Signing HTTP Requests in JAX-RS

I'd like to sign HTTP Requests (similar to this https://tools.ietf.org/html/draft-ietf-oauth-signed-http-request-00).

Therefore I need access to HTTP-Header fields like "Content-Length", "Host", "Date", "User-Agent", etc.

I tried using ClientRequestFilter, WriterInterceptor and AbstractPhaseInterceptor (with a very late phase (Phase.SEND)) to implement this functionality - but it seems the HTTP Headers are set very late - and are out of my reach.

All I get in the PROTOCOL_HEADERS is "Content-Type" and "Accept".

Does CXF provide a functionality to have access to this kind of information (in an interceptor)?

Wheather or not my use case makes sense - imho it generally would be nice to have this functionality ;-)

cheers,
Hermann