Secrecy in Wireless Telegraphy.

Progress, Volume VI, Issue 9, 1 July 1911, Page 717

Secrecy in Wireless Telegraphy.

(Translated for Progress.) As is well known, the waves from a transmission station travel in exactly the same way as the waves caused by a stone dropped into water, i.e., in circles. In many cases this is an advantage, e.g., in communicating standard time and in the case of ships sending out signals of distress, so any station within signalling area ’ could receive the message. For business telegrams, however, and diplomatic correspondence, it is preferable that secrecy

could be obtained if desired;' and for communications in time of war, this is practically imperative. This fact was recognised soon after Marconi had made his discovery, but till quite 'recently no thoroughly satisfactory solution of the difficulty had been arrived at. The invention of “directed” wireless telegraphy proved a step in the right direction, for it meant that the course of the waves was under control to a certain extent. Even then, however, the waves were several miles broad, and so the message could be received by any station . within that area, which happened to be in tune with the transmission station.

Good results were looked for from the introduction of the “undamped oscillation” system, by means of which the receipt of a message was prevented, if there was the slightest discordance between the two stations. Operators working on this system agree upon a definite frequency to be observed in sending the message, and stations at which the wires are not set for exactly the same frequency, cannot receive the message,- in fact, as a rule, the waves do not affect them at all. Apparatus has since been invented, however, by means of which the time of the wires can be changed quickly and easily, until the tune of the transmission station has been found. This done, any unauthorised station can receive the message.

Attempts have been made to prevent such a proceeding. The two stations agree upon, not a single frequency, but a series of different frequencies, which follow one another in a prearranged order. Before an unauthorised station has discovered the first frequency, the transmitting station has passed on to another. This system is, however, not sufficiently reliable, for if the two stations did not set their wires in time for the different frequencies at the exact instant, the message would be unintelligible.

A comparatively simple solution lies in the adoption of a cipher system. Such a system has in many cases proved' successful, but is open to one serious objection. The transcribing of the message from the cipher requires a considerable time, and such a delay might be disastrous in time of Avar, besides involving possibilities of error. Captain Hovland, of the Norwegian Marine Department, set himself the task of inventing a system, which should be as reliable as the cipher system, and yet enable a message to be received as quickly as if sent by the ordinary method of transmission. Results prove that he has been successful. The generator of his apparatus resembles a typewriter, having marked keys corresponding to the letters of the alphabet. Every time a key is pressed, a series of contacts is produced, which gives rise to the current and which transmits the required letter.

The characteristic feature of Hovland’s system is that his apparatus admits of rearrangement, and by means of a special piece of mechanism, called a cryptograph, which consists of several number discs, after the style of a combination lock, a large number of cipher combinations is provided. The effect of the rearrangement of the apparatus is that, instead of the actual contacts corresponding to the individual letters, certain other contacts are made, in accordance with the chosen combination. The receiving station must

set its cryptograph for the same combination, and then the contacts received are automatically changed into the original corresponding letters. An example will make this clearer. Two stations agree to work with the combination 3-724-156. Both set their cryptographs at this number, and the transmitting station, to telegraph the word Berlin, for example, presses one after another the keys BERLIN, By reason of the rearrangement of the apparatus, however, quite different letters will be telegraphed, perhaps SRKH RN. Unauthorised stations could receive these letters, but could make nothing of them. In fact, even of the stations which have Hovland apparatus, only those could translate the message which knew the actual combination used, or which discovered it by trial, and that is practically impossible where there are 720 possible combinations. The apparatus at the receiving station, however, will automatically change the letters according to the combination, and the word “Berlin” will be read.

In the latest machines, as in typewriters, the letters are written on paper, so that the telegram can be read immediately. This accomplishment is all the more remarkable, as exact synchronization of the apparatus at both stations is imperative. It is well known, too, that Charles Y. spent a lot of time and trouble in vain, in trying to make two watches in one and the same room work absolutely regularly. Hovland, however, has succeeded in regulating the action of the two stations. At each of them there is a large revolving cylinder, fitted with contact pieces, which at each revolution make and break the current. Slight irregularities in the working of the apparatus are unavoidable, but by this means they are prevented from becoming greater, as they are reduced to a negligible quantity at each revolution of the discs.

Hovland’s system can be used in ordinary telegraphy, a fact which is certainly of secondary importance, though in time of war it might prove to be of great value. —‘ ‘ Frankfurter Zeitung. ’ J