Manually Signing the APK

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last updated: 2017-09

After the application has been built for release, the APK must be signed prior to distribution so that it can be run on an Android device. This process is typically handled with the IDE, however there are some situations where it is necessary to sign the APK manually, at the command line. The following steps are involved with signing an APK:

  1. Create a Private Key – This step needs to be performed only once. A private key is necessary to digitally sign the APK. After the private key has been prepared, this step can be skipped for future release builds.

  2. Zipalign the APKZipalign is an optimization process that is performed on an application. It enables Android to interact more efficiently with the APK at runtime. Xamarin.Android conducts a check at runtime, and will not allow the application to run if the APK has not been zipaligned.

  3. Sign the APK – This step involves using the apksigner utility from the Android SDK and signing the APK with the private key that was created in the previous step. Applications that are developed with older versions of the Android SDK build tools prior to v24.0.3 will use the jarsigner app from the JDK. Both of these tools will be discussed in more detail below.

The order of the steps is important and is dependent on which tool used to sign the APK. When using apksigner, it is important to first zipalign the application, and then to sign it with apksigner. If it is necessary to use jarsigner to sign the APK, then it is important to first sign the APK and then run zipalign.

Prerequisites

This guide will focus on using apksigner from the Android SDK build tools, v24.0.3 or higher. It assumes that an APK has already been built.

Applications that are built using an older version of the Android SDK Build Tools must use jarsigner as described in Sign the APK with jarsigner below.

Create a Private Keystore

A keystore is a database of security certificates that is created by using the program keytool from the Java SDK. A keystore is critical to publishing a Xamarin.Android application, as Android will not run applications that have not been digitally signed.

During development, Xamarin.Android uses a debug keystore to sign the application, which allows the application to be deployed directly to the emulator or to devices configured to use debuggable applications. However, this keystore is not recognized as a valid keystore for the purposes of distributing applications.

For this reason, a private keystore must be created and used for signing applications. This is a step that should only be performed once, as the same key will be used for publishing updates and can then be used to sign other applications.

It is important to protect this keystore. If it is lost, then it will not be possible to publish updates to the application with Google Play. The only solution to the problem caused by a lost keystore would be to create a new keystore, re-sign the APK with the new key, and then submit a new application. Then the old application would have to be removed from Google Play. Likewise, if this new keystore is compromised or publically distributed, then it is possible for unofficial or malicious versions of an application to be distributed.

Create a New Keystore

Creating a new keystore requires the command line tool keytool from the Java SDK. The following snippet is an example of how to use keytool (replace <my-filename> with the file name for the keystore and <key-name> with the name of the key within the keystore):

$ keytool -genkeypair -v -keystore <filename>.keystore -alias <key-name> -keyalg RSA \
          -keysize 2048 -validity 10000

The first thing that keytool will ask for is the password for the keystore. Then it will ask for some information to help with creating the key. The following snippet is an example of creating a new key called publishingdoc that will be stored in the file xample.keystore:

$ keytool -genkeypair -v -keystore xample.keystore -alias publishingdoc -keyalg RSA \
          -keysize 2048 -validity 10000
Enter keystore password:
Re-enter new password:
What is your first and last name?
  [Unknown]:  Ham Chimpanze
What is the name of your organizational unit?
  [Unknown]:  NASA
What is the name of your organization?
  [Unknown]:  NASA
What is the name of your City or Locality?
  [Unknown]:  Cape Canaveral
What is the name of your State or Province?
  [Unknown]:  Florida
What is the two-letter country code for this unit?
  [Unknown]:  US
Is CN=Ham Chimpanze, OU=NASA, O=NASA, L=Cape Canaveral, ST=Florida, C=US correct?
  [no]:  yes

Generating 2,048 bit RSA key pair and self-signed certificate (SHA1withRSA) with a validity of 10,000 days
        for: CN=Ham Chimpanze, OU=NASA, O=NASA, L=Cape Canaveral, ST=Florida, C=US
Enter key password for <publishingdoc>
        (RETURN if same as keystore password):
Re-enter new password:
[Storing xample.keystore]

To list the keys that are stored in a keystore, use the keytool with the – list option:

$ keytool -list -keystore xample.keystore

Zipalign the APK

Before signing an APK with apksigner, it is important to first optimize the file using the zipalign tool from the Android SDK. zipalign will restructure the resources in an APK along 4-byte boundaries. This alignment allows Android to quickly load the resources from the APK, increasing the performance of the application and potentially reducing memory use. Xamarin.Android will conduct a run-time check to determine if the APK has been zipaligned. If the APK is not zipaligned, then the application will not run.

The follow command will use the signed APK and produce a signed, zipaligned APK called helloworld.apk that is ready for distribution.

$ zipalign -f -v 4 mono.samples.helloworld-unsigned.apk helloworld.apk

Sign the APK

After zipaligning the APK, it is necessary to sign it using a keystore. This is done with the apksigner tool, found in the build-tools directory of the version of the SDK build tools. For example, if the Android SDK build tools v25.0.3 is installed, then apksigner can be found in the directory:

$ ls $ANDROID_HOME/build-tools/25.0.3/apksigner
/Users/tom/android-sdk-macosx/build-tools/25.0.3/apksigner*

The following snippet assumes that apksigner is accessible by the PATH environment variable. It will sign an APK using the key alias publishingdoc that is contained in the file xample.keystore:

$ apksigner sign --ks xample.keystore --ks-key-alias publishingdoc mono.samples.helloworld.apk

When this command is run, apksigner will ask for the password to the keystore if necessary.

See Google's documentation for more details on the use of apksigner.

ℹ️

According to Google issue 62696222, apksigner is "missing" from the Android SDK. The workaround for this is to install the Android SDK build tools v25.0.3 and use that version of apksigner.

Sign the APK with jarsigner

🚫

This section only applies if it is nececssary to sign the APK with the jarsigner utility. Developers are encouraged to use apksigner to sign the APK.

This technique involves signing the APK file using the jarsigner command from the Java SDK. The jarsigner tool is provided by the Java SDK.

The following shows how to sign an APK by using jarsigner and the key publishingdoc that is contained in a keystore file named xample.keystore :

$ jarsigner -verbose -sigalg SHA1withRSA -digestalg SHA1 \
            -keystore xample.keystore mono.samples.helloworld.apk publishingdoc
ℹ️

When using jarsigner, it is important to sign the APK first, and then to use zipalign.

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