amethyst/tools/arti-build/README.md

Arti Android Build Tools

Custom-built Arti (Tor in Rust) native libraries for Amethyst Android. This replaces the Guardian Project's arti-mobile-ex AAR with a minimal JNI wrapper built directly from Arti source.

Why custom build?

	Guardian Project AAR	Custom build
Size	~140MB	~11MB
16KB pages	No	Yes (rustc aligns Android targets to 16 KiB)
Stop/restart	Broken (state file lock)	Works (TorClient persists, only SOCKS proxy stops)
Version	Behind	Pinned to latest (see ARTI_VERSION)

Quick start

Pre-built .so files should be committed to amethyst/src/main/jniLibs/. You only need to rebuild if you want to verify binaries, update the Arti version, or modify the JNI wrapper.

Reproducible builds

The shipped .so is built to be reproducible so anyone — F-Droid, Zapstore, or an independent auditor — can rebuild it from this tag and confirm the committed binary wasn't tampered with. Five things have to be fixed:

Source of non-determinism	Pinned by
rustc / cargo version	rust-toolchain.toml (rustup auto-installs it)
Android NDK revision	ANDROID_NDK_VERSION; build-arti.sh refuses to build with any other revision
transitive dependency versions	committed Cargo.lock; builds run cargo --locked
absolute paths embedded in the binary	--remap-path-prefix in repro-env.sh
codegen/link ordering keyed on the real build path	canonical build path (build-arti.sh builds in /tmp/amethyst-arti-build)

Why the NDK is pinned. It is not just an SDK detail: the NDK supplies the clang that compiles Arti's C dependencies (ring, zstd-sys, libsqlite3-sys) and the lld that links the final cdylib, both of which stamp themselves into the binary's .comment section next to rustc's own version. Swapping the NDK changes the bytes exactly like swapping rustc would. Before this was pinned the build picked the first directory matching ~/Android/Sdk/ndk/*/, so the committed libraries were produced by r25b while this file told everyone to install r27 — two verifiers could both follow the README and get different, equally "correct" results. build-arti.sh now reads each candidate's source.properties and keeps looking until it finds the pinned revision, then re-checks the .note.android.ident stamp of every .so it produced.

CARGO_NDK_VERSION records the cargo-ndk release the pinned output was verified with. cargo-ndk only wraps the NDK, so a mismatch is a warning rather than an error — but it is the next thing to check if your rebuild does not match.

repro-env.sh (sourced by both build scripts) also sets CARGO_INCREMENTAL=0 and a fixed SOURCE_DATE_EPOCH derived from the Arti tag. The size-optimized release profile in Cargo.toml (lto, codegen-units = 1, strip, panic = "abort") is itself deterministic for a fixed toolchain.

Why the canonical path matters. Verified empirically: with the toolchain, lockfile, and path-remapping all in place, two builds at the same path are byte-for-byte identical, but two builds at different paths still differ — not in any embedded string (no path leaks into the binary) but in the order rustc lays out functions/data, which it derives from the real on-disk artifact paths. --remap-path-prefix only rewrites embedded strings, not that internal ordering. So build-arti.sh always compiles in a fixed location (/tmp/amethyst-arti-build, override with ARTI_REPRO_DIR); F-Droid and any verifier must use the same path to get matching bytes. This is the standard way Rust libraries are reproduced (F-Droid builds Rust at a fixed path too).

Verify the committed binary reproduces

From tools/arti-build/, the helper builds twice from clean and diffs the output:

./verify-reproducible.sh            # both ABIs (arm64-v8a + x86_64)
./verify-reproducible.sh --release  # arm64-v8a only (faster)

It prints ✅ REPRODUCIBLE when two clean builds produce identical bytes, then reports whether that matches the committed .so. Both builds compile in the canonical /tmp/amethyst-arti-build, so the result is independent of where the repo is checked out.

Prerequisites

1. Rust toolchain — the exact version is pinned in rust-toolchain.toml; rustup installs it automatically. You only need rustup itself:

   curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
   

2. Android targets

   rustup target add aarch64-linux-android x86_64-linux-android
   

3. cargo-ndk — the release the pinned output was verified with:

   cargo install cargo-ndk --version "$(cat CARGO_NDK_VERSION)" --locked
   

4. Android NDK — the exact revision in ANDROID_NDK_VERSION (currently 30.0.16248370, r30). Any other revision is refused: it would produce a .so that does not match the committed one.

   # Via Android Studio: SDK Manager → SDK Tools → NDK (Side by side)
   # Or via command line:
   sdkmanager "ndk;$(cat ANDROID_NDK_VERSION)"
   

   build-arti.sh finds it automatically under $ANDROID_HOME/ndk/, $ANDROID_SDK_ROOT/ndk/, ~/Android/Sdk/ndk/, ~/Library/Android/sdk/ndk/ or /usr/local/lib/android/sdk/ndk/. ANDROID_NDK_HOME and ANDROID_NDK_ROOT are tried first when set, but they are only hints: every candidate is checked against its own source.properties, and one at the wrong revision is reported and skipped rather than failing the build. CI images (GitHub runners among them) export both at a bundled NDK that is not ours.

Building

cd tools/arti-build

# Build for all targets (arm64 + x86_64)
./build-arti.sh

# Build arm64 only (for release APKs)
./build-arti.sh --release

# Clean rebuild from scratch
./build-arti.sh --clean

The script will:

1. Clone official Arti source from gitlab.torproject.org
2. Check out the version pinned in ARTI_VERSION
3. Copy the JNI wrapper into the source tree
4. Compile with cargo-ndk for each target architecture
5. Output .so files to amethyst/src/main/jniLibs/{arm64-v8a,x86_64}/
6. Verify JNI symbols are exported correctly

Output

amethyst/src/main/jniLibs/
├── arm64-v8a/
│   └── libarti_android.so    (~5-6 MB)
└── x86_64/
    └── libarti_android.so    (~6-7 MB, emulator support)

Verifying 16KB page alignment

Google Play requires 16KB page-aligned native libraries. Verify with:

readelf -l amethyst/src/main/jniLibs/arm64-v8a/libarti_android.so | grep LOAD

The first LOAD segment alignment should be 0x4000 (16384 bytes). This comes from rustc's Android target spec (max-page-size=16384), not from the NDK, so it holds for every NDK revision we could build with.

Checking which toolchain built a .so

The shipped binaries say so themselves — useful when a rebuild does not match, or when auditing a .so you did not build:

# NDK release name + build number (the last component of the pinned revision)
readelf -p .note.android.ident amethyst/src/main/jniLibs/arm64-v8a/libarti_android.so

# clang / lld (from the NDK) and rustc versions
readelf -p .comment amethyst/src/main/jniLibs/arm64-v8a/libarti_android.so

For the pinned toolchain the first command prints r30 and build 16248370. The second prints the rustc version from rust-toolchain.toml, the NDK's clang and LLD, and a second, different clang string that comes from the prebuilt runtime objects the NDK links in — two clang lines there is normal. build-arti.sh runs the first check itself after every build, using the NDK's own llvm-readelf so it works the same on macOS.

Directory structure

tools/arti-build/
├── README.md            # This file
├── ARTI_VERSION         # Pinned Arti git tag (e.g., arti-v2.6.0)
├── ANDROID_NDK_VERSION  # Pinned NDK revision — enforced by build-arti.sh (reproducibility)
├── CARGO_NDK_VERSION    # cargo-ndk release the pinned output was verified with
├── rust-toolchain.toml  # Pinned rustc version + Android targets (reproducibility)
├── Cargo.toml           # Rust dependencies and build profile
├── Cargo.lock           # Pinned transitive dependency versions (reproducibility)
├── repro-env.sh         # Deterministic build env (path remapping, epoch) — sourced by both scripts
├── build-arti.sh        # Build script (Android targets, shipped in APK)
├── build-arti-host.sh   # Build script (host target, for JVM integration tests)
├── verify-reproducible.sh # Builds twice + diffs to prove byte-for-byte reproducibility
└── src/
    └── lib.rs           # JNI bridge (Rust → Kotlin)

# The Arti source is cloned into the canonical build path
# (/tmp/amethyst-arti-build/.arti-source), not under this dir — see
# "Reproducible builds" for why the build location is fixed.

Updating Arti version

1. Check available versions:

   git ls-remote --tags https://gitlab.torproject.org/tpo/core/arti.git | grep 'arti-v' | tail -10
   

2. Update the version file:

   echo "arti-v1.10.0" > ARTI_VERSION
   

3. Update crate versions in Cargo.toml to match the new release. Check the crate versions at:

   https://gitlab.torproject.org/tpo/core/arti/-/raw/arti-v1.10.0/crates/arti-client/Cargo.toml
   

4. Regenerate the committed lockfile so the new versions are pinned (builds run --locked and will fail until this is refreshed):

   ./build-arti.sh --regen-lock     # re-resolves + rewrites ./Cargo.lock, no compile
   

   If you also bump the Rust toolchain, edit channel in rust-toolchain.toml.

5. Rebuild, then re-verify reproducibility (see "Reproducible builds" above) and commit the regenerated .so files together with Cargo.lock / rust-toolchain.toml:

   ./build-arti.sh --clean
   

Architecture: JNI bridge

The Rust wrapper (src/lib.rs) exposes these JNI functions to Kotlin:

JNI function	Kotlin	Purpose
initialize(dataDir)	ArtiNative.initialize()	Create TorClient, bootstrap Tor network
startSocksProxy(port)	ArtiNative.startSocksProxy()	Bind SOCKS5 listener on localhost
stopSocksProxy()	ArtiNative.stopSocksProxy()	Abort listener, release port
getVersion()	ArtiNative.getVersion()	Return Arti version string
setLogCallback(cb)	ArtiNative.setLogCallback()	Register log callback

Key design decisions

• TorClient is created once via initialize() and persists for the app's lifetime. Its state file lock is tied to the object's lifetime and released only on GC/process exit.
• stopSocksProxy() only stops the TCP listener — it does NOT destroy the TorClient. This allows clean stop/start cycles without state file lock conflicts.
• SOCKS5 is implemented in Rust using tokio::net::TcpListener, not delegated to Arti's built-in proxy. This gives us full control over the listener lifecycle.
• Bidirectional forwarding uses tokio::io::copy with tokio::select! for efficiency.

Cargo.toml features

Default features are disabled (default-features = false) to minimize binary size.

Feature	Purpose	Why included
tokio	Async runtime	Required by our SOCKS proxy
rustls	TLS via pure Rust	No OpenSSL dependency, smaller binary
compression	zstd/deflate relay traffic	Reduces bandwidth on Tor circuits
onion-service-client	Access .onion addresses	Amethyst routes .onion relay connections through Tor
static-sqlite	Bundled SQLite	Android native code can't use system SQLite

Not included:

Feature	Why excluded
native-tls	Using rustls instead (smaller, no system dependency)
bridge-client	Amethyst doesn't expose bridge configuration in UI yet. Add back if needed.
pt-client	Pluggable transports — same reason as bridges
onion-service-service	We only connect to .onion, we don't host them

Release profile

[profile.release]
opt-level = "z"       # Optimize for size
lto = true            # Link-time optimization
codegen-units = 1     # Single codegen unit (smaller binary)
strip = true          # Strip debug symbols
panic = "abort"       # No unwinding (smaller binary)

Troubleshooting

cargo-ndk not found

cargo install cargo-ndk --version "$(cat CARGO_NDK_VERSION)" --locked

NDK not found, or "wrong revision"

Install the pinned revision — the build refuses any other, and the error lists every directory it looked at and what it found there:

sdkmanager "ndk;$(cat ANDROID_NDK_VERSION)"

Point ANDROID_NDK_HOME at it only if it lives outside the standard SDK layouts; an ANDROID_NDK_HOME left over from another project is skipped, not fatal.

Rust targets not installed

rustup target add aarch64-linux-android x86_64-linux-android

Build fails with dependency errors

Try a clean build:

./build-arti.sh --clean

JNI symbols missing after build

The build script verifies symbols automatically. If verification fails, check that src/lib.rs function names match the Kotlin package path:

Java_com_vitorpamplona_amethyst_ui_tor_ArtiNative_<methodName>