I don't understand what you are asking.

Sorry it was not clear. I'm using for a Restlet the User Credentials authentication and not the oauth authentication. But this authentication method is not so secured because all the credentials are passed as parameters. I was wondering if it was possible to add a certificate (or a condition) to only allow one application calling my Restlet with the user credentials. So, if another application tries to connect with the User credentials to the Restlet, it will not work.

User credentials are user credentials. For more security you can limit the role it uses to just what it absolutely must have for it to work.

ok thanks