Java Fill array 2 bytes at a time

Question

I am trying to fill a byte array, 2 bytes at a time. My hw is a cryptography hw. I am to find the plain text from the given cipher text using a key. The key is 2 bytes long. The plain text was encrypted with many time pad. The Key is repeated until its as long as the plain text. I got the first 2 bytes of the key, now i just need to repeat it until its as long as the cipher text/plain text.

The cipher text length which is the same as the plain text length is 640138. The key is supposed to be this length as well, but what i got for key length after i applied the solution below was 640144. Doing Math.min below instead of Math.max gives an error String.checkBoundsBeginEnd. I had to convert it to string because the builder is of type StringBuilder. "result" in this instance is key.

How do i make the key as long as the cipher text length?

    byte[] cipherText = Files.readAllBytes(Paths.get("src/cipher3.bmp"));
    byte[] plainText = new byte[cipherText.length];

    byte[] pText = new byte[]{'B', 'M'};
    byte[] key = new byte[pText.length];
    for(int i = 0; i < pText.length; i++){
        key[i] = (byte)(cipherText[i] ^ pText[i]);
    }
    String keyString = Arrays.toString(key);
    System.out.println("The key " + keyString);
    System.out.println("First two in ptext"+ Arrays.toString(pText));
    System.out.println(plainText.length);

    String plainlength = (String) Arrays.toString(new int[]{plainText.length});
    System.out.println(plainlength);
    StringBuilder builder = new StringBuilder(cipherText.length);
    while(builder.length() < cipherText.length){
        builder.append(keyString.substring(0, Math.max(keyString.length(), builder.length() -cipherText.length)));
    }
    String result = builder.toString();
    System.out.println(result);
    System.out.println(result.length());//this gives 640144

I don’t understand how builder.append(keyString.substring(0, Math.max(keyString.length(), builder.length() -cipherText.length))); is supposed to add two bytes at a time, but I do know that you should not be using Strings at all. Instead of StringBuilder, use ByteBuffer, and instead of StringBuilder.append, use ByteBuffer.put. — VGR
– VGR, Commented Sep 8, 2018 at 20:40

samabcde · Accepted Answer · 2018-09-07 01:56:10Z

Math.min should be used to fill correctly. The

java.lang.StringIndexOutOfBoundsException: String index out of range: -640138

gives you clue on what is wrong. Negative input is applied on the subString method, obviously it must be Math.min(keyString.length(), builder.length() -cipherText.length. Since cipherText length must greater than or equals to builder length, you should swap the arguments to give correct result.

public class Fill2ByteToArray {
    public static void main(String[] args) throws IOException {
        int targetLength = 640138;
        byte[] cipherText = new byte[targetLength];
        for (int i = 0; i < cipherText.length; i++) {
            cipherText[i] = Integer.valueOf(i).byteValue();
        }
        byte[] plainText = new byte[cipherText.length];

        byte[] pText = new byte[] { 'B', 'M' };
        byte[] key = new byte[pText.length];
        for (int i = 0; i < pText.length; i++) {
            key[i] = (byte) (cipherText[i] ^ pText[i]);
        }
        String keyString = Arrays.toString(key);
        System.out.println("The key " + keyString);
        System.out.println("First two in ptext" + Arrays.toString(pText));
        System.out.println(plainText.length);

        String plainlength = (String) Arrays.toString(new int[] { plainText.length });
        System.out.println(plainlength);
        StringBuilder builder = new StringBuilder(cipherText.length);
        while (builder.length() < cipherText.length) {
            builder.append(keyString.substring(0, Math.min(keyString.length(), cipherText.length-builder.length())));
        }
        String result = builder.toString();
        System.out.println(result);
        System.out.println(result.length());
    }
}

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