How do you get app_scoped_privkey on the frontend for server side verification?

[JSeam2]

I need some clarifications regarding the app_scoped_privkey as this seems like a crucial detail you need to authenticate the jwt tokens obtained from the idToken

I am following the tutorial found here:
https://web3auth.io/docs/server-side-verification/social-login-users

I know how to obtain the idToken. This is possible using something like const user = await web3AuthInstance.getUserInfo();

However, there seems to be missing documentation regarding the app_scoped_privkey within the web3auth docs.

The documentation states

App scoped private key will be obtained from the frontend once the user is logged in.

and then shows the following snippets

// Incase of ed25519 curve
import { getED25519Key } from "@toruslabs/openlogin-ed25519";
const app_scoped_privkey = "app scoped private key";
const ed25519Key = getED25519Key(Buffer.from(app_scoped_privkey.padStart(64, "0"), "hex"));
const app_pub_key = ed25519Key.pk.toString("hex");

// Incase of secp256k1 curve
import { getPublicCompressed } from "@toruslabs/eccrypto";
const app_scoped_privkey = "app scoped private key";
const app_pub_key = getPublicCompressed(Buffer.from(app_scoped_privkey.padStart(64, "0"), "hex")).toString("hex");

There seems to be missing instructions regarding how app_scoped_privkey can be obtain atm. Would need some help on this

[JSeam2]

Ok I think I have solved this was observing this tutorial found here https://github.com/Web3Auth/examples/blob/main/web-core-sdk/server-side-verification/ssv-via-social-nextjs-core-example/pages/App.tsx

Gist of how to get the private key, you will need to call the web3auth.provider to obtain the private key which then allows you to obtain the public key using the @toruslabs/eccrypto library using the getPublicCompressed() function.

This snippet or link to the sample implementation should be included in the Server Side Verification docs here https://web3auth.io/docs/server-side-verification/social-login-users. It is an important missing step

  const getUserInfo = async () => {
    if (!web3auth) {
      uiConsole('web3auth not initialized yet')
      return
    }
    const user = await web3auth.getUserInfo()

    const privKey = await web3auth.provider?.request({
      method: 'eth_private_key',
    })
    const pubkey = getPublicCompressed(Buffer.from(privKey, 'hex')).toString(
      'hex',
    )

    // Validate idToken with server
    const res = await fetch('/api/login', {
      method: 'POST',
      headers: {
        'Content-Type': 'application/json',
        Authorization: 'Bearer ' + user.idToken,
      },
      body: JSON.stringify({appPubKey: pubkey}),
    })
    if (res.status === 200) {
      console.log('JWT Verification Successful')
      uiConsole(user)
    } else {
      console.log('JWT Verification Failed')
      uiConsole('JWT Verification Failed')
    }
  }

[dangdennis]

Why do we need the appkey for server side verification? We already have your public jwks to verify the jwt (idToken) as a whole. If we don't verify the app scoped public key, what attack vector are we exposing ourselves to?

[JSeam2]

It seems to be something that’s recommended on the docs atm.

I think some kind of signing to validate the signature would make more sense tbh. Now that you’ve mentioned, the flow used does give out the same public key. Would like the web3auth team to clarify on this.

In terms of attack vectors the possibility of calling the priv_key into the memory of some browser app could be the priv_key being stolen by some back door on either the browser or host device.

Another attack vector of not verifying some signature signed using the priv_key is the possibility of a man in the middle where someone sniffs out the public_key stores in the idToken and pretends to be that identity.