C++ Keylogger
Want an easy way to steal passwords? Use a Keylogger! Writing about what a keylogger is, how it works under the hood and how to try this keylogger out for yourself. 2020-05-23
What is a Keylogger?
A keylogger is a relatively simple hardware or software device that logs the keys typed on a keyboard. Here is the definition of a keylogger from Wikipedia:
Keystroke logging, often referred to as keylogging or keyboard capturing, is the action of recording (logging) the keys struck on a keyboard, typically covertly, so that person using the keyboard is unaware that their actions are being monitored. Data can then be retrieved by the person operating the logging program.
In my opinion, in most cases, software keyloggers are better than hardware ones for a few reasons, which is why I opted for the former. Here are some of the reasons why I decided to make a software keylogger:
- They do not require physical access to a computer
- They can be used without specialized hardware
- They have way more options when it comes to retrieving their logs
How it Works
The keylogger I made is very simple; it works in 3 main steps.
First, the keylogger listens for key presses. It does so with the following lines of code:
int keyState = GetAsyncKeyState(k);
if(keyState == -32767){
// [...]
}
It loops through every key on the keyboard and checks whether it has been pressed using the GetAsyncKeyState function. If the function returns -32767, it means that the key it was passed as a parameter was pressed once since the last call to the function. However, this only allows us to know the key code of the key.
Second, the keylogger needs to take the key code of the key that was pressed and transform it into the corresponding ASCII character, so that we can retrieve the actual character that was typed insted of its key code. In order to do so, the keylogger uses the following code:
int ret = ToAsciiEx(k, MapVirtualKeyEx(k, 0, keyboardLayout), keyboardState, buffer, 0, keyboardLayout);
if(ret == 1) str = std::string("") + (char)buffer[0];
else if(ret == 2) str = "?";
else str = "[" + std::to_string((unsigned char)k) + "]";
The first part of the code calls the ToAsciiEx function, which transforms key codes into ASCII characters using the current keyboard layout. The second part of the code is for error handling: if ToAsciiEx throws an error, the key code is displayed inside of square brackets instead (i.e. [116]) to still be able to see that a key was pressed.
Third, the keylogger logs the characters it has just decoded using the log function:
std::cout << str;
log(str, "log.txt");
It simply appends the key that was pressed to a file called log.txt. Click here to see what the logs look like! I didn't implement any complicated logging system so that it could be modified easily. If you wanted, you could modify this keylogger to send the key strokes to a remote server, such as my Raspberry Pi server.
How to Get it
You might be wondering at this point...
How can I get such an awesome program for myself?
You can click here to download the source code of the keylogger, but you will have to compile it yourself using a C++ compiler, such as g++. With that said, keep in mind that it is illegal to use a keylogger without the victim's consent in most countries; use this program at your own risk and make sure to do your own research before using it.
Conclusion
I had already programmed a keylogger before, but it was way worse that this one. For instance, it wasn't able to register capital letters or symbols made using the Shift key, or any other symbols for that matter. I learned a lot about how to translate key codes to actual ASCII characters using the Windows API, which is certainely going to be very useful in the future. This wasn't an easy journey, but I am very happy with the result!