Java BigInteger vs Mono .net BigInteger

Question

I use in .Net project mono implementation of BigInteger (link) In Java I use java.math.BigInteger.

The same code produces different results in Java.

.Net code

    String inputBytes = "8E5BD77F0DCC30864634C134E28BFB42A149675A320786B616F4530708350D270353C30A40450325801B7AFED12BCCA274B8187072A89CC0CC3F95A24A8251243C1835898246F4D64CA3AC61DB841518F0E8FBC8996A40EB626153AE7F0BB87FD713FAC522719431428DE178E780A3FA45788A72C431926AED990E6DA268D2CC";
    String modulus = "00e6b4b4511e0bd1b3d9c82ee189ba6d0c70b1466d94126f99a741af99a92701a789451742a357ddb61a4dea409965ec58dcaa5e30826de871b04700ed0fd46b1693446049734e8f95faba2bf9301347e63ba1771650e71982adef0cca6890b6f7baa7f5421a6533652f4b70c3c4270c480cf54cc06635f22901a42716d1dadf4f";
    String exp = "010001";

    BigInteger mModulus = new BigInteger(hexStringToByteArray(modulus));
    BigInteger mExponent = new BigInteger(hexStringToByteArray(exp));
    BigInteger input = new BigInteger(hexStringToByteArray(inputBytes));
    BigInteger output = input.ModPow(mExponent, mModulus);
    Console.WriteLine("==RESULT==" + byteArray2Hex(output.GetBytes()));   


    public static byte[] hexStringToByteArray(string hexString)
    {
        if (hexString.Length % 2 != 0)
            throw new ArgumentException(String.Format(CultureInfo.InvariantCulture, "The binary key cannot have an odd number of digits: {0}", hexString));

        byte[] HexAsBytes = new byte[hexString.Length / 2];
        for (int index = 0; index < HexAsBytes.Length; index++)
        {
            string byteValue = hexString.Substring(index * 2, 2);
            HexAsBytes[index] = byte.Parse(byteValue, NumberStyles.HexNumber, CultureInfo.InvariantCulture);
        }

        return HexAsBytes; 
    }

    ==RESULT==01FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF003020300C06082A864886F70D02050500041009EB0D996BFC1EFA5675997712A1AB6E

Java code. The same inputBytes array, the same exponent and modulus, but different result.

String inputBytes = "8E5BD77F0DCC30864634C134E28BFB42A149675A320786B616F4530708350D270353C30A40450325801B7AFED12BCCA274B8187072A89CC0CC3F95A24A8251243C1835898246F4D64CA3AC61DB841518F0E8FBC8996A40EB626153AE7F0BB87FD713FAC522719431428DE178E780A3FA45788A72C431926AED990E6DA268D2CC";
    String modulus = "00e6b4b4511e0bd1b3d9c82ee189ba6d0c70b1466d94126f99a741af99a92701a789451742a357ddb61a4dea409965ec58dcaa5e30826de871b04700ed0fd46b1693446049734e8f95faba2bf9301347e63ba1771650e71982adef0cca6890b6f7baa7f5421a6533652f4b70c3c4270c480cf54cc06635f22901a42716d1dadf4f";
    String exp = "010001";

    BigInteger mModulus = new BigInteger(hexStringToByteArray(modulus));
    BigInteger mExponent = new BigInteger(hexStringToByteArray(exp));
    BigInteger input = new BigInteger(hexStringToByteArray(inputBytes));

    BigInteger output = input.modPow(mExponent, mModulus);
    System.out.println("==RESULT==" + Utils.byteArray2Hex(output.getBytes()));

public static byte[] hexStringToByteArray(String s) {
    int len = s.length();
    byte[] data = new byte[len / 2];
    for (int i = 0; i < len; i += 2) {
        data[i / 2] = (byte) ((Character.digit(s.charAt(i), 16) << 4)
                + Character.digit(s.charAt(i+1), 16));
    }
    return data;
}

==RESULT==6ce02bd9536ad76bcfd7633b6a2305ed98b43b0bb5fc2acbf984566f1ab35db02e651e9ed8793bf64b018455872b8ae3a06af082e8d680df407ea1e5df1336a19c6f3e116c6ff1940066396afa1de5633fad814fb42790b3af0e62e6dd53977f78794b2d105cdca9272f9c0feea119fe2c9691b6f6e21db3065fb25d840acea2

I do not understand why the results are different.

P.S. e.g. if I use InputBytes

String inputBytes = "242e35241b85fcfd75a53441ef9fc0941064c16f8e4555dabef5ce8ebc91400c6961b6b607e5dd762dbcabce51b11c8594e7d7183786c8e3c5300c7583c1871fc6f350b817682150b5cd0430ca9a2c3f8315b425c8fea0e7cc18187237ed47d29b082e7e7154888d5fb09f092a6dd5e2d3dac9df8de45837b708b5ae17f03e7f";

the the results in Java and .Net are the same

==RESULT==01ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff003020300c06082a864886f70d02050500041046fd8e86a4833e7141cbe4718e8e92f7

Where is the magic?

Answer 1

From the docs for java.math.BigInteger(byte[]):

Translates a byte array containing the two's-complement binary representation of a BigInteger into a BigInteger. The input array is assumed to be in big-endian byte-order: the most significant byte is in the zeroth element.

From the docs for System.Numerics.BigInteger(byte[]):

The individual bytes in the value array should be in little-endian order, from lowest-order byte to highest-order byte.

So you might want to just try reversing the input bytes for one of the values you've got - it's not clear which set you should reverse, as we don't know what values you're trying to represent. I would suggest adding diagnostics of just printing out the normal decimal representation immediately after construction in each case - if those aren't the same, the rest of the code is irrelevant.

Answer 2

I solved my problem by adding 0 bit at the begining of inputBytes.