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2.0.1 • Public • Published

Module Keys

Communications channels between modules that allow granting different privileges to some modules than others.

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Why Module Keys?

Module Keys provide a way for one module to grant privileges to another module without granting that privilege to every module.

A development team might npm install example-package and trust it to work as advertised, but not trust all of its dependencies or its dependencies' dependencies with unmitigated access to powerful APIs like child_process since subtle bugs can have disastrous consequences when exposed to attacker-controlled inputs.

Module keys enable secure code patterns like those in the examples below. They can combine with module loader hooks and other mechanisms to bound the security consequences of common bugs and the amount of code that might be involved in certain kinds of security failures helping security reviewers to focus their attention.

See also node-sec-patterns which makes these patterns easy to express.

Module keys allow code written in good faith to cooperate while avoiding lowest-common-denominator security problems. It does not allow safely running malicious code within the same process. Potentially malicious code should be sandboxed if it needs to run at all.


$ npm install --save module-keys

Babel Plugin

The babel plugin will add keys to your modules.

Via .babelrc (Recommended)

Add the following line to your .babelrc file:

  "plugins": [
    [ "module-keys/babel", { "rootDir": "/path/to/module/root" } ]

The optional "rootDir" option lets you specify the base URL used to compute relative module identifiers.


babel --plugins module-keys/babel script.js

Via Node API

require("@babel/core").transform("code", {
  plugins: [
    [ "module-keys/babel", { "rootDir": "/path/to/module/root" } ]

CommonJS Modules

Once you've run the Babel plugin over your modules, each module will have its own keys available via require.moduleKeys, and will export its publicKey if doing so would not conflict with an explicit export.

require.moduleKeys.box(...);  // boxes a value.  See API below

ES6 Modules

The Babel plugin will treat any source file with an export declaration as an ES6 module, and instead define a local moduleKeys which has the API below.

moduleKeys.box(...);  // See API below


class Box

A box is a container for a value that may only be opened by an authorized opener.

Boxes are opaque values, and the only way to access the contained value is to use an .unbox method as described below.

const { Box } = require('module-keys');       // CommonJS
import { Box } from './path/to/module-keys';  // ES6


.box(value, mayOpen) creates a Box that may only be opened by an .unbox method.

value - returned when the returned Box is unboxed.

mayOpen - a function that takes a public key. It should return true if the public key identifies an unbox method that should be allowed access to value.

Returns an instance of class Box.

// CommonJS
const { publicKey: fooKey } = require('./foo');
const box = require.moduleKeys.box(value, (k) => k === fooKey && k());
// box may only be opened via ./foo's unboxer.
// ES6
import { publicKey as fooKey } from './foo';
export const box = moduleKeys.box(value, (k) => k === fooKey && k());


.unbox(box, ifFrom, fallback) opens a Box while optionally checking its source.

box - A Box

ifFrom - A function that takes a public key. It should return true if the caller wishes to receive values boxed by the .box method associated with the key.

fallback - A value to return if unboxing fails. Defaults to undefined.

Returns the boxed value if mayOpen(publicKey) is true and ifFrom returns true when passed the boxer's public key. Otherwise returns fallback.

// CommonJS
const { publicKey: barKey } = require('./bar');
function f(box) {
  console.log(`I got ${ require.moduleKeys.unbox(box, () => true, 'a box I cannot open') }`)
// ES6
import { publicKey as barKey } from './bar';
function f(box) {
  console.log(`I got ${ moduleKeys.unbox(box, () => true, 'a box I cannot open') }`)


.unboxStrict(box, ifFrom) is the same as .unbox(box, ifFrom) but raises an Error if unboxing fails.


isPublicKey(x) is true if x is a public key.

This may be called in key predicates to ensure that keys will not themselves perform an operation that enters a private key context.

const { isPublicKey } = require('module-keys');  // CommonJS
import { isPublicKey } from 'module-keys';  // ES6 modules


.privateKey(f) is a function that calls f() and returns its results. Any calls to .publicKey() during the call to f() will return true.

f - a zero argument function

Returns - the result of calling f().

Each private key refers to its corresponding public key via its publicKey property, so to pass a key pair, it is sufficient to pass the private key. Public keys do not refer to private keys.

Warning: Do not export your private keys as that may allow other code to impersonate you. If you need to provide your private key to a module you trust, put it in a box that is only openable by that module.


.publicKey() returns true if called in the context of its corresponding private key or false otherwise.

The Babel plugin auto-exports public keys for all processed modules.

// Getting a public key.
// CommonJS.
const { publicKey: fooPublicKey } = require('./foo');
// Getting a public key.
// ES6 modules
import { publicKey as fooPublicKey } from './foo';

Each publicKey also has a moduleIdentifier property which specifies the location of the module relative to the module root.


makeModuleKeys() returns a new module keys bundle with its own .box, .unbox, .unboxStrict, .publicKey, and .privateKey properties.

const { makeModuleKeys } = require('module-keys');  // CommonJS
import { makeModuleKeys } from 'module-keys';       // ES6 modules


publicKeySymbol is a Symbol that may be used to unambiguously attach a public key to a JavaScript object.

CommonJS export bundles have a symbol property that refers to the public key in addition to any "publicKey" property.

const { publicKeySymbol } = require('module-keys');  // CommonJS
import { publicKeySymbol } from 'module-keys';       // ES6 modules

This is not an official Google product.


npm i module-keys

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