- HTTPS connections by default
- server only works over HTTPS, HTTP is a redirect
- Android enforces that all apps have a valid signature over the entire contents of the APK file
- Android verifies updates based on the signature of the installed app
- file integrity protected by signed metadata
- signed metadata includes hashes of the app and its signing key
- signed metadata generated on a separate machine (which is fully offline for f-droid.org and guardianproject.info)
- public key for verifying metadata signatures built into F-Droid client app
- signed metadata includes timestamp and expiry
- easy Tor support via Settings
While the current setup is already a solid platform, we are implementing a number of improvements:
- The signed metadata will include list of official mirrors, then the client chooses mirrors based on availability and freshness based on local criteria like whether Tor is in use, closest on the internet, etc.
- We are also moving the standard HTTP “etag” cache check from the server to the client so it cannot be abused to track users
- pinned TLS certificate built into the client app
In order to defend against an attacker that holds the signing keys for the app repository, there must be a trustworthy source of information to compare against. Reproducible builds means that anyone with the same source code will produce the exact same binary. When paired with an auditing system, it is easy to catch malware inserted in the build process, rather than the source code, like XCodeGhost. Reproducible builds also makes it possible to have all builds of a release binary have the exact same hash. Then any app repository can build apps only from source code, and have a source of verification data from any other app repository building the same app. Building software from source has become cheap enough that many companies like gitlab.com and Travis CI are offering free, automated build services in the cloud. Since the whole F-Droid toolset is free software and designed to be easy to setup, the barriers to setting up automatic auditing are quite low. People in separate areas of the world with different risk profiles can run verification servers to provide more trustworthy information.