Resolve [#1640] review comments

* `Account` documentation fix

* Make `AccountPurpose.Spending` args not null

As they now live under `AccountPurpose.Spending` where we need them not null
+ `AccountImportSetup` doc fix

* Replace `bytes` with `usk` in `AccountUsk`

* Change test name

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

@@ -69,7 +69,7 @@ internal class TypesafeBackendImpl(private val backend: Backend) : TypesafeBacke
                             treeState = treeState.encoded,
                             ufvk = setup.ufvk.encoding,
                             seedFingerprint = setup.purpose.seedFingerprint,
-                            zip32AccountIndex = setup.purpose.zip32AccountIndex?.index,
+                            zip32AccountIndex = setup.purpose.zip32AccountIndex.index,
                         )
                     AccountPurpose.ViewOnly ->
                         backend.importAccountUfvk(

sdk-lib/src/main/java/cash/z/ecc/android/sdk/model/Account.kt

@@ -28,7 +28,7 @@ import cash.z.ecc.android.sdk.internal.model.JniAccount
  * @param keySource A string identifier or other metadata describing the location of the spending
  *        key corresponding to the account's UFVK. This is set internally by the wallet app based on
  *        its private enumeration of spending methods it supports.
- * @param seedFingerprint The seed fingerprint. Must be length 16.
+ * @param seedFingerprint The seed fingerprint. Must be length 32.
  * @param hdAccountIndex ZIP 32 account index
  */
 data class Account internal constructor(

sdk-lib/src/main/java/cash/z/ecc/android/sdk/model/AccountImportSetup.kt

@@ -1,8 +1,7 @@
 package cash.z.ecc.android.sdk.model
 
 /**
- * Wrapper for the import account API based on viewing key. Set both [seedFingerprint] and [zip32AccountIndex] null
- * when using [AccountPurpose.ViewOnly].
+ * Wrapper for the import account API based on viewing key.
  *
  * @param accountName A human-readable name for the account. This will be visible to the wallet
  *        user, and the wallet app may obtain it from them.

sdk-lib/src/main/java/cash/z/ecc/android/sdk/model/AccountPurpose.kt

@@ -16,8 +16,8 @@ sealed class AccountPurpose {
      * @param zip32AccountIndex The ZIP 32 account-level component of the HD derivation path at which to derive the
      */
     data class Spending(
-        val seedFingerprint: ByteArray?,
-        val zip32AccountIndex: Zip32AccountIndex?,
+        val seedFingerprint: ByteArray,
+        val zip32AccountIndex: Zip32AccountIndex,
     ) : AccountPurpose() {
         override val value = 0
     }

sdk-lib/src/main/java/cash/z/ecc/android/sdk/model/AccountUsk.kt

@@ -1,6 +1,5 @@
 package cash.z.ecc.android.sdk.model
 
-import cash.z.ecc.android.sdk.internal.jni.RustBackend
 import cash.z.ecc.android.sdk.internal.model.JniAccountUsk
 
 /**
@@ -18,57 +17,17 @@ class AccountUsk private constructor(
      */
     val accountUuid: AccountUuid,
     /**
-     * The binary encoding of the [ZIP 316](https://zips.z.cash/zip-0316) Unified Spending
-     * Key for [accountUuid].
-     *
-     * This encoding **MUST NOT** be exposed to users. It is an internal encoding that is
-     * inherently unstable, and only intended to be passed between the SDK and the storage
-     * backend. Wallets **MUST NOT** allow this encoding to be exported or imported.
+     * The [ZIP 316](https://zips.z.cash/zip-0316) Unified Spending Key.
      */
-    private val bytes: FirstClassByteArray
+    private val usk: UnifiedSpendingKey
 ) {
-    internal constructor(uskJni: JniAccountUsk) : this(
-        AccountUuid.new(uskJni.accountUuid),
-        FirstClassByteArray(uskJni.bytes.copyOf())
-    )
-
-    /**
-     * The binary encoding of the [ZIP 316](https://zips.z.cash/zip-0316) Unified Spending
-     * Key for [accountUuid].
-     *
-     * This encoding **MUST NOT** be exposed to users. It is an internal encoding that is
-     * inherently unstable, and only intended to be passed between the SDK and the storage
-     * backend. Wallets **MUST NOT** allow this encoding to be exported or imported.
-     */
-    fun copyBytes() = bytes.byteArray.copyOf()
-
-    // Override to prevent leaking key to logs
-    override fun toString() = "AccountUsk(account=$accountUuid, bytes=***)"
+    override fun toString() = "AccountUsk(account=$accountUuid, usk=$usk)"
 
     companion object {
-        /**
-         * This method may fail if the [bytes] no longer represent a valid key.  A key could become invalid due to
-         * network upgrades or other internal changes.  If a non-successful result is returned, clients are expected
-         * to use [DerivationTool.deriveUnifiedSpendingKey] to regenerate the key from the seed.
-         *
-         * @return A validated AccountUsk.
-         */
-        suspend fun new(
-            accountUuid: AccountUuid,
-            bytes: ByteArray
-        ): Result<AccountUsk> {
-            val bytesCopy = bytes.copyOf()
-            RustBackend.loadLibrary()
-            return runCatching {
-                require(RustBackend.validateUnifiedSpendingKey(bytesCopy))
-                AccountUsk(accountUuid, FirstClassByteArray(bytesCopy))
-            }
-        }
-
-        fun new(jniAccountUsk: JniAccountUsk): AccountUsk =
+        suspend fun new(jniAccountUsk: JniAccountUsk): AccountUsk =
             AccountUsk(
                 accountUuid = AccountUuid.new(jniAccountUsk.accountUuid),
-                bytes = FirstClassByteArray(jniAccountUsk.bytes)
+                usk = UnifiedSpendingKey.new(jniAccountUsk.bytes)
             )
     }
 }

sdk-lib/src/test/java/cash/z/ecc/android/sdk/model/AccountTest.kt

@@ -7,7 +7,7 @@ import kotlin.test.assertFailsWith
 
 class AccountTest {
     @Test
-    fun out_of_bounds() {
+    fun uuid_wrong_length() {
         assertFailsWith(IllegalArgumentException::class) {
             AccountFixture.new(accountUuid = UUID.fromString("random"))
         }