and