@@ -15,6 +15,8 @@ dependencies {
     // This dependency is exported to consumers, that is to say found on their compile classpath.
     api 'org.apache.commons:commons-math3:3.6.1'
 
+    compile group: 'commons-codec', name: 'commons-codec', version: '1.11'
+
     // This dependency is used internally, and not exposed to consumers on their own compile classpath.
     implementation 'com.google.guava:guava:23.0'
 
 
@@ -1,5 +1,6 @@
+#Fri Nov 23 10:30:05 GMT 2018
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-4.10.2-bin.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-4.10.2-all.zip
@@ -1,5 +1,6 @@
 package uk.gov.objecthash;
 
+import java.io.UnsupportedEncodingException;
 import java.nio.charset.StandardCharsets;
 import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
 
@@ -1,5 +1,8 @@
 package uk.gov.objecthash;
 
+import org.apache.commons.codec.DecoderException;
+import org.apache.commons.codec.binary.Hex;
+
 public class StringValue implements ObjectHashable {
     private final String value;
 
@@ -8,6 +11,21 @@ public StringValue(String value) {
     }
 
     public byte[] digest() {
+        if(value.startsWith("**REDACTED**")) {
+            final String hexDigest = value.substring(12);
+            try {
+                return Hex.decodeHex(hexDigest);
+            } catch (DecoderException e) {
+                /* 
+                 * If the redacted value is not valid hex, then treat it as
+                 * a non-redacted string. This ensures that every possible
+                 * data structure has a hash. We do not enforce that the hash
+                 * length matches any particular hashing algorithm, only that
+                 * it is hex encoded.
+                 */
+            }
+        }
+        
         return ObjectHashable.digest(ObjectHashable.STRING_TAG, value);
     }
 }
@@ -1,6 +1,5 @@
 package uk.gov.objecthash;
-import static org.junit.Assert.assertEquals;
-import static org.junit.Assert.assertNotEquals;
+import org.junit.Test;
 
 import java.nio.charset.StandardCharsets;
 import java.time.Instant;
@@ -13,13 +12,14 @@
 import java.util.Map;
 import java.util.Set;
 
-import org.junit.Test;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotEquals;
 
 
 public class ObjectHashTest {
-    
+
     @Test
-    public void hashesStrings()  {
+    public void hashesStrings() {
         assertEquals(ObjectHash.toHexDigest("Hello, Ruby!"), "c92765c1350e6df6800dbadedb3420f398d5f3c7e38a3da48aadb3332280f85f");
     }
 
@@ -45,19 +45,19 @@ public void doesNotNormaliseByteArrays() {
 
     @Test
     public void hashesIntegers() {
-        assertEquals(ObjectHash.toHexDigest(-1), "f105b11df43d5d321f5c773ef904af979024887b4d2b0fab699387f59e2ff01e" );
-        assertEquals(ObjectHash.toHexDigest(0), "a4e167a76a05add8a8654c169b07b0447a916035aef602df103e8ae0fe2ff390" );
-        assertEquals(ObjectHash.toHexDigest(10), "73f6128db300f3751f2e509545be996d162d20f9e030864632f85e34fd0324ce" );
-        assertEquals(ObjectHash.toHexDigest(1000), "a3346d18105ef801c3598fec426dcc5d4be9d0374da5343f6c8dcbdf24cb8e0b" );
-    }    
-    
+        assertEquals(ObjectHash.toHexDigest(-1), "f105b11df43d5d321f5c773ef904af979024887b4d2b0fab699387f59e2ff01e");
+        assertEquals(ObjectHash.toHexDigest(0), "a4e167a76a05add8a8654c169b07b0447a916035aef602df103e8ae0fe2ff390");
+        assertEquals(ObjectHash.toHexDigest(10), "73f6128db300f3751f2e509545be996d162d20f9e030864632f85e34fd0324ce");
+        assertEquals(ObjectHash.toHexDigest(1000), "a3346d18105ef801c3598fec426dcc5d4be9d0374da5343f6c8dcbdf24cb8e0b");
+    }
+
     @Test
     public void hashesTimestamps() {
         var date = LocalDate.parse("2000-01-01");
         Instant instant = date.atStartOfDay(ZoneId.of("UTC")).toInstant();
-        assertEquals(ObjectHash.toHexDigest(instant), "cb34961d3d1a44386a73c37eb64c72a2b61ff40fc108abca92ef07c4954a1645" );
+        assertEquals(ObjectHash.toHexDigest(instant), "cb34961d3d1a44386a73c37eb64c72a2b61ff40fc108abca92ef07c4954a1645");
     }
-    
+
     @Test
     public void hashesDict() {
         Map<String, ObjectHashable> data = new HashMap<>();
@@ -136,4 +136,23 @@ public void hashesDictOfMixedTypes() {
 
         assertEquals("45d9392ad17cead3fa46501eba3e5ac237cb46a39f1e175905f00ef6a6667257", digest);
     }
+
+    @Test
+    public void returnsSuppliedHashIfRedacted() {
+        StringValue original = new StringValue("hello");
+        StringValue redacted = new StringValue("**REDACTED**" + original.hexDigest());
+        
+        assertEquals(original.hexDigest(), redacted.hexDigest());
+    }
+
+    @Test
+    public void hashesKnownRedactedHash() {
+        assertEquals(ObjectHash.toHexDigest("**REDACTED**2a42a9c91b74c0032f6b8000a2c9c5bcca5bb298f004e8eff533811004dea511"), "2a42a9c91b74c0032f6b8000a2c9c5bcca5bb298f004e8eff533811004dea511" );
+    }
+    
+    @Test
+    public void hashesInvalidRedactedValues() {
+        assertEquals(ObjectHash.toHexDigest("**REDACTED**banana"), "525b975c2979f427dcb79db8992035a9c2157a3c54492050753b9af552756c7a");
+    }
+
 }