BitHanko
BitHanko is an app for air-gapped signing. It passes Bitcoin transactions as PSBTs (BIP174) in QR codes, so the private key that signs them stays on a device that never connects to anything. It brings what air-gapped signing needs to a phone: loading the private key, signing, and showing the signed PSBT as a QR code.
What it protects against
How air-gapped the signer is depends on the phone you put it on. Pick the row that matches yours.
| Setup | Air-gapped? | Protects against | Does not protect against |
|---|---|---|---|
| A spare Android phone with Signer Lockdown | Yes, held by the OS: no SIM, no saved Wi-Fi, Wi-Fi and Bluetooth settings locked, and no new apps or accounts | Apps or accounts added later, joining a network by mistake, USB debugging and file transfer | A flaw in Android or the phone's firmware, radio hardware that ignores these settings, a tampered APK |
| A spare phone that you keep offline yourself (Android app or web app) | Only as long as you keep it offline | Remote attacks while it stays offline | Going online by mistake; with the web app, a new build arriving when it goes online |
| The web app or app on the phone you use every day | No. It is an online phone | Other apps and sites reading the key: it is held only while you sign and wiped when you leave the app (the web app also wipes it when the phone goes online) | Malware with control of the phone, a flaw in the browser or OS. Treat it like a hot wallet |
In every setup:
- Check the review screen. Signing is only as safe as your check of the amounts and addresses it shows.
- Saving the key on the phone (Using the app) trades safety for convenience: anyone who has the phone and its passcode can open it. By default nothing is saved.
- The software has not been audited. Keep amounts small.
How it works
The app runs bitwasm-signer, a set of WebAssembly modules made for signing bitcoin. 
With WebAssembly, byte-for-byte the same signing logic runs in every environment, including a dedicated signing device built on a microcontroller. The aim is signing software that is easy to run and maintain because it is portable.
The phone on the right of this page also runs the real WebAssembly modules, so you can try the app's screens and steps on testnet. As you scroll, the phone moves to the screen each section talks about. With Follow app above the phone, tap through it yourself and the text moves to the section for whatever you tap. A few screens, such as the keypad for typing words, are only in the app; for those the phone stays on the previous step. Never send real bitcoin to the demo's addresses.
1. Setup
The signer runs on Android and iOS. How to install it depends on what you are willing to trust; see Setup. On Android, Signer Lockdown makes a dedicated signer: with the SIM card out, it locks the Wi-Fi and Bluetooth settings and, as Device Owner, blocks USB debugging, new apps and accounts.
Once installed, Settings › Device restrictions lets you choose how the signer treats things that could leak what is on the screen or what you type: networks, Bluetooth and NFC, third-party keyboards, USB debugging and accessibility services. For each one you choose whether it is ignored, warns, or stops the key from loading while it is on. The browser demo does not have these checks.
2. Make a key
Your key is a list of 12 or 24 words, the recovery phrase. Tap Create a new wallet. The signer makes the words from secure random numbers, or from dice you roll yourself if you would rather not trust the phone's random numbers.
Have paper and a pen ready, and do this where no one and no camera can see.
Roll dice
To make the words from dice, roll a die and tap the face that came up, once per roll. 12 words take exactly 50 rolls and 24 words 99. The signer warns if the rolls look uneven or follow a pattern that a fair die almost never gives.
Write the words down
Write the words on paper, in order. Anyone who sees them can take your bitcoin, and with the paper you can get it back even if you lose the phone. Show SeedQR shows the words as a SeedQR too.
Check what you wrote
A five-question quiz asks for words from your paper. A slip of the pen shows up here, while you can still fix it. Only after the quiz can the new words be used to sign, and passing it offers to load them as the signing key.
3. Load your key
By default the signer keeps no key. You load it each time you sign, and it is wiped when you close the wallet or leave the app. To keep it on the phone instead, see More features.
There are three ways to load it: scan your backup as a SeedQR, type the words, or, in the browser demo, tap Use the test key. Next comes the passphrase screen; leave it blank if you have none. The wallet screen then shows the key's fingerprint. Compare it with the one in your wallet app to check that both use the same key. The test words give 73c5 da0a.
Type the words
Words are typed on the signer's own keypad, not the phone's keyboard, so the keyboard cannot learn them. Letters that lead to no word cannot be pressed, and the last word only offers words that fit the checksum.
Use a passphrase
A passphrase added to the words makes a different key, and so does a typo in it. The passphrase screen shows the fingerprint without a passphrase and, as you type, with the one you typed. Check that it matches the fingerprint you noted before you tap Open the wallet. With the test words, TREZOR gives b4e3 f5ed.
For 12 or 24 words, Show SeedQR on this screen shows the words as a SeedQR. Opening the wallet clears the words from memory, so this screen is the only place to show the SeedQR of a key you load.
4. Register in your wallet app
Tap Register in your wallet app and choose the keys to share. Usually you keep All at once and tap Show QR, and the signer shows your public keys as an animated QR code. Scan it with the wallet app to add a watch-only wallet. It holds no private keys. You only need to do this once. For a wallet app that only reads an xpub, share one kind.
5. Send
The wallet app builds the transaction, and the signer checks and signs it.
Scan the transaction
In the wallet app, make the transaction and show it as a QR code. Tap Sign a transaction and point the camera at it. The browser demo has Use the test transaction instead of a camera.
Check what it pays
The review shows the amount, the network fee and every output: where the bitcoin goes and the change that comes back to you. Compare each address with the wallet app and tick it. Sign transaction only works after every output is ticked and you have scrolled to the end, so nothing can be skipped.
Return the signature
The signed transaction appears as QR codes. Scan them with the wallet app and send the payment from there. The signer never goes online.
6. When you are done
Tap Close wallet. The key is wiped from memory. The same happens when you leave the app.
More features
Under Advanced on the wallet screen:
- Register a multisig: a wallet that needs several keys (up to three) to spend. The signer then shows its change as change and does not sign inputs from other wallets.
- Sign a message (BIP137) to prove you own an address.
- Derive a child mnemonic (BIP85): new words for another wallet, recoverable from the same backup.
In Settings, Save on this device keeps 12 or 24 word keys encrypted on the phone, opened with the screen lock or biometrics. Each saved key has a name and can be deleted on its own. The passphrase is never saved; you type it at each unlock, so one saved key can open with different passphrases.
Next
- Signer Lockdown: turn an Android phone into a signer only.
- Install over USB without connecting the phone to a network.
- FAQ