The TC-52 was intended for online and offline encryption of
5-bit (baudot) teleprinter signals
(telex). This means that 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 can be seen as a
One-Time Pad (OTP).
The data from the One-Time Pad
(i.e. the key tape) is mixed with the
plaintext by means of the so-called Vernam Cipher.
For this reason, the TC-52 is also known as a
One-Time Tape machine,
or a mixer. Mixing is achieved by applying the binary
XOR (exclusive-OR) operation – also known as modulo-2 addition — to the data.
By applying the same operation again at the receiving end,
the original plaintext can be revealed. When correctly implemented,
this method is unbreakable.
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
It could be used as a fall-back method in case all OTT tape had run out.
The TC-52 is based on the developments of
a cipher machine that was co-developed with
Dr. Edgar Gretener (Gretag)
between 1949 and 1951 for use with
14-bit ETK-47 teleprinters.
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.
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 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 six small screws, allowing the machine
to be adapted for different countries.
Removing the text labels of the machine featured here,
reveals the original French text.
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
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.
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 .
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
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.
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.
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.
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
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
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
and #3 below).
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
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.
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.
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.
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
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 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 hinged box.
This includes a spare XOR unit and some carbon brushes.
The TC-52 connects to the outside world by means of just two cables:
a standard power cable — it connects the machine to the AC mains — and a
special data cable, that connects the TC-52 to a
and to the transmission line, which can be a radio or the teleprinter exchange
Connection to the AC 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.
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.
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.
This kind of Tuchel connectors are extremely rare these days so,
if you have one, hang on to it!
The data cable with the Tuchel connector has 5 coloured wires:
Transmission line (telex)
Transmission line (telex)
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. The necessary power for this is supplied by the TC-52.
The transmission line should also be 40 mA single current operation,
but the power should be supplied by the external party (i.e. the transmission line).