Erlang OpenID client (and maybe server, sometime).

Quickstart workflow: 

[ your website starts ]

0> {ok, _Pid} = openid:start_link([]).

[ one user requested authentification over OpenID. 
  He/she passed an openid URL into form on you site. ]

1> {ok, AuthReq} = openid:prepare(SessionId, UserUrl).
	* SessionId -	Some string. For example "foobar".
	* UserUrl   -	The data, provided by user. Example values:
			"http://john.myopeid.org/"
			"https://www.google.com/o8/id"
			As you can, UserUrl may NOT contain any user name at all.
			Note: this URL is not unique for each user. 
			For example, all google users will provide equal URLs (look at URL above).
	* AuthReq   -	Just a returned context. Used for step 2.

2> Url = openid:authentication_url(AuthReq, ReturnTo, Realm).
	* AuthReq   -	See step 1.
	* ReturnTo  -	URL on YOUR server. User will be redirected here after authentification.
			Resource on this url should be able to handle GET and POST requests and 
			ready to pass all arguments into step 4. Example value:
			"http://yoursite.com/login/openid/verify"

	* Realm	    -	Will be displayed to user by remote OpenID server. 
			Many openid servers wants to see something like "http://yoursite.com/" 
			here and they will display message to user like "Site "yoursite.com" 
			wants to authentificate YOU. Continue?" to user. 
			This value must be immutable for ages. If it changes, user will be
			to reauthentificated again by remote server.

	* Url	    -	URL string for redirection the user. Just send user to this url and wait 
			for him on ReturnUrl.

3> [ user is redirected from your server to remote openID server ]

4> [ user returned to http://yoursite.com/login/openid/verify?openid.data=some_data&openid... ]

5> {ok, OpenId} = verify(SessionId, ReturnTo, HttpParams).
	* SessionId -	See step 1. Same value expected.
	* ReturnTo  -	See step 2. Same value expected for .
	* HttpParams-	Proplist [{string, string}]. It contains returned GET/POST parameters. 
			In zotonic:	z_context:get_q_all_noz(Context).
			In webmachine:	wrq:req_qs(ReqData).
			Constructed from URL on step 4 by mochiweb/webmachine/etc.
	* OpenId    -	Unique OpenID realm of user. String. Sometimes MAY be equal 
			to UserUrl (see step 1). Example values:
			"http://openid.myopenid.org/user/john/yadis/"
			"https://www.google.com/o8/ud/?x=agrg43gwsgsegergerg".
			Do not try to parse it, really. This Realm URL may NOT contain any 
			username. This realm is unique for every user.
			Just store it on your users db as some user_id. This realm helps 
			your's webserver to identity one user in future log-in attempts.


External Links:
http://taizilla.wordpress.com/2010/07/21/erl_openid-an-erlang-openid-2-0-client/ - Tutorial, written by original author.
http://paste.pocoo.org/show/235385/ - code sample (for mochiweb?).