[FIXED] How to read a 32 byte number from a file into a variable?

Issue

I am working with BigInteger, and using my function I write a BigInteger of 256 bits to a file (there are 64 such numbers)

public static byte[] toHH2(BigInteger n) {
    byte[] b = new byte[256];

    for(int i = 0; i < 256; i+=8) {
        b[i] = (byte) (n.longValue() >> (248 - i) & 0xff);
    }
    return b;
}

Then I need to read this 256 bit number and write it to a variable. But for some reason, I end up with a maximum of 64 bit numbers, and I need 256, how to fix the error?

var dataSignatureInt = bytesToIntArray(Files.readAllBytes(Path.of("C:\\Users\\User\\IdeaProjects\\CryptoLab1\\src\\crypto\\DigitalSignatureGOST.png")));
for(int i = 0; i < file.length; i+=256) { //file lenght = 16384 
    BigInteger value = BigInteger.valueOf(dataSignatureInt[i]);

    for(int j = 0; j < 256; j++) {
        System.out.println("i = " + i + " j = " + j);
        value = BigInteger.valueOf((value.longValue() << 8) | dataSignatureInt[i + j]);
    }

Solution

256 bits are 32 bytes, but somehow you do a bit per byte or such.

public static byte[] toHH2(BigInteger n) {
    byte[] b = n.toByteArray(); /// <= 32 bytes in big endian order!
    return Arrays.copyOf(b, 32 - b.length);
    if (b.length < 32) {
        byte[] b2 = new byte[256];
        System.arraycopy(b, 0, b2, 32 - b.length, b.length);
        if (b[0] < 0) { // Do sign extension, if most significant byte negative.
            for (int j = 0; j < 32 - b.length; ++j) {
                b2[j] = -1;
            }
        }
        b = b2;
    }
    return b;
}

For ease I have padded it. Not needed, but it shows that the most significant byte is at index 0 of the bytes.

// Now several such 32 byte arrays are stored in the file.
var dataSignatureInt = Files.readAllBytes(Path.of(
    "C:\\Users\\User\\IdeaProjects\\CryptoLab1\\src\\crypto\\DigitalSignatureGOST.png"));
int numberCount = dataSignatureInt.length / 32;
BigInteger[] numbers = new BigInteger[numberCount];

To split a large byte array into 32 byte subarrays, the class ByteBuffer is ideal.

ByteBuffer buf = ByteBuffer.wrap(dataSignatureInt);
byte[] bytes = new byte[32];
for (int i = 0; i < numberCount; ++i) {
    buf.get(bytes);
    numbers[i] = new BigInteger(bytes);        
}

Last but not least, you could consider using BitSet instead of BigInteger if you are interested in single bits.

BitSet bitSet = new BitSet(256);
byte[] shortenedBytes = bitSet.toByteArray();
bitSet = BitSet.valueOf(shortenedBytes);

Answered By - Joop Eggen
Answer Checked By - Marilyn (JavaFixing Volunteer)