Sign android app with keystore

Creating Keystores and Signing Android Apps

The SK Engineering Team

As a security measure, Android requires that apps be signed in order to be installed. Signing an app first requires creating keystores. A keystore is a storage mechanism for security certificates. A public key certificate is used to sign an APK before deployment to services like the Google Play Store. Signing the APK in this fashion allows Google to provide a high level of certainty that future updates to your APK of the same app come from you and not some malicious third party.

Considerations

There are some things you will need to consider before first deploying your Android app. Primary among these is the expected lifespan of your app. You will not be able to deploy the same app signed by another key at any point in the near future. Android, as well as Google Play, enforces the use of the same key for updates to an APK. If you need to sign your app with another key for any reason, you will have to deploy the app with a new package name. Any ratings your app had on Google Play will be lost. You will also lose touch with your user base unless you have notified them in some way to expect the existing app to be obsolete.

Creating keystores

After you have decided on an app’s lifespan, you’ll want to generate your keystore. Java includes a tool for just this purpose: keytool . keytool is located in your Java JDK installation and should be on your path for the purposes of this article. keytool will quickly generate a public/private key pair and store them in a keystore for you after you answer a few simple questions.

keytool has a number of commands. The most common command used for signing Android builds -genkeypair , commonly abbreviated -genkey . The other commands may be useful to you, but uncommonly so. Again, there are lots of options for this keytool command. The primary -genkey options we are concerned with are in the table below with a brief description:

-keystore	Filename of the generated keystore
-alias	Keypair alias name
-keyalg	Algorithm used to generate keypair
-keysize	Keypair size, in bits
-validity	Keypair validity duration, in days

In other words, running the command

keytool -genkey -v -keystore release.keystore -alias example -keyalg RSA -keysize 2048 -validity 10000

would result in a keystore file called release.keystore which contained an RSA-2048 public/private keypair by the alias name of example and validity of 10,000 days (more than 27 years).

Before running this command, you’ll want to decide on strong passwords for the keystore and key. You’ll need both of these passwords to sign an APK — they can be the same password if you’re into that kind of thing. The tool will also collect some metadata like your name and organization, but all of that is optional.

Signing your APK

After running the command you’ll be the proud owner of a brand new Java Keystore. You probably want to set up your project to use the keystore to sign your APK, so let’s have a look at that.

If you’re using gradle to build your Android project, you will create a android.signingConfig and associate it with one or more android.buildTypes . The two passwords, keystore name, and alias name will all be needed in order to sign an APK. You can handle this in at least a few different ways. The simplest is to enter the relevant information directly into your gradle build script:

If you want to control access to the passwords you can move the information out of the build.gradle file and put it in your local environment or in a properties file to load at build time. To maintain security and control of the information, it’s likely that you would not want to check the keystore properties file into your source control.

Here is an example [from Google] of how to load the information from a file that would be located in your app’s root directory with the project level build.gradle file:

keystore.properties would contain (in this example):

If you prefer the environment variable method, create a script to add the variables to your environment and try something like this:

There are some trade-offs to both of these methods. Figure out what works best for your organization’s methodology and use that one. For the environment variable method, for example, you have to load these variables into your environment somehow. This is less than ideal if you want to generate a signed APK with Android Studio.

If you prefer to sign your APK manually instead of as part of the build process, you’ll want to use apksigner , located at /build-tools//apksigner for build-tools revision 24.0.3 or higher. apksigner uses the public/private key pair stored in your app’s keystore to generate a public key certificate. apksigner then attaches that certificate to the APK. After this is accomplished, the APK is associated with that private key in a unique way. The Android gradle plugin will handle this for you if you configure your build.gradle file with all of the necessary information, as shown above.

You’ll want to zipalign your APK, zipalign will ensure that your app’s uncompressed data starts at a predictable offset inside the APK. zipalign ed APKs are required to publish to the Google Play store.

After your APK is zipalign ed, sign it using apksigner :

You will be prompted at the command line to enter the password for your keystore.

If your keystore and key passwords differ, you’re in for a treat! Using the command above, you will be asked for the keystore password, but will not be asked for the key password. Entering either password results in exceptions and you won’t be having a good time. You’ll need to tell apksigner that you want to specify each password individually. Apparently, this is supposed to be the default behavior, but it hasn’t worked for me. To force apksigner to ask you for the keystore and key password independently, use the —ks-pass and —key-pass options. Following each option with stdin will tell apksigner to capture the password from you at the command line.

I hope this has educated you a bit more about how creating keystores and signing an Android APK works.

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Signing Your Applications

In this document

See also

Android requires that all apps be digitally signed with a certificate before they can be installed. Android uses this certificate to identify the author of an app, and the certificate does not need to be signed by a certificate authority. Android apps often use self-signed certificates. The app developer holds the certificate’s private key.

Signing Overview

You can sign an app in debug or release mode. You sign your app in debug mode during development and in release mode when you are ready to distribute your app. The Android SDK generates a certificate to sign apps in debug mode. To sign apps in release mode, you need to generate your own certificate.

Signing in Debug Mode

In debug mode, you sign your app with a debug certificate generated by the Android SDK tools. This certificate has a private key with a known password, so you can run and debug your app without typing the password every time you make a change to your project.

Android Studio signs your app in debug mode automatically when you run or debug your project from the IDE.

You can run and debug an app signed in debug mode on the emulator and on devices connected to your development manchine through USB, but you cannot distribute an app signed in debug mode.

By default, the debug configuration uses a debug keystore, with a known password and a default key with a known password. The debug keystore is located in $HOME/.android/debug.keystore, and is created if not present. The debug build type is set to use this debug SigningConfig automatically.

For more information about how to build and run apps in debug mode, see Building and Running.

Signing in Release Mode

In release mode, you sign your app with your own certificate:

1. Create a keystore. A keystore is a binary file that contains a set of private keys. You must keep your keystore in a safe and secure place.
2. Create a private key. A private key represents the entity to be identified with the app, such as a person or a company.

Add the signing configuration to the build file for the app module:

Invoke the assembleRelease build task from Android Studio.

The package in app/build/apk/app-release.apk is now signed with your release key.

Note: Including the passwords for your release key and keystore inside the build file is not a good security practice. Alternatively, you can configure the build file to obtain these passwords from environment variables or have the build process prompt you for these passwords.

To obtain these passwords from environment variables:

To have the build process prompt you for these passwords if you are invoking the build from the command line:

After you complete this process, you can distribute your app and publish it on Google Play.

Warning: Keep your keystore and private key in a safe and secure place, and ensure that you have secure backups of them. If you publish an app to Google Play and then lose the key with which you signed your app, you will not be able to publish any updates to your app, since you must always sign all versions of your app with the same key.

The rest of this document provides detailed instructions about how to generate a private key and sign your apps in release mode with Android Studio.

Signing Android Wear Apps

When publishing Android Wear apps, you package the wearable app inside of a handheld app, because users cannot browse and install apps directly on the wearable. Both apps must be signed. For more information on packaging and signing Android Wear apps, see Packaging Wearable Apps.

Signing Your App in Android Studio

To sign your app in release mode in Android Studio, follow these steps:

On the menu bar, click Build >Generate Signed APK.

On the Generate Signed APK Wizard window, click Create new to create a new keystore.

If you already have a keystore, go to step 4.

On the New Key Store window, provide the required information as shown in figure 1.

Your key should be valid for at least 25 years, so you can sign app updates with the same key through the lifespan of your app.

Figure 1. Create a new keystore in Android Studio.

On the Generate Signed APK Wizard window, select a keystore, a private key, and enter the passwords for both. Then click Next.

Figure 2. Select a private key in Android Studio.

On the next window, select a destination for the signed APK and click Finish.

Figure 3. Generate a signed APK in Android Studio.

Automatically Signing Your App

In Android Studio, you can configure your project to sign your release APK automatically during the build process:

1. On the project browser, right click on your app and select Open Module Settings.
2. On the Project Structure window, select your app’s module under Modules.
3. Click on the Signing tab.

Select your keystore file, enter a name for this signing configuration (as you may create more than one), and enter the required information.

Figure 4. Create a signing configuration in Android Studio.

Under Signing Config, select the signing configuration you just created.

Figure 5. Select a signing configuration in Android Studio.

You can also specify your signing settings in Gradle configuration files. For more information, see Configuring Gradle Builds.

Signing Considerations

You should sign all of your apps with the same certificate throughout the expected lifespan of your applications. There are several reasons why you should do so:

• App upgrade: When the system is installing an update to an app, it compares the certificate(s) in the new version with those in the existing version. The system allows the update if the certificates match. If you sign the new version with a different certificate, you must assign a different package name to the application—in this case, the user installs the new version as a completely new application.
• App modularity: Android allows apps signed by the same certificate to run in the same process, if the applications so requests, so that the system treats them as a single application. In this way you can deploy your app in modules, and users can update each of the modules independently.
• Code/data sharing through permissions: Android provides signature-based permissions enforcement, so that an app can expose functionality to another app that is signed with a specified certificate. By signing multiple apps with the same certificate and using signature-based permissions checks, your apps can share code and data in a secure manner.

If you plan to support upgrades for an app, ensure that your key has a validity period that exceeds the expected lifespan of that app. A validity period of 25 years or more is recommended. When your key’s validity period expires, users will no longer be able to seamlessly upgrade to new versions of your application.

If you plan to publish your apps on Google Play, the key you use to sign these apps must have a validity period ending after 22 October 2033. Google Play enforces this requirement to ensure that users can seamlessly upgrade apps when new versions are available.

Securing Your Private Key

Maintaining the security of your private key is of critical importance, both to you and to the user. If you allow someone to use your key, or if you leave your keystore and passwords in an unsecured location such that a third-party could find and use them, your authoring identity and the trust of the user are compromised.

If a third party should manage to take your key without your knowledge or permission, that person could sign and distribute apps that maliciously replace your authentic apps or corrupt them. Such a person could also sign and distribute apps under your identity that attack other apps or the system itself, or corrupt or steal user data.

Your private key is required for signing all future versions of your app. If you lose or misplace your key, you will not be able to publish updates to your existing appn. You cannot regenerate a previously generated key.

Your reputation as a developer entity depends on your securing your private key properly, at all times, until the key is expired. Here are some tips for keeping your key secure:

• Select strong passwords for the keystore and key.
• Do not give or lend anyone your private key, and do not let unauthorized persons know your keystore and key passwords.
• Keep the keystore file containing your private key in a safe, secure place.

In general, if you follow common-sense precautions when generating, using, and storing your key, it will remain secure.

Expiry of the Debug Certificate

The self-signed certificate used to sign your application in debug mode has an expiration date of 365 days from its creation date. When the certificate expires, you will get a build error.

To fix this problem, simply delete the debug.keystore file. The default storage location is in

/.android/ on OS X and Linux, in C:\Documents and Settings\ \.android\ on Windows XP, and in C:\Users\ \.android\ on Windows Vista and Windows 7.

The next time you build, the build tools will regenerate a new keystore and debug key.

Note that, if your development machine is using a non-Gregorian locale, the build tools may erroneously generate an already-expired debug certificate, so that you get an error when trying to compile your application. For workaround information, see the troubleshooting topic I can’t compile my app because the build tools generated an expired debug certificate.

Signing Your App Manually

You do not need Android Studio to sign your app. You can sign your app from the command line using standard tools from the Android SDK and the JDK. To sign an app in release mode from the command line:

Generate a private key using keytool . For example:

This example prompts you for passwords for the keystore and key, and to provide the Distinguished Name fields for your key. It then generates the keystore as a file called my-release-key.keystore . The keystore contains a single key, valid for 10000 days. The alias is a name that you will use later when signing your app.

Compile your app in release mode to obtain an unsigned APK.

Sign your app with your private key using jarsigner :

This example prompts you for passwords for the keystore and key. It then modifies the APK in-place to sign it. Note that you can sign an APK multiple times with different keys.

Verify that your APK is signed. For example:

Align the final APK package using zipalign .

zipalign ensures that all uncompressed data starts with a particular byte alignment relative to the start of the file, which reduces the amount of RAM consumed by an app.

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