C++ XOR encryption

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After reading several white papers on cryptography and runtime PE crypters, I decided to write my own. It's very simple and only for educational purposes.

Here is the GitHub repo: https://github.com/Jyang772/XOR_Crypter

I have two questions.

  • First, why do I have to keep changing my file permissions to start every outputted .exe (File created by Builder.exe not the compiler)? It creates a file that is Shared. I have to right click it and select share with Nobody. Does this have something to do with the File Access and Security Rights? I am using CreateFile() and Readfile to read and write the input and output files.

http://msdn.microsoft.com/en-us/library/windows/desktop/aa363858(v=vs.85).aspx

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  • Second, I can't seem to get XOR encryption to work. It seems pretty straight forward for what I have done. The byte sizes are the same. While I was investigating, I had the Builder and the Stub each output a file with the file data unencrypted. They are the same. Then I tried with the data encrypted. There is no doubt the data is encrypted with the cipher, however it shows up blank when it is decrypted by the stub later on. I'm confused.

Here is my XOR implementation:

fs = byte size Rsize = byte size Should be the same.

Builder:

 char cipher[] ="penguin";
      for (int i = 0; i < fs; i++)
        {   
                FB[i] ^= cipher[i % strlen(cipher)]; // Simple Xor chiper
        }

Stub:

char cipher[] = "penguin";
for (int i = 0; i < Rsize; i++)
    {
        RData[i] ^= cipher[i % strlen(cipher)];
    }

If I were to comment out the encryption function in the Builder and Stub, the crypted file runs fine. Uhh, except with the permissions error.

I'm also trying to include a options menu where the user can select the encryption method used. Perhaps I might have done something wrong there? The Builder.exe adds one byte containing the user's choice to the end of FB buffer. Stub.exe reads that and determines which encryption method is used to decrypt the data.

cryptography encryption c++
2 Answers

Depending on your content data, you write char option in allocated memory pointed by FB or after it (buffer overrun) in "C++ Builder/main.cpp" when calling strcat(FB, choice).

Fix: allocate enough space in FB for data + option char. As you are dealing with binary data you should not use string functions (ex: strcat).

FB = new char[fs + 1];
memcpy(FB +fs, option, 1); // copy the option at end
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