Java number calculation.

Question

hi, i am doing java encryption/decryption and my key is 128 bits. i need to times multiple keys and thus, the value is far exceed the integer and long value.
thus i decide to use math.biginteger.
however, the calculation is weird.
here is my code
BigInteger biPdi = new BigInteger(bPdi); //bPdi is a bytearray
BigInteger biKey = new BigInteger(groupKey);//group key is a bytearray
BigInteger bicKey = new BigInteger(bicGroupKey));//bicgroupkey is a bytearray
System.out.println("derivationkey: " + biPdi.toString(16));
System.out.println("child group key: " + bicKey.toString(16));
b = biPdi.add(bicKey).toByteArray();     
BigInteger bit2 = new BigInteger(b);
System.out.println("derivationkey: " + bit2.toString(16));
this is the result i am getting back.
derivationkey: 30
child group key: 3938393951128f27c3da559f36ea62de
derivationkey: 3938393951128f27c3da559f36ea630e
i know the derivationkey has value "0" which is encode as "30" b/c it is based on 16. thus, the byte value is "48" which is the value of "0".
when i add derviationkey with child group key, i expect to my new diviationkey is same as childgroup key b/c it is childgroup plus 0. however, i am getting a different value. i think the reason is because java byte array try to add 30 to childgroup.
in this case, how can i do a large number calculation? can someone give me any advice??
Thanks in advance.

Answer 1

There's a button what says "code." Hit it and put your code inside the tags.
i know the derivationkey has value "0" which is
encode as "30" b/c it is based on 16. thus, the byte
value is "48" which is the value of "0".0 is zero no matter what the base. And 48 != 0 no matter how much paint you sniff (though it can seem that way).
when i add derviationkey with child group key, i
expect to my new diviationkey is same as childgroup
key b/c it is childgroup plus 0. however, i am
getting a different value. i think the reason is
because java byte array try to add 30 to childgroup.If it's 30 and you add it, the answer will come out as 30 more than the number you added to 30.
in this case, how can i do a large number
calculation? can someone give me any advice??BigInteger works like it should. derivationkey is 30.
~Cheers