push garbage

src/main/java/ch/fhnw/kry/Decrypt.java

@@ -0,0 +1,49 @@
+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);
+        var ctr = new CTR(key);
+
+        int[] data = strToArray(chiffre);
+        byte[] decryptedData = new byte[data.length * 2];
+        for (int i = 0; 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;
+    }
+}