BigInteger toByteArray Question

Question

Hi all,
I want to convert BigInteger into byte array using toByteArray method.
However, some errors occur. The length of byte i get from conversion is not same as the original byte length. some are less than original but some are larger.
I have read a 100 byte from a file each time and then convert the byte array into BigInteger for some modulus operation. Finally, i am able to get back the original BigInteger. when i try to convert these BigInteger into byte array, problem arise. some byte array length is 99, some are 100 and some are 101.
Why these happen?
and how to make the conversion successfully?
Thanks

Answer 1

yes.
// Convert the 100 byte array to BigInteger
// open file
byte[] fileByte = MyUtil.openFile("2.jpg");
// New Add Implementation RSA Block
int fileSize = fileByte.length;
int lastBlockSize = fileSize % blockSize;
int numBlock = fileSize / blockSize;
if (lastBlockSize > 0)
numBlock++;
System.out.println("num Block "+numBlock);
System.out.println("last Block Size " + lastBlockSize);
BigInteger[] fileBI = new BigInteger[numBlock];
int j = 0;
for (int i=0; i<numBlock; i++)
     byte[] RSABlock = null;
     // copy the 100 fileByte into RSABlock
     if (i==numBlock-1)
     RSABlock = new byte[lastBlockSize];
     System.arraycopy(fileByte,j, RSABlock,0,lastBlockSize);
     else
     RSABlock = new byte[blockSize];
     System.arraycopy(fileByte,j, RSABlock,0,blockSize);
     j=j+blockSize;
     fileBI[i] = new BigInteger(1,RSABlock); // absolute BigInteger
     System.out.println("Original " + i + " " + fileBI);
// Convert the BigInteger to byte array
for (int i=0; i<customerDe.length; i++ )
if (customerDe[i].compareTo(BigInteger.ZERO) <0 )
System.out.print("NEGATIVE NUMBER");
     if (customerDe[i].compareTo(fileBI[i]) != 0)
     System.out.println("Not Match " + i);
     System.out.println("diff: "+customerDe[i].subtract(fileBI[i]));
     System.exit(1);
     byte[] decryptSigned = customerDe[i].toByteArray();
     byte[] decryptUnsigned = null;
     // if the decryptSigned doesn't have leading Sign || the length is one
     if (decryptSigned[0] != 0x00 || decryptSigned.length == 1)
     System.out.println("UNSigned BYTE "+i);
     decryptUnsigned = decryptSigned;
     System.out.println(i + " len "+ decryptUnsigned.length);
     else if (decryptSigned[0] == 0x00)
     System.out.println("Signed BYTE "+i);
     decryptUnsigned = new byte[decryptSigned.length-1];
     System.arraycopy(decryptSigned,1,decryptUnsigned,0,decryptUnsigned.length);
     System.out.println(i + " len "+ decryptUnsigned.length);
     if (decryptSigned.length == blockSize-1)
     decryptUnsigned = new byte[decryptSigned.length+1];
     System.arraycopy(decryptSigned,0,decryptUnsigned,1,decryptSigned.length);
     baos.write(decryptUnsigned,0,decryptUnsigned.length);