AES-GCM 仕組みの解説とJavaサンプルコード

import javax.crypto.Cipher;
import javax.crypto.KeyGenerator;
import javax.crypto.SecretKey;
import javax.crypto.spec.GCMParameterSpec;
import javax.crypto.spec.SecretKeySpec;
import java.nio.charset.StandardCharsets;
import java.security.SecureRandom;
import java.util.Base64;

public class AesGcmExample
{
    public static final int AES_KEY_BIT_LENGTH = 256; // Must be one of {128, 192, 256}
    public static final int GCM_NONCE_LENGTH = 12; // Nonce length
    public static final int GCM_TAG_BIT_LENGTH = 128; // Must be one of {128, 120, 112, 104, 96}

    private static String plainText = "This is a plain text which need to be encrypted by Java AES 256 GCM Encryption Algorithm";

    public static void main(String[] args) throws Exception {
        KeyGenerator keyGenerator = KeyGenerator.getInstance("AES");
        keyGenerator.init(AES_KEY_BIT_LENGTH);

        // Generate Key
        SecretKey key = keyGenerator.generateKey();
        byte[] nonce = new byte[GCM_NONCE_LENGTH];
        // Generate nonce ([IMPORTANT] ABSOLUTELY USE RANDOM VALUE EACH TIME!!!)
        SecureRandom random = new SecureRandom();
        random.nextBytes(nonce);
        // Generate AAD (AAD length must be less than 64KiB)
        byte[] aad = "This is an additional authenticated data. It's called the AAD. You can use any value."
                .getBytes(StandardCharsets.UTF_8);

        System.out.println("Original Text : " + plainText);

        byte[] encryptedData = encrypt(plainText.getBytes(), key, nonce, aad);
        System.out.println("Encrypted Text : " + Base64.getEncoder().encodeToString(encryptedData));

        byte[] decryptedData = decrypt(encryptedData, key, nonce, aad);
        System.out.println("Decrypted Text : " + new String(decryptedData, StandardCharsets.UTF_8));
    }

    public static byte[] encrypt(byte[] plainData, SecretKey key, byte[] nonce, byte[] aad) throws Exception
    {
        // Get Cipher Instance
        Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");

        // Create SecretKeySpec
        SecretKeySpec keySpec = new SecretKeySpec(key.getEncoded(), "AES");

        // Create GCMParameterSpec
        GCMParameterSpec gcmParameterSpec = new GCMParameterSpec(GCM_TAG_BIT_LENGTH, nonce);

        // Initialize Cipher for ENCRYPT_MODE
        cipher.init(Cipher.ENCRYPT_MODE, keySpec, gcmParameterSpec);

        if (aad != null) {
            // Update AAD to do additional auth (Optional)
            cipher.updateAAD(aad);
        }

        // Perform Encryption
        byte[] cipherData = cipher.doFinal(plainData);

        return cipherData;
    }

    public static byte[] decrypt(byte[] cipherData, SecretKey key, byte[] nonce, byte[] aad) throws Exception
    {
        // Get Cipher Instance
        Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");

        // Create SecretKeySpec
        SecretKeySpec keySpec = new SecretKeySpec(key.getEncoded(), "AES");

        // Create GCMParameterSpec
        GCMParameterSpec gcmParameterSpec = new GCMParameterSpec(GCM_TAG_BIT_LENGTH, nonce);

        // Initialize Cipher for DECRYPT_MODE
        cipher.init(Cipher.DECRYPT_MODE, keySpec, gcmParameterSpec);

        if (aad != null) {
            // Update AAD to verify additional auth (Optional)
            cipher.updateAAD(aad);
        }

        // Perform Decryption
        byte[] decryptedData = cipher.doFinal(cipherData);

        return decryptedData;
    }
}