MetroVault

MetroVault turns an Android phone into a secure, air-gapped Bitcoin signing device like a cold-storage hardware wallet. It acts as an offline signer for your watch-only wallets (such as Sparrow, BlueWallet, or Electrum) running on an online device. Your private keys are generated and kept on a phone that never connects to the internet; all communication happens through QR codes, and network access is blocked at the Android level. Because the keys never touch an online device, the attack surface for theft and malware is dramatically reduced. MetroVault is fully open source and auditable, built on a custom Kotlin Bitcoin library so every cryptographic operation can be reviewed. Security * Air-gapped operation — designed for devices with no internet (airplane mode, Wi-Fi/Bluetooth off) * Dual-layer encryption — AES-256-GCM with PBKDF2 (210k iterations) plus the Android Keystore * Biometric unlock — hardware-backed fingerprint/face authentication * Plausible deniability — separate "Main" and "Decoy" wallets behind different passwords * Brute-force protection — exponential backoff with lockout, and an optional automatic wipe after repeated failed logins * Screenshots and screen recording are blocked app-wide Wallets * Single-sig and multi-sig (collaborative custody) support * Address types: Native SegWit (bc1q), Taproot (bc1p), Nested SegWit (3...) and Legacy (1...) * BIP-352 Silent Payments support, both to send to or spend from an SP wallet * BIP-39 passphrase support, saved locally or kept in memory only * BIP-85 derivation of child seeds and passwords * Custom entropy from dice rolls or coin flips, plus mnemonic checksum tools * Testnet4 support for testing Transaction signing * PSBT workflow (BIP-174) for single-sig and multi-sig * Air-gapped flow: scan PSBT, verify, sign, export — entirely by QR * Animated QR support: BC-UR (v1/v2) and BBQr for large transactions * On-device address verification and watch-only XPUB/descriptor export * Message signing and verification Permissions MetroVault requests only the camera (optional, for scanning QR codes) and biometric permissions. It has no internet permission and cannot connect to any network.