T-37 ICA
Fjernskriver (Teletypewriter)

by Bjarne Carlsen
SSGT, Royal Danish Air Force  (Ret'd.)

T-37 ICA or the other keytape driven on-line systems never had a system name associated with them.   (Photo by Henrik Teller)
This machine was manufactured by Siemens-Halske of Germany and came in two variants: The T-37 was a 50 baud regular teletype,  while the T-37 ICA was the on-line encryption teletype also running at 50 baud.

A tape reader, to the right of the machine,  was also made by Siemens and was used with some older model T-37's that did not have an integrated tape reader and with older-model T37 ICA's that had no clear text tape reader. It connected to the back of the T-37/T-37 ICA by a cable.

The T-37 ICA uses a 5 level Baudot clear text tape and one-time keytape. The machine in the picture is shown with a blank tape reel in the keytape reel position. On the left side of the machine is the tape punch; the reel should actually be sitting in that position, while the keytape, which came already mounted on plastic reels, should sit in the right hand position. On the left of the machine is the operator's panel with white and red ON and OFF pushbuttons in the top row, and green and red "Cipher" and "Clear" buttons in the bottom row. In addition to lights in the pushbuttons, there are two big lamps - a green "Cipher" lamp and a red "Clear" lamp on the front of the machine.

The T-37 ICA worked just like a normal teletype in "clear" mode and the keytape reader was disengaged. When the green "Cipher" button was pressed, the keytape reader and clear text tape reader, simultaneously read the two inputs which were combined ( exclusive OR'ed) to produce the enciphered output. The keytape reader was also connected to the keyboard in "cipher" mode, so a real-time, secure message exchange could take place across the teletype circuit. This meant that pressing a key on the keyboard while receiving or transmitting a taped message was a definite security violation and it was one sure way of throwing the machine out of synchronization.

The machine itself was unclassified.  In fact, a major Danish supplier of garden seeds also used it to secure their TELEX circuits!

It was quite another matter with the keytapes. These were classified NATO SECRET/CRYPTO, and all the usual precautions, such as the two-man rule, taking full inventories and signing in and out had to be observed when getting new tapes from the crypto vault.

To operate the machine, the operators at either end of the circuit agreed on a setting point on the keytape,  always ensuring that no keytape segment was used twice. This was usually done by tearing the used keytape off as close to the lid of the tape reader as possible after each transmission. (These points were marked and sequentially numbered on the keytape). When the setup was completed, the operators pressed the green cipher button on the left-hand panel and secure communication was now possible.

Most machines were left in the secure setting all day and setup was only performed if synchronization was lost, or in the case of a special transmission. On a few circuits, setup had to be performed at everytransmission because the operator at that location did not work in a cryptosecure environment therefore the keytape from the machine had to be removed and put it in the crypto vault between transmissions.

Normally, teletype machines are loud when they operate, but the T-37 was the loudest of its class and we had 15 of them. Working in the tape relay center when traffic was heavy, especially during big exercises, was like standing in a steel barrel with a couple of machine guns firing at it continuously - it's a wonder we did not all go deaf.

The T-37 ICA was approved for all classifications. In fact, we used to joke that the T-37 ICA's were more secure than our KL-7's because barring keytape compromise or reuse, the T-37 ICA's were absolutely secure.

In spite of the machine's classification level, the duty operators were only required to be cleared for NATO SECRET. The procedure for transmitting or receiving messages with higher classification was to send 20 bells, making the machine sound like an alarm clock gone crazy, then asking the other end for an operator with the proper clearance: "DE RDFQ I HAVE A TS FOR YOU", (in the case of a TOP SECRET message). When the other end acknowledged: "DE RDFP SEND YOUR TS", a new keytape setup was achieved, (whether the circuit already was secure or not), and the message wastransmitted.

In the tape relay center, the machines stood on foam mats, each with a hole cut out in it to take a 100 gram high explosive charge for destroying the machine in case of the bunker being overrun or being attacked. When these charges were put in and wired up during exercises, the mood in the center changed noticeably. It was definitely not fun working in the bunker, knowing that your life depended on the personnel in the guard room on the first floor not hitting the wrong switch by mistake.

The T-37 ICA was retired in the 1980's when NICS and the corresponding Danish FIKS systems were put in place.


1) Henrik Teller <henrik_teller(at)vip.cybercity.dk>
2) Bjarne Carlsen, SSGT Royal Danish Air Force  (Ret'd.)   <bca(at)fakse-ldp.dk>

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Feb 2/06