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MetroVault 4.0.0

Air-gapped Bitcoin signer for offline cold storage. QR-only, no network.

filecart Android Updated Oct 20, 2026
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MetroVault 4.0.0 screenshot

About 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.

License: GPL-3.0-or-later
Release info Update on Oct 19, 2026
Release notes MetroVault v4.0.0 - Release Notes

MetroVault 4.0.0 is a major release. It adds duress unlock, tightens what the app will sign, and moves the app's Bitcoin code into vaultovich, a separate library on Maven Central that can be audited, tested and versioned on its own, extracted from MetroVault.

A few changes affect existing workflows:
• Some PSBTs that v3.9.5 signed are now refused: multi-input segwit v0 spends without previous transactions (a "Sign Anyway" override is offered) and any sighash type other than SIGHASH_ALL or SIGHASH_DEFAULT. See Signing Safety below
• New cosigner key export format: the BIP-48 export is now a BIP-380 key expression ([fp/48h/0h/0h/2h]xpub), which is what coordinators expect

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Duress Unlock
• Duress password: an optional third password, set from the main vault's Security settings, that wipes all app data the moment it is entered on the unlock screen. It works even while the rate limiter has the screen locked, and whether or not "Wipe Data on Failed Login" is on, because someone under coercion must never be told to wait
• Duress fingerprint: the biometric target selector now offers Duress Wipe alongside Main and Decoy; a successful fingerprint unlock then wipes the device instead of opening a vault (the slot holds a random token, never a password)
• Looks like a normal unlock: after the wipe the app opens a throwaway session in decoy mode with one freshly generated wallet, so an attacker sees a working wallet app. Everything in that session lives only in RAM (DuressSession routes both vault files to InMemorySharedPreferences); the first lock or backgrounding ends it and the app shows first-time setup, like a fresh install

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Signing Safety

BIP-143 fee-attack protection
• Previous transactions required for multi-input segwit v0 spends: without PSBT_IN_NON_WITNESS_UTXO the amounts of the other inputs, and so the fee shown on screen, cannot be verified (CVE-2020-14199). When a multi-input PSBT lacks them, MetroVault withholds…
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