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# Android

## Overview

Android requires that all apps be digitally signed with a certificate before they are installed on a device or updated. Android phones enforce a policy that updates to an app must come from the same signing key to validate. This allows the phone to be sure an update is fundamentally the same app, i.e., has the same author.

This documentation covers one way to sign your app where the Google Play Store maintains the authoritative key for your app. This approach is called App Signing by Google Play.

You will need to generate a key on your development workstation to sign an app package before sending it to the Google Play store. If you use app signing by Google Play, the key on your workstation is called the upload key.

## Generate a key

You will need to decide where to store the upload key. A good default is to use one keystore file per app you are creating and to store it in the `.android` folder within your home folder. The folder is automatically created by the Android tools; but if it doesn’t exist, create it.

We recommend using a separate keystore file per app. Below, we use the upload-key-helloworld.jks filename. This assumes you are building an app called “Hello World”; use the (lowercase, no spaces) app name, `helloworld` in the filename for the keystore.

Try not to lose this key; make backups if needed. If you do lose this key, you can contact Google Play support to reset it. If you choose not to use app signing by Google Play, it is absolutely essential that you not lose this key. For this reason, we recommend using App Signing by Google Play.

    $ mkdir -p ~/.android
    $ ~/Library/Caches/org.beeware.briefcase/tools/java/Contents/Home/bin/keytool -keyalg RSA -deststoretype pkcs12 -genkey -v -storepass android -keystore ~/.android/upload-key-helloworld.jks -keysize 2048 -dname "cn=Upload Key" -alias upload-key -validity 10000

    $ mkdir -p ~/.android
    $ ~/.cache/briefcase/tools/java/bin/keytool -keyalg RSA -deststoretype pkcs12 -genkey -v -storepass android -keystore ~/.android/upload-key-helloworld.jks -keysize 2048 -dname "cn=Upload Key" -alias upload-key -validity 10000

    C:\...>IF not exist %HOMEPATH%\.android mkdir %HOMEPATH%\.android
    C:\...>%LOCALAPPDATA%\BeeWare\briefcase\Cache\tools\java\bin\keytool.exe -keyalg RSA -deststoretype pkcs12 -genkey -v -storepass android -keystore %HOMEPATH%\.android\upload-key-helloworld.jks -keysize 2048 -dname "cn=Upload Key" -alias upload-key -validity 10000

    PS C:\...> If (-Not (Test-Path "$env:HOMEPATH/.android")) { New-Item -Path "$env:HOMEPATH\.android" -ItemType Directory }
    PS C:\...> & "$env:LOCALAPPDATA\BeeWare\briefcase\Cache\tools\java\bin\keytool.exe" -keyalg RSA -deststoretype pkcs12 -genkey -v -storepass android -keystore "$env:HOMEPATH\.android\upload-key-helloworld.jks" -keysize 2048 -dname "cn=Upload Key" -alias upload-key -validity 10000

This creates a 2048-bit key and stores it in a Java keystore located in the `.android` folder within your home folder. Since the key’s purpose is to be used as an upload key for the Google Play store, we call the key “upload-key”.

We use a password of `android`. This is the default password for common Android keystores. You should change the password. It is most critical to limit who has access to the keystore file, but changing the password is also important.

See Publishing your app for instructions on using this key to upload an app to the Google Play store.