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Commit b134223f authored by Nick Pelly's avatar Nick Pelly
Browse files

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
parent cc019c0c

core/java/android/nfc/tech/MifareClassic.java

+13 −2
Original line number Diff line number Diff line
@@ -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