using System;

using System.Security.Cryptography;

​

public class Program

{

    static void Main()

    {

        var key = GetRandomData(256 / 8);

        var data = new byte[]{1, 2, 3};

        Console.WriteLine(Convert.ToBase64String(data));

        byte[] associatedData = null; // Optional. If possible, provide something associated to the current context such as the current user id https://crypto.stackexchange.com/a/6716/16630

        var encryptedAesGcmData = EncryptAesGcm(data, key, associatedData, out var tag, out var nonce);

        Console.WriteLine(Convert.ToBase64String(encryptedAesGcmData));

        var decryptedAesGcmData = DecryptAesGcm(encryptedAesGcmData, key, associatedData, tag, nonce);

        Console.WriteLine(Convert.ToBase64String(decryptedAesGcmData));

    }

​

    private static byte[] GetRandomData(int length)

    {

        return RandomNumberGenerator.GetBytes(length);

    }

​

    private static byte[] EncryptAesGcm(byte[] data, byte[] key, byte[] associatedData, out byte[] tag, out byte[] nonce)

    {

        tag = new byte[AesGcm.TagByteSizes.MaxSize];

        nonce = new byte[AesGcm.NonceByteSizes.MaxSize];

        RandomNumberGenerator.Fill(nonce);

        byte[] cipherText = new byte[data.Length];

        using var cipher = new AesGcm(key);

        cipher.Encrypt(nonce, data, cipherText, tag, associatedData);

        return cipherText;

    }

​

    private static byte[] DecryptAesGcm(byte[] data, byte[] key, byte[] associatedData, byte[] tag, byte[] nonce)

    {

        byte[] decryptedData = new byte[data.Length];

        using var cipher = new AesGcm(key);

        cipher.Decrypt(nonce, data, tag, decryptedData, associatedData);

        return decryptedData;

    }

}