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Enigma Machine in C

Implementation of Enigma machine used by German troops to encrypt communications during WWII. The machine is implemented using the C programming language as a header-only library.

NOTE: The current implementation is able to work only with capital letters taken from the english alphabet. That is, we work with the alphabet ABCDEFGHIJKLMNOPQRSTUVWXYZ.

Quick Start

To use the machine compile the examples and in particular the examples/cli.c file

make examples

At this point you can use the CLI as follows

./examples/cli

Within the cli you can use the following commands

Enigma> help
Enigma> List of commands...

        quit – exit from enigma cli
        help – list available commands
        info – print enigma configuration
        set <rotors> | set <reflector> | set <plugboard> – change enigma configuration
        encrypt <plaintext> – encrypt plaintext
        decrypt <ciphertext> – decrypt ciphertext

Some examples are shown below

Enigma> info
Enigma> Current configuration...
        Rotors (from left to right): M3-II, M3-I, M3-III
               Position: 0, 0, 0
                   Ring: 0, 0, 0
        Reflector: M3-B
        Plugboard: 6 plugs
                   (A, M)
                   (F, I)
                   (N, V)
                   (P, S)
                   (T, U)
                   (W, Z)
Enigma> encrypt HELLO
MIJEN

Enigma> set reflector M3-B
Enigma> set rotor left M3-I 1 2
Enigma> set rotor middle M3-II 3 4
Enigma> set rotor right M3-III 5 6
Enigma> set plugboard B-Q C-R
Enigma> info
Enigma> Current configuration...
        Rotors (from left to right): M3-I, M3-II, M3-III
               Position: 1, 3, 5
                   Ring: 2, 4, 6
        Reflector: M3-B
        Plugboard: 2 plugs
                   (B, Q)
                   (C, R)
Enigma> encrypt DSFSDFSDF
SCLMEEYMV

Library Usage

To actually use the library in your own C sources, download the enigma.h file and import it in your code as follows

#define ENIGMA_IMPLEMENTATION
#include "../enigma.h"

First you istantiate an enigma machine with a call to init_enigma

Enigma *e = init_enigma (
			 // rotors model
			 (const char *[]){"M3-II", "M3-I", "M3-III"},
			 // rotor_positions
			 (const uint8_t [ROTORS_N]) {0, 0, 0},
			 // rotor_ring_settings
			 (const uint8_t [ROTORS_N]) {0, 0, 0},
			 // reflector model
			 "M3-B",
			 // plugboard switches
			 (uint8_t [][2]){
			   {'A', 'M'}, {'F', 'I'}, {'N', 'V'},
			   {'P', 'S'}, {'T', 'U'}, {'W', 'Z'}},
			 // plugboard size
			 6 
			 );

And then you can encrypt/decrypt your text.

enigma_encrypt(enigma, plaintext, length, ciphertext);
enigma_decrypt(enigma, ciphertext, length, plaintext);

Notice that before using the machine you have to take care of allocating the memory for the plaintext and the ciphertext buffers. This is shown below

char *str = "HELLO";
size_t length = strlen(str);

char plaintext[length];
char ciphertext[length];
  
memcpy(plaintext, str, length + 1);
memcpy(ciphertext, str, length + 1);

enigma_encrypt(e, plaintext, length, ciphertext);

Finally, when you stop using enigma, you have to call destroy_enigma for freeing up the allocated memory.

destroy_enigma(enigma);