a short & complete class for text encryption, grapped from https://stackoverflow.com/questions/165808/simple-insecure-two-way-data-obfuscation#comment1320154_212707 in Ashkan Sirous' answer

TextEncryption.cs

using System;
using System.Security.Cryptography;
using System.Text;

namespace xxx.ClassLibrary
{
    public static class TextEncryption
    {
        private static TripleDESCryptoServiceProvider GetCryproProvider(string keyStr)
        {
            var md5 = new MD5CryptoServiceProvider();
            var key = md5.ComputeHash(Encoding.UTF8.GetBytes(keyStr));
            return new TripleDESCryptoServiceProvider() { Key = key, Mode = CipherMode.ECB, Padding = PaddingMode.PKCS7 };
        }

        public static string Encrypt(string plainString, string keyStr)
        {
            var data = Encoding.UTF8.GetBytes(plainString);
            var tripleDes = GetCryproProvider(keyStr);
            var transform = tripleDes.CreateEncryptor();
            var resultsByteArray = transform.TransformFinalBlock(data, 0, data.Length);
            return Convert.ToBase64String(resultsByteArray);
        }

        public static string Decrypt(string encryptedString, string keyStr)
        {
            var data = Convert.FromBase64String(encryptedString);
            var tripleDes = GetCryproProvider(keyStr);
            var transform = tripleDes.CreateDecryptor();
            var resultsByteArray = transform.TransformFinalBlock(data, 0, data.Length);
            return Encoding.UTF8.GetString(resultsByteArray);
        }
    }
}