Bitcoin Watch-only Wallet Support

[AndreasGassmann]

AirGap Vault now supports SegWit and internally uses PSBTs to transfer transactions. But we still use our own serializer. If we change the QR codes to URs, AirGap Vault should be compatible with other watch-only wallets.

Coinlib

• Show warnings if request contains suspicious data (eg. strange derivation paths, change address does not match, etc.) https://blog.coinkite.com/troublesome-change/ (We could also store all previous PSBTs to help with that)

Apps

• Show option to use bc-ur QR
• Show info about which inputs we can sign and which have already been signed
• Handle PSBTs with "invalid" masterFingerprint and derivation path (eg. electrum if no fingerprint is available to them): TODO: [PSBT] Support Electrum bip32Derivation style bitcoinjs/bitcoinjs-lib#1664
• Vault: If user scans bc-ur QR, the response QR should automatically select bc-ur as well
• Vault: Mark own addresses (eg. change addresses) in UI

Security

• Make sure the full input is required and verified, to prevent https://blog.trezor.io/details-of-firmware-updates-for-trezor-one-version-1-9-1-and-trezor-model-t-version-2-3-1-1eba8f60f2dd
• Make sure we handle coinjoin PSBTs (eg. disable)
• Make sure we handle multisig PSBTs (eg. disable)

Watch-only wallets

• BlueWallet (Animated QR codes from AirGap Vault do not get recognised BlueWallet/BlueWallet#4229)
• Sparrow
• Specter
• Wasabi
• Electrum
• BTCPayServer

Development

Inspect PSBTs: https://bitcoincore.tech/apps/bitcoinjs-ui/index.html