add (draft of) cash applet

build.gradle

@@ -29,7 +29,10 @@ javacard {
       aid = '0xA0:0x00:0x00:0x08:0x04:0x00:0x01:0x02'
       className = 'NDEFApplet'
     }
-    
+    applet {
+      aid = '0xA0:0x00:0x00:0x08:0x04:0x00:0x01:0x03'
+      className = 'CashApplet'
+    }
     version = '2.3'
   }
 }

src/main/java/im/status/keycard/CashApplet.java

@@ -0,0 +1,113 @@
+package im.status.keycard;
+
+import javacard.framework.*;
+import javacard.security.*;
+
+public class CashApplet extends Applet {
+  private static final short SIGN_OUT_OFF = ISO7816.OFFSET_CDATA + MessageDigest.LENGTH_SHA_256;
+
+  private KeyPair keypair;
+  private ECPublicKey publicKey;
+  private ECPrivateKey privateKey;
+
+  private Crypto crypto;
+  private SECP256k1 secp256k1;
+
+  private Signature signature;
+
+  /**
+   * Invoked during applet installation. Creates an instance of this class. The installation parameters are passed in
+   * the given buffer.
+   *
+   * @param bArray installation parameters buffer
+   * @param bOffset offset where the installation parameters begin
+   * @param bLength length of the installation parameters
+   */
+  public static void install(byte[] bArray, short bOffset, byte bLength) {
+    new CashApplet(bArray, bOffset, bLength);
+  }
+
+  /**
+   * Application constructor. All memory allocation is done here. The reason for this is two-fold: first the card might
+   * not have Garbage Collection so dynamic allocation will eventually eat all memory. The second reason is to be sure
+   * that if the application installs successfully, there is no risk of running out of memory because of other applets
+   * allocating memory. The constructor also registers the applet with the JCRE so that it becomes selectable.
+   *
+   * @param bArray installation parameters buffer
+   * @param bOffset offset where the installation parameters begin
+   * @param bLength length of the installation parameters
+   */
+  public CashApplet(byte[] bArray, short bOffset, byte bLength) {
+    crypto = new Crypto();
+    secp256k1 = new SECP256k1(crypto);
+
+    keypair = new KeyPair(KeyPair.ALG_EC_FP, SECP256k1.SECP256K1_KEY_SIZE);
+    publicKey = (ECPublicKey) keypair.getPublic();
+    privateKey = (ECPrivateKey) keypair.getPrivate();
+    secp256k1.setCurveParameters(publicKey);
+    secp256k1.setCurveParameters(privateKey);
+    keypair.genKeyPair();
+
+    signature = Signature.getInstance(Signature.ALG_ECDSA_SHA_256, false);
+    signature.init(privateKey, Signature.MODE_SIGN);
+
+    register(bArray, (short) (bOffset + 1), bArray[bOffset]);
+  }
+
+  public void process(APDU apdu) throws ISOException {
+    apdu.setIncomingAndReceive();
+
+    // Since selection can happen not only by a SELECT command, we check for that separately.
+    if (selectingApplet()) {
+      selectApplet(apdu);
+      return;
+    }
+
+    byte[] apduBuffer = apdu.getBuffer();
+
+    try {
+      switch (apduBuffer[ISO7816.OFFSET_INS]) {
+        case KeycardApplet.INS_SIGN:
+          sign(apdu);
+          break;
+        default:
+          ISOException.throwIt(ISO7816.SW_INS_NOT_SUPPORTED);
+          break;
+      }
+    } catch (CryptoException ce) {
+      ISOException.throwIt((short)(ISO7816.SW_UNKNOWN | ce.getReason()));
+    } catch (Exception e) {
+      ISOException.throwIt(ISO7816.SW_UNKNOWN);
+    }
+  }
+
+  private void selectApplet(APDU apdu) {
+    byte[] apduBuffer = apdu.getBuffer();
+
+    apduBuffer[0] = KeycardApplet.TLV_PUB_KEY;
+    apduBuffer[1] = (byte) publicKey.getW(apduBuffer, (short) 2);
+
+    apdu.setOutgoingAndSend((short) 0, (short)(apduBuffer[1] + 2));
+  }
+
+  private void sign(APDU apdu) {
+    byte[] apduBuffer = apdu.getBuffer();
+
+    apduBuffer[SIGN_OUT_OFF] = KeycardApplet.TLV_SIGNATURE_TEMPLATE;
+    apduBuffer[(short) (SIGN_OUT_OFF + 3)] = KeycardApplet.TLV_PUB_KEY;
+    short outLen = apduBuffer[(short) (SIGN_OUT_OFF + 4)] = Crypto.KEY_PUB_SIZE;
+
+    publicKey.getW(apduBuffer, (short) (SIGN_OUT_OFF + 5));
+
+    outLen += 5;
+    short sigOff = (short) (SIGN_OUT_OFF + outLen);
+
+    outLen += signature.signPreComputedHash(apduBuffer, ISO7816.OFFSET_CDATA, MessageDigest.LENGTH_SHA_256, apduBuffer, sigOff);
+    outLen += crypto.fixS(apduBuffer, sigOff);
+
+    apduBuffer[(short) (SIGN_OUT_OFF + 1)] = (byte) 0x81;
+    apduBuffer[(short) (SIGN_OUT_OFF + 2)] = (byte) (outLen - 3);
+
+    apdu.setOutgoingAndSend(SIGN_OUT_OFF, outLen);
+  }
+}