Mobile App Authentication with Rails webservice

I am writing webservice for a Mobile app.

Now My Clients requirement is that authentication of API should be done by An Access Key which will come along with Webservice call. And that access Key will change according to session.

I searched many places and found some of the article and railscast that Oauth or Some of the article is suggesting that cancan gem can also act as Role based management.

But I need authentication for that particular access key.

Please give some suggestion.