Add autokey cipher (#5569)

src/test/java/com/thealgorithms/ciphers/AutokeyTest.java

@@ -0,0 +1,36 @@
+package com.thealgorithms.ciphers;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+class AutokeyCipherTest {
+
+    Autokey autokeyCipher = new Autokey();
+
+    @Test
+    void autokeyEncryptTest() {
+        // given
+        String plaintext = "MEET AT DAWN";
+        String keyword = "QUEEN";
+
+        // when
+        String cipherText = autokeyCipher.encrypt(plaintext, keyword);
+
+        // then
+        assertEquals("CYIXNFHEPN", cipherText);
+    }
+
+    @Test
+    void autokeyDecryptTest() {
+        // given
+        String ciphertext = "CYIX NF HEPN";
+        String keyword = "QUEEN";
+
+        // when
+        String plainText = autokeyCipher.decrypt(ciphertext, keyword);
+
+        // then
+        assertEquals("MEETATDAWN", plainText);
+    }
+}