diff --git a/core/src/test/groovy/jenkins/security/CryptoConfidentialKeyTest.groovy b/core/src/test/groovy/jenkins/security/CryptoConfidentialKeyTest.groovy
deleted file mode 100644
index e3816a342c4e76e04469993ab0a9ea61a3af57c3..0000000000000000000000000000000000000000
--- a/core/src/test/groovy/jenkins/security/CryptoConfidentialKeyTest.groovy
+++ /dev/null
@@ -1,37 +0,0 @@
-package jenkins.security
-
-import org.junit.Rule
-import org.junit.Test
-
-/**
- *
- *
- * @author Kohsuke Kawaguchi
- */
-class CryptoConfidentialKeyTest {
-    @Rule
-    public ConfidentialStoreRule store = new ConfidentialStoreRule()
-
-    def key = new CryptoConfidentialKey("test")
-
-    @Test
-    void decryptGetsPlainTextBack() {
-        ["Hello world","","\u0000"].each { str ->
-            assert key.decrypt().doFinal(key.encrypt().doFinal(str.bytes))==str.bytes
-        }
-    }
-
-    @Test
-    void multipleEncryptsAreIdempotent() {
-        def str = "Hello world".bytes
-        assert key.encrypt().doFinal(str)==key.encrypt().doFinal(str)
-    }
-
-    @Test
-    void loadingExistingKey() {
-        def key2 = new CryptoConfidentialKey("test") // this will cause the key to be loaded from the disk
-        ["Hello world","","\u0000"].each { str ->
-            assert key2.decrypt().doFinal(key.encrypt().doFinal(str.bytes))==str.bytes
-        }
-    }
-}
diff --git a/core/src/test/java/jenkins/security/CryptoConfidentialKeyTest.java b/core/src/test/java/jenkins/security/CryptoConfidentialKeyTest.java
new file mode 100644
index 0000000000000000000000000000000000000000..1b302a358bf960c19472fb8e857496de9bf10841
--- /dev/null
+++ b/core/src/test/java/jenkins/security/CryptoConfidentialKeyTest.java
@@ -0,0 +1,35 @@
+package jenkins.security;
+
+import static org.junit.Assert.*;
+import org.junit.Rule;
+import org.junit.Test;
+
+public class CryptoConfidentialKeyTest {
+
+    @Rule
+    public ConfidentialStoreRule store = new ConfidentialStoreRule();
+
+    private CryptoConfidentialKey key = new CryptoConfidentialKey("test");
+
+    @Test
+    public void decryptGetsPlainTextBack() throws Exception {
+        for (String str : new String[] {"Hello world", "", "\u0000"}) {
+            assertArrayEquals(str.getBytes(), key.decrypt().doFinal(key.encrypt().doFinal(str.getBytes())));
+        }
+    }
+
+    @Test
+    public void multipleEncryptsAreIdempotent() throws Exception {
+        byte[] str = "Hello world".getBytes();
+        assertArrayEquals(key.encrypt().doFinal(str), key.encrypt().doFinal(str));
+    }
+
+    @Test
+    public void loadingExistingKey() throws Exception {
+        CryptoConfidentialKey key2 = new CryptoConfidentialKey("test"); // this will cause the key to be loaded from the disk
+        for (String str : new String[] {"Hello world", "", "\u0000"}) {
+            assertArrayEquals(str.getBytes(), key2.decrypt().doFinal(key.encrypt().doFinal(str.getBytes())));
+        }
+    }
+
+}