Fix CTR decryption.

Off by one error in respect to the y blocks.

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

@@ -3,6 +3,11 @@ package ch.fhnw.kry;
 import java.util.Random;
 import java.util.concurrent.ThreadLocalRandom;
 
+import static ch.fhnw.kry.Main.BLOCK_LENGTH;
+
+/**
+ * Implementation of CTR mode for decryption.
+ */
 public class CTR {
     private final SPN spn = new SPN();
     private final int iv;
@@ -13,10 +18,11 @@ public class CTR {
         this.key = key;
     }
 
-    public int getIV() {
-        return iv;
-    }
-
+    /**
+     * Generate a 16 bit long initialisation vector.
+     *
+     * @return Generated initialisation vector.
+     */
     public static int generateIV() {
         Random random = ThreadLocalRandom.current();
         byte[] r = new byte[2];
@@ -25,10 +31,22 @@ public class CTR {
         return r[0] << 8 & r[1];
     }
 
-    public int decrypt(int block, int idx) {
-        int e = (iv + idx) % (1 << 16);
-        e = spn.decryptBlock(key, e);
+    public int getIV() {
+        return iv;
+    }
 
-        return block ^ e;
+    /**
+     * Decrypt a block in CTR mode.
+     *
+     * @param block Encrypted block (only lower 16 bits get looked at).
+     * @param idx   Block index.
+     * @param y     Y at index idx.
+     * @return Decrypted block (in the lower 16 bits of the int).
+     */
+    public int decrypt(int block, int idx, int y) {
+        int e = (iv + idx) % (1 << BLOCK_LENGTH); // iv + i mod 2^16
+        e = spn.encryptBlock(key, e); // yes, we need the encryption function, as this is CTR
+
+        return y ^ e;
     }
 }