How should I specify user/password for client authorization in http request made by httpc:request() function??
For digest, you'll need to do the same thing as basic but more so. Generally you'll hit the page without auth, get the "WWW-Authenticate" header info, then use the realm and nonce there to generate your "Authorization" header. http://en.wikipedia.org/wiki/Digest_access_authentication has a decent example at the bottom.
Generally, HTTPS + Basic is sufficient, if not better, for most use cases.