@digitalbazaar/ezcap-express NPM

ezcap express library (@digitalbazaar/ezcap-express)

zcap's gettin' you down? Get on the Ezcap Express! Woot WOoot! 🚇🎉

Connect middleware that provides easy Authorization Capability (zcap) support for express.js HTTP servers and more.

Background

This library provides Node.js express middleware that can be used to protect resources on HTTP servers using Authorization Capabilities (zcaps). The library is configured with secure and sensible defaults to help developers get started quickly and ensure that their server code is production-ready.

Security

The security characteristics of this library are largely influenced by design decisions made by client and server software. For clients, implementers should pay particular attention to secure private key management. For servers, security characteristics are largely dependent on how carefully the server manages zcap registrations, zcap invocations, and zcap delegations. Bugs or failures related to client key management, or server zcap validity checking will lead to security failures. It is imperative that implementers audit their implementations, preferably via parties other than the implementer.

Install

Node.js 14+ is supported.
Web Crypto API is required by dependencies. Node.js 14 must use a polyfill.

To install from NPM:

npm install @digitalbazaar/ezcap-express

To install development code:

git clone git@github.com:digitalbazaar/ezcap-express.git
cd ezcap-express
npm install

Usage

Define getRootController

// this will only be called if `rootInvocationTarget` matches
// one of the expected root invocation targets specified
async function getRootController({
  req, rootCapabilityId, rootInvocationTarget
}) {
  // get controller for a service object from a database
  let controller;
  try {
    const record = await database.getMyServiceObjectById({
      // typically, root invocation target is a service object ID
      id: rootInvocationTarget
    });
    controller = record.controller;
  } catch(e) {
    if(e.type === 'NotFoundError') {
      const url = `${req.protocol}://${req.get('host')}${req.originalUrl}`;
      throw new Error(
        `Invalid capability identifier "${rootCapabilityId}" ` +
        `for URL "${url}".`);
    }
    throw e;
  }

  // return the service object's controller so it will be
  // added to the root capability for the service object
  return controller;
}

Define documentLoader

import didIo from 'did-io';
import didKeyDriver from 'did-method-key';
import jldl from 'jsonld-document-loader';

const _documentLoader = new jldl.JsonLdDocumentLoader();

// support did:key
didIo.use('key', didKeyDriver.driver());

async function documentLoader(url) {
  let document;
  if(url.startsWith('did:')) {
    document = await didIo.get({did: url, forceConstruct: true});
    return {
      contextUrl: null,
      documentUrl: url,
      document
    };
  }

  // finally, try the base document loader
  return _documentLoader(url);
}

Define authorizeMyZcapInvocation

import {authorizeZcapInvocation} from '@digitalbazaar/ezcap-express';

async function authorizeMyZcapInvocation({expectedAction} = {}) {
  return authorizeZcapInvocation({
    getExpectedValues({req}) {
      const expectedHost = 'ezcap.example';
      const {localId} = req.params;
      const serviceObjectId =
        `https://${expectedHost}/${encodeURIComponent(localId)}`;
      return {
        action: expectedAction,
        host: expectedHost,
        rootInvocationTarget: serviceObjectId
      };
    },
    getRootController
  });
};

Use authorizeMyZcapInvocation

import express from 'express';
import asyncHandler from 'express-async-handler';

const app = express();

app.post('/my-objects/:localId',
  authorizeMyZcapInvocation(),
  asyncHandler(async (req, res) => {
    // your code goes here
    // req.zcap is available to provide authz information
  }));

API Reference

The ezcap approach is opinionated in order to make using zcaps a pleasant experience for developers. To do this, it makes two fundamental assumptions regarding the systems it interacts with:

The systems are HTTP-based and REST-ful in nature.
The REST-ful systems center around reading and writing resources.

If these assumptions do not apply to your system, the zcap library might be a better, albeit more complex, solution for you.

Looking at each of these core assumptions more closely will help explain how designing systems to these constraints make it much easier to think about zcaps. Let's take a look at the first assumption:

The systems are HTTP-based and REST-ful in nature.

Many modern systems tend to have HTTP-based interfaces that are REST-ful in nature. That typically means that most resource URLs are organized by namespaces, collections, and items: /<root-namespace>/<collection-id>/<item-id>. In practice, this tends to manifest itself as URLs that look like /my-account/things/1. The ezcap approach maps the authorization model in a 1-to-1 way to the URL. Following along with the example, the root capability would then be /my-account, which you will typically create and have access to. You can then take that root capability and delegate access to things like /my-account/things to let entities you trust modify the things collection. You can also choose to be more specific and only delegate to /my-account/things/1 to really lock down access. ezcap attempts to keep things very simple by mapping URL hierarchy to authorization scope.

Now, let's examine the second assumption that makes things easier:

The REST-ful systems center around reading and writing resources.

There is an incredible amount of flexibility that zcaps provide. You can define a variety of actions: read, write, bounce, atomicSwap, start, etc. However, all that flexibility adds complexity and one of the goals of ezcap is to reduce complexity to the point where the solution is good enough for 80% of the use cases. A large amount of REST-ful interactions tend to revolve around reading and writing collections and the items in those collections. For this reason, there are only two actions that are exposed by default in ezcap: read and write. Keeping the number of actions to a bare minimum has allowed implementers to achieve very complex use cases with very simple code.

These are the two assumptions that ezcap makes and with those two assumptions, 80% of all use cases we've encountered are covered.

Functions

Typedefs

authorizeZcapInvocation(options) ⇒ function

Authorizes an incoming request.

Kind: global function
Returns: function - Returns an Express.js style middleware route handler.

Param	Type	Default	Description
options	object		Options hashmap.
options.allowTargetAttenuation	boolean	true	Allow the invocationTarget of a delegation chain to be increasingly restrictive based on a hierarchical RESTful URL structure.
options.documentLoader	object		Document loader used to load DID Documents, capability documents, and JSON-LD Contexts.
options.getExpectedValues	GetExpectedValues		Used to get the expected values when checking the zcap invocation.
options.getRootController	function		Used to get the controller of the root capability in the invoked capability's chain.
options.getVerifier	function		An async function to call to get a verifier and verification method for the key ID.
options.inspectCapabilityChain	function		A function that can inspect a capability chain, e.g., to check for revocations.
options.maxChainLength	number	10	The maximum length of the capability delegation chain.
options.maxClockSkew	number	300	A maximum number of seconds that clocks may be skewed when checking capability expiration date-times against `date`, when comparing invocation proof creation time against delegation proof creation time, and when comparing the capability invocation expiration time against `now`.
options.maxDelegationTtl	number	100060602490	The maximum milliseconds to live for a delegated zcap as measured by the time difference between `expires` and `created` on the delegation proof.
options.onError	function		An error handler handler for customizable error handling.
options.suiteFactory	object		A factory for creating the supported suite(s) to use when verifying zcap delegation chains; this is different from `getVerifier` which is used to produce a verifier for verifying HTTP signatures used to invoke zcaps.

authorizeZcapRevocation(options) ⇒ function

Authorizes a request to submit a zcap revocation.

This middleware is opinionated; it MUST be attached to an endpoint that terminates in /zcaps/revocations/:revocationId. This to enable the middleware to automatically generate expected values for running zcap checks and to support a common, conventional revocation API pattern.

The pattern is in support of controlled objects on a service, aka "service objects". Each object's controller is used to populate the root zcap for the object's controller field. This root zcap has an invocation target that matches the URL for the service object, aka its "serviceObjectId".

Therefore, any route that matches an invocation target for a root zcap for a service SHOULD attach this middleware to:

<serviceObjectId>/zcaps/revocations/:revocationId.

This middleware will compute serviceObjectId by combining the expected host with the subpath from the request URL that occurs before /zcaps/revocations/. It assumes that the request URL will have this pattern if the middleware code has been reached. IOW, serviceObjectId will be set using:

https://<expectedHost>/<URL subpath before "/zcaps/revocations/">.

Note: This middleware does NOT support having /zcaps/revocations/ appear multiple places in the request URL.

Attaching this middleware will enable any zcaps delegated from the service object's root zcap to be revoked without having to issue an additional zcap to use the revocation endpoint. This middleware makes that possible by supporting the invocation of a dynamically generated root zcap with an invocation target of:

<serviceObjectId>/zcaps/revocations/:revocationId.

This middleware will set the controller of this root zcap to all controllers in the to-be-revoked zcap's delegation chain, permitting any participant to revoke it. An error will be thrown prior to populating this controller field if the root zcap in the to-be-revoked zcap's chain does not have <serviceObjectId> as its invocation target (or a prefix of it). This ensures that the only zcaps that have been delegated from a root zcap using the service object's ID as part of its invocation target can be revoked at its /zcaps/revocations route, i.e., other zcaps intended for other service objects -- or entirely other services -- cannot be revoked via this middleware.

This middleware will automatically generate two sets of expects values: one for checking the invocation to revoke a capability and one for verifying the delegation chain of the capability that is to be revoked. Only the expected host value can and must be given as a parameter.

The expected values for checking the capability invocation will be:

host: <expectedHost>, rootInvocationTarget: // root zcap with this target, RZ1, can be delegated w/target attenuation // to allow delegates to revoke any zcap, Z1, with RZ1 as the root in its // chain, even if the delegate is not a controller in Z1's chain <serviceObjectId>, // root zcap that this target, RZ2, can be used to revoke a zcap, Z2, // with an "id" of revocationId; RZ2's controller will be populated // using all controllers from Z2's chain, enabling any controller in that // zcap's chain to invoke RZ2 to revoke Z2 <serviceObjectId>/zcaps/revocations/<revocationId>, , action: 'write' .

Kind: global function
Returns: function - Returns an Express.js style middleware route handler.

Param	Type	Description
options	object	Options hashmap.
options.documentLoader	object	Document loader used to load DID Documents, capability documents, and JSON-LD Contexts.
options.expectedHost	string	The expected host header value when checking the zcap invocation.
options.getRootController	function	Used to get the controller of the root capability for the service object.
options.getVerifier	function	An async function to call to get a verifier and verification method for the key ID.
options.inspectCapabilityChain	function	A function that can inspect a capability chain, e.g., to check for revocations; it will be used when verifying the invocation and the delegation chain for the to-be-revoked capability.
options.onError	function	An error handler handler for customizable error handling.
options.suiteFactory	object	A factory for creating the supported suite(s) to use when verifying zcap delegation chains; this is different from `getVerifier` which is used to produce a verifier for verifying HTTP signatures used to invoke zcaps.

GetExpectedValues

Kind: global typedef

GetExpectedValues ⇒ ExpectedValues

A function for returning expected values when checking a zcap invocation.

Kind: global typedef
Returns: ExpectedValues - - The expected values.

Param	Type	Description
options	object	The options passed to the function.
options.req	object	The express request.

ExpectedValues : object

The expected values for checking a zcap invocation performed via an HTTP request.

Kind: global typedef
Properties

Name	Type	Description
action	string	The expected capability action; if no action is specified during an invocation check, then a default action will be determined based on the HTTP method from the request -- which is only safe provided that the handler code path is also determined based on the HTTP method in the request (i.e., typical method-based express/connect routing); if the handler code path is determined by some other means, e.g., the request body, then `action` MUST be set.
host	string	The expected host in the request header.
rootInvocationTarget	string \| Array	The expected invocation target for every acceptable root capability; each string must express an absolute URI.
target	string	The expected invocation target; if no target is specified during an invocation check, then the target will default to the absolute URL computed from the relative request URL and expected host value.