BitTool Signer

BitTool Signer is a free app from the Singapore company behind BitTool Wallet and Ownbit, and its job is narrow: keep crypto keys on a phone that never goes online. You set up a spare iPhone or Android phone in Airplane Mode, create or import a wallet there, and pair it once with a watch-only wallet by showing your public keys as a QR code. From then on, the online wallet shows an unsigned transaction as a QR code, the signer decodes and displays it, and you enter that wallet's password to sign and show the signature as another QR code. It covers Bitcoin, including PSBT and multisig cosigning, Ethereum, BNB Chain, Arbitrum, Base, TRON and Solana, and pairs with wallets such as Sparrow, BlueWallet, MetaMask, Rabby and Solflare as well as BitTool Wallet. The app has no networking code, collects nothing according to its privacy policy, and its iOS and Android source code is on GitHub.