Negative numbers in APDU command

Question

I am writing Java Applet. I need to upload pre-generated RSAPrivateCrtKey to applet with APDU Command.

In my computer I am generating key. Next I serialize this key in privateKeyBuffer :

keyPair = new KeyPair(KeyPair.ALG_RSA_CRT, KeyBuilder.LENGTH_RSA_512);
keyPair.genKeyPair();
rsaPrivateKey = (RSAPrivateCrtKey) keyPair.getPrivate();

byte[] privateKeyBuffer = new byte[165];

short lengthDP1 = rsaPrivateKey.getDP1(privateKeyBuffer, 0);
offset += lengthDP1;

short lengthDQ1 = rsaPrivateKey.getDQ1(privateKeyBuffer, offset);
offset += lengthDQ1;

short lengthP = rsaPrivateKey.getP(privateKeyBuffer, offset);
offset += lengthP;

short lengthQ = rsaPrivateKey.getQ(privateKeyBuffer, offset);
offset += lengthQ;

short lengthPQ = rsaPrivateKey.getPQ(privateKeyBuffer, offset);

After generating a buffer I should convert each array element to hex and then send with APDU and last in applet restore my private key, but in privateKeyBuffer we have negative numbers :

37,65,-96,-110,38,6,-2,73,-37,28,120,-90... (etc)

How I should convert them to hex and keep APDU correct (because as I know only positive numbers allowed) or may be there another way to push key to JavaCard?

Any ideas?

Answer 1

You can simply copy your byte array to the APDU buffer and send it. It will work.

final byte[] apduBuffer = apdu.getBuffer();
Util.arrayCopyNonAtomic(privateKeyBuffer, (short) 0, apduBuffer, (short) 0, (short) privateKeyBuffer.length);
apdu.setOutgoingAndSend((short) 0, (short) privateKeyBuffer.length);

You do not have to think about hex values and positive/negative numbers at all! When you send a byte = 8 bits, you do not deal with its number representation or meaning at all.

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An extra note:

byte[] privateKeyBuffer = new byte[165]; is generally a bad idea. You do not need this buffer at all. You can copy key values directly into the APDU buffer, save some persistent memory and make your applet faster.

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Another note based on dear @Abraham's comment:

The only problem with positive/negative numbers you could face in Java Card is the fact that Java Card byte is signed , unfortunately. That means its values are always in range [-128,127], although people often forget about it:

final byte b = getSomeByteValue(); //from APDU buffer, for example
if (b == 0x80) { //always false!
     //this never happens!
}

If you want to understand your byte as unsigned (range [0, 255]), you have to cast it to short this way:

final byte b = getSomeByteValue(); //from APDU buffer, for example
final short unsignedValue = (short) (b & 0xFF);
if (unsignedValue == 0x80) { //correct!
     //can happen
}