How to transfer RSA public[/private] key outside the card? - javacard

Question

I wrote the below simple program to generate a RSA key pair and transfer the public key to outside the card in the APDU response:
public class CryptoRSA extends Applet {
//Abbreviations
private static final boolean NO_EXTERNAL_ACCESS = false;
//Switch case parameters for selecting instruction = INS in apdu command
private static final byte GENERATE_KEY_PAIR = (byte) 0xC0;
//Create object of keys
RSAPrivateKey thePrivateKey = (RSAPrivateKey) KeyBuilder.buildKey(KeyBuilder.TYPE_RSA_PRIVATE, KeyBuilder.LENGTH_RSA_512, NO_EXTERNAL_ACCESS);
RSAPublicKey thePublickKey = (RSAPublicKey) KeyBuilder.buildKey(KeyBuilder.TYPE_RSA_PUBLIC, KeyBuilder.LENGTH_RSA_512, NO_EXTERNAL_ACCESS);
KeyPair theKeyPair = new KeyPair(thePublickKey, thePrivateKey);
public static void install(byte[] bArray, short bOffset, byte bLength) {
new CryptoRSA();
}
protected CryptoRSA() {
register();
}
public void process(APDU apdu) {
if (selectingApplet()) {
return;
}
byte[] buffer = apdu.getBuffer();
short privateKeySize = 0;
short publicKeySize = 0;
byte[] publicArray;
byte[] privateArray;
try {
switch (buffer[ISO7816.OFFSET_INS]) {
case GENERATE_KEY_PAIR:
theKeyPair.genKeyPair();
PrivateKey thePrivateKey = theKeyPair.getPrivate();
PublicKey thePublicKey = theKeyPair.getPublic();
publicKeySize = thePrivateKey.getSize();
privateKeySize = thePrivateKey.getSize();
byte[] publicKey = JCSystem.makeTransientByteArray((short) (publicKeySize), JCSystem.CLEAR_ON_DESELECT);
((RSAPublicKey) thePrivateKey).getExponent(publicKey, (short) publicKeySize);
Util.arrayCopyNonAtomic(publicKey, (short) 0, buffer, (short) 0, (short) (publicKeySize ));
apdu.setOutgoingAndSend((short) 0, (short) (publicKeySize));
break;
default:
ISOException.throwIt(ISO7816.SW_INS_NOT_SUPPORTED);
}
} catch (Exception e) {
if (e instanceof CryptoException) {
short r = ((CryptoException) e).getReason();
ISOException.throwIt(r);
} else {
ISOException.throwIt((short) 0x8888);
}
}
}
}
But when I send the related APDU command to the card, I receive 0x8888 as below:
OSC:: opensc-tool.exe -s 00a40400060102030405dd -s 00c00000
Using reader with a card: ACS CCID USB Reader 0
Sending: 00 A4 04 00 06 01 02 03 04 05 DD
Received (SW1=0x90, SW2=0x00)
Sending: 00 C0 00 00
Received (SW1=0x88, SW2=0x88)

Answer 1

getSize() returns the bit length of the key, not the byte length. You are probably running out of RAM.
2.((RSAPublicKey) thePrivateKey).getExponent(publicKey, (short) publicKeySize);
This won't work! You are asking for the exponent to be stored at offset publicKeySize in array publicKey -- that is, at the very end of the array, where there are precisely 0 bytes left to store it.
By the way, the next time you come across a problem like this you can use ISOException to send debugging data to the outside world. For instance, ISOException.throwIt(privateKeySize) would have found problem 1.