public class MyAuthorizeAttribute: FilterAttribute, IAuthorizationFilter
{
    public void OnAuthorization(AuthorizationContext filterContext)
    {
        var key = filterContext.HttpContext.Request.QueryString["param_name"];
        if (!IsValid(key))
        {
            // Unauthorized!
            filterContext.Result = new HttpUnauthorizedResult();
        }
    }

    private bool IsValid(string key)
    {
        // You know what to do here => go hit your RavenDb
        // and perform the necessary checks
        throw new NotImplementedException();
    }
}