diff --git a/src/main/java/lc/misc/HelperFunctions.java b/src/main/java/lc/misc/HelperFunctions.java
index f46d49e..446d903 100644
--- a/src/main/java/lc/misc/HelperFunctions.java
+++ b/src/main/java/lc/misc/HelperFunctions.java
@@ -11,9 +11,17 @@ public class HelperFunctions {
         RenderingHints.KEY_FRACTIONALMETRICS, RenderingHints.VALUE_FRACTIONALMETRICS_ON);
   }
 
-  public static String randomString(int n) {
-    String characters = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz23456789$#%@&?";
-    StringBuilder stringBuilder = new StringBuilder();
+  public static final String alphabets = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
+  public static final String numbers = "23456789";
+  public static final String specialCharacters = "$#%@&?";
+  public static final String allCharacters = alphabets + numbers + specialCharacters;
+
+  public static String randomString(final int n) {
+    return randomString(n, allCharacters);
+  }
+
+  public static String randomString(final int n, final String characters) {
+    final StringBuilder stringBuilder = new StringBuilder();
     for (int i = 0; i < n; i++) {
       int index = (int) (characters.length() * Math.random());
       stringBuilder.append(characters.charAt(index));
diff --git a/src/main/scala/lc/captchas/DebugCaptcha.scala b/src/main/scala/lc/captchas/DebugCaptcha.scala
index c581c05..fde6ab4 100644
--- a/src/main/scala/lc/captchas/DebugCaptcha.scala
+++ b/src/main/scala/lc/captchas/DebugCaptcha.scala
@@ -58,7 +58,7 @@ class DebugCaptcha extends ChallengeProvider {
   }
 
   def returnChallenge(): Challenge = {
-    val secret = HelperFunctions.randomString(6)
+    val secret = HelperFunctions.randomString(6, HelperFunctions.alphabets)
     new Challenge(simpleText(secret), "image/png", secret.toLowerCase())
   }
 }