Different CRC32 checksum for the same file - c++

Question

Basically, I have an application which imports a few DLLs and some Python libraries.
I did another application which calculates CRC32 for each of these files and I've added the list on my main application because I want to make it to check these files.
The problem is that sometimes the main application detects a DLL or a library modified even if it wasn't. It don't happens everytime and I am wondering why does that happens? What can determine the CRC32 checksum to differ? It's because my application loads these DLLs or is something external like viruses or anything else?
Here's the code for both main application and the application which does the CRC32 list:
#include "boost/crc.hpp"
#include "boost/filesystem/operations.hpp"
#include "boost/filesystem/path.hpp"
#include "boost/progress.hpp"
#include "boost/integer.hpp"
using namespace std;
namespace fs = boost::filesystem;
int CheckCRC32(LPCSTR szFileName, DWORD dwCRC32)
{
if (!fs::exists(szFileName))
return 1;
DWORD dwResult = 0;
boost::crc_32_type result;
std::streamsize const buffer_size = 1024;
ifstream file(szFileName, std::ios_base::binary);
if (file)
{
do
{
char buffer[buffer_size];
file.read(buffer, buffer_size);
result.process_bytes(buffer, file.gcount());
} while(file);
}
else
return 1;
dwResult = result.checksum();
if (dwResult <= 0)
return 1;
if (dwResult != dwCRC32)
return 2;
return 0;
}
Also, if you have another solution for doing such things or if you have a solution for this method, please let me know.

Answer 1

I am not really trusting your 1024 bytes fixed buffer and the order of calls you chose. Either it is your intention to only use the first 1024 bytes of your files for the crc or this could already be the problem. On the other hand, if the files you crc are smaller than those 1024, it is up to the behavior of file.read() what happens.
Just to give you an option, here the code for a "low level" alternative. If the same crc values will be obtained with this version will depend on whether my initial crc value of 0 is "by the book" or if you need to use another start value.
#include <Windows.h>
#include <cstdint>
#include <string>
#include <vector>
static uint32_t crc32_tab[] = {
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
};
uint32_t
crc32(uint32_t crc, const void *buf, size_t size)
{
const uint8_t *p = static_cast<const uint8_t*>(buf);
crc = crc ^ ~0U;
while (size--)
crc = crc32_tab[(crc ^ *p++) & 0xFF] ^ (crc >> 8);
return crc ^ ~0U;
}
bool Crc32OfFile(const char * pathToFile, uint32_t &result)
{
bool success = false;
HANDLE hFile =
::CreateFileA
( pathToFile
, GENERIC_READ
, FILE_SHARE_READ
, NULL
, OPEN_EXISTING
, FILE_ATTRIBUTE_NORMAL|FILE_FLAG_SEQUENTIAL_SCAN
, NULL
);
if (INVALID_HANDLE_VALUE != hFile)
{
DWORD fileSize = ::GetFileSize(hFile, NULL);
std::vector<uint8_t> buffer;
buffer.resize(fileSize);
DWORD bytesRead = 0UL;
if (::ReadFile(hFile, &buffer[0], buffer.size(), &bytesRead, NULL))
{
if (bytesRead == fileSize)
{
result = crc32(0UL, &buffer[0], buffer.size());
success = true;
}
}
::CloseHandle(hFile);
}
return success;
}