Sign release APKs with a fixed key from CI secrets

README.md

@@ -69,9 +69,11 @@ sideloaded Android app, and needs your own copy of the game data.
     proprietary game assets (ships `res/GET_ASSETS.txt` in their place).
     See "4.1. Packaging a distributable build" below.
   - `apk` - builds `dist/questshock-<version>-android-arm64.apk` for
-    Android-based OpenXR headsets. Unlike every other target, this needs
-    network access at build time (Gradle/AGP's own dependency
-    resolution). See "5. Android / Quest build" below.
+    Android-based OpenXR headsets (a debug build, `...-arm64-debug.apk`,
+    when there is no signing key - see "4.3. Releases and signing" below).
+    Unlike every other target, this needs network access at build time
+    (Gradle/AGP's own dependency resolution). See "5. Android / Quest
+    build" below.
   - `android-studio` - stages `engine/`/`android/gl4es-src/` (scratch,
     patched copies) and the Android prebuilts with host-resolvable paths,
     so Android Studio can compile and deploy `android/` natively instead
@@ -215,7 +217,7 @@ package on every push, using the build-image as its container (so no
 extra setup is needed in CI beyond the image itself), and publishes the
 resulting tarball to dl.ladkau.de.
 
-### 4.3. GitHub releases
+### 4.3. Releases and signing
 
 The repository is developed on Gitea, and the GitHub repository is a mirror of
 it. `.github/workflows/release.yml` is GitHub's own workflow (the Gitea build
@@ -230,9 +232,33 @@ is `.gitea/workflows/build.yml`):
   `docker-entrypoint.sh` and `build-engine.sh`, so a change to any of them
   builds a new image (`build-image/VERSION` is not used). The first run is slow
   because it builds the image.
-- The APK is built with `assembleDebug`, so it is signed with the debug key of
-  the machine that built it. Android only updates an installed app from an APK
-  with the same signing key.
+- The APK is signed with the release key described below.
+
+**Signing.** Android only updates an installed app from an APK signed with the
+same key, so every distributed APK is signed with one fixed key. `make apk`
+(`build-image/build-apk.sh`) builds and signs the release APK when
+`QUESTSHOCK_KEYSTORE` is set, together with `QUESTSHOCK_KEYSTORE_PASSWORD` and
+optionally `QUESTSHOCK_KEY_ALIAS` (default `questshock`) and
+`QUESTSHOCK_KEY_PASSWORD` (default: the keystore password). Without a keystore
+it builds a debug APK, `questshock-<version>-android-arm64-debug.apk`, signed
+with a per-machine debug key. That is what Android Studio's Run uses too; an
+installed build has to be uninstalled before one signed with another key can be
+installed.
+
+Create the key once and keep the file and its password safe: without them,
+users have to uninstall the app to get an update.
+
+```sh
+keytool -genkeypair -v -storetype PKCS12 -keystore questshock-release.jks \
+    -alias questshock -keyalg RSA -keysize 4096 -validity 10000
+base64 -w0 questshock-release.jks    # the value of ANDROID_KEYSTORE_BASE64
+```
+
+Both CI systems read the key from these secrets: `ANDROID_KEYSTORE_BASE64`,
+`ANDROID_KEYSTORE_PASSWORD`, and optionally `ANDROID_KEY_ALIAS` and
+`ANDROID_KEY_PASSWORD`. Without `ANDROID_KEYSTORE_BASE64`, the Gitea build
+produces a debug APK and the GitHub build fails on a tag. `*.jks` and
+`*.keystore` are git-ignored.
 
 ## 5. Android / Quest build
 
@@ -248,7 +274,8 @@ is `.gitea/workflows/build.yml`):
   apk` needs its own separate toolchain - see "5.2. Building natively in
   Android Studio" below.
 
-`make apk` builds `dist/questshock-<version>-android-arm64.apk` - an
+`make apk` builds `dist/questshock-<version>-android-arm64.apk` (or
+`...-arm64-debug.apk`, see "4.3. Releases and signing" above) - an
 immersive OpenXR app (see `android/app/src/main/cpp/xr_session.c`) that
 can be sideloaded onto any Android-based VR headset with OpenXR support
 (Meta Quest, Pico, etc. - see "1. Design principles" above), not just one