How to get an unsigned byte array from a BigInteger in Java?

Question

I need to convert a BigInteger to an unsigned integer encoded in big-endian format but I am having issues since BigInteger.toByteArray returns a signed representation. How can I convert this value to an unsigned format?

(Relatively) Helpful Background

I am working on some code that uses JNI to have c++ call some Java methods to handle some cryptographic functionality (this is a Microsoft CNG provider that offloads some functionality to Java). I have the public key in Java and the BigInteger values that I need to convert are the coordinates of the Elliptic Curve Public Key. According to the CNG documentation I need to provide these points as "unsigned integers encoded in big-endian format".

Edit

In hindsight, this might have been a silly post. I was getting confused with negative and positive numbers and how to handle that (and because it's late and my mind has turned to mush) but it turns out that I don't need to deal with that since the elliptic curve points won't be negative. Thank you to everyone who responded on here! I will leave this up in case it helps anyone else.

If integer in question is positive, what is the difference? If it is negative, then what should be the result of conversion to unsigned? — Artur Biesiadowski
– Artur Biesiadowski, Commented Sep 3, 2016 at 6:26
In other words, this is a duplicate of stackoverflow.com/q/38918190/95954 — Rudy Velthuis
– Rudy Velthuis, Commented Sep 3, 2016 at 7:30
Short answer:prepend a 0 byte to the byte array and load it back into a BigInteger. Then it is positive. — Rudy Velthuis
– Rudy Velthuis, Commented Sep 3, 2016 at 7:32

Mark Jeronimus · Accepted Answer · 2016-09-03 09:23:58Z

1

With the help of a 2's complement reference value we can do this like below

private static final BigInteger TWO_COMPL_REF = BigInteger.ONE.shiftLeft(64);

    public static byte[] parseBigIntegerPositive(BigInteger b) {
        if (b.compareTo(BigInteger.ZERO) < 0)
            b = b.add(TWO_COMPL_REF);

       byte[] unsignedbyteArray= b.toByteArray();
        return unsignedbyteArray;
    }

edited Sep 3, 2016 at 9:23

Mark Jeronimus

9,6163 gold badges41 silver badges56 bronze badges

answered Sep 3, 2016 at 6:55

Jekin Kalariya

3,5072 gold badges22 silver badges32 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Rudy Velthuis Over a year ago

That does not return the same value, does it?

Jekin Kalariya Over a year ago

@rudy I think yes beacause it will just give unsigned version of byte array

Rudy Velthuis Over a year ago

By adding 1^64 to a negative BigInteger? Yes, well, if you truncate it to an unsiged integer, then perhaps. Otherwise, certainly not.

Collectives™ on Stack Overflow

How to get an unsigned byte array from a BigInteger in Java?

1 Answer 1

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related