Un po' di esempi su come cifrare e decifrare dati in .NET:

using System;
using System.Collections;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Security.Cryptography;
using System.Text;
 
namespace TestEncryptDecrypt
{
   class Program
   {
      const String PASSWORD_HASH = "p@ssW0rd";
      const String SALT_KEY = "s@l7_K&y";
      const String VI_KEY = "@!2#&46%%64!@";
      const String TRIPLE_DES_KEY = "23jiefjneu5474bcaio48rhe";
      const String RSA_PRIVATE_KEY = "" +
         "wVe/eITmOuNLD1oSUrOsQHvG11m/kQpxkbAPMZxXGNDmKqdLj3BdRYp35hjNGh" + 
         "CNR4l2dGy4+Y9HVo+tHwHiuoBoSYm3/79C2Ars3mue+MkWZ6gCdn21WHtUmMOLtqeNLrKzX" + 
         "WpihI7MzZbK9bFfHK9f4w+O/ssuGo2628VYE2M=" +
         "AQAB" +
         "
7Tqlq/oXjTHWzmhNzbNOnBRaKotYZC/p4584ZSvlGjVkG7vdvuWFRe6gMkaOA1Zc9VVP" + 
         "pd+jHi6ihYN/WZbtKw==
" +
         "0KQhxIC0zSQJKM2736fm8tiupnS9bTF/rg5YiY4NTbKLMNwJsg8rTWl27+knS6QDBUu/u" + 
         "u182aTa6hPzozOGqQ==" +
         "LiyAGp0vAFh3Cu3aVvNTlaWQdz9wEPASgi93dpm4mgVSWuOSIZni+t2baK267oNczJOy" + 
         "UQLaaQGCuKIDo5hwyQ==" +
         "qMbNRh9zoqru+eILg65YJcTg62huP+GVzqBuQL/3xQ2UiW7GLRMWTWXuP77U+dxwUR1o" + 
         "uPcaUxmHuWlwYmnxGQ==" +
         "etPo41x9UGcYrsVaMeMsTTvUT9l5bYFigkWfkP1nGHwlzi/7FJjs38D6KoglPd" + 
         "eU7HYhnc76ud3VMDBbXldIZw==" +
         "MgsHSSeE++sG65NRuMw+onYnMmhNeDouyRm75QhlVswDNzkUCq13xt8fR5RVyhBMsR54Z" + 
         "YwamKWoPlRz59myGEZqFx13RM0ztMZNhWU13ts4CA8401LnaqhZjmJcZSJgIDfKlc+YyMyGG" + 
         "g+2e4S8NEfIOLw7+/PSuWYqnh2C4rE=" +
         "";
      const String RSA_PUBLIC_KEY = "" + 
         "wVe/eITmOuNLD1oSUrOsQHvG11m/kQpxkbAPMZxXGNDmKqdLj3BdRYp35hjNGhCN" + 
         "R4l2dGy4+Y9HVo+tHwHiuoBoSYm3/79C2Ars3mue+MkWZ6gCdn21WHtUmMOLtqeNLrKzXWpih" + 
         "I7MzZbK9bFfHK9f4w+O/ssuGo2628VYE2M=" + 
         "AQAB" + 
         "";
 
      static void Main(string[] args)
      {
 
         String plainText = "prova di cifratura stringa 1 &^'-_=";
 
         String encrypt = EncryptRijndael(plainText);
 
         String decrypt = DecryptRijndael(encrypt);
 
         encrypt = EncryptTripleDES(plainText);
 
         decrypt = DecryptTripleDES(encrypt);
 
         encrypt = EncryptRSA(plainText);
 
         decrypt = DecryptRSA(encrypt);
 
         String hash = GetMd5Hash(plainText);
      }
 
      public static string EncryptRijndael(string plainText)
      {
         byte[] plainTextBytes = Encoding.UTF8.GetBytes(plainText);
 
         using (Rfc2898DeriveBytes rfc2898DeriveBytes = 
            new Rfc2898DeriveBytes(PASSWORD_HASH, Encoding.ASCII.GetBytes(SALT_KEY)))
         {
            byte[] keyBytes = rfc2898DeriveBytes.GetBytes(256 / 8);
 
            using (var symmetricKey = 
               new RijndaelManaged() { Mode = CipherMode.CBC, Padding = PaddingMode.Zeros })
            {
               var encryptor = symmetricKey.CreateEncryptor(keyBytes, Encoding.ASCII.GetBytes(VI_KEY));
 
               byte[] cipherTextBytes;
 
               using (var memoryStream = new MemoryStream())
               {
                  using (var cryptoStream = 
                     new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write))
                  {
                     cryptoStream.Write(plainTextBytes, 0, plainTextBytes.Length);
 
                     cryptoStream.FlushFinalBlock();
 
                     cipherTextBytes = memoryStream.ToArray();
                  }
               }
               return Convert.ToBase64String(cipherTextBytes);
            }
         }
      }
 
      public static string DecryptRijndael(string encryptedText)
      {
         byte[] cipherTextBytes = Convert.FromBase64String(encryptedText);
         using (Rfc2898DeriveBytes rfc2898DeriveBytes = 
            new Rfc2898DeriveBytes(PASSWORD_HASH, Encoding.ASCII.GetBytes(SALT_KEY)))
         {
            byte[] keyBytes = rfc2898DeriveBytes.GetBytes(256 / 8);
 
            using (var symmetricKey = 
               new RijndaelManaged() { Mode = CipherMode.CBC, Padding = PaddingMode.None })
            {
               var decryptor = symmetricKey.CreateDecryptor(keyBytes, Encoding.ASCII.GetBytes(VI_KEY));
 
               using (var memoryStream = new MemoryStream(cipherTextBytes))
               {
                  using (var cryptoStream = 
                     new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Read))
                  {
                     byte[] plainTextBytes = new byte[cipherTextBytes.Length];
 
                     int decryptedByteCount = cryptoStream.Read(plainTextBytes, 0, plainTextBytes.Length);
 
                     return Encoding.UTF8.GetString(plainTextBytes, 0, decryptedByteCount).TrimEnd("\0".ToCharArray());
                  }
               }
            }
         }
      }
 
      public static string EncryptTripleDES(string plainText)
      {
         using (TripleDESCryptoServiceProvider des = 
            new TripleDESCryptoServiceProvider())
         {
            des.Mode = CipherMode.ECB;
            des.Key = System.Text.Encoding.ASCII.GetBytes(TRIPLE_DES_KEY);
 
            des.Padding = PaddingMode.PKCS7;
            using (ICryptoTransform DESEncrypt = des.CreateEncryptor())
            {
               Byte[] Buffer = ASCIIEncoding.ASCII.GetBytes(plainText);
 
               return Convert.ToBase64String(DESEncrypt.TransformFinalBlock(Buffer, 0, Buffer.Length));
            }
         }
      }
 
      public static string DecryptTripleDES(string encryptedText)
      {
         using (TripleDESCryptoServiceProvider des = 
            new TripleDESCryptoServiceProvider())
         {
            des.Mode = CipherMode.ECB;
            des.Key = System.Text.Encoding.ASCII.GetBytes(TRIPLE_DES_KEY);
 
            des.Padding = PaddingMode.PKCS7;
            using (ICryptoTransform DESEncrypt = des.CreateDecryptor())
            {
               Byte[] Buffer = Convert.FromBase64String(encryptedText.Replace(" ", "+"));
 
               return Encoding.UTF8.GetString(DESEncrypt.TransformFinalBlock(Buffer, 0, Buffer.Length));
            }
         }
      }
 
      static String EncryptRSA(String plainText)
      {
         Byte[] bytes = System.Text.Encoding.Unicode.GetBytes(plainText);
 
         using (RSACryptoServiceProvider rsa = new RSACryptoServiceProvider())
         {
            rsa.FromXmlString(RSA_PUBLIC_KEY);
            return Convert.ToBase64String(rsa.Encrypt(bytes, false));
         }
      }
 
      static String DecryptRSA(String ciphertext)
      {
         using (RSACryptoServiceProvider rsa = new RSACryptoServiceProvider())
         {
            rsa.FromXmlString(RSA_PRIVATE_KEY);
 
            Byte[] decrypt = rsa.Decrypt(Convert.FromBase64String(ciphertext), false);
 
            return System.Text.Encoding.Unicode.GetString(decrypt);
         }
      }
 
      static string GetMd5Hash(string input)
      {
         using (MD5 md5Hash = MD5.Create())
         {
            byte[] data = md5Hash.ComputeHash(Encoding.UTF8.GetBytes(input));
 
            StringBuilder sBuilder = new StringBuilder();
 
            for (int i = 0; i < data.Length; i++)
               sBuilder.Append(data[i].ToString("x2"));
 
            return sBuilder.ToString();
         }
      }
   }
}