ShieldedKeyDeriver

JNI bridge to Midnight's Rust cryptography for deriving shielded keys.

Architecture:

Kotlin (Android) → JNI → Rust FFI → Midnight Ledger (Rust)

Why JNI? Midnight's shielded key derivation uses:

• Blake2b hashing (90% of operations)

• JubJub elliptic curve (10% of operations - ZKP-friendly)

Reimplementing JubJub curve in Kotlin is risky for a wallet. Instead, we use Midnight's battle-tested Rust implementation via JNI (98% confidence vs 85% for pure Kotlin).

Native Library: This class loads libkuira_crypto_ffi.so (Android) which is compiled from:

• Location: kuira-crypto-ffi/

• Dependencies: midnight-zswap, midnight-serialize (version-abstract — see Cargo.toml)

Version Compatibility: The Rust FFI is the version abstraction boundary — Kotlin depends on stable function signatures, not Midnight version internals. See MIDNIGHT_GUIDELINES.md for version details.

Thread Safety: This object is thread-safe. The underlying Rust functions are pure and stateless.

Memory Safety:

• The JNI bridge allocates C strings for results

• Native memory is freed automatically after copying to Kotlin strings

• The seed ByteArray is NOT cleared by this function - caller must wipe it

Usage:

val seed = deriveSeedFromBIP32()  // 32 bytes at m/44'/2400'/0'/3/0
try {
    val keys = ShieldedKeyDeriver.deriveKeys(seed)
    println("Coin PK: ${keys.coinPublicKey}")
    println("Enc PK: ${keys.encryptionPublicKey}")
} finally {
    MemoryUtils.wipe(seed)  // CRITICAL: Always wipe seed
}

Error Handling:

• Returns null if FFI call fails (invalid seed, native library error, etc.)

• Logs error details to stderr in the native library

• On Android, check Logcat for native error messages

References:

• Rust FFI: kuira-crypto-ffi/src/lib.rs

• Algorithm: docs/SHIELDED_ADDRESS_ALGORITHM.md

• POC Results: docs/SHIELDED_JNI_POC_RESULTS.md