Shade/android/shade-android/README.md

# shade-android

Kotlin implementation of the Shade E2EE protocol for Android apps. Byte-for-byte compatible with `@shade/core` (TypeScript), so messages encrypted on a TS backend can be decrypted on Android and vice versa.

## Status

**M-Cross 1 ✅** — keys + HKDF + X3DH + fingerprint.
**M-Cross 2 ✅** — full ratchet step (encrypt + decrypt roundtrip) + wire 0x02 (RatchetMessage and PreKeyMessage with/without OTPK).
**M-Cross 3 ✅** — streams 0x11 (KDF labels with embedded NULs, deterministic chunk nonce/AAD, wire 0x11 encode/decode).
**M-Cross 4 ✅** — backup-key HKDF + AES-GCM, group sender-key step (`kdfChainKey` + Ed25519 sign over `aad ‖ ct`), storage HKDF (storageKey/fieldKey/rowNonce). Pending: scrypt master-key, argon2id swap, Android KeystoreStorage (sibling module).

Cross-platform test vectors in `/test-vectors/` are loaded by both the TS
and Kotlin test suites; any byte-divergence fails CI within 60 s. See
`ROADMAP-ANDROID.md` for the parity-checkpoint matrix and
`/docs/cross-platform.md` for how to add a new vector.

## Usage (target API)

```kotlin
import no.zyon.shade.ShadeSessionManager
import no.zyon.shade.crypto.TinkProvider
import no.zyon.shade.storage.KeystoreStorage

val crypto = TinkProvider()
val storage = KeystoreStorage(context)
val manager = ShadeSessionManager(crypto, storage)
manager.initialize()

// Establish a session with a peer
val bundle = fetchBundleFromServer("bob@example.com")
manager.initSessionFromBundle("bob@example.com", bundle)

// Encrypt
val envelope = manager.encrypt("bob@example.com", "hello")

// Decrypt
val plaintext = manager.decrypt("alice@example.com", incomingEnvelope)
```

## Crypto primitives

Backed by Google Tink:
- X25519 for Diffie-Hellman (via `X25519.generatePrivateKey()` / `computeSharedSecret`)
- Ed25519 for signing (via `Ed25519Sign` / `Ed25519Verify`)
- AES-256-GCM (via `AesGcmJce`)
- HKDF-SHA256 (via `Hkdf.computeHkdf`)
- HMAC-SHA256 (via `MacFactory`)

## Building

Requires JDK 17. The module compiles as a pure-JVM Kotlin library so the
parity gate runs without an Android SDK install. The Android-specific
storage adapter (Keystore + EncryptedSharedPreferences) will land as a
sibling Gradle module in M-Cross 4.

```bash
cd android
./gradlew :shade-android:test
```

The Gradle wrapper downloads Gradle 8.10.2 on first run.

## Compatibility

The Kotlin implementation must produce byte-identical output to `@shade/core` for:
- KDF chain derivations (root key ratchet, chain key ratchet)
- X3DH shared secrets
- Ratchet message keys and ciphertext (given the same keys)
- Fingerprints (safety numbers)
- Binary wire format (`@shade/proto`)

Shared test vectors in `test-vectors/` are loaded by both the TS and Kotlin test suites. Any divergence fails the CI immediately.