The Turing/Welchman Bombe simulator (in Python)

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Bombe Simulator in Python

Introduction

This software, in Python language, emulates in text mode, a Turing/Welchman three rotors Bombe. This model allows you to find the daily key for messages encrypted with the Enigma I or M3 which only uses three rotors.

I wrote another simulator but in C language which is 100 times faster. Question: Is this program in Python of interest?

Yes, because the source code is much easier to read. On the other hand, since it is written in Python, it can run on any system that has the Python interpreter installed.

Algorithm

  1. We build the electric circuit from the menu given in argument. This circuit is composed of a series of Enigmas linked together and linked to the input register and the Test Register which displays the Stecker associated with this register in the event of a Stop.
  2. Big loop on Grundstellung (from AAA to ZZZ). For each GrundStellung (For one Walzenlage given as argument):
    1. The registers are reset (absence of current)
    2. We advance the rotors
    3. Current is injected into the input register(s). As a result, electricity flows throughout the circuits. If the Grundstellung does not correspond to a Stop, all 26 elements that make up the Test Register are activated by the electric current. We then move on to the next GrundStellung. If the Test Register contains only one element activated by the electric current or if all elements are activated except one, we have a Stop, I.E. a plausible position which can correspond to the sought solution.
    4. If there is a Stop, the Bombe stops if the program is running in Debug mode. In this mode, in addition, each Grundstellung tested is displayed. If the Debug mode is not active only the Stop are displayed but the program continues (we go to the next GrundStellung).

Remarks:

  • Most often, only one register is used (The Test Register).
  • There is no option regarding the use of the Diagonal Board. This one is automatically used if the main wiring is connected to one of its entry points.

Tips:

  • At each Grundstellung tested, I loop 20 times to simulate the flow of electricity in the different Enigmas and the cables that connect them. In reality, electricity flows instantaneously.
  • To simulate the passage of current everywhere, I run the Enigmas first in one direction and then in the other direction. Unfortunately, this doubles the calculation time.
  • I use a lot of registers. Thus, I use a register to connect each Enigma to another Enigma. In the real Bombe, the Enigmas were connected together simply by cables. Registers were actually used associated with the "Machine Gun". We could thus know a large part of the steckers and not only the stecker shown by the Test Register. In my software, if the Debug mode is activated, we display all registers (this emulate the Machine Gun).

Download the program and untar it

On Windows or Linux, you can download my program with the CURL command: 

$ curl -O http://www.jfbouch.fr/crypto/enigma/bombe/BOMBE_PY.tar
$ tar -xf  BOMBE_PY.tar
$ cd BOMBE_PY
$ more README.txt

Show syntax

Just activate my program with no arguments: 

$ python3 turing_welchman.py
Usage:
 -h          Online Help
 -d          Debug
 -D a:1,d:2  Diagonal Board
             letter 'a' linked to register '1', ...
 -W X,Y,Z    The Walzenlage, default: -W I,II,III
 -U UKW      The Reflector, default: B
 -T l[,...]  The terminal letter(s) (A by default)
 -i r[,...]  The Input Register(s) (1 by default)
 -M menu     The menu, list of blocs. One bloc:
             input_register:Grund:output_register,...
             for example:
             -M 01:ZZA:02,02:ZZB:03,03:ZZC:01
             Note: The input register is the first register
 -I XYZ      The indicator position
             by default, the first entry of the menu

A simple example

I use the Ellsbury menu described in the page dedicated to my simulator in C (link).

The program displays for each Stop:

  • The Walzenlage
  • The GrundStellung
  • The Stecker associated with the Entry letter 
$ cat MENUS/ellsbury_01.sh
#!/bin/sh
# Ellsbury menu
python3 turing_welchman.py\
    -i 1 \
    -T A \
    -W II,I,III \
    -D E:1,B:2,A:3,H:4,D:5 \
    -M 1:ZZA:2,2:ZZC:3,3:ZZF:4,4:ZZG:1,1:ZZB:5,5:ZZI:2,5:ZZH:3,1:ZZE:4

$ sh MENUS/ellsbury_01.sh
stop II,I,III BGX, E:X