Example of DES encryption code

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/******************************************************************************
  desex.c, an example of how to use the openssl DES implementation... Data
  is input on stdin, and the enrypted information is output to stdout. Prompts
  appear on stderr...

  This is an example of the Electronic Code Book mode, which is predictable
  for known input, and is not recommended.

  Copyright (c) Michael Still 2001
  Released under the terms of the GNU GPL
******************************************************************************/

#include 
#include 

int main(int argc, char *argv[]){
  des_cblock key, input, output;
  des_key_schedule sched;
  int c, i;

  fprintf(stderr, "Setting up the DES library...\n");
  des_string_to_key("Mary had a little lamb, it's fleece as white as snow. Everywhere that Mary went, the lamb would surely go...", &key);

  fprintf(stderr, "Schedualling the key...\n");
  switch(des_set_key_checked(&key, sched)){
  case -1:
    fprintf(stderr, "Bad parity\n");
    _exit(42);
    break;

  case -2:
    fprintf(stderr, "Key is weak\n");
    _exit(42);
    break;
  }

  fprintf(stderr, "Start typing and hit ctrl-d to finish...\n");

  i = 0;
  while((c = fgetc(stdin)) != EOF){
    input[i] = c;

    i++;
    if(i == 8){
      des_ecb_encrypt(&input, &output, sched, DES_ENCRYPT);
      fprintf(stdout, "%c%c%c%c%c%c%c%c", output[0], output[1], output[2],
	      output[3], output[4], output[5], output[6], output[7]);
      i = 0;
    }
  }
}
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DES examples

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This page contains some DES example code I wrote in reaction to a crypto tutorial I attended a while ago. I finally got around to putting the code online…

The basic point of the code is it shows you how to use the DES calls within openssl. It also shows how the electronic code book for of DES is a bad choice for vaguely predictable data such as images.

A visual demonstration of this is these two images. The one on the right is an encrypted version of the one on the left… The vertical flip of the image is because of the way libtiff reads colour images, and to be honest isn’t worth fixing for this example.

   

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Example of DES decryption code

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/******************************************************************************
  desex.c, an example of how to use the openssl DES implementation... Data
  is input on stdin, and the enrypted information is output to stdout. Prompts
  appear on stderr...

  This is an example of the Electronic Code Book mode, which is predictable
  for known input, and is not recommended.

  Copyright (c) Michael Still 2001
  Released under the terms of the GNU GPL
******************************************************************************/

#include 
#include 

int main(int argc, char *argv[]){
  des_cblock key, input, output;
  des_key_schedule sched;
  int c, i;

  fprintf(stderr, "Setting up the DES library...\n");
  des_string_to_key("Mary had a little lamb, it's fleece as white as snow. Everywhere that Mary went, the lamb would surely go...", &key);

  fprintf(stderr, "Schedualling the key...\n");
  switch(des_set_key_checked(&key, sched)){
  case -1:
    fprintf(stderr, "Bad parity\n");
    _exit(42);
    break;

  case -2:
    fprintf(stderr, "Key is weak\n");
    _exit(42);
    break;
  }

  fprintf(stderr, "Start typing and hit ctrl-d to finish...\n");

  i = 0;
  while((c = fgetc(stdin)) != EOF){
    input[i] = c;

    i++;
    if(i == 8){
      des_ecb_encrypt(&input, &output, sched, DES_DECRYPT);
      fprintf(stdout, "%c%c%c%c%c%c%c%c", output[0], output[1], output[2],
	      output[3], output[4], output[5], output[6], output[7]);
      i = 0;
    }
  }
}
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