πŸ”“Backing up a wallet

This guide will walk you through how backups of a user's wallet are set up with Portal.

MPC backups allow your users to recover their MPC wallets in the event that their device is lost, stolen, or is bricked.

Backups are handled in two pieces: user MPC backups and custodian MPC backups.

At the time of recovery, these two backups are used together to generate new shares to be stored on-device, allowing the user to migrate their wallet with a new device.

User MPC backups

Storing a user MPC backup is done by storing an encryption key with the user's cloud storage provider and the encrypted share within your infrastructure.

In order to support user MPC backups, three main dependencies must be met:

  1. You must have cloud storage configured for storing the user's share of the MPC backup

  2. Your server must be able to store the encrypted share

  3. Your mobile app must send the encrypted share to your server for storage

Setting up your cloud storage adapter

With our kotlin implementation, we support Google Drive.

See Configure Gdrive Storage for more info on how to configure google drive.

Storing encrypted shares

On your server, create an endpoint to accept the encrypted share and store it.

server.js
/*
 * This endpoint can be defined by you however you want
 */
app.post('/portal/clients/:clientId/backup', async (req, res) => {
  const { clientId } = req.params
  const { backupShare } = req.body
  
  // Example db function updating a "users" table with the backupShare
  await db.users.update({
    where: {
      clientId,
    },
    data: {
      backupShare,
    },
  })
  res.status(200).send()
})

Sending shares to your server

In order to add support for user MPC backups to your mobile app, you must perform two steps:

  1. Generate an encrypted client backup share using portal.mpc.backup.

  2. Send the resulting backup share to your API and store it.

  3. Call portal.api.storedClientBackupShare to notify Portal that the encrypted client backup share was saved successfully. (Alternatively you can make an HTTP request to our API directly.)

In the example below, we back up a wallet with a Backup Wallet button that utilizes Google Drive storage:

package io.portal.android.app

// Imports...

class MainActivity : AppCompatActivity() {
    lateinit var portal: Portal
    lateinit var backupButton: Button
    
    // Your API instance.
    private val exchangeApi: Api = Api()
    
    // The user from your API instance.
    lateinit var user: User
    
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        backupButton = findViewById(R.id.backupButton)
        backupButton.setOnClickListener { handleBackup() }
    }
    
    override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
        super.onActivityResult(requestCode, resultCode, data)

        if (portal.backup.drive.auth.onActivityResult != null) {
            portal.backup.drive.auth.onActivityResult!!(requestCode, resultCode, data)
        }
    }

    private fun handleBackup() {
        CoroutineScope(Dispatchers.IO).launch {
            try {
                // Get an encryped client backup share.
                val backupShare = portal.mpc.backup() { status ->
                    // Do something with the status, such as update a progress bar
                    // or log the progress
                    Log.println(Log.INFO, "[PortalEx]", "Backup status: ${status.status} is done: ${status.done}")
                }
                
                // Send the backup share to your API and store it.
                exchangeApi.storeCipherText(user.id, backupShare)
                
                // βœ… Notify Portal that the backup share was stored! πŸ™Œ
                portal.api.storedClientBackupShare(true)
            } catch (err: Throwable) {
                // ❌ Notify Portal that the backup share was not stored.
                portal.api.storedClientBackupShare(false)
            }
        }
    }
}

Status Flow

ReadingShare("Reading share")
GeneratingShare("Generating share")
ParsingShare("Parsing share")
EncryptingShare("Encrypting share")
StoringShare("Storing share")
Done("Done")

Custodian MPC backups

Storing a custodian MPC backup is done by Portal generating a custodian backup share and sending the share – via webhook – to be stored within your infrastructure.

In order to support custodian MPC backups, two main dependencies must be met:

  1. Register a webhooks_root with Portal

  2. Implement the /{webhooks_root}/backup route in your server to store your backup share

Registering your webhook root

You can register your webhooks_root via the Portal REST API.

Implementing the webhook

Portal will send the custodian MPC backup share to /{webhooks_root}/backup via a POST request. The body of this POST request will contain two fields:

  • clientId - The Portal clientId for the user

  • share - a JSON.stringified version of the backup share

server.js
import express from 'express'

const app = express()

app.post('/webhook/backup', async (req, res) => {
  // Destructure the provided fields from the request body
  const { 
    clientId, // The Portal clientId for the user
    share, // a JSON.strinfigied version of the backup share
  } = req.body
  
  // TODO: Store the backup share within your infrastructure
  
  res.status(200).send()
})

When Portal makes a request to your /backup webhook, another immediate request is made to /backup/fetch in order to validate the backup share was stored successfully.

Next steps

Now that you've successfully implemented MPC backups, your app can support MPC recovery. In the next section, we will walk through the recovery process.

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