Question

So. Let’s say I were to make a hex editor to edit… oh… let’s say a .DLL file. How can I edit a .DLL file’s hex by using C# or C++? And for the “fixed part”, I want to make it so that I can browse from the program for a specific .DLL, have some pre-coded buttons on the programmed file, and when the button is pressed, it will automatically execute the requested action, meaning the button has been pre-coded to know what to look for in the .DLL and what to change it to. Can anyone help me get started on this?

Also, preferably C#. Thank you!

Answer 1

The basics are very simple.

A DLL, or any file, is a stream of bytes.

Basic file operations allow you to read and write arbitrary portions of a file. The term of art is basically “Random Access Files Operations”.

In C, the fundamental operations are read(), write(), and lseek().

read allows you to read a stream of bytes in to a buffer, write allows you to write a buffers of bytes to a file, lseek allows you to position anywhere you want in the file.

Example:

int fd = open("test.dat", O_RDWR);
off_t offset = lseek(fd, 200, SEEK_SET);
if (off_t == -1) {
    printf("Boom!\n");
    exit(1);
}    
char buf[1024];
ssize_t bytes_read = read(fd, buf, 1024);
offset = lseek(fd, 100, SEEK_SET);
ssize_t bytes_written = write(fd, buf, 1024);
flush(fd);
close(fd);

This reads 1024 bytes from a file, starting at the 200th byte of the file, then writes it back to the file at 100 bytes.

Once you can change random bytes in a file, it’s a matter of choosing what bytes to change, how to change them, and doing the appropriate reads/lseeks/writes to make the changes.

Note, those are the most primitive I/O operations, there are likely much better ones you can use depending on your language etc. But they’re all based on those primitives.

Interpreting the bytes of a file, displaying them, etc. That’s an exercise for the reader. But those basic I/O capabilities give you the fundamentals of changing files.