Add Tests for HillCipher (#5562)

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

@@ -0,0 +1,36 @@
+package com.thealgorithms.ciphers;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+class HillCipherTest {
+
+    HillCipher hillCipher = new HillCipher();
+
+    @Test
+    void hillCipherEncryptTest() {
+        // given
+        String message = "ACT"; // Plaintext message
+        int[][] keyMatrix = {{6, 24, 1}, {13, 16, 10}, {20, 17, 15}}; // Encryption key matrix
+
+        // when
+        String cipherText = hillCipher.encrypt(message, keyMatrix);
+
+        // then
+        assertEquals("POH", cipherText);
+    }
+
+    @Test
+    void hillCipherDecryptTest() {
+        // given
+        String cipherText = "POH"; // Ciphertext message
+        int[][] inverseKeyMatrix = {{8, 5, 10}, {21, 8, 21}, {21, 12, 8}}; // Decryption (inverse key) matrix
+
+        // when
+        String plainText = hillCipher.decrypt(cipherText, inverseKeyMatrix);
+
+        // then
+        assertEquals("ACT", plainText);
+    }
+}