Simple algorithm to modify a payload in such a way to have a specific CRC32 value

crc32-extender.c

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

const uint32_t ui_crc32_table[] =
  {
   0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419, 0x706af48f,
   0xe963a535, 0x9e6495a3, 0x0edb8832, 0x79dcb8a4, 0xe0d5e91e, 0x97d2d988,
   0x09b64c2b, 0x7eb17cbd, 0xe7b82d07, 0x90bf1d91, 0x1db71064, 0x6ab020f2,
   0xf3b97148, 0x84be41de, 0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7,
   0x136c9856, 0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9,
   0xfa0f3d63, 0x8d080df5, 0x3b6e20c8, 0x4c69105e, 0xd56041e4, 0xa2677172,
   0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b, 0x35b5a8fa, 0x42b2986c,
   0xdbbbc9d6, 0xacbcf940, 0x32d86ce3, 0x45df5c75, 0xdcd60dcf, 0xabd13d59,
   0x26d930ac, 0x51de003a, 0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423,
   0xcfba9599, 0xb8bda50f, 0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924,
   0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d, 0x76dc4190, 0x01db7106,
   0x98d220bc, 0xefd5102a, 0x71b18589, 0x06b6b51f, 0x9fbfe4a5, 0xe8b8d433,
   0x7807c9a2, 0x0f00f934, 0x9609a88e, 0xe10e9818, 0x7f6a0dbb, 0x086d3d2d,
   0x91646c97, 0xe6635c01, 0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e,
   0x6c0695ed, 0x1b01a57b, 0x8208f4c1, 0xf50fc457, 0x65b0d9c6, 0x12b7e950,
   0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3, 0xfbd44c65,
   0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2, 0x4adfa541, 0x3dd895d7,
   0xa4d1c46d, 0xd3d6f4fb, 0x4369e96a, 0x346ed9fc, 0xad678846, 0xda60b8d0,
   0x44042d73, 0x33031de5, 0xaa0a4c5f, 0xdd0d7cc9, 0x5005713c, 0x270241aa,
   0xbe0b1010, 0xc90c2086, 0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
   0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4, 0x59b33d17, 0x2eb40d81,
   0xb7bd5c3b, 0xc0ba6cad, 0xedb88320, 0x9abfb3b6, 0x03b6e20c, 0x74b1d29a,
   0xead54739, 0x9dd277af, 0x04db2615, 0x73dc1683, 0xe3630b12, 0x94643b84,
   0x0d6d6a3e, 0x7a6a5aa8, 0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1,
   0xf00f9344, 0x8708a3d2, 0x1e01f268, 0x6906c2fe, 0xf762575d, 0x806567cb,
   0x196c3671, 0x6e6b06e7, 0xfed41b76, 0x89d32be0, 0x10da7a5a, 0x67dd4acc,
   0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5, 0xd6d6a3e8, 0xa1d1937e,
   0x38d8c2c4, 0x4fdff252, 0xd1bb67f1, 0xa6bc5767, 0x3fb506dd, 0x48b2364b,
   0xd80d2bda, 0xaf0a1b4c, 0x36034af6, 0x41047a60, 0xdf60efc3, 0xa867df55,
   0x316e8eef, 0x4669be79, 0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236,
   0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f, 0xc5ba3bbe, 0xb2bd0b28,
   0x2bb45a92, 0x5cb36a04, 0xc2d7ffa7, 0xb5d0cf31, 0x2cd99e8b, 0x5bdeae1d,
   0x9b64c2b0, 0xec63f226, 0x756aa39c, 0x026d930a, 0x9c0906a9, 0xeb0e363f,
   0x72076785, 0x05005713, 0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38,
   0x92d28e9b, 0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21, 0x86d3d2d4, 0xf1d4e242,
   0x68ddb3f8, 0x1fda836e, 0x81be16cd, 0xf6b9265b, 0x6fb077e1, 0x18b74777,
   0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c, 0x8f659eff, 0xf862ae69,
   0x616bffd3, 0x166ccf45, 0xa00ae278, 0xd70dd2ee, 0x4e048354, 0x3903b3c2,
   0xa7672661, 0xd06016f7, 0x4969474d, 0x3e6e77db, 0xaed16a4a, 0xd9d65adc,
   0x40df0b66, 0x37d83bf0, 0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
   0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6, 0xbad03605, 0xcdd70693,
   0x54de5729, 0x23d967bf, 0xb3667a2e, 0xc4614ab8, 0x5d681b02, 0x2a6f2b94,
   0xb40bbe37, 0xc30c8ea1, 0x5a05df1b, 0x2d02ef8d
  };

#define crc32(b,s) (uint32_t)(pre_crc32(b,s) ^ 0xffffffff)

uint32_t pre_crc32(const uint8_t* ui_buffer, size_t sz_buffer_len) {
  uint32_t ui_crc32 = 0xffffffff;
  for (int i = 0; i < sz_buffer_len; i++) {
    int i_lookup_index = (ui_crc32 ^ ui_buffer[i]) & 0xff;
    ui_crc32 = (ui_crc32 >> 8) ^ ui_crc32_table[i_lookup_index];
  }
  return ui_crc32;
}

uint32_t unwind_crc32(const uint8_t* ui_pre, size_t sz_pre_len,
                      const uint8_t* ui_post, size_t sz_post_len,
                      uint32_t ui_target_crc32) {

  uint32_t ui_pre_crc32 = pre_crc32(ui_pre, sz_pre_len);

  /* step backwards reversing the crc calculation */
  ui_target_crc32 ^= 0xffffffff;
  for (int i = 0; i < sz_post_len; i++) {
    int i_lookup_index = 0;
    while (((ui_crc32_table[i_lookup_index] ^ ui_target_crc32) >> 24) & 0xff) {
      i_lookup_index++;
    }

    ui_target_crc32 ^= ui_crc32_table[i_lookup_index];
    ui_target_crc32 <<= 8;
    ui_target_crc32 |= i_lookup_index ^ ui_post[sz_post_len - i - 1];
  }

  for (int i = 0; i < 4; i++) {
    int i_lookup_index = 0;
    while (((ui_crc32_table[i_lookup_index] ^ ui_target_crc32) >> 24) & 0xff) {
      i_lookup_index++;
    }

    ui_target_crc32 ^= ui_crc32_table[i_lookup_index];
    ui_target_crc32 <<= 8;
    ui_target_crc32 |= i_lookup_index;
  }

  return ui_target_crc32 ^ ui_pre_crc32;
}

const char* p_c_original = (
 "int main() {\n"
 "  printf(\"hello\\n\");\n"
 "  return 0;\n"
 "}\n"
                            );
const char* p_c_new_message = (
 "int main() {\n"
 "  printf(\"hello\\n\");\n"
 "  system(\"cat /etc/shadow\");\n"
 "  /*"
                               );
const char* p_c_trailer = (
 "*/\n"
 "  return 0;\n"
 "}\n"
                           );

int main(int argc, char** argv) {
  const uint32_t ui_original_crc32 = crc32((const uint8_t*)p_c_original,
                                           strlen(p_c_original));
  printf("Original message:\n%s\nCRC32: %08X\n",
         p_c_original,
         ui_original_crc32);

  const size_t sz_new_message = strlen(p_c_new_message);
  const size_t sz_trailer = strlen(p_c_trailer);
  uint32_t ui_jibberish = unwind_crc32(
    (const uint8_t*)p_c_new_message, sz_new_message,
    (const uint8_t*)p_c_trailer, sz_trailer,
    ui_original_crc32);

  printf("Computed jibberish to insert: %08X\n", ui_jibberish);

  char* p_c_payload = (char*)malloc(sz_new_message + sz_trailer + 4 + 1);
  p_c_payload[sz_new_message + sz_trailer + 4] = '\0';
  memcpy(p_c_payload, p_c_new_message, sz_new_message);
  memcpy(p_c_payload + sz_new_message, &ui_jibberish, 4);
  memcpy(p_c_payload + sz_new_message + 4, p_c_trailer, sz_trailer);

  printf("Computed payload as:\n%s\n", p_c_payload);

  printf("CRC32: %08X\n",
         crc32((const uint8_t*)p_c_payload, sz_new_message + sz_trailer + 4));

  FILE* f = fopen("message.txt", "w");
  fwrite(p_c_payload, sz_new_message + sz_trailer + 4, 1, f);
  fclose(f);

  free(p_c_payload);
  return 0;
}