Ok, so I just tried this out locally (replacing values in my handler) and I got a key back. So your configuration works ok.
Can you try removing the slash from your call (it 404’ed on mine), so instead of
http://{your-tib-host}:{tib-port}/auth/{profile-id}/proxy/
do:
http://{your-tib-host}:{tib-port}/auth/{profile-id}/proxy
M.