Access Control with Objects

In our third smart contract, we will demostrate how to control access to different functions of a smart contract. So far, we have only used publicFacet to expose all functions. There is an other facet, called creatorFacet that is provided only to the caller who creates the contract instance. In this smart contract, we limit the publicFacet API to a read-only function get(), and use creatorFacet API to expose the set() method to the caller who creates the contract instanace.

Here is the complete code for 03-access.js smart contract:

import { Far } from '@endo/far';

export const start = () => {
  let value = 'Hello, World!';
  const get = () => value;
  const set = v => (value = v);

  return {
    publicFacet: Far('ValueView', { get }),
    creatorFacet: Far('ValueCell', { get, set }),
  };
};

We can write a simple test as below to make sure that trying to set using the publicFacet throws an exception, but using the creatorFacet works:

test('access control', async t => {
  const { publicFacet, creatorFacet } = access.start();
  t.is(await E(publicFacet).get(), 'Hello, World!');
  await t.throwsAsync(E(publicFacet).set(2), { message: /no method/ });
  await E(creatorFacet).set(2);
  t.is(await E(publicFacet).get(), 2);
});

Note that the set() method has no access check inside it. Access control is based on separation of powers between the publicFacet, which is expected to be shared widely, and the creatorFacet, which is closely held. We'll discuss this object capabilities approach more later. If you're having trouble, check out the tut-03-access branch in the example repo.

Object Access Rules

The object access rules include introduction, parenthood, endowment, and initial conditions, which help manage and control access to objects within the contract:

• Introduction: Objects can only reference other objects that they have been introduced to. This prevents unauthorized access by ensuring that only known objects can interact.
• Parenthood: Objects can create child objects. The parent object has control over the child object’s initial state and capabilities, defining what the child can and cannot do.
• Endowment: Objects can be endowed with certain capabilities or resources upon creation. This allows the contract to control what actions an object can perform based on its endowments.
• Initial Conditions: Objects are initialized with certain conditions or states. These initial conditions define the starting point for the object’s behavior and interactions.

Also see Object Capability Model

Next, let's look at minting and trading assets with Zoe.