using System;

using System.Security.Cryptography;

using System.Text;

using System.Linq;

​

public class Program

{

    private const string SALT_KEY = "@VietinbankNeverDie@2020";

    private static readonly byte[] SALT_BYTES = Encoding.UTF8.GetBytes(SALT_KEY);

    public const int KEY_BYTES = 16;

    public const int NONCE_BYTES = 12;

    public static void Main()

    {

        var text = "Hello world";

        var encrypted = EncryptAesGcmText(text);

        var decrypted = DecryptAesGcmText(encrypted);

        var isEqual = decrypted == text;

        Console.WriteLine("Is Equal {0}", isEqual);

    }

​

    public static string EncryptAesGcmText(string input)

    {

        try

        {

            var bytesToBeEncrypted = Encoding.UTF8.GetBytes(input);

            var bytesEncrypted = AesGcmEncrypt(bytesToBeEncrypted);

            var base64 = Convert.ToBase64String(bytesEncrypted);

            var text = Encoding.Default.GetString(bytesEncrypted);

            return base64;

        }

        catch (Exception)

        {

            return null;

        }

    }

​

    public static string DecryptAesGcmText(string input)

    {

        try

        {

            var bytesToBeDecrypted = Convert.FromBase64String(input);

            var bytesDecrypted = AesGcmDecrypt(bytesToBeDecrypted);

            return Encoding.UTF8.GetString(bytesDecrypted);

        }

        catch (Exception)

        {

            return null;

        }

    }

​

    private static byte[] AesGcmEncrypt(byte[] bytesToBeEncrypted)

    {

        var tag = new byte[KEY_BYTES];

        var nonce = new byte[NONCE_BYTES];

        var cipherText = new byte[bytesToBeEncrypted.Length];

        using var cipher = new AesGcm(SALT_BYTES);

        cipher.Encrypt(nonce, bytesToBeEncrypted, cipherText, tag, null);

        return ConcatArrays(tag, ConcatArrays(nonce, cipherText));

    }

​

    private static byte[] AesGcmDecrypt(byte[] bytesToBeDecrypted)

    {

        var tag = SubArray(0, KEY_BYTES, bytesToBeDecrypted);

        var nonce = SubArray(KEY_BYTES, NONCE_BYTES, bytesToBeDecrypted);

        var toDecrypt = SubArray(KEY_BYTES + NONCE_BYTES, bytesToBeDecrypted.Length - tag.Length - nonce.Length, bytesToBeDecrypted);

        var decryptedData = new byte[toDecrypt.Length];

        using var cipher = new AesGcm(SALT_BYTES);

        cipher.Decrypt(nonce, toDecrypt, tag, decryptedData, null);

        return decryptedData;

    }

​

    public static T[] ConcatArrays<T>(params T[][] arrays)

    {

        var position = 0;

        var outputArray = new T[arrays.Sum(array => array.Length)];

        foreach (var array in arrays)

        {

            Array.Copy(array, 0, outputArray, position, array.Length);

            position += array.Length;

        }

​

        return outputArray;

    }

​

    public static T[] SubArray<T>(int start, int length, params T[] data)

    {

        start = start >= 0 ? start : 0;

        length = length >= start || length <= data.Length ? length : data.Length;

        var result = new T[length];

        Array.Copy(data, start, result, 0, length);

        return result;

    }

}