First create class : encrypt_decrypt.cs
inside:

using System;
using System.Security.Cryptography;
using System.Text;
 
namespace RestWebinarSample
{
 
    class Encryptor
    {
        public static string IV = "1a1a1a1a1a1a1a1a";
        public static string Key = "1a1a1a1a1a1a1a1a1a1a1a1a1a1a1a13";
 
        public static string Encrypt(string decrypted)
        {
            byte[] textbytes = ASCIIEncoding.ASCII.GetBytes(decrypted);
            AesCryptoServiceProvider endec = new AesCryptoServiceProvider();
            endec.BlockSize = 128;
            endec.KeySize = 256;
            endec.IV = ASCIIEncoding.ASCII.GetBytes(IV);
            endec.Key = ASCIIEncoding.ASCII.GetBytes(Key);
            endec.Padding = PaddingMode.PKCS7;
            endec.Mode = CipherMode.CBC;
            ICryptoTransform icrypt = endec.CreateEncryptor(endec.Key, endec.IV);
            byte[] enc = icrypt.TransformFinalBlock(textbytes, 0, textbytes.Length);
            icrypt.Dispose();
            return Convert.ToBase64String(enc);
        }
 
        public static string Decrypted(string encrypted)
        {
            byte[] textbytes = Convert.FromBase64String(encrypted);
            AesCryptoServiceProvider endec = new AesCryptoServiceProvider();
            endec.BlockSize = 128;
            endec.KeySize = 256;
            endec.IV = ASCIIEncoding.ASCII.GetBytes(IV);
            endec.Key = ASCIIEncoding.ASCII.GetBytes(Key);
            endec.Padding = PaddingMode.PKCS7;
            endec.Mode = CipherMode.CBC;
            ICryptoTransform icrypt = endec.CreateDecryptor(endec.Key, endec.IV);
            byte[] enc = icrypt.TransformFinalBlock(textbytes, 0, textbytes.Length);
            icrypt.Dispose();
            return System.Text.ASCIIEncoding.ASCII.GetString(enc);
        }
    }
}
 

then 2 buttons encrypt and decrypt

private void encrypt_Click(object sender, EventArgs e)
{
 
    Directory.CreateDirectory("data\\");
 
    var sw = new StreamWriter("data\\" + "data.ls");
 
    string enctxt = Encryptor.Encrypt("willbeencrypted");
 
    sw.WriteLine(enctxt);
 
    sw.Close();
}
 
private void decrypt_Click(object sender, EventArgs e)
{
    StreamReader sr = new StreamReader(Application.StartupPath + "\\data\\" + "data.ls");
    string line = sr.ReadLine();
 
    MessageBox.Show(Encryptor.Decrypted(Convert.ToString(line)));
}