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Below is a sample class showing how to encrypt and decrypt text when we have only password. It uses symmetric key algorithm.

using
System;
using System.Collections.Generic;
using System.Text;
using System.Security.Cryptography;
using System.IO; 

namespace EncryptionTester
{
  class RijndaelWrapper
  {
    // Retruns salt from password.
    private static byte[] Password2Salt(String password)

    {
      int saltLength = password.Length * ((int)password[0] % 3 + 2);
      saltLength = Math.Max(8, saltLength);
      byte[] resSalt = new byte[saltLength];
      for (int i = 0; i < saltLength; ++i)
        resSalt[i] =
          Convert.ToByte(password[i % password.Length] ^ (i + 17));
      return resSalt;
    } 

    // Returns symmetric algorithm with key, IV and salt extracted from
    //  password string.
    private static SymmetricAlgorithm GetAlgorithm(string password)
    {
      // Init key
      byte[] saltBytes = Password2Salt(password);
      Rfc2898DeriveBytes rfc2898 =
       new Rfc2898DeriveBytes(password, saltBytes);

      // Init algorithm
      SymmetricAlgorithm sa = new RijndaelManaged();
      sa.Key = rfc2898.GetBytes(sa.KeySize / 8);
      sa.IV = rfc2898.GetBytes(sa.BlockSize / 8);
      return sa;
    }

    // Encrypts text using password, returns encrypted string
    //  in base64 representation.
    public static String Encrypt(String text, String password)
    {
      SymmetricAlgorithm sa = GetAlgorithm(password);

      // Encrypt text
      ICryptoTransform ict = sa.CreateEncryptor();
      using (MemoryStream mso = new MemoryStream())
      {
        using (CryptoStream cs = new CryptoStream(mso, ict,
                                              CryptoStreamMode
.Write))
        {
          byte[] textByteArray = Encoding.UTF8.GetBytes(text);
          cs.Write(textByteArray, 0, textByteArray.Length);
        }
        // Convert encrypted text from bytes to base64 representation
        byte[] encryptedTextBytesArray = mso.ToArray();
        string base64EncryptedText =
             Convert
.ToBase64String(encryptedTextBytesArray);
        return base64EncryptedText;
      }
    }

    // Decrypts text in base64 representation (thats what Encrypt() returns)
    // using password,
returns decrypted string. Throws CryptographicException
    // in case of bad password, input text or
other errors.
    public static String Decrypt(String base64EncryptedText, String password)
    {
     SymmetricAlgorithm sa = GetAlgorithm(password);

      // Decrypt text
      ICryptoTransform ict = sa.CreateDecryptor();
      using (MemoryStream mso = new MemoryStream())
      {
        using (CryptoStream sc = new CryptoStream(mso, ict, 
                                      CryptoStreamMode
.Write))
        {
          byte[] base64EncryptedTextByteArray =  
                   Convert
.FromBase64String(base64EncryptedText);

          sc.Write(base64EncryptedTextByteArray, 0,  
                   base64EncryptedTextByteArray.Length);

        }
        // Return decrypted string
        return Encoding.UTF8.GetString(mso.ToArray());
      }
    }
  }
 

  class Program
  {
    static void Main(string[] args)
    {
      // Input from user
      string text = "a text we want to keep secret";
      string password = "ourpass";
 

      // Text to be sent over network, put to database, etc...
      string encryptedText = RijndaelWrapper.Encrypt(text, password);
 

      // Now we want to get decrypt text back
      string decryptedText = string.Empty;
      try
      {
        decryptedText = RijndaelWrapper.Decrypt(encryptedText, password);
      }
      catch (CryptographicException)
      {
        // Wrong password??
        decryptedText = "??";
      }
 

      // Output results
      Console.WriteLine("Text to encrypt       : " + text);
      Console.WriteLine("Base64 encrypted text : " + encryptedText);
      Console.WriteLine("Decrypted text        : " + decryptedText);
   
}
  }
}

Posted on Saturday, September 22, 2007 4:12 PM 70-536 preparation | Back to top


Comments on this post: .NET Symmetric encryption sample wrapper (Ch12)

# re: .NET Symmetric encryption sample wrapper (Ch12)
Requesting Gravatar...
Hi

you work ids great at the same time your code not work properly , because if after write to stream, you need flush to all block from the crytosteam

sc .FlushFinalBlock()

thank you
Left by RRaveen on Mar 22, 2009 12:38 PM

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