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1import javax.crypto.*;
2import javax.crypto.spec.GCMParameterSpec;
3import java.nio.ByteBuffer;
4import java.nio.charset.StandardCharsets;
5import java.security.SecureRandom;
6import java.util.Arrays;
7import javax.crypto.spec.SecretKeySpec;
8
9public class Crypto{
10    public static final int AES_KEY_SIZE = 128; // in bits
11    public static final int GCM_NONCE_LENGTH = 12; // in bytes
12    public static final int GCM_TAG_LENGTH = 16; // in bytes
13    public static final String SECRET_KEY = "5508805566732388";
14
15    public static void main(String[] args)throws Exception{
16        String plainText = "Hello there!";
17        byte[] plainTextBytes = plainText.getBytes();
18
19        Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding", "SunJCE");
20
21        SecureRandom random = SecureRandom.getInstanceStrong();
22        byte[] iv = new byte[GCM_NONCE_LENGTH];
23        random.nextBytes(iv);
24        GCMParameterSpec spec = new GCMParameterSpec(GCM_TAG_LENGTH * 8, iv);
25
26        byte[] secretKeyBytes = SECRET_KEY.getBytes();
27        SecretKey key = new SecretKeySpec(secretKeyBytes, 0, secretKeyBytes.length, "AES"); 
28
29        //
30        System.out.println("Before encryption");
31        System.out.println("Plain text is: " + new String(plainTextBytes, StandardCharsets.UTF_8));
32        System.out.println("Iv is: " + new String(iv, StandardCharsets.UTF_8));
33        //
34
35        //Encryption
36        cipher.init(cipher.ENCRYPT_MODE, key, spec);
37        cipher.updateAAD(iv);
38        byte[] encryptedData = cipher.doFinal(plainTextBytes);
39        
40        byte[] authTag = Arrays.copyOfRange (encryptedData, encryptedData.length-16, encryptedData.length);
41        
42        //
43        System.out.println("After encryption");
44        System.out.println("Encrypted data is: " + new String(encryptedData, StandardCharsets.UTF_8));
45        System.out.println("Iv is: " + new String(iv, StandardCharsets.UTF_8));
46        System.out.println("authTag is: " + new String(authTag, StandardCharsets.UTF_8));
47        //
48
49        //Decryption
50        cipher.init(cipher.DECRYPT_MODE, key, spec);
51        cipher.updateAAD(iv);
52        byte[] decryptedData = cipher.doFinal(encryptedData);
53
54        //
55        System.out.println("After Decryption");
56        System.out.println("decrypted data is: " + new String(decryptedData, StandardCharsets.UTF_8));
57        //
58    }
59}