Javacard send RSA Public Key in APDU

Question

What would be a good solution to send a JavaCard RSAPublicKey via APDU? Get exponent and modules and pack them into a byte array?

Answer 1

Yes, you need to send both exponent and modulus serialized together as a byte array. These two methods solve your issue:

//reads the key object and stores it into the buffer
private final short serializeKey(RSAPublicKey key, byte[] buffer, short offset) {
    short expLen = key.getExponent(buffer, (short) (offset + 2));
    Util.setShort(buffer, offset, expLen);
    short modLen = key.getModulus(buffer, (short) (offset + 4 + expLen));
    Util.setShort(buffer, offset + 2 + expLen, modLen);
    return (short) (4 + expLen + modLen);
}

//reads the key from the buffer and stores it inside the key object
private final short deserializeKey(RSAPublicKey key, byte[] buffer, short offset) {
    short expLen = Util.getShort(buffer, offset);
    key.setExponent(buffer, (short) (offset + 2), expLen);
    short modLen = Util.getShort(buffer, (short) (offset + 2 + expLen));
    key.setModulus(buffer, (short) (offset + 4 + expLen), modLen);
    return (short) (4 + expLen + modLen);
}