As mentioned earlier, the traffic on the German hired telecommunication lines through Sweden was intercepted. It was quickly discovered that apart from plaintext, encrypted text was also transmitted. When Beurling found how the Geheimschreiber functioned, Swedish technicians under the leadership of Viggo Bergström started to construct special machines for decryption after directions from Beurling.[7] These machines very soon made it possible to follow the frequent changes in cipher keys and subsequently quickly extract the plaintext. The machines were later built in quite large numbers in L.M. Ericson's workshop for precision mechanics.
Photo 3 - Decryption unit for German line traffic, ex. ``APP''.
The German Geheimschreiber traffic was broken and deciphered from June 1940 until May 1943. This went on even when new models were introduced and the key procedures were gradually changed.
The intercepts came from a number of teleprinters in a room in Karlaplan 4 which were connected to the different lines between Germany - Norway, Sweden and Finland. The machines, which were very noisy, were supervised 24 hours a day. The texts came out in a never-ending stream of paper tapes, which were then glued on to big sheets of paper.
The daily routine was the following: In the morning (after 9 o'clock, when the daily key was changed) the cryptanalysts examined the incoming traffic, waiting for a case with sufficiently many ``parallel texts'' to occur. As soon as possible, the cipher key extraction (``avvecklingen'') took place. When the new key settings were produced they were given to the staff who worked with the deciphering machines, and the deciphering of the day's harvest could start. Subsequently the plaintexts were cleaned up and typed. They were later given to the various consumers of intelligence.
Photo 4 - The decryption units, ``APPARNA'', at their working
places.Photo 5 - In the machine room. (Room with teleprinter-receiving machines connected to the intercepted lines.)
As mentioned above, two types of traffic were observed and studied. The most extensive were the military traffic and the traffic between Berlin and the embassy in Stockholm. The diplomatic traffic had the highest priority, as this concerned Swedish-German relations. At least two key settings therefore had to be determined every day.
At first, the teleprinters used in Karlaplan 4 were machines from the American firm Teletype. Teleprinters were, however, in short supply as importing them was difficult during the war. However, the Royal Telecommunication Administration (Kgl. Telegrafverket) were persuaded to surrender a number of their teleprinters, which resulted in a return to Morse telegraphy on some lines. When the crypto department later succeeded in obtaining a batch of Siemens teleprinters, these replaced the Teletype machines, which was certainly reasonable considering their use.
As mentioned earlier, when newer versions of the Geheimschreiber were later introduced, attachment units were constructed and connected to some of the deciphering machines that were used for the C model traffic. For the Z-traffic only one deciphering machine was built.[8]
Large quantities of messages were decrypted and distributed.
Number of distributed messages.
| Year | Encrypted | Unspecified | Unencrypted | Total |
| 1940 | 7100 | 7100 | ||
| 1941 | 41400 | 41400 | ||
| 1942 | 101000 | 19800 | 120800 | |
| 1943 | 86600 | 13000 | 99600 | |
| 1944 | 29000 | 29000 | ||
| Total | 187600 | 77500 | 32800 | 297900 |
The highest number of messages distributed in one day (October 1943): 678.
This extensive traffic resulted in an increase in the number of staff. The number of teleprinters and deciphering machines also increased until they reached a total of 32.
The decryption of the Geheimschreiber traffic developed gradually into a real industry needing a lot of people. In 1941 the staff increased to 500 people and later on it became even bigger from time to time.
Photo 6 - View from the rack room. (Room with equipment racks where the intercepted telecommunication lines entered.)Photo 7 - Another view from the rack room.
The breaking of the Geheimschreiber was a large contributory factor to the establishment of FRA (formed from the Defence Staff's signals and crypto departments) as an independent authority on 1 July 1942.
In May 1943 the keying principle was changed with the result that further deciphering became impossible. A small group stayed on to handle those messages that for different reasons had not been deciphered earlier.