|
|
|
|
|
|
Like the Enigma machine, the
Fialka uses a reflector (UKW) in order to achieve
reciprocity (reversibility). The reflector connects
pairs of wires together. When a current hits one contact,
it will be reflected back into the drum via the other one
and vice versa. Many people have tried to measure the
loops in the Fialka's reflector, but to no avail.
Of the 30 contacts, 26 are connected together in pairs,
just like expected, but there are 4 'special' wires that
seem to go nowhere.
|
For a long time it has been unknown what the 4 'special'
wires of the reflector were doing. Unlike the other 26 wires,
they are not connected to any of the other wires.
Instead they are all directed to the so-called Magic Circuit
that is located inside the large red 'blob' to the right of the drum.
Inside the MODE switch assemby is a circuit board with 3 transistors
and 7 diodes and it took us a little while to uderstand their operation.
It is in fact a smal circuit, consisting of 3 transistors and 6 diodes,
plus a single diode. The single diode is not connected to the other
circuit at all. All 4 'special' wires from the reflector go to this
circuit board.
|
|
| |
|
Magic circuit inside the MODE switch
|
|
One wire (13) is connected only to a diode that is in turn connected
to the mechanical 5-bit plain-text encoder (under the keyboard).
This line is used to override the coded letter and replace it with
the original letter. When the current (through the drum) reaches
pin 13 of the reflector, no signal is returned and the plain-text
letter is used instead.
This results in a 1:30 chance for a letter to be enciphered as itself.
The problem is now that we're left with an odd number of wires,
making it impossible to create pairs. They could, of course, have
used two wires to override the diode matrix, but that would have
doubled the chance of a letter becoming itself, which was probably
regarded as a cryptologic risk. So, they came up with a clever solution.
|
The other 3 wires are connected to a transistor circuit,
that can best be described as a rotational switcher.
When a current hits pin 18 of the reflector in Coding mode (3),
it will be returned via pin 24. However, when the current hits pin 24,
it will be reflected via pin 16. Finally, if the current comes in at pin 16,
the return path will be via pin 18.
This rotational switching has the side-effect that the Fialka loses
its reciprocity.
|
|
|
However, the solution is equally simple.
Pin 16 and 24 are routed via the MODE switch and are swapped
in decoding mode (P). This results in the triangle rotating
in reverse direction, wich effectively reverses the
non-linearity of the above process.
The drawing above shows the translation sequence for both
Coding (3) and
Decoding (P) mode.
|
The circuit diagram of the Magic Circuit is given below.
Three similar circuits, based around T6, T7 and T8, are cascaded.
The output of each circuit is connected to the input of the next one, via a diode.
The output of the third stage (T8) is looped back to the input of the first stage (T6).
This results in fact in a triangular circuit. All three lines (pins 4, 5 and 6
of the Magic Circuit) can be inputs as well as outputs.
Pin 18 of the reflector is connected directly to pin 6 of the Magic Circuit.
Pins 16 and 24 of the reflector are routed via unit 8 and 9 of the MODE switch.
In plain-text mode (switch to 0) they are left unconnected. In Decoding mode
(switch to P) pin 16 of the reflector is connected to pin 5 of the Magic Circuit
and pin 24 of the reflector goes to pin 4 of the Magic Circuit. In Coding mode
(switch to 3) reflector pins 16 and 24 are swapped.
When pin 5 of the Magic Circuit is pulled low by a current from pin 18 of
the reflector, the NPN transistor T6 will conduct as it's base is pulled
high by the 560 ohm resistor. As a result, the collector of T6 will be
pulled low, which also pulls the base of T7 low. T7 will therefore not
conduct and the diode (D79) will pull the emitter down. So, pin 5 will
be low (i.e. the signal from pin 6 is 'reflected' via pin 5).
As T7 is not conducting, the base of T8 will be high,
so T8 will be conducting, resulting in a high on pin 4.
As the end of the circuit (D82) is looped back to the beginning,
the above is true for any of the three lines. In other words: a low
on line n will result in a low on line n+1 and a high on the remaining line.
Diode D83 is not connected to the rest of the circuit.
It is probably only part of the Magic Circuit for convience of wiring.
The cathode of D83 is connected to pin 13 of the reflector.
When this line is driven low, it pulls the plain-text enable line
of the mechanical 5-bit encoder (under the keyboard) low,
which will cause the plain-text 5-bit code to override the diode matrix.
As no signal is returned into the drum, this doesn't interfere with
the rest of the diode matrix. It is this signal that can cause a
letter to be enciphered as itself.
|
|
|
