Added Autokey cipher and AutokeyTest.java

Author: bennybebo

src/main/java/com/thealgorithms/ciphers/Autokey.java

@@ -0,0 +1,57 @@
+package com.thealgorithms.ciphers;
+
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * The Autokey Cipher is an interesting and historically significant encryption method, 
+ * as it improves upon the classic Vigenère Cipher by using the plaintext itself to 
+ * extend the key. This makes it harder to break using frequency analysis, as it 
+ * doesn’t rely solely on a repeated key.
+ * 
+ * @author bennybebo
+ */
+public class AutokeyCipher {
+
+    // Encrypts the plaintext using the Autokey cipher
+    public String encrypt(String plaintext, String keyword) {
+        plaintext = plaintext.toUpperCase().replaceAll("[^A-Z]", ""); // Sanitize input
+        keyword = keyword.toUpperCase();
+        
+        StringBuilder extendedKey = new StringBuilder(keyword);
+        extendedKey.append(plaintext); // Extend key with plaintext
+        
+        StringBuilder ciphertext = new StringBuilder();
+        
+        for (int i = 0; i < plaintext.length(); i++) {
+            char plainChar = plaintext.charAt(i);
+            char keyChar = extendedKey.charAt(i);
+            
+            int encryptedChar = ((plainChar - 'A') + (keyChar - 'A')) % 26 + 'A';
+            ciphertext.append((char) encryptedChar);
+        }
+        
+        return ciphertext.toString();
+    }
+
+    // Decrypts the ciphertext using the Autokey cipher
+    public String decrypt(String ciphertext, String keyword) {
+        ciphertext = ciphertext.toUpperCase().replaceAll("[^A-Z]", ""); // Sanitize input
+        keyword = keyword.toUpperCase();
+        
+        StringBuilder plaintext = new StringBuilder();
+        StringBuilder extendedKey = new StringBuilder(keyword);
+        
+        for (int i = 0; i < ciphertext.length(); i++) {
+            char cipherChar = ciphertext.charAt(i);
+            char keyChar = extendedKey.charAt(i);
+            
+            int decryptedChar = ((cipherChar - 'A') - (keyChar - 'A') + 26) % 26 + 'A';
+            plaintext.append((char) decryptedChar);
+            
+            extendedKey.append((char) decryptedChar); // Extend key with each decrypted char
+        }
+        
+        return plaintext.toString();
+    }
+}

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 {
+
+    AutokeyCipher autokeyCipher = new AutokeyCipher();
+
+    @Test
+    void autokeyEncryptTest() {
+        // given
+        String plaintext = "MEET AT DAWN";
+        String keyword = "QUEEN";
+
+        // when
+        String cipherText = autokeyCipher.encrypt(plaintext, keyword);
+
+        // then
+        assertEquals("QKUC YT DVZN", cipherText);
+    }
+
+    @Test
+    void autokeyDecryptTest() {
+        // given
+        String ciphertext = "QKUC YT DVZN";
+        String keyword = "QUEEN";
+
+        // when
+        String plainText = autokeyCipher.decrypt(ciphertext, keyword);
+
+        // then
+        assertEquals("MEET AT DAWN", plainText);
+    }
+}