identifying caller in capnproto RPC - c++

Question

I am implementing a service in CapnProto. The service follows these steps (roughly):
authenticate on the server
do operations through a Service interface (object-capability) once authenticated.
I want to achieve something like the following:
interface Authorization {
login #0 (userName :User) -> (result :Service);
}
interface Service {
# doOperation needs userName in order to operate, as an implicit
# parameter, when the RPC arrives. I do not want to use an explicit
# userName parameter. Otherwise, a user could claim to
# be someone else in a function parameter. To achieve this I need
# to know who is the userName that holds this capability from inside
# doOperation. I want to avoid token authentication
# for each operation.
# Thus, the RPC I am implementing is stateful.
doOperation #0 (param1 :A);
#...
}
What I want is, that from doOperation, I can identify the user that is using that capability (I want to know her userName). Namely:
What I have solved is that the user using the Service capability is known to have that permission (since the Service is the result of calling login)
The problem is that I have many of those users, and, for each of them, I want to do the matching between the user of the Service capability and her login in the first step.

Answer 1

It turns out that this was very simple.
When creating Service interface in code, just pass the authentication information and save it in the Service object, like this:
class ServiceImpl : public Service::Server {
string userId_;
public:
explicit ServiceImpl(string userId) : userId_(move(userId)) {}
protected:
kj::Promise<void> doOperatoration(DoOperationContext ctx) override {
//use userId_ here
}
};
class AuthorizationImpl : public Authorization::Server {
protected:
kj::Promise<void> login(LoginContext ctx) override {
std::string user = ctx.getParams().getUserName();
//Here I forward the authentication state to the service
ctx.getResults().setService(kj::heap<ServiceImpl>(user);
//..
}
};