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Hagelin T-52 and TC-52   Telecrypto
Hybrid online/offline cipher machine

The T-52 or TC-52 1 was an online/offline cipher machine for teleprinter circuits, developed by Boris Hagelin at AB Cryptoteknik in Stockholm (Sweden) around 1952, and manufactured from 1953 onwards at Crypto AG in Zug (Switzerland) [1]. The machine is a hybrid between a C-38 mechanical pin-wheel cipher machine and a One-Time Tape (OTT) cipher machine (mixer).

The TC-52 was intended for online and offline encryption of 5-bit (baudot) teleprinter signals (telex). As such, the mechanics of the machine had to be fast enough to keep up with the common telex speed of approx. 50 baud.

For secure data exchange, the TC-52 had two paper tape readers on top of its body. One reader would contain the clear text, whilst the other one contained the cipher tape. When the cipher tape was a truely randomly created data stream, the cipher would be unbreakable, as the key tape acts as a One-Time Pad (OTP).
Hagelin TC-52

Such systems are commonly called mixer machines and are based on the Vernam Cipher. The key stream is 'mixed' with the cleartext stream by means of a digital XOR operation (modulo-2 addition). The resulting ciphertext is then sent to the other end via telephone lines or by radio. By applying the same cipher stream again at the other end, the original clear text is revealed.

Unlike other OTT systems however, the TC-52 also contained a standard Hagelin mechanical wheel-based cipher machine that was converted for online encryption of the electric data signals. This additional cipher machine was based on the M209 that was also used by the American Army during WWII. It could be used as a fall-back method in case all OTT tape had ran out. The TC-52 is based on the developments of Telekrypto-Gerät 35, a cipher machine that was co-developed with Dr. Edgar Gretener (Gretag) between 1949 and 1951 for use with 14-bit ETK-47 teleprinters.

  1. The difference between T-52 and TC-52 is currently unknown. In his biography, Hagelin only talks about T-52 and the later T-55, but on the actual machines they are designated as TC-52 and TC-55. On this page, we will be using the TC-52 designator, in order to avoid confusion with the Siemens T-52 Geheimschreiber.

The TC-52 in its transit case The TC-52 after opening the transit case Hagelin TC-52 The two tape readers on top of the machine Controls on the front panel The interior of the TC-52, after the top lid has been removed. Close-up of the pin-and-lug machine inside the TC-52 Looking at the contacts of a scrambler plug

Hybrid cipher machine
As mentioned in the introduction on this page, the TC-52 is in fact a combination of two cipher machines in one case. In theory, it is 'just' a mixer machine that works according to the Vernam Cipher principle. The letters of the cleartext message are 'mixed' with the contents of a paper tape that contains fully random symbols. With the right precoutions, this cipher is unbreakable. This situation is given in the simplified block diagram below.

Simplified block diagram of the TC-52 in transmit-mode
If no random key tape is available, it is possible to 'fall back' to a less secure pseudo-random key generator, based on a mechanical Hagelin cipher machine, similar to the M-209 (bottom right). The mechanical movements of the wheel based M-209 pin/lug machine, is converted into a 5-bit digital code, similar to that generated by the tape reader.

On the input side, the user can choose between typing the text directly on the teletype (on-line) or sending a previously prepaired paper tape (off-line). The latter is more secure as it avoids making mistakes and does not reveal the typing habits of the operator (Traffic Analysis).

On-line and off-line use
Contrary to the earlier T-52, The TC-52 can be used for the transmission of both on-line messages and messages that have been prepaired off-line. When used as an on-line ciphering machine, a teletype machine (Telex) is connected directly to the TC-52. When in transmission mode (TX), the message is typed on the teleprinter and the TC-52 will process the data, encipher it and transmit it on the line.

In off-line mode, the the message is first prepared on a (separate) teletype machine and stored on a punched paper tape. The pre-recorded message is then entered into the rightmost tape reader on top of the TC-52, after which the user presses the Start button.

The image on the right shows the two tape readers. The rightmost one is used for the plaintext tape. The leftmost one was used for the OTT key tape. Off-line operation was not possible with the earlier T-52 as it lacked the rightmost tape reader.
The two tape readers on top of the machine

The image above reveals that this machine was probably used in Denmark, as the text below the two tape reader is in Danish. The text label is bolted onto the cover of the machine with 6 tiny little screws, allowing the machine to be adapted for different countries. Removing the text label reveals the original French text, as can be seen in image #5 below.

The two tape readers on top of the machine The tape readers after opening the lid Opening the lid over the paper readers The two tape readers on top of the machine Close-up of the two paper-tape readers, with French text plates Close-up of the two tape readers. The leftmost one is opened, ready to accept a tape. Close-up of the two tape readers. The leftmost one is opened, ready to accept a tape. Closing the tape reader again

Removing the top lid of the TC-52 reveals a true marvel of electro-mechanical engineering. Considering the (small) size of the machine and the fact that the basics of this machine were developed in 1951, Hagelin must have been way ahead of his time.

The image on the right shows the interior of the TC-52 after the top cover has been removed. At the centre of the machine are the two 5-bit tape readers. In front of that is a more or less standard mechanical Hagelin cipher machine (see below).

At the left are two large black contact cylinders, used for the conversion of a mechanical position into a 5-bit digital code. At the rear right is a notched axle that acts as a data serialiser for the main output. The driving motor is at the rear.
Close-up of the interior, seen from the front right of the machine.

Other parts, such as the clever XOR circuit, are hidden in the bottom compartment of the machine, together with some other relays and contacts. All parts of the machine are covered in detail in the machine's operating instructions [2] .

The interior of the TC-52, after the top lid has been removed. Close-up of the interior, seen from the front right of the machine. Another view of the interior, seen from the right. The interior of the TC-52, seen from the front left. The interior of the TC-52, seen from the left. The interior of the TC-52, seen from the right. 5-bits encoder/scrambler Two valves and three capacitors

Electro-mechanical XOR circuit
At the heart of every mixer-machine is an XOR circuit. The output of the XOR circuit is the logical exclusive OR (modulo-2 addition) of the two input signals. In modern electronics, this is easily achieved by using a small electronic Integrated Circuit (IC) or, even more modern, purely in software. Hagelin however, solved this problem with just two electro-mechanical relays.

In the TC-52, five XOR circuits are used, as there are 5 digital bits to make up the 32 possible characters of the International Telegraph Alphabet (25 = 32). The five XOR circuits are mounted at the bottom side of the machine, spread over the left hand side.

The two relays of a single XOR unit also act as a memory store. When reading the incoming serial data from a teletype machine, each bit is stored in a single relay. The other relay is then loaded with the corresponding bit from the key tape, after which the XOR operation is read out.
XOR circuit with relais

A spare XOR unit is included with the spare parts in a special compartment inside the storage case. The image on the right shows the spare XOR unit. The two relays are mounted, together with two resistors, on a black connector, so that they can be exchanged easily without the need for soldering. Repair of the XOR units was done in the field.

Spare XOR circuit XOR circuit with relais XOR circuit with relais XOR circuit with relais

Pseudo Random Number Generator (PRNG)
If no secure (random) cipher tape was available, the TC-52 could 'fall-back' to a less secure method of encryption. In that case, the key stream was generated by a so-called Pseudo Random Number Generator (PRNG). Like everything else in the TC-52, Hagelin solved this problem mechanically by using parts of his well-proven mechanical cipher machines.

At the heart of the PRNG is a common Hagelin Pin & Lug machine, similar to the M-209 and the C-38, complete with the cipher wheels (and pins), the drum with the movable lugs and the coupling cog-wheels.

The image on the right shows the pin-wheel machine that is situated at the front/centre of the TC-52. The resemblance with the well-known mechanical Hagelin cipher machines is striking. Each of the 6 cipher wheels can engage a contact that is mounted in front of it, behind a plexglass cover. The red line is the index mark.
Close-up of the pin-and-lug machine inside the TC-52

Contrary to the M-209, that had 27 bars in its drum, the drum of the TC-52 contains 24 bars. For each encoded character, the drum makes just half a revolution, using only 12 of the 24 bars. This was done because of the mechanical speed limitations when encoding characters at 50 baud (7 characters per second). The other half of the drum was used for the next character, etc. This divided the drum in an even and odd half, which was one of the internal key-settings. The lugs of the bars would only be reset after a full revolution.

As a result of the movement of the cipher wheels and the drum, the output disc (axle) can be at 32 different positions (rather than just 26 on the M-209). This is because the digital 5-bit telegraph alphabet allows 32 different characters. The output axle of the PRNG is connected directly to the arm of a contact cylinder, similar to the stepping switch of an old fashioned telephone exchange.

Inside the contact cylinder are five contact arms that each represent a digital bit. When rotating, they scan the contacts of 32 interchangeable plugs, each of which represents one of the 32 possible bit combinations.

The image on the right shows the front contact cylinder, that is mounted directly to the left of the cipher wheels. Rather than just converting a character into a 5-bit digital code, the switch also acts as a scrambler as all 32 character codes are interchangeable. This was also part of the key-settings (images #2 and #3 below).
5-bits encoder/scrambler

In order to compensate for the lower number of active bars in the drum (12 instead of 27), a second contact cylinder (scrambler) has been added. It is mounted directly behind the primary scrambler (image #4 below). Operation is further explained in this simplified diagram:

Simplified diagram of the 'Converter' and scramblers
To make matters even more complicated, the operation of the rear cylinder is inverted. In other words: the contact arm of the front cylinder is moved by a displaced bar in the drum, whilst the contact arm of the rear cylinder is activated by an undisplaced bar. The total number of steps (of both cylinders added together) is always 12.

As a result, the machine produces two key-series and we have to choose between them. This is done by means of a change-over switch that is activated by one of the six key wheels. The operating point of the keywheel that is used, is displaced about 90° from the operating point of the bar-shifting lever. The absense of a pin selects the front cylinders, whilst the presence of a pin selects the rear (inverted) cylinder.

A movable plug at the front of the machine is used for selection of the wheel that is used for switching between the two cylinders. It can be inserted (and fixed) in only one of the six possible positions at any given time. The position of the change-over plug was part of the key-settings.

The contact cylinder to the left of the cipher wheels Removing one of the plugs Looking at the contacts of a scrambler plug Clear view of both contact cylinders (scramblers) Close-up of the alphabet disc that was used for settings the start position of the rear contact cylinder (scrambler). The font panel of the TC-52 showing the position of the change-over selection plug Removing the plug that selects the wheel that activated the change-over contact The selector plug removed from the the socket. It can be inserted into any of 6 positions.

Setting the key
When using the internal PRNG (i.e. the mechanical Converter machine), there are two sets of key elements that should remain secret. These are the internal and external key settings.

Internal keys
  • The positions of the 2 x 32 five-contact plugs on the two contact cylinders.
  • The positions of the lugs on the 24 bars of the drum.
  • The positions of the pins in the 6 key wheels.
External keys
  • The starting positions of the 6 key wheels.
  • The starting position of the drum (even or odd half turn).
  • The starting position of the selector scanning arms of the contact cylinders.
  • The position of the change-over contact plug.

Below is a quick overview of the various controls on the front panel of the TC-52. The two main power switches are on the right. The rightmost one is the mains power switch and the one next to it allows the motor to be switched off when pausing transmission.

At the centre are three lever-operated switches, similar to the switches of an old-fashioned telephone. These switches are used to control the mode of operation of the TC-52. The counter is used for counting the number of transmitted characters. The big knob at the far left can be used to advance the drum by half a cycle (when in receive mode), so that the correct starting position (even or odd half of the drum) can be selected.

More information about the operation of the TC-52 can be found in the original instruction manual that was supplied with the machine. As this manual was stencilled with rather poor quality ink on thin paper, it is nearly impossible to make high-quality scans. We have therefore made a complete transcript of the manual, which is available for download below.

The TC-52 was supplied in a sturdy black transit case, or sometimes in a so-called letherette. The black sturdy case allows the machine to be transported safely, and prevents it from getting damaged. The machine is seated on a horizontal base plate and is held in place by a rubber frame. The frame itself rests on a rubber shock-mount.

After opening the case, the machine is easily accessible, as can be seen in the picture on the right. For operation, the machine must be taken out of the case, whilst the base plate remains attached to the bottom of the machine.

The base plate is attached to the case by means of a short black ribbon, so that it can not be separated from the case. The machine is either operated whilst standing on the base plate, or is released from the rubber frame prior to being installed. The ribbon is visible in images #6 and #7 below.
The TC-52 after opening the transit case

The cables and spare parts for the TC-52 are stored in the bottom of the transit case, below the machine. The cables are wound on a set of spools and are held in place by a couple of straps. The spare parts are stored inside a long rectangular box, as can be seen in images #7 and #8 below. This includes a spare XOR unit and some carbon brushes.

The TC-52 in its transit case The TC-52 in its transit case The TC-52 in its transit case The TC-52 after opening the transit case The transit case with the machine's mounting base still in it The transit case after taking out the machine's mounting base The spare parts cabinet hidden at the bottom of the storage case Contents of the spare parts cabinet

The TC-52 connects to the outside world by means of only two cables: a standard power cable, that connects the machine to the mains, and a very special data cable, that is used for connection to a teleprinter and the transmission line (radio or telex).

Connection to the mains is rather straight forward. Depending on the version, the TC-52 can be connected to a range of different mains voltages. When connecting a unit, always check the position of the power selector first. it is located at the rear of the machine, close to the motor (check the rightmost images below).

The supplied (rubber) cable has a 'standard' Hagelin mains plug on one end (see image #3 below). This plug should be entered into the mains socket at the rear of the TC-52. The other end should have a local mains plug.
The data cable that connects to the teleprinter and the telex line

Connection to a teleprinter and the teletype exchange (telex) is slightly more complicated. They are both connected via a single cable that should be supplied with the machine. The cable has a 5-way Tuchel connector at one end, that plugs into the interface socket at the rear of the TC-52. Tuchel connectors are extremely rare these days so, if you have one, hang on to it!

The cables are stored inside a compartment at the bottom of the transit case Mains power cable Mains connector (machine-end) The data cable that connects to the teleprinter and the telex line Close-up of the 5-pin Tuchel connector of the data cable Mains voltage selector Taking the voltage selector out Changing for 110V operation

The data cable with the Tuchel connector has 5 coloured wires:

Colour Polarity Direction Description
Red + IN Teleprinter
Grey - IN Teleprinter
White + OUT Transmission line (telex)
Black - OUT Transmission line (telex)
Yellow - DC Ground

Wiring of the Tuchel connector when looking into the socket of the TC-52
The TC-52 is intended for connection to teleprinter equipment which has the magnet and the sending contact connected in series, and 40 mA single current operation. Power for this is supplied by the TC-52. The transmission line should also be 40 mA single current operation, where power is supplied by the external party (i.e. the transmission line).

Other Hagelin on-line Telecrypto machines
  • T-52
  • T-55
  • TC-55
  • TL-520
  • TMX-53
  • CBX-53
  1. Crypto AG. Crypto Magazine 2009, number 1.
    p. 12: Crypto AG's family tree.
    p. 14: T-52 and T-55 Telex-based cipher machines.

  2. Crypto AG/Crypto Museum, Operating Instructions for Telecrypto TC-52
    Original document: November 1955. Transcribed 9 August 2010.
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