Reproducible Builds

Technical details and current coverage of reproducible builds for Gem Wallet Android and iOS releases.

Reproducible builds provide a verifiable link between published source code and a release binary. Our current reproducible-build coverage applies to the Android F-Droid release. We intend to expand this coverage to additional Android and iOS distribution channels.

Android

F-Droid

Gem Wallet uses the independent F-Droid build infrastructure to rebuild the F-Droid APK and verify it before publication.

Verification model

Gem Wallet publishes a signed F-Droid APK for each release. Separately, the F-Droid build servers execute the public build recipe from a clean environment:

  1. The F-Droid metadata selects an exact Git commit and Android NDK version.
  2. F-Droid checks out that commit from the public Gem Wallet repository.
  3. The public reproducible build scripts install the pinned native toolchain and run :app:assembleFdroidRelease.
  4. The build produces an unsigned APK. F-Droid downloads the corresponding developer-signed APK and confirms that it is signed by the certificate fingerprint declared in AllowedAPKSigningKeys.
  5. F-Droid copies the APK signature to the rebuilt artifact and verifies it. Android APK Signature Scheme v2/v3 signatures cover the complete APK outside the signing block, so successful verification requires the rebuilt APK payload to match the Gem Wallet release byte-for-byte.
  6. F-Droid publishes the developer-signed APK only when verification succeeds. A release that does not reproduce is not published.

Deterministic build controls

Gem Wallet includes Rust native libraries, which makes the output sensitive to compiler, linker, NDK, host-platform, and optimizer differences. The F-Droid build controls these inputs by:

  • pinning the Rust toolchain and cargo-ndk versions in android/reproducible/versions.sh;
  • installing cargo-ndk with --locked, while application Rust dependencies are resolved by the committed Cargo.lock;
  • pinning the exact Android NDK revision in both the Android project and the F-Droid metadata;
  • building on a Linux x86_64 host with explicit NDK linker and archiver paths;
  • restricting native APK targets to armeabi-v7a and arm64-v8a, independently of the x86_64 build host;
  • using the dedicated fdroid product channel, without Google push and review modules;
  • disabling R8 optimization with -dontoptimize to prevent non-deterministic DEX output and mapping identifiers;
  • running Gradle without the daemon or configuration cache and clearing transformation caches before the release build.

The exact source commit, build commands, output path, NDK revision, upstream APK URL, and allowed signing certificate are public in the F-Droid metadata. The scripts that configure Rust, Cargo, the NDK toolchain, and Gradle are maintained with the application source under android/reproducible.

Security properties and limitations

A successful F-Droid verification demonstrates that the signed APK published by Gem Wallet can be reproduced from the declared source commit and build recipe. The rebuild is performed outside Gem Wallet infrastructure, reducing reliance on our release pipeline as a single point of trust.

Reproducibility does not prove that the source code is vulnerability-free, that every dependency is trustworthy, or that the selected compiler toolchain is free from compromise. Those risks require source review, dependency controls, and toolchain security in addition to reproducible builds.

Build status and verified releases are available on the Gem Wallet F-Droid page.

Universal APK and Google Play

The Gem Wallet universal APK and Google Play release are not currently covered by the F-Droid verification described above. Reproducible-build support for these Android distribution channels is planned for future work.

iOS

The App Store release is not currently reproducible. Reproducible-build support for iOS and independent verification of the App Store binary are planned for future work.

Frequently Asked Questions

Reproducible Builds allow anyone to build Gem Wallet from the public source code and verify that the resulting app matches the official release. This provides additional transparency and helps confirm that the published app was built from the same source code available on GitHub.
Open source means that the Gem Wallet source code is publicly available and can be reviewed by anyone. Reproducible Builds go one step further by allowing independent developers and services such as F-Droid to build the app from the declared source code and verify that the resulting Android APK matches the official Gem Wallet release byte-for-byte.
Yes. You can build both the Android and iOS apps from the public Gem Wallet source code. The Android app supports reproducible builds using the public build scripts and pinned toolchain setup. You can also build and run the iOS app locally.
The reproducible build scripts and pinned toolchain setup are public. The F-Droid metadata declares the exact source commit, Android NDK revision, build commands, and expected APK output for each release. F-Droid independently rebuilds the Android app and publishes it only when the APK payload matches the Gem Wallet release byte-for-byte.
If you have a question, encounter a build issue, or find a discrepancy, please submit an issue in the Gem Wallet GitHub repository. Include information about your environment, build steps, and any relevant error messages to help us investigate the issue.