using System;

using System.Security.Cryptography;

using System.Text;

​

public class Program

{

        private static readonly string _key = "~ineedtochangethiskeybeacauseeveryoneknowsit~";

​

        // We can use a fixed pepper for the PBKDF2 salt.

        private static readonly byte[] _pepper = new byte[] { 0x04, 0xC5, 0x02, 0xF8, 0xD3, 0xD4, 0x23, 0xB9 };

    public static void Main()

    {

        Console.WriteLine("Input encrypted string:");

        var input = Console.ReadLine();

        Console.WriteLine(Decrypt(input));

    }

        public static string Decrypt(string encryptedText)

        {

            if (encryptedText == null) return null;

            if (encryptedText == "") return "";

​

            var ivAndCiphertext = Convert.FromBase64String(encryptedText);

            if (ivAndCiphertext.Length >= 16)

            {

                var iv = new byte[16];

                var ciphertext = new byte[ivAndCiphertext.Length - 16];

                Array.Copy(ivAndCiphertext, 0, iv, 0, iv.Length);

                Array.Copy(ivAndCiphertext, iv.Length, ciphertext, 0, ciphertext.Length);

​

                using var aes = Aes.Create();

                using var pbkdf2 = new Rfc2898DeriveBytes(_key, _pepper, 32767);

                var key = pbkdf2.GetBytes(32);

​

                aes.Mode = CipherMode.CBC;

                aes.Padding = PaddingMode.PKCS7;

                aes.Key = key;

                aes.IV = iv;

​

                // create a new Decryptor

                using var aesTransformer = aes.CreateDecryptor();

                // get a byte array of the plain text

                var plaintext = aesTransformer.TransformFinalBlock(ciphertext, 0, ciphertext.Length);

​

                // set the return value

                return Encoding.UTF8.GetString(plaintext);

            }

​

            throw new ArgumentException("Failed to decrypt string, it is not a valid encrypted string.");

        }

}