subtracks/signing/README.md

Verifying a subtracks download

subtracks ships through four channels:

• GitHub releases — the APK and the AAB are signed with the release key. The public certificate is committed here as release-certificate.pem and attached to each release, along with a SHA256SUMS.
• Google Play — uses Play App Signing, so Google re-signs the app it delivers. The release key is only the upload key there: it signs the AAB we send to Play, not the APK users install. The installed app verifies against Google's app signing certificate (Play Console → App integrity), and the upload key only proves the AAB we uploaded.
• F-Droid — re-signed by F-Droid with its own key; see F-Droid's signing keys.
• nightly builds (the subtracks-nightly artifact from CI) — signed with the public debug key committed at app/debug.keystore, whose certificate is nightly-certificate.pem.

GitHub releases

The APK and the AAB are signed with the release key. Certificate SHA-256:

# keytool -printcert, and Play Console → App integrity (upload key)
02:DA:2C:B7:B8:21:4E:53:BD:BF:B4:55:69:8D:CB:96:52:22:F9:1C:95:4B:52:10:AE:AC:96:FE:32:9C:8F:01
# apksigner verify --print-certs
02da2cb7b8214e53bdbfb455698dcb965222f91c954b5210aeac96fe329c8f01

Each release attaches the APK, the AAB, the checksums and the certificate (committed here as release-certificate.pem):

sha256sum -c SHA256SUMS
apksigner verify --print-certs subtracks.apk              # compare with the .pem
keytool -printcert -jarfile subtracks.aab                 # the AAB
keytool -printcert -file release-certificate.pem          # prints its SHA-256

The .aab is the exact bundle uploaded to Google Play (install it by generating device APKs with bundletool); the .apk is the universal APK for sideloading. The signing certificate SHA-256 is also listed in the release notes.

Google Play

The AAB we upload is signed with the release (upload) key, and that is what Play checks; Google then re-signs the delivered APKs with its own app signing key. So a Play-installed app is verified against Play's app signing certificate (shown in Play Console → App integrity), not release-certificate.pem.

To check the AAB that was sent:

keytool -printcert -jarfile app-release.aab

nightly builds

The signing key is the committed app/debug.keystore (a public key: alias androiddebugkey, password android). Certificate SHA-256:

# keytool -printcert
DF:31:3F:18:58:E9:AD:F4:7F:44:65:B2:80:8F:DC:E5:B9:F8:8A:48:35:DC:D0:79:95:19:61:C3:05:05:08:37
# apksigner verify --print-certs
df313f1858e9adf47f4465b2808fdce5b9f88a4835dcd079951961c305050837

To check a nightly APK:

apksigner verify --print-certs subtracks-nightly.apk

The SHA-256 digest line must equal the fingerprint above (the certificate is committed at nightly-certificate.pem).