Storing Your Data Securely on NFC Tags from Android

Near Field Communication (NFC) is a set of short-range wireless technologies, typically requiring a distance of 4 cm or less to initiate a connection. NFC allows you to share small payloads of data between an NFC tag and an Android-powered device, or between two Android-powered devices.

Android-powered devices with NFC simultaneously support three main operation modes:

1. Reader / writer mode, allowing the NFC device to read and / or to write passive NFC tags and stickers.
2. P2P mode, allowing the NFC device to exchange data with other NFC peers; this operation mode is used by Android Beam.
3. Card emulation mode, allowing the NFC device itself to act as an NFC card. The emulated NFC card can then be accessed by an external NFC reader, such as an NFC point-of-sale terminal.

NFC Tags, sometimes referred to as smart tags, are programmed with just about any sort of information and letting you read them with a smartphone or another NFC-capable device. For example, they can look as plastic cards or can be embedded in different things. (Pic. 1)

Pic. 1 NFC tag

Tags can be of the following types:

1. Tag 1 Type: based on the ISO14443A standard. Is read and re-write capable. Can be configured to be read only. Memory availability is 96 bytes, but memory size can be expandable up to 2 kbyte. 
2. Tag 2 Type: also based on the ISO14443A standard. This tag is read, re-write, set as read only capable. Basic memory – 48 bytes, can be expanded to 2 bytes.
3. Tag 3 Type: based on the Sony FeliCa system. Has 2 kbyte memory. More applicable for complex apps. Has a higher cost.
4. Tag 4 Type: defined to be compatible with ISO14443A and B standards. Is a pre-configured at manufacture. Cab be either read / re-write / read only. Memory can be up to 32 kbytes.

Today we are going to demonstrate how to use reader / writer mode.

Our application has following capabilities:

• writing encrypted data on tag,
• reading encrypted data on tag.

DES algorithm implementation is used for encryption / decryption.

About NFC using

First of all you should add this permission to AndroidManifest file for NFC functionality use.

<uses-permission android:name="android.permission.NFC">
</uses-permission> 

Then initialize NfcAdapter.

 NfcAdapter nfcAdapter = NfcAdapter.getDefaultAdapter(this);       

Note: not all devices support NFC functionality. To prevent crashes we’d like to recommended to check  NfcAdapter.

if (nfcAdapter == null) {
   Toast.makeText(this, "This device doesn't support NFC.", 
      Toast.LENGTH_LONG).show();
   finish();
} elseif(!nfcAdapter.isEnabled()) {
   Toast.makeText(this, "NFC is disabled.", 
      Toast.LENGTH_LONG).show();
   finish();
}

If you want to read the tag and launch the appropriate Activity when the app is closed, you should add such intent filter for this Activity to AndroidManifest file. 

<intent-filter>
   <action android:name="android.nfc.action.NDEF_DISCOVERED"/>
   <category android:name="android.intent.category.DEFAULT"/>
   <data android:mimeType="text/plain"/>
</intent-filter>

For switching between read tag mode and write tag mode, you can create PendingIntent.

You should also create intent-filters with the following Actions.

Then you should create PendingIntent for using intent-filters with these actions. PendingIntent will be used to enable foreground dispatch to the given Activity. For enable foreground dispatch we need IntentFilter arrays to handle NFC intents.  

privatevoidsetIntentFilterForWritingToATag(){
   // Intent to start an activity when a tag is discovered.
   IntentFilter tagForWriteDetectedIntentFilter = 
      new IntentFilter(NfcAdapter.ACTION_TAG_DISCOVERED);
   writeTagIntentFiltersArray = 
      new IntentFilter[]{ tagForWriteDetectedIntentFilter };
}

privatevoidsetIntentFilterForReadingNoteFromTagOrExchangingOverP2P(){
   // Intent to start an activity when a tag with NDEF payload 
   // is discovered.
   IntentFilter ndefDetectedIntentFilter = 
      new IntentFilter(NfcAdapter.ACTION_NDEF_DISCOVERED);
   try {
      ndefDetectedIntentFilter.addDataType("text/plain");
   } catch (MalformedMimeTypeException e) {
      e.printStackTrace();
   }
   ndefExchangeFiltersArray = 
      new IntentFilter[]{ndefDetectedIntentFilter};
}     

If you want to activate read tag mode, use the method:

privatevoidenableNdefReadTagMode(){
   nfcAdapter.enableForegroundDispatch(this, nfcPendingIntent, 
      ndefExchangeFiltersArray, null);
} 

If you want to activate write tag mode, use the method:

privatevoidenableTagWriteMode(){
   mWriteTagModeActive = true;
   IntentFilter tagDetected = 
      new IntentFilter(NfcAdapter.ACTION_TAG_DISCOVERED);
   writeTagIntentFiltersArray = new IntentFilter[]{tagDetected};
   nfcAdapter.enableForegroundDispatch(this, nfcPendingIntent, 
      writeTagIntentFiltersArray, null);
}

How to write string to TAG

1) Convert your String to byte array and use it for  NdefRecord initialization.

byte[] textBytes = text.getBytes();
NdefRecord textRecord = new NdefRecord(NdefRecord.TNF_MIME_MEDIA, 
   "text/plain".getBytes(), new byte[]{}, textBytes);

2) Than add  NdefRecord to NdefRecord array and after this set this array in NdefMessage.     

NdefRecord[] records = {textRecord};
NdefMessage message = new NdefMessage(records);

3) Thereafter you get Ndef from tag and write a message using Ndef.

Ndef ndef = Ndef.get(tag);
ndef.writeNdefMessage(message); 

How to read NdefMessage from Tag 

1)       Parse intent with action.

Parcelable[] messagesArrayInTag = 
   intent.getParcelableArrayExtra(NfcAdapter.EXTRA_NDEF_MESSAGES);            

2) Get NdefMessage array from Parcelable[] array.

NdefMessage messages = new NdefMessage[messagesArrayInTag.length];
for (int i = 0; i < messagesArrayInTag.length; i++)
   messages[i] = (NdefMessage) messagesArrayInTag[i]; 

3) Then you can parse NdefMessage with this code example.

NdefMessage currentNdefMessage = messagesArray[j];
NdefRecord[] recordForCurrentMessage = currentNdefMessage.getRecords();
NdefRecord currentRecord = recordForCurrentMessage[0];
String currentRecordPayload = new String(currentRecord.getPayload());             

How to get information about TAG 

You can use the following methods for Tag object:

byte[] getId() - get tag id. Should be converted to String for better reading.

String[] getTechList() - get the technologies available in this tag, as fully qualified class names.

String toString () - Human-readable description of the tag, for debugging.

To get information from Ndef object use this snippet:

Ndef ndef = Ndef.get(detectedTag);  
String ndefType = ndef.getType();
int ndefMaxSize = ndef.getMaxSize();
boolean isWritable = ndef.isWritable();
boolean isCanMakeReadOnly = ndef.canMakeReadOnly();

Encryption / Decryption

We use Des Java implementation in JDK by Sun. These packages aren’t available in Android, but we inserted some classes manually. While implementing this we used the password with 8 symbols length.

DesEncryptor class provides encryption and decryption with using password.

You may download the sample project here.

Thanks for reading our article. We hope this information was helpful for you.