Open the app

The wallet that
signs offline.

An air-gapped Bitcoin & Ethereum wallet, an encrypted vault, and a secret-splitter, in one HTML file. Nothing to install, no account, nothing uploaded, nothing shipped to your door.

Open gapbit Download the file
▪ The whole app is one readable file, verify its hash
WindowsmacOSLinuxiPhoneAndroid
650 KB
App size
Any device
Runs anywhere
No telemetry
Ever
MIT
Open source
gapbit Bitcoin wallet showing a native segwit receive address, its BIP-84 derivation path and QR code
bitcoin wallet · receive · m/84'/0'/0'/0/0
01

Entropy

Randomness you can watch being made.

Seeds are minted from measured physical sources (microphone, camera, timing jitter, pointer motion) with live SP800-90B-style estimates on screen while you harvest.

  • Per-source min-entropy readouts with continuous health tests: a failing source is dropped, not accepted
  • Fold → Toeplitz extractor → HKDF-SHA-512, every stage readable in the file
  • XORed with your OS generator: a flaw in either source cannot weaken the key
  • HKDF, BIP-39 and seed test vectors must pass before generation is allowed
Live entropy harvest: five physical sources with min-entropy estimates, health tests passing and a certified-bits counter
physical sources · certified entropy · 4,124 bits
02

Air-gapped signing

Sign on a machine the internet cannot reach.

Your online device builds the transaction from a watch-only key that can spend nothing. This device, offline by design, checks it and signs it. A QR code carries it out.

  • The app probes real endpoints and hard-blocks key creation and signing while any network is reachable
  • Bitcoin: native segwit, PSBT, fee warnings when a fee looks like an attack or a typo
  • Ethereum & ERC-20 on five networks: the signer decodes the call data and shows the real recipient and amount
  • Broadcast from any phone or browser: a signed transaction carries no keys
Air-gapped signing screen: OFFLINE - SAFE TO CREATE AND SIGN banner and the four-step online/offline workflow
air-gapped · offline · safe to create and sign
03

Splitting

The backup no single event can take.

Any secret becomes N pieces of which any M restore it, and fewer than M reveal nothing at all. Not a weaker guess. Nothing, by mathematics.

  • Up to 8 pieces, any threshold: Shamir Secret Sharing over GF(256)
  • ~400 bytes each, as a file, a QR code, or words: printable, engravable, cloud-storable
  • Every piece carries a plain-language note saying what it is and what it needs
  • Duplicates, corruption and mixed sets are caught before anything opens
Backup screen: a vault file plus three key files created 2-of-3, each with QR and download: any 2 of 3 open the wallet, 1 reveals nothing
key files · 2 of 3 · store them in different places
04

Vault

Any file. Any size. Same guarantee.

A will, a deed, a password list, a 2 GB archive: encrypted once, and the key is what gets split. The encrypted file is safe to store anywhere.

  • AES-256-GCM with Argon2id, up to 256 MB memory-hard
  • Pieces stay ~400 bytes whether the payload is a note or two gigabytes
  • Cloud, email, USB: without enough key files, the file opens nothing
  • Byte-identical recovery, verified on binary payloads
Files & secrets vault: encrypting a private note whose key will be split into key files
files & secrets · encrypt · key files go somewhere else
05

The hidden wallet

A wallet nobody can prove exists.

Full 25th-word support: the same seed words plus a different passphrase open a completely different, fully working wallet, and nothing anywhere shows whether one exists.

  • Hand over the ordinary wallet at a checkpoint; the real balance sits behind a word in your head
  • Split the words, keep the word: even all piece-holders together reach only the ordinary wallet
  • Standard BIP-39: recoverable in any wallet, forever
  • Stated honestly: this defeats searches, not violence. If your safety is threatened, comply.
Import screen: 12 or 24 seed words and the optional 25th-word passphrase that opens a different wallet
import · 24 words · optional 25th word

And the rest

Everything else in the file.

Watch-only

An account xpub shows balances and builds transactions: the online machine can spend nothing.

Five EVM networks

Ethereum, Base, Arbitrum One, Optimism, Polygon. USDT, USDC, DAI, WBTC presets plus any contract.

No blind signing

The signer re-derives the address, rebuilds the transaction and decodes the call data before you sign.

Mistake refusal

An Ethereum address in the Bitcoin field is refused by name, and vice versa. Fresh change address every send.

Phone-ready

Installs to the Home Screen on iPhone and Android and runs in airplane mode. No app store.

Hardware-wallet backup

Import a Ledger or Trezor seed, split it, keep using the device. Its paper sheet stops being one object.

Self-tests

HKDF, BIP-39 and extractor vectors run before every session, visible in the footer of every screen.

Verify what you run

Published SHA-256 for every file, and a verify button in the app itself. View-source shows every line.

Nothing stored

No disk writes, no localStorage, no cookies, no telemetry. Close the tab and nothing remains.

No purchase record

Vendor customer lists have leaked: addresses, then extortion. A downloaded file puts you on no list.

Standard derivation

BIP-39 / BIP-84 / BIP-44. Recovery needs no manufacturer to still exist.

Works in 2040

No dependencies, no build step, no company that has to survive. The file is the product.

Get it

Two builds. Pick by what is at stake.

Standard

The everyday wallet: check balances, receive, send, broadcast, encrypt files. One machine, online. For low and moderate amounts.

Open gapbit →
Download the file

Air-gapped signer

For savings and high-stakes keys. A separate build with no network at all — it cannot connect and cannot update itself. Runs on an offline device; signs by QR.

What you need & download →
sha-256 · gapbit-wallet.html
2bf70d924173c4b875031b66… · full hashes in VERIFY.txt. Verify before you trust: shasum -a 256 gapbit-wallet.html

Questions

Asked and answered.

A wallet in a browser: is that safe?
Browser wallets earned their bad name from tools that generated keys with weak randomness or phoned home. gapbit is built against exactly that history: entropy is measured from physical sources and XORed with the OS generator so neither can weaken the key alone, the app refuses to create keys or sign while any network is reachable, nothing is ever stored or transmitted, and the entire application is one file you can hash against a published value. The honest limit is printed in the app too: a fully compromised machine at signing time beats any software. That is why keys are made and used on a machine that never goes online.
What happens if gapbit.com disappears?
Nothing. The file you downloaded keeps working: it has no server, no dependencies and no license check. Wallets use standard BIP-39/84/44 derivation, so any other wallet can recover them forever. The file is the product; the website is just where you first got it.
Can you see my keys or my files?
No, structurally: there is no account, no telemetry and no server to send anything to. Keys are created and used on your machine while it is offline. You can confirm this in the source: it is one file, and every line is in it.
What if I lose some of the pieces?
That is the point of the threshold. Split 2-of-3 and any two pieces restore everything: you can lose any one of them, to a fire, a dead USB stick or a bad memory, and nothing is lost. Choose the threshold to match how much loss you want to survive. And test it: split, recombine, confirm the addresses match. Five minutes.
Can the people holding pieces steal from me?
Any M holders acting together can rebuild the wallet: that is what a threshold means, and the app says so before you create one. If that matters, keep the pieces with people who do not know each other, or use the 25th word: split the seed, keep the passphrase in your head, and even all the pieces together open only the ordinary wallet.
Does it replace my hardware wallet?
It can, but it does not have to. Keep the device for signing if you like it, and use gapbit to fix the part the manufacturer left unsolved: the 24-word sheet in your drawer, which is one object one event can destroy. Import the seed, split it, store the pieces apart, destroy the sheet.
Is it audited?
Not yet independently, and the app says so on its own pages. It is built on audited libraries; the layer above them is tested and cross-verified byte-for-byte against reference implementations. Until a formal review exists, treat it accordingly: start with amounts you are comfortable with, and read the file. That is what it is for.
How is this free? What is the catch?
There is no catch inside the file: it is complete, and collects nothing: there is no account to upsell and no data to sell. It is MIT licensed, so it is free for anyone, personal or commercial, forever. It costs nothing to distribute, so it costs nothing to use.