F-Droid supports reproducible builds of apps, so that anyone can run the build process again and reproduce the same APK as the original release. This means that F-Droid can verify that an app is 100% free software while still using the original developer’s APK signatures. Ideally, all of the built APKs will have the exact same hash, but that is a more difficult standard with less payoff. Right now, F-Droid verifies reproducible builds using the APK signature.

This concept is occasionally called “deterministic builds”. That is a much stricter standard: that means that the whole process runs with the same ordering each time. The most important thing is that anyone can run the process and end up with the exact same result.

How it is implemented as of now

Publishing signed binaries from elsewhere (e.g. the upstream developer) is now possible after verifying that they match ones built using a recipe. Publishing only takes place if there is a proper match. (Which seems very unlikely to be the case unless the exact same tool-chain is used, so I would imagine that unless the person building and signing the incoming binaries uses fdroidserver to build them, probably the exact same buildserver ID, they will not match. But at least we have the functionality to support that.)

This procedures are implemented as part of fdroid publish. At the publish step, the reproducibility check will follow this logic:

Publish both (upstream-)developer signed and F-Droid signed APKs

Use this approach for shipping a version of an app, with both (upstream-)developer signed and F-Droid signed APKs. This enables us to ship updates for users who installed apps from other sources than F-Droid (eg. Play Store) which are therefore signed by the app-developers, while also shipping updates for apps which were built and signed by F-Droid.

This requires to put (upstream-)developer signatures into fdroiddata. We provide a command for easily extracting signatures from APKs:

$ cd /path/to/fdroiddata
$ fdroid signatures F-Droid.apk

You may also supply HTTPS-URLs directly to fdroid signatures instead of local files. The signature files are extracted to the according metadata directory ready to be used with fdroid publish. A signature consists of 3 files and the result of extracting one will resemble this file listing:

$ ls metadata/org.fdroid.fdroid/signatures/1000012/
CIARANG.RSA  CIARANG.SF  MANIFEST.MF

Exclusively publishing (upstream-)developer signed APKs

For using this older approach, everything in the metadata should be the same as normal, with the addition of the Binaries: directive to specify where to get the binaries from. In this case F-Droid will never attempt to ship APKs signed by F-Droid. Should fdroid publish manage to verify that a downloaded APK can be built reproducibly, the downloaded APK will be published. Otherwise F-Droid will skip publishing this version of the app.

Here is an example for a Binaries directive:

Binaries: https://foo.com/path/to/myapp-%v.apk

Also see: Build Metadata Reference - Binaries

Verification builds

Many people or organizations will only be interested in reproducing builds to make sure that the f-droid.org builds match the original source and nothing has been inserted in. In that case, the resulting APKs are not published for installation. The Verification Server automates this process.

Reproducible Builds

An awful lot of builds already verify with no extra effort since Java code is often compiled into the same bytecode by a wide range of Java versions. The Android SDK’s build-tools will create differences in the resulting XML, PNG, etc. files, but this is usually not a problem since the build.gradle includes the exact version of build-tools to use.

Anything built with the NDK will be much more sensitive. For example, even for builds that use the exact same version of the NDK (e.g. r13b) but on different platforms (.e.g OSX version Ubuntu), the resulting binaries will have differences.

Additionally, we’ll have to look out for anything that includes timestamping information, is sensitive to sort order, etc.

Google is also working towards reproducible builds of Android apps, so using recent versions of the Android SDK helps. One specific case is starting with Gradle Android Plugin v2.2.2, timestamps in the APK file’s ZIP header are automatically zeroed out.

platform Revisions

The Android SDK tools were changed in 2014 to stick two data elements in AndroidManifest.xml as part of the build process: platformBuildVersionName and platformBuildVersionCode. platformBuildVersionName includes the “revision” of the platforms package built against (e.g. android-23), however different “revisions” of the same platforms package cannot be installed in parallel. Plus the SDK tools do not support specifying the required revision as part of the build process. This often results in an otherwise reproducible build where the only difference is the platformBuildVersionName attribute.

The “platform” is part of the Android SDK that represents the standard library that is installed on the phone. They have two parts to their version: “version code”, which is an integer that represents the SDK release, and the “revision”, which represents bugfix versions to each platform. These versions can be seen in the included build.prop file. Each revision has a different number in ro.build.version.incremental. Gradle has no way to specify the revision in compileSdkVersion or targetSdkVersion. Only one “platform-23” can be installed at a time, unlike build-tools, where every release can be installed in parallel.

Here are two examples where I think all the differences came from just different revisions of the platform:

• https://verification.f-droid.org/de.nico.asura_12.apk.diffoscope.html
• https://verification.f-droid.org/de.nico.ha_manager_25.apk.diffoscope.html

PNG Crush/Crunch

A standard part of the Android build process is to run some kind of PNG optimization tool, like aapt singleCrunch or pngcrush. These do not provide deterministic output, it is still an open question as to why. Since PNGs are normally committed to the source repo, a workaround to this problem is to run the tool of your choice on the PNG files, then commit those changes to the source repo (e.g. git). Then, disable the default PNG optimization process by adding this to build.gradle:

android {
    aaptOptions {
        cruncherEnabled = false
    }
}

R8 Optimizer

It appears that some R8 optimizations done in nondeterministic way, producing different bytecode on different build runs.

For instance, R8 tries to optimize ServiceLoader usage making a static list of all services in the code. The order of this list may be different (or even incomplete) on each build run. The only way to avoid this behavior is disabling such optimizations declaring optimized classes in proguard-rules.pro:

-keep class kotlinx.coroutines.CoroutineExceptionHandler
-keep class kotlinx.coroutines.internal.MainDispatcherFactory

Be careful with R8. Always test your builds multiple times and disable optimizations which produce a nondeterministic output.

Resource Shrinker

It’s possible to reduce the APK file size removing unused resources from the package. This is useful when a project depends on some bloat libraries such as AppCompat, especially when R8/ProGuard code shrinking is used.

However, it might be possible that resource shrinker will increase the APK size on different platforms, especially if there is not too many resources to shrink, in which case then original APK will be used instead of the shrinked one (nondeterministic behavior of Gradle plugin). Avoid using resource shrinker unless it decreases the APK file size significantly.

Build Server IDs

To describe the build environment used by F-Droid builds, APKs have two files inserted into them:

• META-INF/fdroidserverid - git commit hash of fdroidserver used for the build
• META-INF/buildserverid - git commit hash of makebuildserver used for the build

To ensure reproducibility, use the exact same revision of the ./makebuildserver and fdroid build. You can find the commit hash of fdroidserver by going to your git clone and running git log -n1. The build server instance is stamped with the git commit hash on creation, and that ID is included in builds.

ZIP entry info

The ZIP format was originally designed around the MSDOS FAT filesystem. UNIX file permissions were added as an extension. APKs only need the most basic ZIP format, without any of the extensions. These extensions are often stripped out in the final release signing process. But the APK build process can add them. For example:

--- a2dp.Vol_137.apk
+++ sigcp_a2dp.Vol_137.apk
@@ -1,50 +1,50 @@
--rw----     2.0 fat     8976 bX defN 79-Nov-30 00:00 AndroidManifest.xml
--rw----     2.0 fat  1958312 bX defN 79-Nov-30 00:00 classes.dex
--rw----     1.0 fat    78984 bx stor 79-Nov-30 00:00 resources.arsc
+-rw-rw-rw-  2.3 unx     8976 b- defN 80-000-00 00:00 AndroidManifest.xml
+-rw----     2.4 fat  1958312 b- defN 80-000-00 00:00 classes.dex
+-rw-rw-rw-  2.3 unx    78984 b- stor 80-000-00 00:00 resources.arsc

Migration to reproducible builds

TODO

• NDK inserts changing build-id, probably via ld
• jar sort order for APKs
• aapt versions produce different results (XML and res/ subfolder names)

Sources

• https://gitlab.com/fdroid/fdroidserver/commit/8568805866dadbdcc6c07449ca6b84b80d0ab03c
• Verification Server
• https://verification.f-droid.org
• https://reproducible-builds.org
• https://wiki.debian.org/ReproducibleBuilds
• https://gitian.org/
• Google Issue #70292819 platform-27_r01.zip was overwritten with a new update (Google login and JavaScript required)
• Google Issue #37132313 platformBuildVersionName makes builds difficult to reproduce, creates unneeded diffs (Google login and JavaScript required)
• Google Issue #110237303 resources.arsc built with non-determism, prevents reproducible APK builds (Google login and JavaScript required)