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, Sep 03 '16 at 06:26
Take a look at [the answer I gave to a similar qustion](http://stackoverflow.com/questions/38918190/how-to-get-the-2s-complement-value-of-a-biginteger-of-arbitrary-length/38931124#38931124). — Rudy Velthuis, Sep 03 '16 at 07:26
In other words, this is a duplicate of http://stackoverflow.com/q/38918190/95954 — Rudy Velthuis, Sep 03 '16 at 07:30
Short answer:prepend a 0 byte to the byte array and load it back into a BigInteger. Then it is positive. — Rudy Velthuis, Sep 03 '16 at 07:32

score 2 · Accepted Answer · edited Sep 03 '16 at 09:23

2

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 03 '16 at 09:23

Mark Jeronimus

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answered Sep 03 '16 at 06:55

Jekin Kalariya

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That does not return the same value, does it? – Rudy Velthuis Sep 03 '16 at 12:03
@rudy I think yes beacause it will just give unsigned version of byte array – Jekin Kalariya Sep 03 '16 at 12:05
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By adding 1^64 to a negative BigInteger? Yes, well, if you truncate it to an unsiged integer, then perhaps. Otherwise, certainly not. – Rudy Velthuis Sep 03 '16 at 12:15

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

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