using System;

using System.Text;

using Org.BouncyCastle.Crypto.Engines;

using Org.BouncyCastle.Crypto.Modes;

using Org.BouncyCastle.Crypto.Parameters;

using Org.BouncyCastle.Security;

​

public class AesGcmExample

{

    public static void Main(string[] args)

    {

        string plaintext = "This is a secret message.";

        byte[] key = GenerateRandomKey(256); // 256ビットキー

        byte[] iv = GenerateRandomIV(); // 96ビットIV

        byte[] associatedData = Encoding.UTF8.GetBytes("Associated Data");

​

        byte[] ciphertext = Encrypt(plaintext, key, iv, associatedData);

        string decryptedText = Decrypt(ciphertext, key, iv, associatedData);

​

        Console.WriteLine("Plaintext: " + plaintext);

        Console.WriteLine("Ciphertext: " + Convert.ToBase64String(ciphertext));

        Console.WriteLine("Decrypted Text: " + decryptedText);

    }

​

    public static byte[] Encrypt(string plaintext, byte[] key, byte[] iv, byte[] associatedData)

    {

        try

        {

            GcmBlockCipher cipher = new GcmBlockCipher(new AesEngine());

            AeadParameters parameters = new AeadParameters(new KeyParameter(key), 128, iv, associatedData);

            cipher.Init(true, parameters);

​

            byte[] plaintextBytes = Encoding.UTF8.GetBytes(plaintext);

            byte[] ciphertext = new byte[cipher.GetOutputSize(plaintextBytes.Length)];

            int len = cipher.ProcessBytes(plaintextBytes, 0, plaintextBytes.Length, ciphertext, 0);

            cipher.DoFinal(ciphertext, len);

​

            return ciphertext;

        }

        catch (Exception ex)

        {

            Console.WriteLine("Encryption error: " + ex.Message);

            return null;

        }

    }

​

    public static string Decrypt(byte[] ciphertext, byte[] key, byte[] iv, byte[] associatedData)

    {

        try

        {

            GcmBlockCipher cipher = new GcmBlockCipher(new AesEngine());

            AeadParameters parameters = new AeadParameters(new KeyParameter(key), 128, iv, associatedData);

            cipher.Init(false, parameters);

​

            byte[] plaintext = new byte[cipher.GetOutputSize(ciphertext.Length)];

            int len = cipher.ProcessBytes(ciphertext, 0, ciphertext.Length, plaintext, 0);

            cipher.DoFinal(plaintext, len);

​

            return Encoding.UTF8.GetString(plaintext, 0, len);

        }

        catch (Exception ex)

        {

            Console.WriteLine("Decryption error: " + ex.Message);

            return null;

        }

    }

​

    public static byte[] GenerateRandomKey(int keySize)

    {

        SecureRandom random = new SecureRandom();

        byte[] key = new byte[keySize / 8];

        random.NextBytes(key);

        return key;

    }

​

    public static byte[] GenerateRandomIV()

    {

        SecureRandom random = new SecureRandom();

        byte[] iv = new byte[12]; // 96ビットIV

        random.NextBytes(iv);

        return iv;

    }

}