Technical details
=================

### FILES

Your wallet contains your private keys and various transaction related metadata. It is stored in app-private
storage:

    Mainnet: /data/data/de.schildbach.wallet/files/wallet-protobuf
    Testnet: /data/data/de.schildbach.wallet_test/files/wallet-protobuf-testnet

The wallet file format is not compatible to wallet.dat (Satoshi client). Rather, it uses a custom protobuf format
which should be compatible between clients using bitcoinj.

Certain actions cause automatic rolling backups of your wallet to app-private storage:

    Mainnet: /data/data/de.schildbach.wallet/files/key-backup-protobuf
    Testnet: /data/data/de.schildbach.wallet_test/files/key-backup-protobuf-testnet

Your wallet can be manually backed up to and restored from a share of the storage access framework (likely Google Drive):

    Mainnet: bitcoin-wallet-backup-<yyyy-MM-dd-HH-mm>
    Testnet: bitcoin-wallet-backup-testnet-<yyyy-MM-dd-HH-mm>

If you want to recover coins from manual backups and for whatever reason you cannot use the app
itself to restore from the backup, see the separate [README.recover.md](README.recover.md) guide.

The current fee rate for each of the fee categories (economic, normal, priority) is cached in
app-private storage:

    Mainnet: /data/data/de.schildbach.wallet/files/fees.txt
    Testnet: /data/data/de.schildbach.wallet_test/files/fees-testnet.txt


### DEBUGGING

Wallet file for Testnet can be pulled from an (even un-rooted) device using:

    adb pull /data/data/de.schildbach.wallet_test/files/wallet-protobuf-testnet

Log messages can be viewed by:

    adb logcat

The app can send extensive debug information. Use **Options > Settings > Report Issue** and follow the dialog.
In the generated e-mail, replace the support address with yours.


### BUILDING THE DEVELOPMENT VERSION

If you haven't done already, follow the **Prerequisites for Building** section in the [top-level README](../README.md).

It's important to know that the development version uses Testnet, is debuggable and the wallet file
is world readable/writeable. The goal is to be able to debug easily.

Finally, you can build Bitcoin Wallet and sign it with your development key. Again in your workspace,
use:

    # each time
    gradle clean test :wallet:assembleDevDebug

You'll find the signed APK under this path:

    wallet/build/outputs/apk/dev/debug/bitcoin-wallet-dev-debug.apk

To install the app on your Android device, use:

    gradle :wallet:installDevDebug

If installation fails, make sure "Developer options" and "USB debugging" are enabled on your Android device, and an ADB
connection is established.


### BUILDING THE PRODUCTIVE VERSION

At this point I'd like to remind that you continue on your own risk. According to the license,
there is basically no warranty and liability. It's your responsibility to audit the source code
for security issues and build, install and run the application in a secure way.

The production version uses Mainnet, is built non-debuggable, space-optimized with ProGuard and the
wallet file is protected against access from non-root users. It is built from the same branch (or
tag) as the development version. After you have cloned/updated the git repository as described above,
use:

    # each time
    gradle clean test :wallet:assembleProdRelease

You'll find the unsigned APK under this path:

    wallet/build/outputs/apk/prod/release/bitcoin-wallet-prod-release-unsigned.apk

Apart from the missing signature and checksums in `META-INF/`, it should be identical to the APKs
provided via the app stores.


### SETTING UP FOR DEVELOPMENT

You can import the project into IntelliJ IDEA or Android Studio, as it uses Gradle for building.


### TRANSLATIONS

The source language is English. Translations for all languages except German [happen on Transifex](https://explore.transifex.com/bitcoin-wallet/bitcoin-wallet/).

The English resources are pushed to Transifex. Changes are pulled and committed to the git
repository from time to time. It can be done by manually downloading the files, but using the `tx`
command line client is more convenient:

    # first time only
    sudo apt install transifex-client

If strings resources are added or changed, the source language files need to be pushed to
Transifex. This step will probably only be executed by the maintainer of the project, as special
permission is needed:

    # push source files to Transifex
    tx push -s

As soon as a translation is ready, it can be pulled:

    # pull translation from Transifex
    tx pull -f -l <language code>

Note that after pulling, any bugs introduced by either translators or Transifex itself need to be
corrected manually.


### NFC (Near field communication)

Bitcoin Wallet supports reading Bitcoin requests via NFC, either from a passive NFC tag or from
another NFC capable Android device that is requesting coins.

For this to work, just enable NFC in your phone and hold your phone to the tag or device (with
the "Request coins" dialog open). The "Send coins" dialog will open with fields populated.

Instructions for preparing an NFC tag with your address:

- We have successfully tested [this NFC tag writer](https://play.google.com/store/apps/details?id=com.nxp.nfc.tagwriter).
  Other writers should work as well, let us know if you succeed.

- Some tags have less than 50 bytes capacity, those won't work. 1 KB tags recommended.

- The tag needs to contain a Bitcoin URI. You can construct one with the "Request coins" dialog,
  then share with messaging or email. You can also construct the URI manually. Mainnet example:
  `bitcoin:1G2Y2jP5YFZ5RGk2PXaeWwbeA5y1ZtFhoL`

- The type of the message needs to be URI or URL (not Text).

- If you put your tag at a public place, don't forget to enable write protect. Otherwise, someone
  could overwrite the tag with his own Bitcoin address.


### BITCOINJ

Bitcoin Wallet uses [bitcoinj](https://bitcoinj.org) for Bitcoin specific logic.


### EXCHANGE RATES

Bitcoin Wallet reads this feed from "CoinGecko" for getting exchange rates:

    https://api.coingecko.com/api/v3/exchange_rates

We chose this feed because it is not dependent on a single exchange. This feature can be disabled
with the compile-time flag

    Constants.ENABLE_EXCHANGE_RATES


### SWEEPING WALLETS

When sweeping wallets, Bitcoin Wallet uses a set of Electrum servers to query for unspent transaction
outputs (UTXOs). This feature can be disabled with the compile-time flag:

    Constants.ENABLE_SWEEP_WALLET