Expose ZIP 32 arbitrary key derivation

CHANGELOG.md

@@ -6,6 +6,10 @@ and this library adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+- `DerivationTool.deriveArbitraryWalletKey`
+- `DerivationTool.deriveArbitraryAccountKey`
+
 ## [2.2.5] - 2024-10-22
 
 ### Added

backend-lib/Cargo.toml

@@ -19,6 +19,7 @@ zcash_client_backend = { version = "0.14", features = ["orchard", "tor", "transp
 zcash_client_sqlite = { version = "0.12.2", features = ["orchard", "transparent-inputs", "unstable"] }
 zcash_primitives = "0.19"
 zcash_proofs = "0.19"
+zip32 = "0.1.2"
 
 # Infrastructure
 prost = "0.13"

backend-lib/src/main/java/cash/z/ecc/android/sdk/internal/Derivation.kt

@@ -38,6 +38,37 @@ interface Derivation {
         numberOfAccounts: Int
     ): Array<String>
 
+    /**
+     * Derives a ZIP 32 Arbitrary Key from the given seed at the "wallet level", i.e.
+     * directly from the seed with no ZIP 32 path applied.
+     *
+     * The resulting key will be the same across all networks (Zcash mainnet, Zcash
+     * testnet, OtherCoin mainnet, and so on). You can think of it as a context-specific
+     * seed fingerprint that can be used as (static) key material.
+     *
+     * @param contextString a globally-unique non-empty sequence of at most 252 bytes that
+     *        identifies the desired context.
+     * @return an array of 32 bytes.
+     */
+    fun deriveArbitraryWalletKey(
+        contextString: ByteArray,
+        seed: ByteArray
+    ): ByteArray
+
+    /**
+     * Derives a ZIP 32 Arbitrary Key from the given seed at the account level.
+     *
+     * @param contextString a globally-unique non-empty sequence of at most 252 bytes that
+     *        identifies the desired context.
+     * @return an array of 32 bytes.
+     */
+    fun deriveArbitraryAccountKey(
+        contextString: ByteArray,
+        seed: ByteArray,
+        networkId: Int,
+        accountIndex: Int
+    ): ByteArray
+
     companion object {
         const val DEFAULT_NUMBER_OF_ACCOUNTS = 1
     }

backend-lib/src/main/java/cash/z/ecc/android/sdk/internal/jni/RustDerivationTool.kt

@@ -40,6 +40,18 @@ class RustDerivationTool private constructor() : Derivation {
         networkId: Int
     ): String = deriveUnifiedAddressFromViewingKey(viewingKey, networkId = networkId)
 
+    override fun deriveArbitraryWalletKey(
+        contextString: ByteArray,
+        seed: ByteArray
+    ): ByteArray = deriveArbitraryWalletKeyFromSeed(contextString, seed)
+
+    override fun deriveArbitraryAccountKey(
+        contextString: ByteArray,
+        seed: ByteArray,
+        networkId: Int,
+        accountIndex: Int
+    ): ByteArray = deriveArbitraryAccountKeyFromSeed(contextString, seed, accountIndex = accountIndex, networkId = networkId)
+
     companion object {
         suspend fun new(): Derivation {
             RustBackend.loadLibrary()
@@ -79,5 +91,19 @@ class RustDerivationTool private constructor() : Derivation {
             key: String,
             networkId: Int
         ): String
+
+        @JvmStatic
+        private external fun deriveArbitraryWalletKeyFromSeed(
+            contextString: ByteArray,
+            seed: ByteArray
+        ): ByteArray
+
+        @JvmStatic
+        private external fun deriveArbitraryAccountKeyFromSeed(
+            contextString: ByteArray,
+            seed: ByteArray,
+            accountIndex: Int,
+            networkId: Int
+        ): ByteArray
     }
 }