kry-ctr-spn/src/main/java/ch/fhnw/kry/Decrypt.java

package ch.fhnw.kry;

import java.nio.charset.StandardCharsets;

import static ch.fhnw.kry.Main.BLOCK_LENGTH;

public class Decrypt {
    public String decrypt(String keyString, String chiffre) {
        int key = Integer.parseInt(keyString, 2);

        int[] data = strToArray(chiffre);
        byte[] decryptedData = new byte[data.length * 2];
        int iv = data[0];
        var ctr = new CTR(iv, key);

        for (int i = 1; i < data.length; i++) {
            int block = data[i];
            int decryptedBlock = ctr.decrypt(block, i);

            decryptedData[i * 2] = (byte)(decryptedBlock >>> 8);
            decryptedData[i * 2 + 1] = (byte)(decryptedBlock & 0x0000_00FF);
        }

        String decryptedString = new String(decryptedData, StandardCharsets.US_ASCII);

        return decryptedString;
    }

    /**
     * Convert a bit string into an integer array.
     *
     * Because later we only ever look at the lower 16 bits,
     * we stuff every block of 16 bits into the lower half of an int
     * (meaning the igh 16 bits of the ints in the array are always 0).
     *
     * @param bits Bit string. Its length must be a multiple of 16.
     * @return int array, with every int's lower 16 bits set to 16 bits of the input string.
     */
    public int[] strToArray(String bits) {
        int[] data = new int[bits.length() / BLOCK_LENGTH];

        for (int i = 0; i < data.length; i++) {
            int startIdx = i * BLOCK_LENGTH;
            String wordBits = bits.substring(startIdx, startIdx + BLOCK_LENGTH);
            int word = Integer.parseInt(wordBits, 2);
            data[i] = word;
        }

        return data;
    }
}