Android

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0.1.0

The first release: Gerfaut for Android, as a public beta. Gerfaut watches bitcoin wallets it cannot spend from. There is no key generation, no seed handling and no signing anywhere in the code, and no Send button in the interface.

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Watching a wallet

• Watch a wallet from what you already have: a descriptor, an extended or single public key, a plain address, a multisig setup, a BSMS record or another wallet’s export file. Paste it, pick a file, or scan its QR code, static or animated. A long animated code reads as fast at its last frame as at its first.
• Gerfaut works out the format on its own and shows what it understood, with the first address, before it stores anything. It refuses a private key, with a sentence that says what to bring instead.
• Watch several wallets on mainnet, signet, testnet4 or regtest. Hold a wallet card and drag it into the order you want, on the home screen and in the settings.
• For each wallet: the balance, the transaction history and the UTXOs, with one action to hide every amount on screen.
• Receive addresses with their QR code. Receive offers only addresses nobody has paid yet, and a fresh one after every sync, up to 200 past the next unused one. The gap limit warning counts the unused addresses in a row.
• A Policy page for every wallet: what the descriptor allows, branch by branch, primary paths first, with each timelock counted down in blocks and in days. A branch open now shows in green, and one that opens within 30 days in amber. Its Descriptor section shows and copies the whole wallet as one multipath descriptor, with /<0;1>/*.
• Export the history of a wallet as CSV, filtered by date and direction, to a file or through the share sheet.
• Home screen widgets for the price, the balance (hidden by default) and the block height.

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Broadcasting a signed transaction

• Broadcast a signed transaction or a PSBT, read back in plain language before it goes anywhere. When no backend could confirm a coin it spends and only the PSBT declares its amount, Gerfaut marks the amounts and the fee as the PSBT claims and says what to check before you send.
• A signature that leaves the outputs open (SIGHASH_NONE or SIGHASH_SINGLE) gets a red warning, since whoever relays such a transaction can send that money elsewhere. A transaction the network is sure to refuse, such as one that spends a coin already spent in a block, reads Will be refused and cannot be sent.
• On a PSBT still missing signatures, the fee rate reads ≈ … once signed, the rate it pays once your signing device signs it. A transaction already waiting in the mempool reads Already in the mempool, and sending it again does no harm.

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Talking to the chain

• Public servers by default, or your own node over Electrum or Esplora. Gerfaut shows a self-signed certificate and trusts it once, then refuses it if it ever changes.
• A Tor client built in, so a .onion backend works with no Orbot installed. While Test the connection starts it, the Tor card shows its progress. When a .onion server stays silent for 30 seconds, the built-in Tor opens a second route to it alongside the first one, and Gerfaut uses whichever answers.
• A sync downloads only what a wallet lacks, so a payment that Live notices costs a few kilobytes. Gerfaut reads a wallet in full when the vault opens and once a day. Past the last used address, a sync asks the server about as many unused addresses as the gap limit, and no more.
• A This is my node switch under the address of your own server, in Settings > Network. Turned on, it lets Live follow up to 20,000 addresses, against 2,000 otherwise. Leave it off for a server you do not run, which would refuse most of them and learn every one.
• With Live on and the Automatic backend, syncs, transaction previews and broadcasts go first to the operator Live talks to, Blockstream on mainnet. A single operator then sees your addresses. When Gerfaut opens, the first sync waits up to 20 seconds for Live to connect.
• A preview or a broadcast moves to another server only when one cannot be reached. When a node refuses a transaction, Gerfaut asks no other node and shows the node’s own words.
• A server that is down while another one answers goes to the end of the list for five minutes, then twice as long at each new failure, up to an hour. When the server Live talks to sees no new block for two hours, Gerfaut compares it with another server of the same operator. Live moves there if its own server lags two blocks or more.

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Getting told when something moves

• Check for transactions, in Settings > Notifications, syncs your wallets while Gerfaut is closed, every 15 minutes, every hour or every six hours. Gerfaut posts a local notification when a sync finds something new.
• Live, another choice under Check for transactions, keeps a connection open to the backend your syncs already use. It tells you when a transaction reaches the mempool, then again at its first confirmation. An Electrum server tells Live within seconds. An Esplora server takes longer, because Live polls it and goes through every address in turn.
• Live runs with Gerfaut closed and comes back after the phone restarts, behind the small permanent notification Android requires, which has a Stop button. A check every 15 minutes stays scheduled for whenever Android stops Live.
• Before Live starts, a sheet explains what it does and what the server learns, then asks for the battery exemption, which you can refuse. On a Xiaomi, Redmi, POCO, Huawei, Honor, Samsung, OnePlus, Oppo or realme phone, it also lists the switches to turn on in the maker’s battery manager. A button opens Gerfaut’s app info page.
• Gerfaut announces each transaction once when it appears and once when it confirms, whichever part of Gerfaut saw it first, in the same words as on desktop. A fee bump brings no second notification. An incoming payment that leaves the mempool with nothing in its place gets one more notification saying it is no longer coming. Past three transactions in one wallet at once, one line counts the rest, and notifications stop at 20 a minute. If Gerfaut cannot word a notification, it still posts one with no wallet and no amount, which says more is waiting in Gerfaut.
• Show wallet and amount, a switch in Settings > Notifications, decides whether a notification names the wallet and gives the amount. It is on by default and applies whether Gerfaut is locked or not. With the switch off, a notification only says what happened, as in New transaction · pending. Hidden balances keep the amount out either way.
• When Android stops letting Gerfaut’s notifications through, Settings > Notifications says so in amber, with a button to the system settings. Live waits meanwhile and starts again once the notifications get through.
• When Live cannot follow every address, Settings > Notifications says how many addresses of how many wallets wait for the next sync, and how to lift the limit. Each wallet shows Live, Partly live or At next sync on its card, on its page and in Settings > Wallets. Always watch live first, under Settings > Wallets > Advanced, picks the wallets Live follows first.
• When a server takes only part of your addresses, Live asks it again for all of them a day later, or an hour later when it took none. On your own node, when the server itself refuses addresses, the note names the server setting that lets it follow more.

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Keeping it yours

• An encrypted backup file, and the same backup as an animated QR code, so another device gets your wallets with no server in between. The export page asks for a passphrase of several words, since someone who copies the file can try passwords offline. It can also add your node settings. The restore page lists what the file puts in place, wallet by wallet and node setting by node setting, before it writes anything, and never replaces a certificate you already accepted. A wallet whose keys belong to another network says which one. A wrong password, a file that is not a Gerfaut backup and a backup made by a newer Gerfaut each get their own sentence. Gerfaut never blames a password that was right. A wallet Gerfaut cannot watch stays on the restore list, unticked, with the reason, and the confirmation names it once the others are restored.
• The vault stays out of Android’s own backups and device-to-device transfers. It leaves the phone only as a Gerfaut backup, sealed under your password.
• Gerfaut never overwrites a vault this phone cannot open, such as one restored without its key. It sets the vault aside under another name and offers to start over, with the restore page first.
• Fiat value beside the balances and the transaction amounts, in the currency you select, from mempool.space, the default, or CoinGecko. With CoinGecko, Powered by CoinGecko shows once on each screen that uses its prices, and on the price widget. No price source hears from Gerfaut until you turn on Fiat value or place the price widget. A wallet on signet, testnet4 or regtest shows 0 in your currency, since test coins are worth nothing.
• Amounts in BTC or in sats, and a light, dark or system theme.
• A PIN or password lock with biometrics. Attempts slow down after three tries and stay slowed after a restart. Coming back from a file picker, a save dialog or a share sheet more than ten minutes after it opened asks for the lock again.
• Only a biometric that Android rates as strong opens the lock. On a phone whose face or fingerprint unlock is rated weak, the switch stays greyed out and says why. When the fingerprints or faces on the phone change, biometrics stop opening Gerfaut until you turn them on again.
• A calculator disguise for the icon. While Gerfaut is disguised, it posts no notification, Live stays off and no update check goes out. From Android 13, the recent apps keep no picture of Gerfaut while it is disguised. The sheet that turns the disguise on also says what Android keeps: an older thumbnail below Android 13, and Gerfaut’s earlier notifications in the notification history, if you turned it on.
• A notice on the home screen when a newer release is out, with a button to the release page and a Later button that closes it for that release. Gerfaut asks GitHub at most once a day, when you open it, and never announces a pre-release. The first check waits a day after you first open Gerfaut. Settings > About turns the automatic check off.
• Settings > About marks a 0.x version as a beta, with a Report a problem button that opens the issues of the Android repository. The first page of the welcome tour says that Gerfaut is in public beta.
• One APK per processor architecture, so a phone downloads only the code it runs. Gerfaut installs on a phone without a camera, and scanning stays optional.
• TalkBack names every switch, field and checkbox and reads errors aloud. Everything you can select has a touch area of at least 48 by 48 dp.

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Security

• Gerfaut stores the wallet list encrypted, under a key that Android’s secure storage keeps apart from the vault.
• Gerfaut marks a descriptor or a policy you copy as sensitive on the clipboard. Android shows no preview of it and keeps it out of its history. Gerfaut clears it from the clipboard a minute later, unless you copied something else since. If Android closes Gerfaut within that minute, Gerfaut clears it the next time it opens.
• A server cannot make up a confirmation. Gerfaut checks that the transaction is in the block the server names and, on mainnet, that the block carries the work the network asked for. Over Electrum, a confirmation in an older block must also link to the checkpoints built into Gerfaut. When no other server answers in place of one whose proof does not hold, the sync says that a server sent a false proof, whether you started it, it covered every wallet or Live started it. Over Esplora, a confirmation from before March 2014 must also link to the checkpoints, and every block height a server gives must carry the work of its block. A server cannot take a confirmation away either: to replace a block that holds one, it must send the headers of its new chain, each linked to the one before and, on mainnet, carrying the work of its height.
• When one of your nodes is a .onion address, the price and the update check go through Tor, on any network. Gerfaut sends neither when Tor is out of reach. Neither the price widget nor the automatic update check starts Tor for itself. When the check finds no Tor running, the day does not count, and it tries again the next time Gerfaut comes back on screen.
• The built-in Tor client is arti 2.7.0, with its security fixes. A relay can no longer fill memory through half-closed streams, and a directory mirror can no longer do it through the consensus it sends, or stall Tor by holding back its answer.
• While an app lock is set, the balance widget shows no wallet name and no amount, and Show balances on widgets stays greyed out.
• Under the calculator disguise, a vault that fails to open shows the calculator, not Gerfaut’s error page.
• Gerfaut refuses a file far too large to be a wallet, a transaction or a backup, and an animated QR code that announces more than 5,000 parts.
• A history or a backup you share reaches only the app you select in the share sheet. Gerfaut deletes its copy from the cache an hour later, or when it next starts.
• From Android 12, Android hides the windows other apps draw over Gerfaut, so none can cover what a button does.
• A release build writes no error text to the system log, where it could quote an address.
• Every release carries a SHA256SUMS manifest signed with minisign, and Verify a download gives the key and the commands. CI builds the APKs unsigned, and the author signs them on his own machine, so the release key never reaches GitHub.