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I'm trying to achieve what seems like a simple goal but having trouble finding a simple solution to it. Here's my problem:

Given a private key, I would like the simplest way to "inject" this key into a web app context so that all subsequent usage of the web3 object uses this as web3.eth.defaultAccount.

Before someone brings up security issues, I would like to emphasize that we can ignore security implications for now. It's a special case and I have solutions to deal with the security issue as long as this is possible.

The only solution I've found so far has been to create a custom provider and instantiate the Web3 instance with this custom provider which overrides certain methods like sendTransaction, etc.--this is how MetaMask seems to work using https://github.com/MetaMask/provider-engine.

But setting up all these custom providers just for what I'm trying to achieve--take a private key string and turn it into web3.eth.defaultAccount--feels like an overkill.

Any help would be appreciated. Thank you!

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    Did you research web3 v1.0? It has web3.eth.accounts.privateKeyToAccount (I didn't try that api yet). For web3 v0.20 I've used truffle-hdwallet-provider, it uses MetaMask's provider-engine but it easier to setup (Although you have to provide the mnemonic instead of the raw private key). – Ismael Nov 6 '17 at 21:21
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    @Ismael Yes indeed! I actually found the solution after I posted and just managed to get it to work just now. I can confirm this works. The trick is to inject it into web3.eth.wallet – Vlad Nov 6 '17 at 21:35
  • It should be good if you write what are you doing so it can help others in similar situation. – Ismael Nov 6 '17 at 21:42
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Using web3js 1.0.0:

const privateKey = 'e0f34403.................................29c8c861937';
const account = web3.eth.accounts.privateKeyToAccount('0x' + privateKey);
web3.eth.accounts.wallet.add(account);
web3.eth.defaultAccount = account.address;

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