Awesome
webview-crypto
This repo provides some helper tools to run the Web Cryptography API in a WebView.
It is used in
react-native-webview-crypto
,
nativescript-webview-crypto
, and
nativescript-angular-webview-crypto
. It is not meant to be used directly, but simply serves as a common building
block for those libraries.
Why?
The Web Cryptography API is implemented in all major browsers and provides performant and secure way of doing client side encryption in JavaScript. However it is not supported in NativeScript or React Native, which limits them from using Javascript libraries that depend on Web Crypto.
Luckily, the iOS and Android browser engines do support this API. We can use their implementations by creating a WebView and communicating with it asynchronously.
Usage
We provide two entrypoints in this repo.
Main Thread
MainWorker
is used in your main thread. It communicates to the WebView
asynchronously with string messages, providing a crypto
attribute
that fulfills the Crypto
interface. If you set this to be globally defined, all applications that depend
on window.crypto
will work transperently.
import {MainWorker} from "webview-crypto";
function sendToWebView(message: string): void {
// sends `message` to the webview
}
var mw = new MainWorker(sendToWebView); // optional second argument for debug on or off
// call `mw.onWebViewMessage` whenever you get a message from the WebView
onWebViewMessage(mv.onWebViewMessage.bind(mv));
mw.crypto.subtle.generateKey(
// whatever
)
window.crypto = mw.crypto;
WebView
WebViewWorkerSource
is a string that contains the source defining
a WebViewWorker
constructor that should be used in your WebView.
After loading that Javascript in the WebView, initialize
WebViewWorker
so that it can communicate with the main thread and do the
work of executing the cryptography.
function sendToMain(message: string): void {
// send `message` to the main thread
}
var wvw = new WebViewWorker(sendToMain);
// call `wvw.onMainMessage` whenever you get a message from the main thread
onMainMessage(wvw.onMainMessage.bind(wvw));
Tests
We have some unit tests for basic behavior here.
Run npm run test:local
to run them in a local browser. You also need to run
npm run build:watch
to recompute the webViewWorkerString
injected as needed.
In Travis CI, they run on iOS, Android, and Chrome through SauceLabs.
While these tests do help catch some bugs, they do not provide any strong reassurance that this library will work in React Native and Typescript. That's because on those platforms, half the code is running in a WebView and the other half in their native JavaScript engine, which is either JavaScriptCore or V8. I haven't come up with a way to test this in an automated fashion.
So in addition to local unit tests, all code changes that might break something should be tested against the example repos (React Native and NativeScript) on both iOS and Android.
I welcome suggestions on improving this process and making it more automated.
Caveats
While this attempts to as stick to the Web Cryptography API as possible, this is impossible in a few situations due to the differing browser implementations.
Incomplete Support
This library is limited by the mobile browser's support. On iOS, the WebView's use WebKit, which has limited and incomplete support (example). If something isn't working, that might be why. Try it on Safari and see if it works there.
getRandomValues
Since this uses an asynchronous bridge to execute the crypto logic it
can't quite execute crypto.getRandomValues
correctly, because that method
returns a value synchronously. It is simply impossible (as far as I know,
please let me know if there any ways to get around this) to wait for the
bridge to respond asynchronously before returning a value.
Instead, we add a _promise
attribute to the TypedArray
you passed in. This resolves
when the TypedArray
has been filled with random values.
Also, on all crypto.subtle
methods that takes in
TypedArray
s, we will automatically wait for it to resolve. This means that if you
are using the TypedArray
in further cryptographic code, it will work transparently.
So hopefully existing code that uses the Web Cryptography API will continue to work
without modification.
CryptoKey
Since JavaScriptCore
does not support window.Crypto
, it also doesn't have a CryptoKey
interface.
So instead of returning an actual CryptoKey
from
subtle.generateKey()
we instead return an object that confirms to the CryptoKey
interface and has
a _import
property that has the value of the key exported as jwk
or using
the value for importing the key. This allows
you to treat the CryptoKey
as you would normally, and whenever you need to use
it in some subtle
method, we will automatically convert it back to a real
CryptoKey
from the _import
string and the metadata.
This project was funded by Burke Software and Consulting LLC for passit.