Introduction
This article shows how to implement an Enigma like cryptography using C#. This kind of cryptography system was used until the 70's. The most famous use of it was by the German army during the WWII. The breaking of this code led the allies to victory. This cryptography method can still be secure enough for use in non critical systems. If the Germans would have used the method with care, maybe the allies would not have discovered their secrets.
Background
The reader might be interested in some general information about Enigma. These links could help you:
Using the code
This code is a complete program that shows the inner workings of such a machine. If the user wants to understand more about the Enigma machine, then he/she would need to Google for more information. This code shows how simple it is to emulate such complicate wirings as those of the Enigma machine, using a modern programming language.
The machine has at least three rotors, each one having the alphabet written on it. The rotors has a visible part which consists of the alphabet and a part that has other letters which are linked to the first one by wires. The three rotors are arranged one after the other from right to left. The rotor is not static.
This kind of machine was a combination of electrical and mechanical cryptography system. Basically, when the user of the machine pressed a key, the current would go to the first rotor to the letter corresponding to that key (let's say 'A'). The wirings of the wheel would lead to an output letter from the first rotor with a different value, let's say 'D', this impulse would then go to the second rotor, and the same thing would happen there, and in the third rotor. From there, the impulse would go to a last wheel - that is named the 'reflector'. This wheel (rotor) is the only non rotating wheel. The impulse would go to this wheel and from here it would return to the third rotor, and make its way backwards. From the first rotor, the current would illuminate a LED under a panel. This would be the encrypted letter.
This is not all. At each key press, the first rotor would rotate with one position so that the output letter would not be the same when you press the same key twice. The second rotor would rotate one position once at every 28 rotations of the first rotor, and the third one once every 28 rotations of the second rotor. As I've said earlier, the reflector is the only static rotor. On some models, the second and third rotor would rotate more than once at every 28 rotations of the previous rotor.
To decrypt the data, you just had to set the rotors at the exact initial position as the rotors on the machine that produced that output and then type in the encrypted data.
There were several rotors and they could be changed. To complicate things even more, the military version had a front panel with letters were you could interconnect two letters so that each time a letter would occur, it would be replaced with the other one. To show this kind of complicated inner workings, the following C# code would suffice:
void Button1Click(object sender, System.EventArgs e)
{
char[] chIn = txtInit.Text.ToUpper().ToCharArray();
txtFinal.Text = "";
for(int i=0;i<chIn.Length;i++){
if(chIn[i]>=65 && chIn[i]<=90){
rr.Move();
rr.PutDataIn(chIn[i]);
txtFinal.AppendText(""+rr.GetDataOut());
}
}
}
You can see that we only use the upper letters. That's because the existence of punctuation and spaces would help in the decryption of the message.
I've also created a rotor class which practically represents one rotor of the enigma machine. In this class, we have the methods Move which moves the rotor one position and the PutDataIn which emulates the sending of the electric signal to the first rotor. As you can see, we only send the data to the first rotor, which will send the data further down the chain.
Here is the code for this class:
using System;
using System.Text;
using System.Windows.Forms;
namespace Enigma
{
public class Rotor
{
private string layout;
private byte offset;
private Rotor previous, next;
private Label lbl;
private char cIn = '\0', notchPos;
public Rotor(string layout,Label lbl,char notchPos)
{
this.layout = layout;
this.previous = previous;
this.next = next;
this.lbl = lbl;
this.notchPos = notchPos;
offset = 0;
}
public string GetLayout(){
return layout;
}
public void SetNextRotor(Rotor next){
this.next = next;
}
public void SetPreviousRotor(Rotor previous){
this.previous = previous;
}
public char GetInverseCharAt(string ch){
int pos = layout.IndexOf(ch);
if(offset>pos){
pos = 26 - (offset-pos);
}else{
pos = pos - offset;
}
if(previous!=null){
pos = (pos+previous.GetOffset())%26;
}
return (char)(65+pos);
}
public int GetOffset(){
return offset;
}
public char GetNotchPos(){
return notchPos;
}
public void ResetOffset(){
offset = 0;
}
public bool HasNext(){
return next!=null;
}
public bool HasPrevious(){
return previous!=null;
}
public void Move(){
if(next==null){
return;
}
offset++;
if(offset==26){
offset = 0;
}
if(next!=null && (offset+65) == ((notchPos-65)%26)+66){
next.Move();
}
lbl.Text = ""+((char)(65+offset));
}
public void MoveBack(){
if(offset==0){
offset = 26;
}
offset--;
lbl.Text = ""+((char)(65+offset));
}
public void PutDataIn(char s){
cIn = s;
char c = s;
c = (char)(((c - 65) + offset) % 26 + 65);
if(next!=null){
c = layout.Substring((c-65),1).ToCharArray()[0];
if((((c-65)+(-offset))%26 + 65)>=65){
c = (char)(((c-65)+(-offset))%26 + 65);
}else{
c = (char)(((c-65)+(26+(-offset)))%26 + 65);
}
next.PutDataIn(c);
}
}
public char GetDataOut(){
char c = '\0';
if(next!=null){
c = next.GetDataOut();
c = GetInverseCharAt(""+c);
}else{ c = layout.Substring((cIn-65),1).ToCharArray()[0];
c = (char)(((c - 65) + previous.offset)%26+65);
}
return c;
}
}
}
Points of Interest
Although it seems peculiar, the older the cryptographic algorithm, the safer it is. This is because it means that people have tested it and they did not found any way in. Maybe in a later article, I will show you how to implement an instant messenger that uses this cryptography technique.
History
- Apr 29, 2011: Updated source code.
