feat: add card number encryption utility

src/main/java/com/example/bankcards/util/CardEncryptionUtil.java

@@ -0,0 +1,56 @@
+package com.example.bankcards.util;
+
+import java.util.Base64;
+import javax.crypto.Cipher;
+import javax.crypto.spec.SecretKeySpec;
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.stereotype.Component;
+
+@Component
+public class CardEncryptionUtil {
+
+    @Value("${app.card.encryption-key}")
+    private String encryptionKey;
+
+    private static final String ALGORITHM = "AES";
+
+    public String encrypt(String cardNumber) {
+        try {
+            SecretKeySpec keySpec = new SecretKeySpec(
+                encryptionKey.getBytes(),
+                ALGORITHM
+            );
+            Cipher cipher = Cipher.getInstance(ALGORITHM);
+            cipher.init(Cipher.ENCRYPT_MODE, keySpec);
+            byte[] encrypted = cipher.doFinal(cardNumber.getBytes());
+            return Base64.getEncoder().encodeToString(encrypted);
+        } catch (Exception e) {
+            throw new RuntimeException("Failed to encrypt card number", e);
+        }
+    }
+
+    public String decrypt(String encryptedCardNumber) {
+        try {
+            SecretKeySpec keySpec = new SecretKeySpec(
+                encryptionKey.getBytes(),
+                ALGORITHM
+            );
+            Cipher cipher = Cipher.getInstance(ALGORITHM);
+            cipher.init(Cipher.DECRYPT_MODE, keySpec);
+            byte[] decoded = Base64.getDecoder().decode(encryptedCardNumber);
+            byte[] decrypted = cipher.doFinal(decoded);
+            return new String(decrypted);
+        } catch (Exception e) {
+            throw new RuntimeException("Failed to decrypt card number", e);
+        }
+    }
+
+    public String mask(String cardNumber) {
+        if (cardNumber == null || cardNumber.length() < 4) {
+            return "****";
+        }
+        return (
+            "**** **** **** " + cardNumber.substring(cardNumber.length() - 4)
+        );
+    }
+}