Deterministic, Reproducible Builds

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Revision as of 19:21, 7 January 2015 by Hans (Talk | contribs) (document current Binaries/publis process)

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The Dream

If projects have deterministic build systems that reproduce the exact same APK whenever and wherever its built, then there will no longer be a need for the FDroid per-app signing key. FDroid will be able to build the exact same APK, then it can copy the original developer's signature out of the APK they produce, and it will verify. Ideally, the FDroid-generated APK will have the exact same hash as well.

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. Everything in the metadata should be the same as normal, with the addition of the Binaries: directive to specify where (with pattern substitution) to get the binaries from.

Publishing only takes place if there is a proper match. (Which seems very unlikely to be the case unless the exact same toolchain 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 procedure is currently implemented as part of fdroid publish. At the publish step, there are two options: a) sign the built apk, or b) download a signed binary from elsewhere, compare it with the built one, and if they match publish the downloaded (externally signed) one. Option b) is enabled by having a Binaries: directive in the metadata, to provide a URL where the binaries are to be retrieved from (with substitutions for the version number in the APK file name).

General Plan

short term

  1. get Lil' Debi's build to be deterministic
  2. download Guardian Project's APK from link in recipe
  3. make it verify as part of fdroid publish
  4. verify against Guardian Project GPG signature in recipe
  5. sign Guardian Project APK using FDroid's GPG key
  6. install Guardian Project APK in FDroid repo

Open question: how to migrate users to new signing key?

long term

  1. get Lil' Debi's build to be deterministic
  2. make it produce the exact same APK, minus signature
  3. copy signature from Guardian Project's APK, included in build recipe
  4. use FDroid APK with Guardian Project's APK signature
  5. sign FDroid APK with FDroid's GPG key
  6. merge FDroid's GPG signature with Guardian Project's GPG signature (if the cipher prefs are the same, gpg will check multiple signatures in a single .sig or .asc file


  • faketime <timestamp> ant clean release
  • jar sort order for APKs
  • faketime integrated into FDroid build?
  • aapt versions produce different results (XML and res/ subfolder names)