Java AES Encryption and Decryption Example

Below is a complete example of AES (Advanced Encryption Standard) encryption and decryption in Java using the Java Cryptography Extension (JCE). AES is a symmetric encryption algorithm, meaning the same key is used for both encryption and decryption.

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AES Encryption and Decryption Example

import javax.crypto.Cipher;
import javax.crypto.KeyGenerator;
import javax.crypto.SecretKey;
import javax.crypto.spec.SecretKeySpec;
import java.util.Base64;

public class AESExample {
    public static void main(String[] args) throws Exception {
        // Step 1: Generate a secret key
        SecretKey secretKey = generateAESKey();
        System.out.println("Generated Key: " + Base64.getEncoder().encodeToString(secretKey.getEncoded()));

        // Step 2: Encrypt a message
        String originalMessage = "Hello, AES Encryption!";
        String encryptedMessage = encrypt(originalMessage, secretKey);
        System.out.println("Encrypted Message: " + encryptedMessage);

        // Step 3: Decrypt the message
        String decryptedMessage = decrypt(encryptedMessage, secretKey);
        System.out.println("Decrypted Message: " + decryptedMessage);
    }

    // Generate a 128-bit AES key
    public static SecretKey generateAESKey() throws Exception {
        KeyGenerator keyGen = KeyGenerator.getInstance("AES");
        keyGen.init(128); // Key size (128, 192, or 256 bits)
        return keyGen.generateKey();
    }

    // Encrypt a message using AES
    public static String encrypt(String message, SecretKey secretKey) throws Exception {
        Cipher cipher = Cipher.getInstance("AES");
        cipher.init(Cipher.ENCRYPT_MODE, secretKey);
        byte[] encryptedBytes = cipher.doFinal(message.getBytes());
        return Base64.getEncoder().encodeToString(encryptedBytes); // Encode to Base64 for readability
    }

    // Decrypt a message using AES
    public static String decrypt(String encryptedMessage, SecretKey secretKey) throws Exception {
        Cipher cipher = Cipher.getInstance("AES");
        cipher.init(Cipher.DECRYPT_MODE, secretKey);
        byte[] decryptedBytes = cipher.doFinal(Base64.getDecoder().decode(encryptedMessage));
        return new String(decryptedBytes);
    }
}

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Explanation of the Code

1. Key Generation:

• A 128-bit AES key is generated using KeyGenerator.
• The key is encoded to a Base64 string for easy storage or transmission.

1. Encryption:

• The Cipher class is initialized in ENCRYPT_MODE with the secret key.
• The message is encrypted and encoded to a Base64 string for readability.

1. Decryption:

• The Cipher class is initialized in DECRYPT_MODE with the same secret key.
• The encrypted message is decoded from Base64 and decrypted back to the original message.

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Output Example

Generated Key: 3q2+7w==
Encrypted Message: 5f4dcc3b5aa765d61d8327deb882cf99
Decrypted Message: Hello, AES Encryption!

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Key Points

1. Key Size:

• AES supports key sizes of 128, 192, or 256 bits. Ensure your Java environment supports the desired key size (e.g., install the Unlimited Strength Jurisdiction Policy Files for 256-bit keys).

1. Padding and Mode:

• By default, AES uses ECB mode and PKCS5Padding. You can specify these explicitly:
  java Cipher cipher = Cipher.getInstance("AES/ECB/PKCS5Padding");
• For better security, use CBC (Cipher Block Chaining) mode with an initialization vector (IV).

1. Base64 Encoding:

• Encrypted data is binary, so it is encoded to Base64 for easy storage or transmission.

1. Security Considerations:

• Securely store and manage the secret key.
• Avoid hardcoding keys in your code.
• Use secure random number generators for key generation.

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Using AES with CBC Mode and IV

For better security, use CBC mode with an Initialization Vector (IV). Here’s an example:

import javax.crypto.Cipher;
import javax.crypto.KeyGenerator;
import javax.crypto.SecretKey;
import javax.crypto.spec.IvParameterSpec;
import java.security.SecureRandom;
import java.util.Base64;

public class AESCBCExample {
    public static void main(String[] args) throws Exception {
        // Generate a secret key
        SecretKey secretKey = generateAESKey();
        System.out.println("Generated Key: " + Base64.getEncoder().encodeToString(secretKey.getEncoded()));

        // Generate a random IV
        byte[] iv = generateIV();
        System.out.println("Generated IV: " + Base64.getEncoder().encodeToString(iv));

        // Encrypt a message
        String originalMessage = "Hello, AES CBC Mode!";
        String encryptedMessage = encrypt(originalMessage, secretKey, iv);
        System.out.println("Encrypted Message: " + encryptedMessage);

        // Decrypt the message
        String decryptedMessage = decrypt(encryptedMessage, secretKey, iv);
        System.out.println("Decrypted Message: " + decryptedMessage);
    }

    // Generate a 128-bit AES key
    public static SecretKey generateAESKey() throws Exception {
        KeyGenerator keyGen = KeyGenerator.getInstance("AES");
        keyGen.init(128);
        return keyGen.generateKey();
    }

    // Generate a random IV
    public static byte[] generateIV() {
        byte[] iv = new byte[16]; // 16 bytes for AES
        new SecureRandom().nextBytes(iv);
        return iv;
    }

    // Encrypt a message using AES in CBC mode
    public static String encrypt(String message, SecretKey secretKey, byte[] iv) throws Exception {
        Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
        cipher.init(Cipher.ENCRYPT_MODE, secretKey, new IvParameterSpec(iv));
        byte[] encryptedBytes = cipher.doFinal(message.getBytes());
        return Base64.getEncoder().encodeToString(encryptedBytes);
    }

    // Decrypt a message using AES in CBC mode
    public static String decrypt(String encryptedMessage, SecretKey secretKey, byte[] iv) throws Exception {
        Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
        cipher.init(Cipher.DECRYPT_MODE, secretKey, new IvParameterSpec(iv));
        byte[] decryptedBytes = cipher.doFinal(Base64.getDecoder().decode(encryptedMessage));
        return new String(decryptedBytes);
    }
}

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Output Example (CBC Mode)

Generated Key: 3q2+7w==
Generated IV: 5f4dcc3b5aa765d61d8327deb882cf99
Encrypted Message: 5f4dcc3b5aa765d61d8327deb882cf99
Decrypted Message: Hello, AES CBC Mode!

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