Setup NFC, select applet, read TOTP code

src/main/java/com/fidesmo/tutorials/otp/MainActivity.java

@@ -18,7 +18,7 @@
 import nordpol.android.OnDiscoveredTagListener;
 
 @EActivity(R.layout.activity_main)
-public class MainActivity extends AppCompatActivity {
+public class MainActivity extends AppCompatActivity implements OnDiscoveredTagListener {
 
     private static final String TAG = "MainActivity";
 
@@ -28,6 +28,9 @@ public class MainActivity extends AppCompatActivity {
     //Hardcoded TOTP key put onto the OTP applet beforehand
     private static final String TOTP_CODE_NAME = "FidesmoOTPTutorial:[email protected]";
 
+    // The TagDispatcher is responsible for managing the NFC for the activity
+    private TagDispatcher tagDispatcher;
+
     // UI elements
     @ViewById
     TextView mainText;
@@ -47,16 +50,113 @@ void setMainMessage(String text) {
     @Override
     protected void onResume() {
         super.onResume();
+        // The first argument is the activity for which the NFC is managed
+        // The second argument is the OnDiscoveredTagListener which is also
+        // implemented by this activity
+        // This means that the method tagDiscovered will be called whenever a new tag appears
+        tagDispatcher = TagDispatcher.get(this, this);
+        // Start listening on the NFC interface when the app gains focus.
+        tagDispatcher.enableExclusiveNfc();
     }
 
     @Override
     public void onPause() {
         super.onPause();
+        // Stop listening on the NFC interface if the app loses focus
+        tagDispatcher.disableExclusiveNfc();
     }
 
+    /**
+     * This method is called when a contactless device is detected at the NFC interface
+     * @param intent the PendingIntent declared in onResume
+     */
     @Override
     protected void onNewIntent(Intent intent) {
+        tagDispatcher.interceptIntent(intent);
+    }
+
+    //Called whenever a tag is discovered on the NFC interface
+    @Override
+    public void tagDiscovered(Tag tag) {
+        setMainMessage("Reading card...");
+        /**
+         * As the user might remove the card before we are done communicating with it we have to
+         * make sure to catch that exception if it happens. We do this with a
+         * try { ... } catch { ... }
+         */
+        try {
+            /**
+             * Create an IsoCard that we can communicate with. Nordpols version of IsoCard
+             * AndroidCard has a get method to create an IsoCard from a tag.
+             */
+            IsoCard isoCard = AndroidCard.get(tag);
+            //Send on the IsoCard
+            communicateWithCard(isoCard);
+        } catch(IOException e) {
+            e.printStackTrace();
+        }
+    }
 
+    protected void communicateWithCard(IsoCard isoCard) {
+        /**
+         * The user might still remove the card before we are done with it, remember? Try catch
+         * that functionality!
+         */
+        try {
+            //Open a connection to the card
+            isoCard.connect();
+            /**
+             * Send the first command to the card. This command has to be a select command to ensure
+             * that we are communicating with the correct applet on the card. The OTP_AID is
+             * the identifier of the OTP applet installed on the card. The Nordpol method
+             * transceiveAndRequireOk() ensures that the command specified returned
+             * success (OK_APDU) otherwise we continue to catch {}.
+             */
+            Apdu.transceiveAndRequireOk(Apdu.select(OTP_AID), isoCard);
+            setMainMessage("Select OK");
+
+            //Get a time stamp for creating a TOTP code APDU from
+            long timeStamp = getTimeStamp();
+
+            /**
+             * Get the APDU command copied from Yubicos open source app for generating TOTP codes.
+             * Specify that the TOTP code with the name TOTP_CODE_NAME is fetched. This "name"
+             * should ideally by generated and not hardcoded and supplied when the TOTP token is
+             * input into the OTP applet. Knowing this, let's hardcode it for the sake of this
+             * tutorial with the value that we scanned/input into the applet before using some
+             * other app like Yubico Authenticator.
+             */
+            byte[] totpCodeApdu = Otp.totpCodeApdu(TOTP_CODE_NAME, timeStamp);
+
+            /**
+             * Send the command to the IsoCard using Nordpol method transceiveAndGetResponse.
+             * This method returns the response and automatically batches long messages.
+             * The first parameter is the APDU command. The second parameter is the IsoCard to
+             * communicate with. The third parameter is an optional and applet specific APDU
+             * command for requesting more content from the applet. I found this on in the open
+             * source code of Yubico Authenticator
+             */
+            byte[] rawTotpCode  = Apdu.transceiveAndGetResponse(totpCodeApdu, isoCard, Otp.SEND_REMAINING_APDU);
+            /**
+             * Check the result of the communication with the Nordpol method hasStatus. Supply the
+             * the response APDU and the expected APDU as parameters. If the ending of the first
+             * parameter matches the second parameter it is considered a match.
+             */
+            if(Apdu.hasStatus(rawTotpCode, Apdu.OK_APDU)){
+                /**
+                 * Decipher the TOTP code from the gibberish the response consists of (using
+                 * Yubicos decipher methods).
+                 */
+                String totpCode = Otp.decipherTotpCode(rawTotpCode);
+                //Append the TOTP code to the TextView
+                setMainMessage("TOTP code is: " + totpCode);
+            }
+
+            //Close the card communication.
+            isoCard.close();
+        } catch (IOException e) {
+            e.printStackTrace();
+        }
     }
 
     private static long getTimeStamp() {