diff --git a/src/main/java/jota/utils/Checksum.java b/src/main/java/jota/utils/Checksum.java
index 0a303db..39d620f 100644
--- a/src/main/java/jota/utils/Checksum.java
+++ b/src/main/java/jota/utils/Checksum.java
@@ -38,7 +38,11 @@ public class Checksum {
     }
 
     private static String calculateChecksum(String address) {
-        // TODO
-        throw new NotImplementedException(address);
+        Curl curl = new Curl();
+        curl.reset();
+        curl.setState(Converter.copyTrits(address, curl.getState()));
+        curl.transform();
+        String checksum = Converter.trytes(curl.getState()).substring(0, 9);
+        return checksum;
     }
 }
diff --git a/src/main/java/jota/utils/Converter.java b/src/main/java/jota/utils/Converter.java
index b9cccbd..a43388f 100644
--- a/src/main/java/jota/utils/Converter.java
+++ b/src/main/java/jota/utils/Converter.java
@@ -91,6 +91,16 @@ public class Converter {
         }
     }
 
+    public static int[] copyTrits(final String input, final int[] destination) {
+        for (int i = 0; i < input.length(); i++) {
+            int index = Constants.TRYTE_ALPHABET.indexOf(input.charAt(i));
+            destination[i * 3] = TRYTE_TO_TRITS_MAPPINGS [index][0];
+            destination[i * 3 + 1] = TRYTE_TO_TRITS_MAPPINGS[index][1];
+            destination[i * 3 + 2] = TRYTE_TO_TRITS_MAPPINGS[index][2];
+        }
+        return destination;
+    }
+
     public static String trytes(final int[] trits, final int offset, final int size) {
 
         StringBuilder trytes = new StringBuilder();
diff --git a/src/main/java/jota/utils/Curl.java b/src/main/java/jota/utils/Curl.java
index b07236f..2c296e8 100644
--- a/src/main/java/jota/utils/Curl.java
+++ b/src/main/java/jota/utils/Curl.java
@@ -13,7 +13,7 @@ public class Curl {
     private static final int NUMBER_OF_ROUNDS = 27;
     private static final int[] TRUTH_TABLE = {1, 0, -1, 1, -1, 0, -1, 1, 0};
 
-    private final int[] state = new int[STATE_LENGTH];
+    private int[] state = new int[STATE_LENGTH];
 
     public void absorb(final int[] trits, int offset, int length) {
 
@@ -35,7 +35,7 @@ public class Curl {
         return state;
     }
 
-    private void transform() {
+    public void transform() {
 
         final int[] scratchpad = new int[STATE_LENGTH];
         int scratchpadIndex = 0;
@@ -56,4 +56,5 @@ public class Curl {
     public int[] getState() {
         return state;
     }
+    public void setState(int[] state) { this.state = state; }
 }
diff --git a/src/main/java/jota/utils/InputValidator.java b/src/main/java/jota/utils/InputValidator.java
index 327812a..9ce67b7 100644
--- a/src/main/java/jota/utils/InputValidator.java
+++ b/src/main/java/jota/utils/InputValidator.java
@@ -1,11 +1,5 @@
 package jota.utils;
 
-import jdk.nashorn.internal.runtime.regexp.joni.Regex;
-import jota.model.Transfer;
-import org.apache.commons.lang3.NotImplementedException;
-
-import java.util.Arrays;
-
 /**
  * Created by pinpong on 02.12.16.
  */
@@ -26,10 +20,7 @@ public class InputValidator {
     }
 
     public static boolean isTrytes(String trytes, int length) {
-
         // If no length specified, just validate the trytes
-
-        Regex regexTrytes = new Regex("^[9A-Z]{" + (length == 0 ? "0," : length) + "}$");
-        return trytes.matches(regexTrytes.toString());
+        return trytes.matches("^[9A-Z]{" + (length == 0 ? "0," : length) + "}$");
     }
 }
diff --git a/src/test/java/jota/ChecksumTest.java b/src/test/java/jota/ChecksumTest.java
index d67afef..e56644e 100644
--- a/src/test/java/jota/ChecksumTest.java
+++ b/src/test/java/jota/ChecksumTest.java
@@ -11,15 +11,16 @@ import static org.junit.Assert.assertEquals;
  */
 public class ChecksumTest {
 
-    private static final String TEST_ADDRESS = "RVORZ9SIIP9RCYMREUIXXVPQIPHVCNPQ9HZWYKFWYWZRE9JQKG9REPKIASHUUECPSQO9JT9XNMVKWYGVAZETAIRPTM";
+    private static final String TEST_ADDRESS_WITHOUT_CHECKSUM = "RVORZ9SIIP9RCYMREUIXXVPQIPHVCNPQ9HZWYKFWYWZRE9JQKG9REPKIASHUUECPSQO9JT9XNMVKWYGVAZETAIRPTM";
+    private static final String TEST_ADDRESS_WITH_CHECKSUM = "RVORZ9SIIP9RCYMREUIXXVPQIPHVCNPQ9HZWYKFWYWZRE9JQKG9REPKIASHUUECPSQO9JT9XNMVKWYGVAZETAIRPTMLSDUKRPBM";
 
     @Test
     public void shouldAddChecksum() {
-        assertEquals(Checksum.addChecksum(TEST_ADDRESS), true);
+        assertEquals(Checksum.addChecksum(TEST_ADDRESS_WITHOUT_CHECKSUM), TEST_ADDRESS_WITH_CHECKSUM);
     }
 
     @Test
     public void shouldRemoveChecksum() {
-        assertEquals(Checksum.addChecksum(TEST_ADDRESS), Checksum.isValidChecksum(Checksum.addChecksum(TEST_ADDRESS)));
+        assertEquals(Checksum.removeChecksum(TEST_ADDRESS_WITH_CHECKSUM), TEST_ADDRESS_WITHOUT_CHECKSUM);
     }
 }