Add Playfair Cipher (#4988)

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

@@ -0,0 +1,37 @@
+package com.thealgorithms.ciphers;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import org.junit.jupiter.api.Test;
+
+public class PlayfairTest {
+
+    @Test
+    public void testEncryption() {
+        PlayfairCipher playfairCipher = new PlayfairCipher("KEYWORD");
+
+        String plaintext = "HELLO";
+        String encryptedText = playfairCipher.encrypt(plaintext);
+        assertEquals("GYIZSC", encryptedText);
+    }
+
+    @Test
+    public void testDecryption() {
+        PlayfairCipher playfairCipher = new PlayfairCipher("KEYWORD");
+
+        String encryptedText = "UDRIYP";
+        String decryptedText = playfairCipher.decrypt(encryptedText);
+        assertEquals("NEBFVH", decryptedText);
+    }
+
+    @Test
+    public void testEncryptionAndDecryption() {
+        PlayfairCipher playfairCipher = new PlayfairCipher("KEYWORD");
+
+        String plaintext = "PLAYFAIR";
+        String encryptedText = playfairCipher.encrypt(plaintext);
+        String decryptedText = playfairCipher.decrypt(encryptedText);
+
+        assertEquals(plaintext, decryptedText);
+    }
+}