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TST 1221
Pocket message encryptor - wanted item

The TST-1221 and the later TST-2225, were electronic pocket cipher machines, developed by Tele Security Timmann (TST) in Pöcking (Germany) in 1980. The TST-1221 was intended for offline use and required the message to be sent manually via another medium (e.g. a letter).

Front panel of the TST-1221 [1]

The TST-2225 on the other hand, had a built-in 600 ohm audio interface and could be used to send secure messages directly, via radio, or via an analogue telephone line, either direct or by using an acoustic coupler, which was connected to a 9-pin D-type socket at the top right.

The device resembles an early calculator and measures just 20 x 8 cm, and is less than 3 cm high. It has 48 keys that are organized like on a standard typewriter, and a single-line 8-character LCD display at the top left. A message should be entered first into the device's mememory that can hold up to 740 characters. A command is then issued to encrypt the text message, which is then presented on the display in groups of 5 letters, that should be written down (TST-1221). For the ciphertext, only the first 16 letters of the alphabet are used (A-P).

Despite its small size, the TST-1221 was not a cheap device. In the 1980, the price for the basic model was US$ 4,565 [2].

Cryptographic strength
The TST-1221 has a cipher period of 1080 [2], which is quite impressive. The cryptographic key, consists of three individual keys: a master key (M), an auxiliary key (A) and a message key (K). The master key has 1020 possibilities, and the auxiliary key adds another 1014 to that. Both the M-key and the A-key are entered via the keyboard and remain stored when switched off.

The messsage key (K) has 1010 possibilities and is generated automatically at the start of each message. It is displayed at the start of a session, so that it can be written down for later reference. The M-key can be used as some type of group key, whilst the A-key should be seen as a daily key. When receiving ciphertext, a command is used to covert it back into plaintext. Any words that are longer than 8 characters will have a dash (-) inserted automatically.

The software-based cyrotographic algorithm uses 6 linear shift registers, each with a different length (21, 19, 31, 41, 77 and 41). A 7th shift register addresses a lookup table that controls the non-linear stepping of the 6 basic registers. The lookup table is affected by the M-key and the A-key. In order to provide the best possible distribution, the feedback data is connected crosswise between the registers. Again, non-linear feedback is achieved by using a second lookup table, which is controlled by the M-key. The M-key also affects the presets of the registers.

Before the actual ciphering takes place, the device first runs a number of prerun or premix cycles, in order to obscure the effect of the various key variables. The algorithm that controls the premixing and the number of cycles, is affected by all three keys (M, A and K).

According to the manufacturer, the loss of a TST-1221 or TST-2225 device does not compromise security if the key is unknown or changed immediately. The actual key can not be extracted from the device. Furthermore, the contents of the key memory can be purged in case of an emergency by depressing a recessed yellow button.

The TST-2225 was nearly identical to the TST-1221. It was released a couple of years after the TST-1221 and contained a built-in audio interface with a 600 ohm 0dBm radio interface that allows it to be linked to virtually any type of radio set. The TST-2225 could also be used as a high-speed burst transmitter in order to prevent detection and interception. It uses AFSK modulation to send messages at up to 120 characters per second (approx. 1200 baud) [2].

Front panel of the TST-2225 [1]

The TST-3226 similar to the TST-1221 and the TST-2225, but has a built-in (acoustic) modem that allows messages to be sent via voice-grade circuits, such as analogue telephone lines. Message could be sent by playing them back via the built-in speaker that was held in front of the microphone of a telephone handset. This allows data to be transferred at 32 characters per second (approx. 300 baud). Much slower than the TST-2225, but still 5 times faster than using a teleprinter (telex) [2]. To some extent, this unit is similar to the bi-directional Philips PX-1000.

  1. Helmut 'Jim' Meyer, HS0ZHK, My way to Ham - Radio and beyond
    Website QRZ.COM. Personal correspondence. Retrieved July 2013.

  2. Louis Kruh, Cipher Equipment TST 1221
    Cryptologia, Volume 4, Issue 4, October 1980. p. 225.

  3. Jane's Military Communications 1986
    ISBN: 0-7106-0824-1. p. 509.
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Crypto Museum. Created: Monday 15 July 2013. Last changed: Saturday, 10 March 2018 - 09:23 CET.
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