How to decrypt file using XOR in c++?

Question

I was learning about cryptography using c++, so i get started with very simple XOR, i want to open file 0x1.txt and encrypt it using XOR with 3 keys, and create a new file named 0x2.txt and put the encrypted data into it , and decrypt it and put its content in 0x3.txt:

encrpyt -> 0x1.txt -->put encrypted data in 0x2.txt ; decrypt 0x2.txt --> put decryped data in 0x3.txt

and here is my code :

encrypt code :

LPVOID Crypt(HANDLE hFile, DWORD dwFileSize) {
    // allocate buffer for file contents
    LPVOID lpFileBytes = malloc(dwFileSize);
    // read the file into the buffer
    ReadFile(hFile, lpFileBytes, dwFileSize, NULL, NULL);

    // apply XOR encryption
    int i;
    char key[3] = {'*', '~', '#'};
    for (i = 0; i < dwFileSize; i++) {
        *((LPBYTE)lpFileBytes + i) ^= key[i % sizeof(key)];
    }

    return lpFileBytes;
}

calling the function to encrypt the file:

HANDLE hFile = CreateFile("0x1.txt", GENERIC_READ, 0, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);

        // get file size
        DWORD dwFileSize = GetFileSize(hFile, NULL);
        CloseHandle(hFile);
        // crypt and get crypted bytes
        LPVOID lpFileBytes = Crypt(hFile, dwFileSize);

then put the encrpyed data in 0x2.txt :

HANDLE hCryptedFile = CreateFile("0x2.txt", GENERIC_WRITE, 0, NULL, CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);

        // write to crypted file
        WriteFile(hCryptedFile, lpFileBytes, dwFileSize, NULL, NULL);

Now i want to decrypt the content of 0x2.txt file i made this :

HANDLE hFile = CreateFile("0x2.txt", GENERIC_READ, 0, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);

        // get file size
        DWORD dwFileSize = GetFileSize(hFile, NULL);

        // decrypt and obtain decrypted bytes
        LPVOID lpFileBytes = Crypt(hFile, dwFileSize);
        CloseHandle(hFile);

create file 0x3.txt:

HANDLE hTempFile = CreateFile("0x3.txt", GENERIC_WRITE, 0, NULL, CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);
        // write to temporary file
        WriteFile(hTempFile, lpFileBytes, dwFileSize, NULL, NULL);
        // clean up
        CloseHandle(hTempFile);
        free(lpFileBytes);

but the file, it encrypt more , not decrypt !. so what its the problem ? here is my full code:

https://pastebin.com/6WZX5J1K

Comments are not for extended discussion; this conversation has been moved to chat. — Samuel Liew

Surt Surt · Accepted Answer · 2020-05-12T13:52:09

I think you should test your encrypt/decrypt first without all the file stuff.

Beware totally not tested code

void Encrypt( std::string& txt) {
  // apply XOR encryption
  int i;
  char key[3] = {'*', '~', '#'};
  for (i = 0; i < txt.length(); i++) {
    txt[i] ^= key[i % sizeof(key)];
  }
}

bool Test() {
  std::string org { "123" };
  std::string encrypted = org;
  Encrypt(encrypted, encrypted.length());
  std::string decrypted = encrypted;
  Encrypt(decrypted, decrypted.length());
  // std::cout << org << " " << encrypted << " " << decrypted << std::endl;
  return org == decrypted;
}

If Test returns true your encode/decode works if not you can concentrate on that part else you need to start debugging that first.

How to decrypt file using XOR in c++?

1 Answers