C Generate a header file via a Binary file

Question

I'm trying to make a simple encryption type of stuff. So what I wanna do is read an executable's contents, encrypt it and generate a header file which will contain a variable with the encrypted bytes/binaries, then it will decrypt it etc. So the question is how can I export that encrypted stuff onto a header file. Because for example if you try to print a byte representation of the contents you can do it with

printf("%x", byte);

But I don't think that you can use that kind of format to store the bytes in an unsigned char, since the usual format is

unsigned char bytes[] = {0x010, 0x038, 0x340 etc...}

In Python I can do it, but I can't seem to figure it out how to do it directly in C.

If you have recommendations of sources, please share them.

I'm trying to focus on Windows Executables at the moment, most likely I'll try to execute the binary code on a Virtually Allocated Memory, I've seen some code that does it, so I wanna try doing it myself.

Answer 1

Quick and dirty, unsafe and untested. Reads the file defined in INPUT_FILE, and outputs it to OUTPUT_FILE in the format of: unsigned char var[] = { 0xXX, 0xXX ... }; The name of the variable is controlled by VARIABLE_NAME. You should add your own sanity checks, i.e. check the returns from fopen() and the likes.

#include <stdio.h>
#include <stdlib.h>

#define INPUT_FILE "file.exe"
#define OUTPUT_FILE "out.txt"
#define VARIABLE_NAME "bytes"

int main(int argc, char *argv[]) {
    FILE *fp = fopen(INPUT_FILE, "rb");

    // Get file size
    fseek(fp, 0, SEEK_END);
    long size = ftell(fp);
    fseek(fp, 0, SEEK_SET);

    // Alloc, read
    unsigned char *buf = malloc(size);
    fread(buf, size, 1, fp);
    fclose(fp);

    // Write the data out
    fp = fopen(OUTPUT_FILE, "wb");
    fprintf(fp, "unsigned char %s[] = { ", VARIABLE_NAME);
    for (long i = 0; i < size; i++) {
        fprintf(fp, "0x%02x%s", buf[i], (i == size-1) ? " };" : ", ");
    }
    fclose(fp);
    free(buf);
    return 0;
}

Answer 2

Do you want something like that :

#include <stdio.h>

int encode(int c)
{
  return (unsigned char) (c ^ 0xf);
}

int main(int argc, char ** argv)
{
  if (argc != 3) {
    fprintf(stderr, "usage: %s <file in> <file out>\n", *argv);
  }
  else {
    FILE * fpin;
    FILE * fpout;
    
    if ((fpin = fopen(argv[1], "rb")) == NULL) /* under Windows 'b' is necessary to read binary */
      perror("cannot open inpout file");
    else if ((fpout = fopen(argv[2], "w")) == NULL)
      perror("cannot open inpout file");
    else {
      const char * sep = "unsigned char bytes[] = {";
      int c;
     
      while ((c = fgetc(fpin)) != EOF) {
        fprintf(fpout, "%s0x%x", sep, encode(c));
        sep = ", ";
      }
      
      fputs("};\n", fpout);
      fclose(fpin);
      fclose(fpout);
    }
  }
  
  return 0;
}

of course modifying encode

Compilation and execution :

pi@raspberrypi:/tmp $ gcc -Wall e.c
pi@raspberrypi:/tmp $ ./a.out ./a.out h
pi@raspberrypi:/tmp $ cat h
unsigned char bytes[] = {0x70, 0x4a, 0x43, 0x49, 0xe, 0xe, 0xe, 0xf ... 0xf, 0xf, 0xf, 0xf, 0xf};
pi@raspberrypi:/tmp $ ls -l h
-rw-r--r-- 1 pi pi 43677 juil.  4 18:44 h

(I cut cat h result to only show its begin and end)