Why App Signing Matters for Your Android Apps Somkiat Khitwongwattana Android GDE @akexorcist

What’s App Signing in Android? Part of the Android app development process to ensure the integrity and authenticity of your app, and it's required for publishing your app on Google Play. .aab

Benefit Ensure that the app you release is the same app that you developed Security Integrity Ensure that the app you release is the same app that you developed

Signing Process .aab Signing Key

APK Signature Scheme This process verifies the authenticity and integrity of Android application packages (APKs), ensuring they haven't been tampered with and originate from the expected developer.

APK Signature Scheme ● Developed specifically for APK files ● Covers all files within the APK v2+ Scheme v1 Scheme ● Based on JAR Signing ● Doesn't cover all files in the APK

APK Signature Scheme ● Version 2 ● Version 3 ● Version 3.1 ● Version 4 v2+ Scheme v1 Scheme ● Version 1

v2+ Scheme Before Signing After Signing Contents of ZIP entries Central Directory End of Central Directory APK Signing Block Contents of ZIP entries Central Directory End of Central Directory

Signature Algorithm ● RSASSA-PSS with SHA2-256 digest, 32 bytes of salt ● RSASSA-PSS with SHA2-512 digest, 64 bytes of salt ● RSASSA-PKCS1-v1_5 with SHA2-256 digest ● RSASSA-PKCS1-v1_5 with SHA2-512 digest ● ECDSA with SHA2-256 digest ● ECDSA with SHA2-512 digest ● DSA with SHA2-256 digest

Signature Algorithm Supported keys sizes and EC curves ● RSA 1024, 2048, 4096, 8192, 16384 ● EC NIST P-256, P-384, P-521 ● DSA 1024, 2048, 3072

APK Signature Scheme v1 Supported across all Android versions Based on a technology called JAR signing, which has been used for a long time to secure Java programs. Not recommended for security and performance reasons

APK Signature Scheme v2 Android 7.0 (Nougat) or higher More robust way to verify the authenticity and integrity of Android application packages (APKs). It's like a tamper-evident seal for your app, ensuring that it hasn't been modified and truly comes from the developer you expect. ● Speeds up app installation times ● Verifies the integrity of the entire APK file ● Smaller APK file size

APK Signature Scheme v2 Android 7.0 (Nougat) or higher ● APK Signing Block ● Hashing and Signing ● Verification

APK Signature Scheme v3 Android 9 (Pie) or higher Builds upon the strong foundation of v2, adding a critical feature: key rotation. This allows developers to change their signing keys without disrupting app updates for users. ● Strengthens security and enables future cryptographic upgrades. ● Seamless updates from key rotation without any disruption or reinstallation

APK Signature Scheme v3 Android 9 (Pie) or higher ● Proof-of-Rotation ● Multiple Signers ● Verification with Key Rotation

APK Signature Scheme v4 Android 11 (R) or higher Employs a detached signature format. This means the signature is stored in a separate file, offering several advantages. Streaming installations, where apps can be used even while they are still being downloaded.

APK Signature Scheme v4 Android 11 (R) or higher ● Merkle Hash Tree ● Streaming-compatible Signing Scheme ● Incremental App Update Supports ● Verification with .idsig file

APK Signature Scheme v3.1 Android 13 (T) or higher Builds upon the key rotation capabilities of v3, addressing some of its limitations and offering a smoother experience for developers and users. ● Strengthens security and enables future cryptographic upgrades. ● Specify a target SDK version for each signing key in the rotation chain. ● Backward compatible with v3 ● v4 compatible

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// build.gradle.kts (:app) android { ... signingConfigs { create("release") { ... enableV3Signing = true enableV4Signing = true } } }

Play App Signing Google manages and protects your app's signing key for you and uses it to sign optimized distribution APKs that are generated from your app bundles. Keys are protected by Google’s Key Management Service.

Signing Process Upload Key .aab

Signing Process .aab App Signing Key

The key you use to sign your app bundle before you upload it on Google Play. Upload Key App Signing Key The key Google Play uses to sign the APKs that are delivered to a user's device.

Play Encrypt Private Key tool PEPK tool A tool to export private keys from a Java keystore and encrypt them for transfer to Google Play.

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RSA key with 4096 bits (RSASSA_PKCS1V15) Play App Signing RSA key with 1024 bits (RSASSA_PKCS1V15) Android Studio Up to you keytool (CLI) For strong encryption strength, RSASSA-PSS with 4096 bits is recommended

● Android 13 (T) and above ● Android 7.0 (Nougat) to Android 12 (S) ● Below Android 7.0 (Nougat) Key upgrades on Google Play

Key upgrades on Google Play Android 13 (T) and above The Android platform enforces the usage of the upgraded key.

Key upgrades on Google Play Android 7.0 (Nougat) to Android 12 (S) The Android platform does not enforce the usage of this upgraded key and still recognizes the legacy signing key as the app signing key. Google Play Protect will check that app updates are signed with your upgraded key, unless turned off by the user.

Key upgrades on Google Play Below Android 7.0 (Nougat) Use the legacy signing key as the app signing key.

Dual Signing Android 13 and higher will use the new key, older versions will use the old key. Google Play Protect Verifies that app updates are signed with the upgraded key. Provide an extra layer of security without fully enforcing the new key at the platform level. Google Play’s approach to key upgrades

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Source Stamp Signer A security feature introduced by Google Play to enhance the trustworthiness and security of Android apps. It essentially adds verifiable metadata to an app's manifest, indicating its origin and distribution channel.

Signer #1 DN: CN=Somkiat, L=Bangkok, ST=Bangkok, C=TH key algorithm: RSA key size (bits): 2048 Source Stamp Signer DN: CN=Android, OU=Android, O=Google Inc., L=Mountain View, ST=California, C=US key algorithm: RSA key size (bits): 4096

Post-key rotation After rotating your app signing key, you must update the SHA-1 or SHA-256 fingerprint for any services used within your app, such as Firebase, Google APIs, or Digital Asset Links. Be sure to use the fingerprint of the new app signing key.

Create an keystore with keytool a command-line tool that comes bundled with the Java Development Kit (JDK). It's a powerful utility for managing cryptographic keys and certificates. Example directory on macOS /Applications/Android Studio.app/Contents/jbr/Contents/Home/bin/keytool

keytool -genkeypair \ -alias \ -keyalg RSA \ -sigalg RSASSA-PSS \ -keystore .jks \ -keysize 4096 \ -validity 3650 \ # 10 years -storepass \ -keypass \ -dname "CN=Somkiat, L=Bangkok, ST=Bangkok, C=TH"

APK Signature analysis apksigner A command-line tool provided by Google as part of the Android SDK apksigtool a tool for parsing android APK Signing Blocks and verifying APK signatures https://github.com/obfusk/apksigtool

apksigner verify --print-certs -v .apk Verified using v1 scheme (JAR signing): true Verified using v2 scheme (APK Signature Scheme v2): true Verified using v3 scheme (APK Signature Scheme v3): true Verified using v3.1 scheme (APK Signature Scheme v3.1): true Verified using v4 scheme (APK Signature Scheme v4): false Verified for SourceStamp: true Number of signers: 1 Signer (minSdkVersion=33, maxSdkVersion=2147483647) certificate DN: #### Signer (minSdkVersion=33, maxSdkVersion=2147483647) certificate SHA-256 digest: #### Signer (minSdkVersion=33, maxSdkVersion=2147483647) key algorithm: RSA Signer (minSdkVersion=33, maxSdkVersion=2147483647) key size (bits): 4096 Signer (minSdkVersion=33, maxSdkVersion=2147483647) public key SHA-256 digest: #### Signer (minSdkVersion=24, maxSdkVersion=32) certificate DN: #### Signer (minSdkVersion=24, maxSdkVersion=32) certificate SHA-256 digest: #### Signer (minSdkVersion=24, maxSdkVersion=32) key algorithm: RSA Signer (minSdkVersion=24, maxSdkVersion=32) key size (bits): 2048 Signer (minSdkVersion=24, maxSdkVersion=32) public key SHA-256 digest: #### Source Stamp Signer certificate DN: #### Source Stamp Signer certificate SHA-256 digest: #### Source Stamp Signer key algorithm: RSA

apksigtool parse .apk PAIR ID: 0x1b93ad61 APK SIGNATURE SCHEME v3.1 BLOCK SIGNER 0 SIGNED DATA DIGEST 0 SIGNATURE ALGORITHM ID: 0x104 (RSASSA-PKCS1-v1_5 with SHA2-512 digest) CERTIFICATE 0 X.509 SUBJECT: #### PUBLIC KEY ALGORITHM: RSA PUBLIC KEY BIT SIZE: 4096 MIN SDK: 33 MAX SDK: 2147483647 MIN SDK: 33 MAX SDK: 2147483647 SIGNATURE 0 SIGNATURE ALGORITHM ID: 0x104 (RSASSA-PKCS1-v1_5 with SHA2-512 digest) PUBLIC KEY PUBLIC KEY ALGORITHM: RSA PUBLIC KEY BIT SIZE: 4096

Conclusion ● Google Play manages key rotation for developers ● Use a robust signature algorithm for your key ● Keep your app signing key and upload key separate ● Enable v3 and v4 scheme to leverage App Signing features ● Disabling v1 scheme requires setting your app's minimum supported Android version to 7.0 (Nougat) or higher ● Even with strong signing keys, apps can still be reverse engineered

App Signing https://source.android.com/docs/security/features/apksigning Use Play App Signing https://support.google.com/googleplay/android-developer/answer/9842756 apksigner https://developer.android.com/tools/apksigner APK Signing Block considerations https://gist.github.com/obfusk/31c332b884464cd8aa06ce1ba1583c05 Resources

Thank You! Somkiat Khitwongwattana Android GDE @akexorcist