Ciphering equipment used by the Swiss Army

Gilbert Vernam, an engineer of the American AT&T, developed the idea to encrypt a teletype punched paper tape using a second "key tape". Both punched tapes were combined together adding the elements "hole" and "no hole" corresponding to "1" and "0", the sum is used as the product of the encryption. When the punched tape generated at the receiving site is combined with the "key tape", a tape with the original code can be generated. This can be fed to a teletype printer and the original message can be read in plain text.
The Einton - Kombinationsschreiber ETK developed by Dr. E. Gretener AG was introduced in the year 1947. In the ETK, a unique fourteen segment code is used to compose the characters.
Together with cryptology specialists of the Federal Military Department, Dr. E. Gretener developed a ciphering machine based on the Vernam principle. In contrast to the original system using a "key tape", a sequence of numbers generated by a ciphering machine is used to be combined with the ETK printer code. So no physical punched paper tape had to be transmitted to the receiver site, a second ciphering machine using the same setting of controls (decryption key) can be used to decode the message - as long as both machines operate with all steps synchroneously. If the key numbers generated of both ciphering machines loose synchronisation (even one missing step), the machine at the reception site has to be set ahead the missing number of steps. When both machines are at the same step number, decoding can continue.

The Telecrypto 53 set is inserted between a transmitting ETK set and a telephone landline; at the receiving site, the pulses coming from the receiver are fed to the ciphering machine which will generate the signal to drive the ETK printer. This arrangement can be used for real time encryption.

The key consists of 12 characters and a number code with four out of eight numbers. The four permutation wheels at the left side and the advancement wheels at the right side of the key display window are moved electrically to set the key to start decryption. According to the number key, the switches 1 - 8 are set to the correct position.
When the key is set to the start position, it is ready to transmit or receive. If the machines get out of synchronisation, which will rarely happen when landlines are used and will happen quizte frequently on wireless connections with fading, the machine at the receiving side has to set one or more steps ahead, to be in synchronicity again, th estep counter has to be used for this purpose.
The situation improved slightly with the improved teleprinter ETK-R (Radio) Model 55, which is less sensitive to transmission failures due to improved filtering.

The SPG (Schlüsselproduktionsgerät) is nothing else then a mechanical password generator. It is used to generate a true random message key, so that the operators did not use SUN, RAIN or the name of the unit commander every day. Similar password generators are used in the computer era, as man - made passwords are usually easy to crack with a "dictionnary attack".

The Telecrypto 53 has been produced by Dr. E. Gretener AG located at Zurich.

The Telecrypto 53 together with the printer ETK-R 55 has been in use over landlines with the signal troops, it turned out to be quite unreliable for shortwave communications and has been replaced by the KFF 58 for this purpose. In 1983, the ciphering machines and teleprinters have been withdrawn from troop use and most of them have been destroyed as the had been classified as secret equipment.

Telekryptogerät 53

Gretag, Dr. Edgar Gretener AG, Zürich