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Phone as a wallet

A guide to store bitcoin on a clean Android or iPhone secured with multisignature in the Blockstream Green Wallet.
A recommendation to people who are looking into how to take custody of their first satoshis and not running their own node yet.

Why a phone?

  • generic hardware (there is no suggestion of it containing valuable keys)
  • minimised supply chain attack (many are used without bitcoin involved)
  • robust architecture with a secure chip (in the SoC or separate as the Titan M in Google Pixels from 3/3A onwards)
  • capable of connecting to the internet and initiate a bitcoin transaction (vs a hardware wallet needs another computer and additional software)

Why the Blockstream Green wallet?

Phone specs

  • use a dedicated device to minimise the attack surface
  • can be a used phone sitting in the drawer, but the more recent model it is the better
  • locked bootloader (true to all factory firmwares - see the notes on custom firmwares below)
  • encrypted storage - default on iPhones and newer Androids when a screen lock with a PIN is set

Steps done on the phone

  • set up in a safe environment and network
  • apply the latest security update
  • perform a factory reset
  • apply the most strict privacy settings and log in to only the bare minimum services
  • apply the strongest possible PIN or passphrase to the lock screen, store it in a password manager, avoid biometrics
  • download the Blockstream Green wallet from the App Store / Play Store / F-droid / GitHub
  • set up Green Wallet with the 6 character PIN and Two-Factor Authentication (2FA) with email and Google Authenticator

Further advice

  • Set up a password manager. Bitwarden is a good open-source option with free, encrypted cloud storage and self hosting ability.
  • the Time-Based One Time Password (TOTP) authentication (Google Authenticator / Authy) is the best 2FA option for most. Don't run it on the same device and store the backup secret in a password manager
  • if using email don't login to the email account on the same device - use a dedicated, unique, pseudonymous email with end-to-end encrypted providers like Protonmail or TutaNota - store the login in a password manager
  • carefully note the 24 words seed down to paper (pencil lasts more then ink) / etch into metal and store in a safe place
  • it is the safest to store the phone switched off (the encryption key leaves the memory and not only protected by the lockscreen)
  • the 2FA creates a 2-of-2 multisig with Blockstream which can only be accessed after 365 days with only the seed (in case of losing access to the 2FA or Blockstream disappearing)
  • connect always through Tor, it is built in to Green and helps avoiding connecting the IP address with the bitcoin stored while querying Blockstream's server

Watch-only wallet

  • on an other device (can be a day-to-day used phone or desktop) set up Blockstream Green in watch only mode
  • the watch only wallet can be used to generate addresses and monitor the funds on the blockchain
  • there is no risk of losing the funds if the watch-only device is lost, stolen or compromised (it only contains the public keys, not touching the private ones)
  • note the privacy implications of the labels, addresses and xpub being stored on Blockstream's server

Using a custom firmware

  • a locked bootloader is a must - check on Android with:
    fastboot oem device-info
  • Two hardened, privacy focused custom firmwares which allow relocking the bootloader:

Resources

Level up

  • Single seed multi-location backup schemes
    The aim is to create 3 packages of cryptographically secure backups where the funds cannot be recovered from any single package, but can be recovered with the combination of any two. Can be thought of as a physical 2-of-3 multisig solution.

  • 10x Security Bitcoin Guide
    How to store bitcoin without any single point of failure. Multisig security is a difference in kind and not in degree. It affords you the ability to avoid loss while making 1 (or more) catastrophic failures in securing your bitcoin. By using a security system that is fault-tolerant, you can move much faster (with less caution) through each step and still attain far higher levels of security vs any single-key system. This guide will show you how.