The first event in the Shark world was the reusing in December of November keys; this did not happen on all Shark but only for traffic to and from the U Boat authorities in the Biscay ports who had run out of keys. We had hoped that this would produce reencodements into Shark proper but we hoped in vain.
The second and far more important event was the introduction of a new Kurier procedure. Kurier, known to us by the cover name Squash, had first been mentioned in Enigma on June 22nd and was at this time still in the experimental stage. At first it was simply a technical development, not directly connected with cryptography, a method of ultra high speed transmission taking place on a frequency deviating by a given amount from that on which normal traffic was being sent. Until December 9th this deviation frequency was fixed in a daily Shark signal but of the 9th a new system came into force. The frequency deviation was now designated by a bigram; this bigram was set down twice and 4 dummy letters were added anywhere provided that the repeated bigrams were not separated; the resulting 8 letters were encyphered on the machine and transmitted in 2 four letter groups which changed completely every hour. Two of these messages were broken on the assumption that they gave the same bigram and it was then simple to spot the indicating system; the message set-up was taken from the K book, one column of which was allotted to each day: the 24 trigrams in the column had the last letter doubled and the resulting tetragrams were used successively for each hour of the day. Actual Kurier transmissions were encyphered in the same way and used the normal short Signal book.
We were now in a position whenever current to supply the bigram giving the frequency deviation but the bigram table itself and the deviation equivalents were unknown. Arrangements were made to put special pressure on Shark when there was hope of currency with a view to trying out various suggested tables but the table was never in fact reconstructed.
Though Squash never came into general operational use it had considerable importance as a method of breaking Shark. Breaking a paired day on Squash was a comparatively simple matter and led to a considerable increase of the number of hours during which we were current. Study of the shift system and operators' characteristics also enabled us on one occasion to break a first day of a pair but there was not normally sufficient information to make this practicable.
Squash transmissions stopped on February 2nd when the apparatus was removed from the Berlin area. The apparatus was eventually reinstalled else where about 2 days before the end of the war but no further transmissions were made.
December's new key was Bounce which was used in the N. Italian area by authorities similar to those using Bonito. For some time there had been a small number of messages in 4 letter groups apparently on the Bonito services and which had failed to come out. It soon