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?
- open source, reproducible build
- available on multiple platforms: Android, iOS and desktops
- easy, self explanatory interface and detailed documentation
- unique Two-Factor Authentication (2FA) capability
- note that this is not a private way to use bitcoin since the wallet is connecting to the server of Blockstream
- 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
- 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
- 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:
- GrapheneOS compatible with Pixel devices
- CalyxOS for Pixels and the Xiaomi Mi A2
How secure is full-disk encryption technology on LineageOS, or Android phones in general?
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.