Added a Simple XOR Cipher

DIRECTORY.md

@@ -52,6 +52,7 @@
             * [RSA](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/ciphers/RSA.java)
             * [SimpleSubCipher](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/ciphers/SimpleSubCipher.java)
             * [Vigenere](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/ciphers/Vigenere.java)
+            * [XORCipher](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/ciphers/XORCipher.java)
           * conversions
             * [AffineConverter](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/conversions/AffineConverter.java)
             * [AnyBaseToAnyBase](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/conversions/AnyBaseToAnyBase.java)
@@ -613,6 +614,7 @@
             * [RSATest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/ciphers/RSATest.java)
             * [SimpleSubCipherTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/ciphers/SimpleSubCipherTest.java)
             * [VigenereTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/ciphers/VigenereTest.java)
+            * [XORCipherTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/ciphers/XORCipherTest.java)
           * conversions
             * [AnyBaseToDecimalTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/conversions/AnyBaseToDecimalTest.java)
             * [BinaryToDecimalTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/conversions/BinaryToDecimalTest.java)

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

@@ -0,0 +1,39 @@
+package com.thealgorithms.ciphers;
+
+import java.nio.charset.Charset;
+import java.nio.charset.StandardCharsets;
+import java.util.Base64;
+
+/**
+ * A simple implementation of XOR cipher that, given a key, allows to encrypt and decrypt a plaintext.
+ *
+ * @author <a href="https://github.com/lcsjunior">lcsjunior</a>
+ *
+ */
+public final class XORCipher {
+
+    private static final Charset CS_DEFAULT = StandardCharsets.UTF_8;
+
+    private XORCipher() {
+    }
+
+    private static byte[] xor(final byte[] inputBytes, final byte[] keyBytes) {
+        byte[] outputBytes = new byte[inputBytes.length];
+        for (int i = 0; i < inputBytes.length; ++i) {
+            outputBytes[i] = (byte) (inputBytes[i] ^ keyBytes[i % keyBytes.length]);
+        }
+        return outputBytes;
+    }
+
+    public static String encrypt(final String plaintext, final String key) {
+        byte[] plaintextBytes = plaintext.getBytes(CS_DEFAULT);
+        byte[] keyBytes = key.getBytes(CS_DEFAULT);
+        return Base64.getEncoder().encodeToString(xor(plaintextBytes, keyBytes));
+    }
+
+    public static String decrypt(final String cipher, final String key) {
+        byte[] cipherBytes = Base64.getDecoder().decode(cipher);
+        byte[] keyBytes = key.getBytes(CS_DEFAULT);
+        return new String(xor(cipherBytes, keyBytes), CS_DEFAULT);
+    }
+}

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

@@ -0,0 +1,19 @@
+package com.thealgorithms.ciphers;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+class XORCipherTest {
+
+    @Test
+    void shouldEncryptAndDecryptTest() {
+        // given
+        String plaintext = "My t&xt th@t will be ençrypted...";
+        String key = "My ç&cret key!";
+        // when
+        String cipherText = XORCipher.encrypt(plaintext, key);
+        // then
+        assertEquals(XORCipher.decrypt(cipherText, key), plaintext);
+    }
+}