read iv from chiffre (still not correct but at least less wrong now)

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

@@ -8,8 +8,8 @@ public class CTR {
     private final int iv;
     private final int key;
 
-    public CTR(int key) {
-        this.iv = generateIV();
+    public CTR(int iv, int key) {
+        this.iv = iv;
         this.key = key;
     }
 
@@ -17,7 +17,7 @@ public class CTR {
         return iv;
     }
 
-    public int generateIV() {
+    public static int generateIV() {
         Random random = ThreadLocalRandom.current();
         byte[] r = new byte[2];
         random.nextBytes(r);

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

@@ -7,11 +7,13 @@ 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 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);
 

src/test/java/ch/fhnw/kry/CTRTest.java

@@ -8,11 +8,9 @@ class CTRTest {
 
     @Test
     void generateIV() {
-        var ctr = new CTR(0);
-
         int count = 0;
         while(count < Integer.MAX_VALUE) {
-            int iv = ctr.generateIV();
+            int iv = CTR.generateIV();
             assert (iv < 1 << 16);
             count++;
         }