Make Mifare Classic increment/decrement operands little endian

Also make sure they are non-negative.

This is not documented in Mifare Classic spec, but based on findings from NXP:
- Operand should be stored in little-endian format in the transceive buffer
- Tag ignores the sign bit on the operand, its effectively 31-bit unsigned
- Overflow and underflow generates an error.

Change-Id: Id3389b3894ded732c4b00d564ca53f5df651359e

diff --git a/core/java/android/nfc/tech/MifareClassic.java b/core/java/android/nfc/tech/MifareClassic.java
index 1b383f1..34fd7cf 100644 (file)
--- a/core/java/android/nfc/tech/MifareClassic.java
+++ b/core/java/android/nfc/tech/MifareClassic.java
@@ -22,6 +22,7 @@ import android.os.RemoteException;
 
 import java.io.IOException;
 import java.nio.ByteBuffer;
+import java.nio.ByteOrder;
 
 /**
  * Technology class representing MIFARE Classic tags (also known as MIFARE Standard).
@@ -328,12 +329,14 @@ public final class MifareClassic extends BasicTagTechnology {
      */
     public void increment(int blockIndex, int value) throws IOException {
         validateBlock(blockIndex);
+        validateValueOperand(value);
         checkConnected();
 
         ByteBuffer cmd = ByteBuffer.allocate(6);
+        cmd.order(ByteOrder.LITTLE_ENDIAN);
         cmd.put( (byte) 0xC1 );
         cmd.put( (byte) blockIndex );
-        cmd.putInt(value);  // ByteBuffer does the correct big endian translation
+        cmd.putInt(value);
 
         transceive(cmd.array(), false);
     }
@@ -345,16 +348,24 @@ public final class MifareClassic extends BasicTagTechnology {
      */
     public void decrement(int blockIndex, int value) throws IOException {
         validateBlock(blockIndex);
+        validateValueOperand(value);
         checkConnected();
 
         ByteBuffer cmd = ByteBuffer.allocate(6);
+        cmd.order(ByteOrder.LITTLE_ENDIAN);
         cmd.put( (byte) 0xC0 );
         cmd.put( (byte) blockIndex );
-        cmd.putInt(value);  // ByteBuffer does the correct big endian translation
+        cmd.putInt(value);
 
         transceive(cmd.array(), false);
     }
 
+    private void validateValueOperand(int value) {
+        if (value < 0) {
+            throw new IllegalArgumentException("value operand negative");
+        }
+    }
+
     /**
      * Copy from temporary memory to value block.
      * @param blockIndex