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Delegating and restricting authority

Any interesting program will need to interact with the outside world, like accessing the network or the file system, or by creating and communicating with other programs. We call ambient authority all those rights implicitly granted to the program to make these things possible, and because Pony makes this concept explicit, we need to take the time to talk about what it means and how it works. In other languages like for example C, you can always attempt to do an operation like I/O, and it will usually succeed save some runtime checks (your disk being full might make an operation fail, for example). But in Pony, any piece of code interacting with the outside world needs the authority to do so.

The operating system essentially knows nothing about the structure of any program it runs, in particular it is ignorant of any Pony specific concepts. In order to impose that some code is authorized for an operation like writing to disk, Pony requires a special argument to be passed, a capability bearing the authority required for the task at hand. The type system guarantees that inadequate capabilities for a task fail at compile time.

Recall the definition of capability from the Object Capabilities section:

A capability is an unforgeable token that (a) designates an object and (b) gives the program the authority to perform a specific set of actions on that object.

In Pony, the Main actor is created with an Env object, which holds the unforgeable AmbientAuth token in its root field. This value is the capability that represents the ambient authority given to us by the system.

Here is a program that connects to via TCP on port 80 and quits:

use "net"

class MyTCPConnectionNotify is TCPConnectionNotify
  let _out: OutStream

  new iso create(out: OutStream) =>
    _out = out

  fun ref connected(conn: TCPConnection ref) =>

  fun ref connect_failed(conn: TCPConnection ref) =>

actor Connect
  new create(out: OutStream, auth: AmbientAuth) =>
    TCPConnection(auth, MyTCPConnectionNotify(out), "", "80")

actor Main
  new create(env: Env) =>
    try Connect(env.out, env.root as AmbientAuth) end

The Main actor authorizes the Connect actor by passing the ambient authority token on to it. This token is unforgeable since the AmbientAuth constructor is private and the only existing instance is provided by the runtime itself.

The Connect actor uses this authority when it creates a TCP connection:

TCPConnection(auth, MyTCPConnectionNotify(out), "", "80")

The TCPConnection requires an authority as first parameter, and since the compiler checks that the correct type was passed, this guarantees that a TCPConnection can only be created by an actor holding the required authorization.

The implementation of the TCPConnection constructor does not even use the authorization parameter at run time, all it does is require it to be of the right type. The type checking done by the compiler is sufficient for this guarantee.

Restrict, then delegate your authority

In order to handle our own code and that of others more safely, and also to understand our code better, we want to split up the authority, and only grant the particular authority a piece of code actually requires.

Imagine we don’t trust the Connect actor, so we don’t want to provide it with more authority than needed. For example, there is no point in granting it filesystem access, since it is supposed to do network things (specifically, TCP), not access the filesystem. To understand how we can achieve this, let’s look at the authority types involved.

The first parameter of the TCPConnection constructor has the type

type TCPConnectionAuth is (AmbientAuth | NetAuth | TCPAuth | TCPConnectAuth)

so instead of passing the entire AmbientAuth (the root of all authority), we can “downgrade” that to a TCPConnectAuth (the most restrictive authority in net), pass it to the Connect actor, and have that pass it to the TCPConnection constructor:

actor Connect
  new create(out: OutStream, auth: TCPConnectionAuth) =>
    TCPConnection(auth, MyTCPConnectionNotify(out), "", "80")

actor Main
  new create(env: Env) =>
    try Connect(env.out, TCPConnectAuth(env.root as AmbientAuth)) end

Now we are sure it cannot access the filesystem or listen on a TCP or UDP port.

Authorization-friendly interface

Consider the above example again, but this time let’s think of the Connect actor being part of a 3rd party package that we are building. Our goal is to write the actor in such a way that users of our package can grant it only the authority necessary for it to function.

As the package author, it is then our responsibility to realize that the minimal authority possible is the TCPConnectAuth. But anything that the TCPConnection constructor accepts is valid, and a user of our package might be strict and pass a TCPConnectAuth, or might be lax and just pass the AmbientAuth.

Our current implementation already satisfies all this, because TCPConnectionAuth is a union type of all possible authorization types and we use that. If we want, we could also write

type ConnectActorAuth is TCPConnectionAuth

and use this type alias instead, signalling to the user that anything that is a TCPConnectionAuth is allowed as the authority token passed to the Connect actor.

Authority hierarchies

Looking again at the type of the authorization parameter to TCPConnection,

type TCPConnectionAuth is (AmbientAuth | NetAuth | TCPAuth | TCPConnectAuth)

you might notice that this looks like a hierarchy of authorities:

AmbientAuth >> NetAuth >> TCPAuth >> TCPConnectAuth

where in this paragraph, “>>” means “grants at least as much authority as”. In fact, the AmbientAuth encompasses all ambient authority and is a strictly larger authority than NetAuth, which grants access to the network, which is more powerful than TCPAuth which is restricted to the TCP protocol. Finally, TCPConnectAuth is good only for creating a TCPConnection.

This hierarchy is established by means of the constructor of the weaker authority accepting one of the stronger authorities, for example

new val create(from: (AmbientAuth val | NetAuth val)) : TCPAuth val^