Encriptar/Decriptar cadenas con Base64 en C#

Me he encontrado un codigo para encriptar y decriptar cadenas con Base64:

static class EncryptClass
{

    // http://www.codeproject.com/KB/cs/Cryptography.aspx

    /// 

    /// Encrypt a string using dual encryption method. Return a encrypted cipher Text
    /// 
    /// string to be encrypted
    /// use hashing? send to for extra secirity
    /// 
    public static string Encrypt(string toEncrypt, bool useHashing)
    {
        byte[] keyArray;
        byte[] toEncryptArray = UTF8Encoding.UTF8.GetBytes(toEncrypt);

        string key = Properties.Settings.Default.SecurityKey; //<-- crealo en tu configuracion

        if (useHashing)
        {
            MD5CryptoServiceProvider hashmd5 = new MD5CryptoServiceProvider();
            keyArray = hashmd5.ComputeHash(UTF8Encoding.UTF8.GetBytes(key));
            hashmd5.Clear();
        }
        else
            keyArray = UTF8Encoding.UTF8.GetBytes(key);

        TripleDESCryptoServiceProvider tdes = new TripleDESCryptoServiceProvider();
        tdes.Key = keyArray;
        tdes.Mode = CipherMode.ECB;
        tdes.Padding = PaddingMode.PKCS7;

        ICryptoTransform cTransform = tdes.CreateEncryptor();
        byte[] resultArray = cTransform.TransformFinalBlock(toEncryptArray, 0, toEncryptArray.Length);
        tdes.Clear();
        return Convert.ToBase64String(resultArray, 0, resultArray.Length);
    }
    /// 

    /// DeCrypt a string using dual encryption method. Return a DeCrypted clear string
    /// 
    /// encrypted string
    /// Did you use hashing to encrypt this data? pass true is yes
    /// 
    public static string Decrypt(string cipherString, bool useHashing)
    {
        byte[] keyArray;
        byte[] toEncryptArray = {0};

        try
        {
            /*
             * La longitud de cadena enritptada con Hashing es de 24
             * si es menor de 24 lanza una excepcion, asi que hay que pasar de ella
             */
            toEncryptArray = Convert.FromBase64String(cipherString);
        }
        catch (Exception)
        {
            return null;
        }

        string key = Properties.Settings.Default.SecurityKey; 

        if (useHashing)
        {
            MD5CryptoServiceProvider hashmd5 = new MD5CryptoServiceProvider();
            keyArray = hashmd5.ComputeHash(UTF8Encoding.UTF8.GetBytes(key));
            hashmd5.Clear();
        }
        else
            keyArray = UTF8Encoding.UTF8.GetBytes(key);

        TripleDESCryptoServiceProvider tdes = new TripleDESCryptoServiceProvider();
        tdes.Key = keyArray;
        tdes.Mode = CipherMode.ECB;
        tdes.Padding = PaddingMode.PKCS7;

        ICryptoTransform cTransform = tdes.CreateDecryptor();
        byte[] resultArray = cTransform.TransformFinalBlock(toEncryptArray, 0, toEncryptArray.Length);

        tdes.Clear();
        return UTF8Encoding.UTF8.GetString(resultArray);
    }
}

Creas un valor de configuracion llamado SecurityKey en tu proyecto y le asignas un valor aleatorio

Fuente: http://www.codeproject.com/KB/cs/Cryptography.aspx

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