How to use a strong random source on Expo

[klaop57]

I'm trying to use ethers.js inside Expo, but whatever I do it keeps telling me:

WARNING: Missing strong random number source; using weak randomBytes

Is there any way to use a strong random source. I know expo has expo-random, but I see no clear way of making ethers.js use that.

[ricmoo]

I need to experiment with expo to figure out how to do this, but haven’t had a chance. I don’t use Expo myself, but this is a common question.

For most operations you do not need this though, as it is only used for Wallet.createRandom.

[klaop57]

thanks for the quick response.

that's actually the thing i need, creating a random wallet! together with signing a transaction.

I've forked ether.js and am trying to get it to work. Will post here on success.

[ricmoo]

I see. In the meantime, you can implement createRandom easily. :)

function myCreateRandom() {
    const randomBytes = ... // Use expo's method to create 16 bytes of random data
    const mnemonic = ethers.utils.HDNode.entropyToMnemonic(randomBytes);
    return Wallet.fromMnemonic(mnemonic);
}

Which should get you up an running without needing to fork for now. :)

[klaop57]

thanks, that worked 🎉

for the next person trying to get this to work:

import * as Random from 'expo-random';
import 'ethers/dist/shims.js';
import { ethers } from "ethers";

const createWallet = async () => {
  const randomBytes = await Random.getRandomBytesAsync(16);
  const mnemonic = ethers.utils.HDNode.entropyToMnemonic(randomBytes);
  const wallet = ethers.Wallet.fromMnemonic(mnemonic);
  return wallet;
}

[ricmoo]

Thanks! I’ll add that in the docs for Expo. :)

[billtlee]

In my case, I have

import * as Random from 'expo-random';
import 'ethers/dist/shims.js';
import { ethers } from 'ethers';

export async function generateMnemonics() {
  // return utils.HDNode.entropyToMnemonic(utils.randomBytes(16)).split(' ');
  const randomBytes = await Random.getRandomBytesAsync(16);
  return utils.HDNode.entropyToMnemonic(randomBytes).split(' ');
}

But I am still getting that warning

[klaop57]

yes, i am also still getting the warning. But I think the unsafe random source is not used since this function takes in random bytes directly from the caller.

[pcowgill]

Here's a version of TweetNaCl.js for React Native that uses expo-random as the source of randomness https://github.com/rajtatata/react-native-expo-tweet-nacl

[pcowgill]

Thanks! I’ll add that in the docs for Expo. :)

@ricmoo Did this land anywhere in the v5 docs? Thanks!

[ricmoo]

I've referenced this loosely here and think the better way to support this is to use the crypto native package it mentions. Does that work in Expo?

[ricmoo]

I'm going to close this now, but if you'd like anything more added to the docs please continue discussing or re-open the issue.

Thanks! :)

[Albert-Gao]

@ricmoo expo indeed has a Crypto polyfill, but using this still gives me a warning

https://www.npmjs.com/package/expo-standard-web-crypto

[ricmoo]

@Albert-Gao The import statements in ESM will all occur before any code is executed except in files which also import the same library.

So in your above example, all 3 imports occur before the call to polyFillCrypto(). It is somewhat of a nuisance for polyfilling.

Can you verify the window.random is the secure random functions after the import? To see which shim took priority? I wonder if adding side-effects: true to the crypto shim will help the bundler?