Android signing

Android Signing

Instructions

Signing is the single most brittle step of an Android release. Get it right once, lock it down, and never touch it again.

1. Keystore Generation

Generate a 25-year RSA 2048 upload key (never commit the file):

keytool -genkey -v \
  -keystore upload-keystore.jks \
  -keyalg RSA -keysize 2048 -validity 9125 \
  -alias upload \
  -storetype JKS \
  -storepass "$KEYSTORE_PASSWORD" \
  -keypass   "$KEY_PASSWORD" \
  -dname "CN=My Company, OU=Mobile, O=My Company, L=City, S=State, C=US"

Store the .jks file, KEYSTORE_PASSWORD, KEY_ALIAS, KEY_PASSWORD in your CI secret store and 1Password. Losing the key means losing the ability to update the app unless you're on Play App Signing.

2. Gradle Wiring

android/key.properties (git-ignored):

storeFile=../upload-keystore.jks
storePassword=...
keyAlias=upload
keyPassword=...

android/app/build.gradle.kts:

val keystoreProperties = Properties().apply {
    val f = rootProject.file("key.properties")
    if (f.exists()) load(FileInputStream(f))
}

android {
    signingConfigs {
        create("release") {
            storeFile = file(keystoreProperties["storeFile"] as String)
            storePassword = keystoreProperties["storePassword"] as String
            keyAlias = keystoreProperties["keyAlias"] as String
            keyPassword = keystoreProperties["keyPassword"] as String
        }
    }
    buildTypes {
        release { signingConfig = signingConfigs.getByName("release") }
    }
}

3. Play App Signing (mandatory for new apps)

Google holds the app signing key; you hold the upload key. Benefits:

• Lost upload key can be reset by Play support (app signing key cannot be).
• Google re-signs each install, enabling automatic APK optimisation.
• Required for Android App Bundles, which are the only accepted format for new apps since August 2021.

Enrol at Play Console → Setup → App integrity → App signing. Upload your upload certificate (.pem), not the private key.

4. Signature Schemes v1 / v2 / v3 / v4

Scheme	Protects	Added
v1 (JAR)	Individual files inside the APK	Android 1.0
v2 (APK)	The entire APK (faster install, tamper-proof)	Android 7.0
v3 (APK)	v2 + key rotation	Android 9.0
v4 (APK)	Incremental install (.apk.idsig)	Android 11

AGP 8.4+ signs with v1+v2+v3 by default. v4 is emitted automatically for bundletool build-apks --local-testing.

5. Extracting the Upload Certificate

keytool -export -rfc \
  -keystore upload-keystore.jks \
  -alias upload \
  -file upload_certificate.pem

Upload this .pem to Play Console when enrolling.

6. Verify a Signed AAB / APK

apksigner verify --verbose --print-certs app-release.apk
bundletool validate --bundle=app-release.aab

Confirm the SHA-256 matches the fingerprint Play shows under App integrity.

7. CI: Decoding the Keystore

- name: Decode keystore
  run: |
    echo "$ANDROID_KEYSTORE_BASE64" | base64 -d > android/upload-keystore.jks
    cat > android/key.properties <<EOF
    storeFile=../upload-keystore.jks
    storePassword=${{ secrets.KEYSTORE_PASSWORD }}
    keyAlias=${{ secrets.KEY_ALIAS }}
    keyPassword=${{ secrets.KEY_PASSWORD }}
    EOF

8. Key Rotation (v3)

If you suspect the upload key is compromised:

1. Generate a new keystore.
2. Play Console → App integrity → Request upload key reset (upload new .pem).
3. Wait for Google's approval (typically 1–2 business days).
4. Update CI secrets and delete the old .jks everywhere.

The app signing key is never rotated for existing apps.

9. Common Gotchas

• jarsigner alone is insufficient for modern Android; always use apksigner (invoked by AGP).
• storeFile paths are relative to android/, not the project root — a leading ../ is usually required.
• Keystore type mismatch (JKS vs PKCS12) after keytool upgrades. Convert with keytool -importkeystore -srcstoretype JKS -deststoretype PKCS12.
• Different upload key per variant (e.g. dev/staging/prod) — staging builds are often signed with a debug-like keystore checked into the repo.

10. Checklist

• Upload keystore generated with RSA 2048+, 25-year validity.
• Enrolled in Play App Signing; upload certificate uploaded to Play Console.
• key.properties and *.jks git-ignored; CI decodes from base64 secret.
• signingConfig wired to release build type only.
• apksigner verify run on CI artifact before upload.
• Fingerprints recorded in 1Password alongside the keystore.
• Rotation procedure documented in docs/signing-rotation.md.