wallet: support offset in key image export and import

diff --git a/docs/monero-spec.drawio b/docs/monero-spec.drawio
index c6bbbf1..5d06c5c 100644
--- a/docs/monero-spec.drawio
+++ b/docs/monero-spec.drawio
@@ -945,10 +945,10 @@
         <mxCell id="QYUe6zCbC_x82nWfNGpH-329" parent="QYUe6zCbC_x82nWfNGpH-248" style="text;strokeColor=none;fillColor=none;align=left;verticalAlign=top;spacingLeft=4;spacingRight=4;overflow=hidden;rotatable=0;points=[[0,0.5],[1,0.5]];portConstraint=eastwest;fontFamily=Verdana;" value="importOutputs(string outputsHex): uint" vertex="1">
           <mxGeometry height="26" width="814" y="318" as="geometry" />
         </mxCell>
-        <mxCell id="QYUe6zCbC_x82nWfNGpH-330" parent="QYUe6zCbC_x82nWfNGpH-248" style="text;strokeColor=none;fillColor=none;align=left;verticalAlign=top;spacingLeft=4;spacingRight=4;overflow=hidden;rotatable=0;points=[[0,0.5],[1,0.5]];portConstraint=eastwest;fontFamily=Verdana;" value="exportKeyImages(bool all=false): MoneroKeyImage[]" vertex="1">
+        <mxCell id="QYUe6zCbC_x82nWfNGpH-330" parent="QYUe6zCbC_x82nWfNGpH-248" style="text;strokeColor=none;fillColor=none;align=left;verticalAlign=top;spacingLeft=4;spacingRight=4;overflow=hidden;rotatable=0;points=[[0,0.5],[1,0.5]];portConstraint=eastwest;fontFamily=Verdana;" value="exportKeyImages(bool all=false): MoneroKeyImageExportResult" vertex="1">
           <mxGeometry height="26" width="814" y="344" as="geometry" />
         </mxCell>
-        <mxCell id="QYUe6zCbC_x82nWfNGpH-331" parent="QYUe6zCbC_x82nWfNGpH-248" style="text;strokeColor=none;fillColor=none;align=left;verticalAlign=top;spacingLeft=4;spacingRight=4;overflow=hidden;rotatable=0;points=[[0,0.5],[1,0.5]];portConstraint=eastwest;fontFamily=Verdana;" value="importKeyImages(MoneroKeyImage[] keyImages): MoneroKeyImageImportResult" vertex="1">
+        <mxCell id="QYUe6zCbC_x82nWfNGpH-331" parent="QYUe6zCbC_x82nWfNGpH-248" style="text;strokeColor=none;fillColor=none;align=left;verticalAlign=top;spacingLeft=4;spacingRight=4;overflow=hidden;rotatable=0;points=[[0,0.5],[1,0.5]];portConstraint=eastwest;fontFamily=Verdana;" value="importKeyImages(MoneroKeyImage[] keyImages, uint offset=0): MoneroKeyImageImportResult" vertex="1">
           <mxGeometry height="26" width="814" y="370" as="geometry" />
         </mxCell>
         <mxCell id="9wusJIeZhnCERMAPth61-1" parent="QYUe6zCbC_x82nWfNGpH-248" style="text;strokeColor=none;fillColor=none;align=left;verticalAlign=top;spacingLeft=4;spacingRight=4;overflow=hidden;rotatable=0;points=[[0,0.5],[1,0.5]];portConstraint=eastwest;fontFamily=Verdana;" value="freezeOutput(String keyImage): void" vertex="1">
diff --git a/docs/monero-spec.pdf b/docs/monero-spec.pdf
index a5f2e8c..405587a 100644
Binary files a/docs/monero-spec.pdf and b/docs/monero-spec.pdf differ
diff --git a/src/main/cpp/monero_jni_bridge.cpp b/src/main/cpp/monero_jni_bridge.cpp
index 90e8346..d7d2262 100644
--- a/src/main/cpp/monero_jni_bridge.cpp
+++ b/src/main/cpp/monero_jni_bridge.cpp
@@ -1236,17 +1236,7 @@ JNIEXPORT jstring JNICALL Java_monero_wallet_MoneroWalletFull_exportKeyImagesJni
   MTRACE("Java_monero_wallet_MoneroWalletFull_exportKeyImagesJni");
   monero_wallet* wallet = get_handle<monero_wallet>(env, instance, JNI_WALLET_HANDLE);
   try {
-
-    // fetch key images
-    vector<shared_ptr<monero_key_image>> key_images = wallet->export_key_images(all);
-    MTRACE("Fetched " << key_images.size() << " key images");
-
-    // wrap and serialize key images
-    rapidjson::Document doc;
-    doc.SetObject();
-    doc.AddMember("keyImages", monero_utils::to_rapidjson_val(doc.GetAllocator(), key_images), doc.GetAllocator());
-    string key_images_json = gen_utils::serialize(doc);
-    return env->NewStringUTF(key_images_json.c_str());
+    return env->NewStringUTF(wallet->export_key_images(all)->serialize().c_str());
   } catch (...) {
     rethrow_cpp_exception_as_java_exception(env);
     return 0;
@@ -1260,14 +1250,10 @@ JNIEXPORT jstring JNICALL Java_monero_wallet_MoneroWalletFull_importKeyImagesJni
   string key_images_json = string(_key_images_json ? _key_images_json : "");
   env->ReleaseStringUTFChars(jkey_images_json, _key_images_json);
 
-  // deserialize key images to import
-  vector<shared_ptr<monero_key_image>> key_images = monero_key_image::deserialize_key_images(key_images_json);
-  //MTRACE("Deserialized " << key_images.size() << " key images from java json");
-
-  // import key images
-  shared_ptr<monero_key_image_import_result> result;
+  // import key images at their offset
   try {
-    result = wallet->import_key_images(key_images);
+    shared_ptr<monero_key_image_export_result> exported = monero_key_image_export_result::deserialize(key_images_json);
+    shared_ptr<monero_key_image_import_result> result = wallet->import_key_images(exported->m_key_images, exported->m_offset == boost::none ? 0 : exported->m_offset.get());
     return env->NewStringUTF(result->serialize().c_str());
   } catch (...) {
     rethrow_cpp_exception_as_java_exception(env);
diff --git a/src/main/java/monero/wallet/MoneroWallet.java b/src/main/java/monero/wallet/MoneroWallet.java
index e7a46c8..70da958 100644
--- a/src/main/java/monero/wallet/MoneroWallet.java
+++ b/src/main/java/monero/wallet/MoneroWallet.java
@@ -38,6 +38,7 @@ import monero.wallet.model.MoneroCheckReserve;
 import monero.wallet.model.MoneroCheckTx;
 import monero.wallet.model.MoneroIncomingTransfer;
 import monero.wallet.model.MoneroIntegratedAddress;
+import monero.wallet.model.MoneroKeyImageExportResult;
 import monero.wallet.model.MoneroKeyImageImportResult;
 import monero.wallet.model.MoneroMessageSignatureResult;
 import monero.wallet.model.MoneroMessageSignatureType;
@@ -778,17 +779,17 @@ public interface MoneroWallet {
   /**
    * Export key images since the last export.
    * 
-   * @return signed key images since the last export
+   * @return signed key images since the last export and their offset among the wallet's outputs
    */
-  List<MoneroKeyImage> exportKeyImages();
+  MoneroKeyImageExportResult exportKeyImages();
   
   /**
    * Export signed key images.
    * 
    * @param all exports all key images if true, else exports the key images since the last export
-   * @return signed key images
+   * @return signed key images and their offset among the wallet's outputs
    */
-  public List<MoneroKeyImage> exportKeyImages(boolean all);
+  public MoneroKeyImageExportResult exportKeyImages(boolean all);
   
   /**
    * Import signed key images and verify their spent status.
@@ -798,6 +799,15 @@ public interface MoneroWallet {
    */
   public MoneroKeyImageImportResult importKeyImages(List<MoneroKeyImage> keyImages);
   
+  /**
+   * Import signed key images and verify their spent status.
+   * 
+   * @param keyImages are key images to import and verify (requires hex and signature)
+   * @param offset offset of the first key image among the wallet's outputs
+   * @return results of the import
+   */
+  public MoneroKeyImageImportResult importKeyImages(List<MoneroKeyImage> keyImages, long offset);
+  
   /**
    * Get new key images from the last imported outputs.
    * 
diff --git a/src/main/java/monero/wallet/MoneroWalletDefault.java b/src/main/java/monero/wallet/MoneroWalletDefault.java
index 4ec0496..6a30c6e 100644
--- a/src/main/java/monero/wallet/MoneroWalletDefault.java
+++ b/src/main/java/monero/wallet/MoneroWalletDefault.java
@@ -41,6 +41,8 @@ import monero.wallet.model.MoneroAccount;
 import monero.wallet.model.MoneroAddressBookEntry;
 import monero.wallet.model.MoneroIncomingTransfer;
 import monero.wallet.model.MoneroIntegratedAddress;
+import monero.wallet.model.MoneroKeyImageExportResult;
+import monero.wallet.model.MoneroKeyImageImportResult;
 import monero.wallet.model.MoneroMessageSignatureType;
 import monero.wallet.model.MoneroOutgoingTransfer;
 import monero.wallet.model.MoneroOutputWallet;
@@ -365,10 +367,15 @@ abstract class MoneroWalletDefault implements MoneroWallet {
   }
   
   @Override
-  public List<MoneroKeyImage> exportKeyImages() {
+  public MoneroKeyImageExportResult exportKeyImages() {
     return exportKeyImages(false);
   }
   
+  @Override
+  public MoneroKeyImageImportResult importKeyImages(List<MoneroKeyImage> keyImages) {
+    return importKeyImages(keyImages, 0);
+  }
+  
   @Override
   public MoneroTxWallet createTx(MoneroTxConfig config) {
     if (config == null) throw new MoneroError("Send request cannot be null");
diff --git a/src/main/java/monero/wallet/MoneroWalletFull.java b/src/main/java/monero/wallet/MoneroWalletFull.java
index 3c28de5..fdc50a4 100644
--- a/src/main/java/monero/wallet/MoneroWalletFull.java
+++ b/src/main/java/monero/wallet/MoneroWalletFull.java
@@ -50,6 +50,7 @@ import monero.wallet.model.MoneroCheckReserve;
 import monero.wallet.model.MoneroCheckTx;
 import monero.wallet.model.MoneroIncomingTransfer;
 import monero.wallet.model.MoneroIntegratedAddress;
+import monero.wallet.model.MoneroKeyImageExportResult;
 import monero.wallet.model.MoneroKeyImageImportResult;
 import monero.wallet.model.MoneroMessageSignatureResult;
 import monero.wallet.model.MoneroMessageSignatureType;
@@ -876,19 +877,22 @@ public class MoneroWalletFull extends MoneroWalletDefault {
   }
 
   @Override
-  public List<MoneroKeyImage> exportKeyImages(boolean all) {
+  public MoneroKeyImageExportResult exportKeyImages(boolean all) {
     assertNotClosed();
-    String keyImagesJson = exportKeyImagesJni(all);
-    List<MoneroKeyImage> keyImages = JsonUtils.deserialize(MoneroRpcConnection.MAPPER, keyImagesJson, KeyImagesContainer.class).keyImages;
-    return keyImages;
+    String resultJson = exportKeyImagesJni(all);
+    MoneroKeyImageExportResult result = JsonUtils.deserialize(MoneroRpcConnection.MAPPER, resultJson, MoneroKeyImageExportResult.class);
+    if (result.getKeyImages() == null) result.setKeyImages(new ArrayList<MoneroKeyImage>());
+    return result;
   }
 
   @Override
-  public MoneroKeyImageImportResult importKeyImages(List<MoneroKeyImage> keyImages) {
+  public MoneroKeyImageImportResult importKeyImages(List<MoneroKeyImage> keyImages, long offset) {
     assertNotClosed();
     
-    // wrap and serialize key images in container for jni
-    KeyImagesContainer keyImageContainer = new KeyImagesContainer(keyImages);
+    // wrap and serialize offset and key images for jni
+    MoneroKeyImageExportResult keyImageContainer = new MoneroKeyImageExportResult();
+    keyImageContainer.setOffset(offset);
+    keyImageContainer.setKeyImages(keyImages);
     String importResultJson = importKeyImagesJni(JsonUtils.serialize(keyImageContainer));
     
     // deserialize response
@@ -1794,12 +1798,6 @@ public class MoneroWalletFull extends MoneroWalletDefault {
     public List<MoneroTxSet> txSets;
   }
   
-  private static class KeyImagesContainer {
-    public List<MoneroKeyImage> keyImages;
-    @SuppressWarnings("unused") public KeyImagesContainer() { } // necessary for serialization
-    public KeyImagesContainer(List<MoneroKeyImage> keyImages) { this.keyImages = keyImages; };
-  }
-  
   private static DeserializedBlocksContainer deserializeBlocks(String blocksJson) {
     DeserializedBlocksContainer deserializedBlocksContainer = new DeserializedBlocksContainer();
     deserializedBlocksContainer.blocks = new ArrayList<MoneroBlock>();
diff --git a/src/main/java/monero/wallet/MoneroWalletRpc.java b/src/main/java/monero/wallet/MoneroWalletRpc.java
index 607b5c9..f767c08 100644
--- a/src/main/java/monero/wallet/MoneroWalletRpc.java
+++ b/src/main/java/monero/wallet/MoneroWalletRpc.java
@@ -64,6 +64,7 @@ import monero.wallet.model.MoneroCheckTx;
 import monero.wallet.model.MoneroDestination;
 import monero.wallet.model.MoneroIncomingTransfer;
 import monero.wallet.model.MoneroIntegratedAddress;
+import monero.wallet.model.MoneroKeyImageExportResult;
 import monero.wallet.model.MoneroKeyImageImportResult;
 import monero.wallet.model.MoneroMessageSignatureResult;
 import monero.wallet.model.MoneroMessageSignatureType;
@@ -1109,14 +1110,14 @@ public class MoneroWalletRpc extends MoneroWalletDefault {
   }
 
   @Override
-  public List<MoneroKeyImage> exportKeyImages(boolean all) {
+  public MoneroKeyImageExportResult exportKeyImages(boolean all) {
     return rpcExportKeyImages(all);
   }
 
   @SuppressWarnings("unchecked")
   @Override
-  public MoneroKeyImageImportResult importKeyImages(List<MoneroKeyImage> keyImages) {
-    
+  public MoneroKeyImageImportResult importKeyImages(List<MoneroKeyImage> keyImages, long offset) {
+
     // convert key images to rpc parameter
     List<Map<String, Object>> rpcKeyImages = new ArrayList<Map<String, Object>>();
     for (MoneroKeyImage keyImage : keyImages) {
@@ -1125,10 +1126,11 @@ public class MoneroWalletRpc extends MoneroWalletDefault {
       rpcKeyImage.put("signature", keyImage.getSignature());
       rpcKeyImages.add(rpcKeyImage);
     }
-    
+
     // send rpc request
     Map<String, Object> params = new HashMap<String, Object>();
     params.put("signed_key_images", rpcKeyImages);
+    params.put("offset", offset);
     Map<String, Object> resp = rpc.sendJsonRequest("import_key_images", params);
     Map<String, Object> result = (Map<String, Object>) resp.get("result");
     
@@ -1142,7 +1144,7 @@ public class MoneroWalletRpc extends MoneroWalletDefault {
 
   @Override
   public List<MoneroKeyImage> getNewKeyImagesFromLastImport() {
-    return rpcExportKeyImages(false);
+    return rpcExportKeyImages(false).getKeyImages();
   }
   
   @Override
@@ -1989,20 +1991,23 @@ public class MoneroWalletRpc extends MoneroWalletDefault {
    * Common method to get key images.
    * 
    * @param all specifies to get all xor only new images from last import
-   * @return {MoneroKeyImage[]} are the key images
+   * @return {MoneroKeyImageExportResult} the key images and their offset among the wallet's outputs
    */
   @SuppressWarnings("unchecked")
-  private List<MoneroKeyImage> rpcExportKeyImages(boolean all) {
+  private MoneroKeyImageExportResult rpcExportKeyImages(boolean all) {
     Map<String, Object> params = new HashMap<String, Object>();
     params.put("all", all);
     Map<String, Object> resp = rpc.sendJsonRequest("export_key_images", params);
     Map<String, Object> result = (Map<String, Object>) resp.get("result");
+    MoneroKeyImageExportResult exportResult = new MoneroKeyImageExportResult();
+    if (result.containsKey("offset")) exportResult.setOffset(((BigInteger) result.get("offset")).longValue());
     List<MoneroKeyImage> images = new ArrayList<MoneroKeyImage>();
-    if (!result.containsKey("signed_key_images")) return images;
+    exportResult.setKeyImages(images);
+    if (!result.containsKey("signed_key_images")) return exportResult;
     for (Map<String, Object> rpcImage : (List<Map<String, Object>>) result.get("signed_key_images")) {
       images.add(new MoneroKeyImage((String) rpcImage.get("key_image"), (String) rpcImage.get("signature")));
     }
-    return images;
+    return exportResult;
   }
   
   @SuppressWarnings("unchecked")
diff --git a/src/main/java/monero/wallet/model/MoneroKeyImageExportResult.java b/src/main/java/monero/wallet/model/MoneroKeyImageExportResult.java
new file mode 100644
index 0000000..6a5f9b6
--- /dev/null
+++ b/src/main/java/monero/wallet/model/MoneroKeyImageExportResult.java
@@ -0,0 +1,30 @@
+package monero.wallet.model;
+
+import java.util.List;
+
+import monero.daemon.model.MoneroKeyImage;
+
+/**
+ * Models results from exporting signed key images.
+ */
+public class MoneroKeyImageExportResult {
+
+  private Long offset;
+  private List<MoneroKeyImage> keyImages;
+
+  public Long getOffset() {
+    return offset;
+  }
+
+  public void setOffset(Long offset) {
+    this.offset = offset;
+  }
+
+  public List<MoneroKeyImage> getKeyImages() {
+    return keyImages;
+  }
+
+  public void setKeyImages(List<MoneroKeyImage> keyImages) {
+    this.keyImages = keyImages;
+  }
+}
diff --git a/src/test/java/TestMoneroWalletCommon.java b/src/test/java/TestMoneroWalletCommon.java
index 78fd1dd..4adc8e5 100644
--- a/src/test/java/TestMoneroWalletCommon.java
+++ b/src/test/java/TestMoneroWalletCommon.java
@@ -48,6 +48,7 @@ import monero.wallet.model.MoneroCheckTx;
 import monero.wallet.model.MoneroDestination;
 import monero.wallet.model.MoneroIncomingTransfer;
 import monero.wallet.model.MoneroIntegratedAddress;
+import monero.wallet.model.MoneroKeyImageExportResult;
 import monero.wallet.model.MoneroKeyImageImportResult;
 import monero.wallet.model.MoneroMessageSignatureResult;
 import monero.wallet.model.MoneroMessageSignatureType;
@@ -2408,17 +2409,19 @@ public abstract class TestMoneroWalletCommon {
   @Test
   public void testExportKeyImages() {
     assumeTrue(TEST_NON_RELAYS);
-    List<MoneroKeyImage> images = wallet.exportKeyImages(true);
+    MoneroKeyImageExportResult result = wallet.exportKeyImages(true);
+    assertEquals(0, (long) result.getOffset());
+    List<MoneroKeyImage> images = result.getKeyImages();
     assertTrue(images.size() > 0, "No signed key images in wallet");
     for (MoneroKeyImage image : images) {
       assertTrue(image instanceof MoneroKeyImage);
       assertTrue(image.getHex().length() > 0);
       assertTrue(image.getSignature().length() > 0);
     }
-    
+
     // wallet exports key images since last export by default
-    images = wallet.exportKeyImages();
-    List<MoneroKeyImage> imagesAll = wallet.exportKeyImages(true);
+    images = wallet.exportKeyImages().getKeyImages();
+    List<MoneroKeyImage> imagesAll = wallet.exportKeyImages(true).getKeyImages();
     assert(imagesAll.size() > images.size());
   }
   
@@ -2451,9 +2454,10 @@ public abstract class TestMoneroWalletCommon {
   @Test
   public void testImportKeyImages() {
     assumeTrue(TEST_NON_RELAYS);
-    List<MoneroKeyImage> images = wallet.exportKeyImages();
+    MoneroKeyImageExportResult exportResult = wallet.exportKeyImages();
+    List<MoneroKeyImage> images = exportResult.getKeyImages();
     assertTrue(images.size() > 0, "Wallet does not have any key images; run send tests");
-    MoneroKeyImageImportResult result = wallet.importKeyImages(images);
+    MoneroKeyImageImportResult result = wallet.importKeyImages(images, exportResult.getOffset());
     assertTrue(result.getHeight() > 0);
     
     // determine if non-zero spent and unspent amounts are expected
@@ -2544,11 +2548,11 @@ public abstract class TestMoneroWalletCommon {
     assertTrue(numOutputsImported > 0, "No outputs imported");
     
     // export key images from offline wallet
-    List<MoneroKeyImage> keyImages = offlineWallet.exportKeyImages();
-    
+    MoneroKeyImageExportResult keyImageResult = offlineWallet.exportKeyImages();
+
     // import key images to view-only wallet
     assertTrue(viewOnlyWallet.isConnectedToDaemon());
-    viewOnlyWallet.importKeyImages(keyImages);
+    viewOnlyWallet.importKeyImages(keyImageResult.getKeyImages(), keyImageResult.getOffset());
     assertEquals(wallet.getBalance(), viewOnlyWallet.getBalance());
     
     // create unsigned tx using view-only wallet
diff --git a/src/test/java/TestMoneroWalletFull.java b/src/test/java/TestMoneroWalletFull.java
index 0a65cec..817f8a5 100644
--- a/src/test/java/TestMoneroWalletFull.java
+++ b/src/test/java/TestMoneroWalletFull.java
@@ -19,11 +19,11 @@ import java.util.concurrent.TimeUnit;
 import monero.common.MoneroError;
 import monero.common.MoneroRpcConnection;
 import monero.common.MoneroUtils;
-import monero.daemon.model.MoneroKeyImage;
 import monero.daemon.model.MoneroNetworkType;
 import monero.wallet.MoneroWallet;
 import monero.wallet.MoneroWalletFull;
 import monero.wallet.MoneroWalletRpc;
+import monero.wallet.model.MoneroKeyImageExportResult;
 import monero.wallet.model.MoneroMultisigInfo;
 import monero.wallet.model.MoneroMultisigInitResult;
 import monero.wallet.model.MoneroOutputQuery;
@@ -153,8 +153,8 @@ public class TestMoneroWalletFull extends TestMoneroWalletCommon {
         List<MoneroTransfer> inTransfers1 = wallet.getTransfers(new MoneroTransferQuery().setIsIncoming(true));
         
         // export and re-import key images
-        List<MoneroKeyImage> keyImages = wallet.exportKeyImages();
-        wallet.importKeyImages(keyImages);
+        MoneroKeyImageExportResult keyImageResult = wallet.exportKeyImages();
+        wallet.importKeyImages(keyImageResult.getKeyImages(), keyImageResult.getOffset());
         
         // get incoming transfers after importing
         List<MoneroTransfer> inTransfers2 = wallet.getTransfers(new MoneroTransferQuery().setIsIncoming(true));