diff --git a/src/main/java/ch/fhnw/kry/CTR.java b/src/main/java/ch/fhnw/kry/CTR.java
new file mode 100644
index 0000000..7987cc9
--- /dev/null
+++ b/src/main/java/ch/fhnw/kry/CTR.java
@@ -0,0 +1,34 @@
+package ch.fhnw.kry;
+
+import java.util.Random;
+import java.util.concurrent.ThreadLocalRandom;
+
+public class CTR {
+    private final SPN spn = new SPN();
+    private final int iv;
+    private final int key;
+
+    public CTR(int key) {
+        this.iv = generateIV();
+        this.key = key;
+    }
+
+    public int getIV() {
+        return iv;
+    }
+
+    public int generateIV() {
+        Random random = ThreadLocalRandom.current();
+        byte[] r = new byte[2];
+        random.nextBytes(r);
+
+        return r[0] << 8 & r[1];
+    }
+
+    public int decrypt(int block, int idx) {
+        int e = (iv + idx) % (1 << 16);
+        e = spn.decryptBlock(key, e);
+
+        return block ^ e;
+    }
+}
diff --git a/src/main/java/ch/fhnw/kry/Decrypt.java b/src/main/java/ch/fhnw/kry/Decrypt.java
new file mode 100644
index 0000000..c4c85e8
--- /dev/null
+++ b/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;
+    }
+}
diff --git a/src/main/java/ch/fhnw/kry/Main.java b/src/main/java/ch/fhnw/kry/Main.java
index e260c3d..20ad38f 100644
--- a/src/main/java/ch/fhnw/kry/Main.java
+++ b/src/main/java/ch/fhnw/kry/Main.java
@@ -5,10 +5,12 @@ package ch.fhnw.kry;
  */
 public class Main {
 
-    private static final String KEY = "00111010100101001101011000111111";
-    private static final String CHIFFRE =
+    static final String KEY = "00111010100101001101011000111111";
+    static final String CHIFFRE =
         "00000100110100100000101110111000000000101000111110001110011111110110000001010001010000111010000000010011011001110010101110110000";
 
+    static final int BLOCK_LENGTH = 16;
+
     public static void main(String[] args) {
 
     }
diff --git a/src/main/java/ch/fhnw/kry/SPN.java b/src/main/java/ch/fhnw/kry/SPN.java
index 8c39bee..da2da63 100644
--- a/src/main/java/ch/fhnw/kry/SPN.java
+++ b/src/main/java/ch/fhnw/kry/SPN.java
@@ -3,6 +3,7 @@ package ch.fhnw.kry;
 import java.util.HashMap;
 import java.util.Map;
 
+import static ch.fhnw.kry.Main.BLOCK_LENGTH;
 import static java.util.Map.entry;
 
 public class SPN {
@@ -62,37 +63,9 @@ public class SPN {
         entry(15, 15)
     );
 
-    private static final int BLOCK_LENGTH = 16;
+
     private static final int ROUNDS = 4;
 
-    public String decrypt(int key, String chiffre) {
-
-        return null;
-    }
-
-    /**
-     * 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;
-    }
-
     /**
      * Encrypt a block with the defined SPN.
      *
diff --git a/src/test/java/ch/fhnw/kry/CTRTest.java b/src/test/java/ch/fhnw/kry/CTRTest.java
new file mode 100644
index 0000000..2aa54f2
--- /dev/null
+++ b/src/test/java/ch/fhnw/kry/CTRTest.java
@@ -0,0 +1,20 @@
+package ch.fhnw.kry;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class CTRTest {
+
+    @Test
+    void generateIV() {
+        var ctr = new CTR(0);
+
+        int count = 0;
+        while(count < Integer.MAX_VALUE) {
+            int iv = ctr.generateIV();
+            assert (iv < 1 << 16);
+            count++;
+        }
+    }
+}
diff --git a/src/test/java/ch/fhnw/kry/DecryptTest.java b/src/test/java/ch/fhnw/kry/DecryptTest.java
new file mode 100644
index 0000000..139b406
--- /dev/null
+++ b/src/test/java/ch/fhnw/kry/DecryptTest.java
@@ -0,0 +1,28 @@
+package ch.fhnw.kry;
+
+import org.junit.jupiter.api.Test;
+
+import static ch.fhnw.kry.Main.CHIFFRE;
+import static ch.fhnw.kry.Main.KEY;
+import static org.junit.jupiter.api.Assertions.*;
+
+class DecryptTest {
+
+    @Test
+    void strToArray() {
+        var decrypt = new Decrypt();
+
+        String input = "00000100110100100000101110111000000000101000111110001110011111110110000001010001010000111010000000010011011001110010101110110000";
+        int[] data = decrypt.strToArray(input);
+
+        assertEquals(8, data.length);
+        assertEquals(0x04D2, data[0]);
+        assertEquals(0x2BB0, data[data.length - 1]);
+    }
+
+    @Test
+    void decrypt() {
+        var decrypt = new Decrypt();
+        String decrypted = decrypt.decrypt(KEY, CHIFFRE);
+    }
+}
diff --git a/src/test/java/ch/fhnw/kry/SPNTest.java b/src/test/java/ch/fhnw/kry/SPNTest.java
index dfbccd3..4c18b3f 100644
--- a/src/test/java/ch/fhnw/kry/SPNTest.java
+++ b/src/test/java/ch/fhnw/kry/SPNTest.java
@@ -73,16 +73,4 @@ class SPNTest {
         assertEquals(y, spn.encryptBlock(key, x));
         assertEquals(x, spn.decryptBlock(key, y));
     }
-
-    @Test
-    void strToArray() {
-        var spn = new SPN();
-
-        String input = "00000100110100100000101110111000000000101000111110001110011111110110000001010001010000111010000000010011011001110010101110110000";
-        int[] data = spn.strToArray(input);
-
-        assertEquals(8, data.length);
-        assertEquals(0x04D2, data[0]);
-        assertEquals(0x2BB0, data[data.length - 1]);
-    }
 }