Question

The following program attempts to encrypt a string and save in to a text file, and open the file, decrypt and show the message.

Here is the code-

private: System::Void saveToolStripMenuItem_Click(System::Object^  sender, System::EventArgs^  e) {
             SaveFileDialog^ dlg = gcnew SaveFileDialog();
             dlg->Filter = "Text Files|*.txt";
             char key = 'a';
             if(dlg->ShowDialog()==Windows::Forms::DialogResult::OK)
             {
                 String^ stream = txtOutput->Text;
                 char* num = new char[stream->Length];
                 char* xorchar = (char*)(void*)Marshal::StringToHGlobalAnsi(stream);
                 int i=0;
                 for(i=0;i<stream->Length;++i){
                     num[i] = *xorchar ^ key;
                     *xorchar++;
                 }

                 num[i] = '\0';  //add trailing NULL
                 //Marshal::FreeHGlobal((System::IntPtr)(void*)xorchar);  THIS WAS GIVING AN ERROR, NOT SURE WHY
                 String^ save = gcnew String(num);
                 System::IO::File::WriteAllText(dlg->FileName, save);
             }

         }
private: System::Void openToolStripMenuItem_Click(System::Object^  sender,      System::EventArgs^  e) {
         OpenFileDialog^ dlg = gcnew OpenFileDialog();
         dlg->Filter = "Text Files|*.txt";
         String^ stream;
         char key = 'a';
         if(dlg->ShowDialog()==Windows::Forms::DialogResult::OK)
         {
            stream = System::IO::File::ReadAllText(dlg->FileName);


            char* num = (char*)(void*)Marshal::StringToHGlobalAnsi(stream);
            int i=0;
            for (i=0;i<stream->Length;++i)
            {
                num[i] = num[i] ^ key;   //DECRYPT
            }

            String^ orig_stream = gcnew String(num);
            txtOutput->Text = orig_stream;

         }
     }

Now the issue is, when i input a string, it works only for characters which are not equal to the key.
Example, Let key = ‘a’

eg: INPUT: “I Like This” // This will decrypt correctly.

INPUT: “I Like apples” //Only upto ‘I Like’ will decrypt, rest doesnt show up.

ie, If it encounters the character ‘key’ (in this case ‘a’), it stops decryption.

Any help is appreciated. Thanks!

Answer 1

A value xor’d with itself will always produce 0. A 0 char is also called the null-terminator (0 == '\0'). Strings in C++ are null-terminated, aka they stop at the null-terminator.

Simple example:

#include <iostream>

int main(){
  char str[] = "hi there!\0this will not get printed.";
  std::cout << str;
}