Rijndael cryptography encryption

Question

I have the sample code using C# .NET Rijndael cryptography encryption, which can decrypt ciphertext "FGgsUwQ1gToX40dfgdfgfdgdfgdfCzEr3wOenxTPMN9jJC" to plain "75.117.87.87".

enter image description here

I have tried to write the reserved method to encrypt plain text "75.117.87.87" to "FGgsUwQ1gToX40dfgdfgfdgdfgdfCzEr3wOenxTPMN9jJC" but not successful (encrypted text is not as same as expectation - , but the result likes in the attached image . Please help me, thank you.

static int Main(string[] args)
    {
        Decrypt();
        Console.WriteLine("########################");
        Console.WriteLine("########################");
        Encrypt();
        Console.ReadLine();
        return 0;
    }
public static void Decrypt()
    {
        string ciphertext = "LkbsUwQ1gToX40dYXizgc0BCCCzEr3wOenxTPMN9jJC";
        string secretkey = "11111111";
        string saltkey = "thekey";

        string result =  rijndaelDecrypt(ciphertext, secretkey, saltkey);

        Console.WriteLine("Cipher text: " + ciphertext);
        Console.WriteLine("Decrypted text: " + result);
        return;
    }
public static string  rijndaelDecrypt(string ciphertext, string secretkey, string saltkey)
    {
        if (string.IsNullOrEmpty(ciphertext))
        {
            return null;
        }
        if (string.IsNullOrEmpty(secretkey))
        {
            return null;
        }

        ciphertext = "EAAAA" + ciphertext;

        RijndaelManaged rijndaelManaged = null;
        string result = null;
        try
        {
            Rfc2898DeriveBytes rfc2898DeriveBytes = new Rfc2898DeriveBytes(secretkey, Encoding.ASCII.GetBytes(saltkey));
            byte[] buffer = Convert.FromBase64String(ciphertext);
            MemoryStream memoryStream = new MemoryStream(buffer);
            rijndaelManaged = new RijndaelManaged();
            rijndaelManaged.Key = rfc2898DeriveBytes.GetBytes(rijndaelManaged.KeySize / 8);
            rijndaelManaged.IV = rb(memoryStream);
            ICryptoTransform transform = rijndaelManaged.CreateDecryptor(rijndaelManaged.Key, rijndaelManaged.IV);
            CryptoStream cryptoStream = new CryptoStream(memoryStream, transform, CryptoStreamMode.Read);
            StreamReader streamReader = new StreamReader(cryptoStream);
            result = streamReader.ReadToEnd();
        }
        catch (Exception)
        {
            if (rijndaelManaged != null)
            {
                rijndaelManaged.Clear();
            }
            return null;
        }
        finally
        {
            if (rijndaelManaged != null)
            {
                rijndaelManaged.Clear();
            }
        }
        return result;
    }
public static void Encrypt()
    {
        string ciphertext = "75.117.87.87";
        string secretkey = "1111111";
        string saltkey = "thekey";
        string result;
        RijndaelManaged rijndaelManaged = new RijndaelManaged();

        Rfc2898DeriveBytes rfc2898DeriveBytes = new Rfc2898DeriveBytes(secretkey, Encoding.ASCII.GetBytes(saltkey));

        rijndaelManaged.Key = rfc2898DeriveBytes.GetBytes(rijndaelManaged.KeySize / 8);

        byte[] buffer = EncodeToBytes(ciphertext);
        MemoryStream memoryStream = new MemoryStream(buffer);
        rijndaelManaged.IV = rfc2898DeriveBytes.GetBytes(rijndaelManaged.BlockSize / 8);
        //rijndaelManaged.IV = rb(memoryStream);

        ICryptoTransform transform = rijndaelManaged.CreateEncryptor(rijndaelManaged.Key, rijndaelManaged.IV);
        CryptoStream cryptoStream = new CryptoStream(memoryStream, transform, CryptoStreamMode.Read);
        StreamReader streamReader = new StreamReader(cryptoStream);
        result = streamReader.ReadToEnd();


        var encryptor = rijndaelManaged.CreateEncryptor(rijndaelManaged.Key, rijndaelManaged.IV);
        var msEncrypt = new MemoryStream();

        using (var csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write))
        using (var swEncrypt = new StreamWriter(csEncrypt))
            swEncrypt.Write(result);

        result = Convert.ToBase64String(msEncrypt.ToArray());

        Console.WriteLine("Plain text: " + ciphertext);
        Console.WriteLine("Encrypted text: " + result);
        return;
    }
private static byte[] rb(Stream s)
    {
        byte[] array = new byte[4];
        if (s.Read(array, 0, array.Length) != array.Length)
        {
            return null;
        }
        byte[] array2 = new byte[BitConverter.ToInt32(array, 0)];
        if (s.Read(array2, 0, array2.Length) != array2.Length)
        {
            return null;
        }
        return array2;
    }
 private static byte[] EncodeToBytes(string str)
    {
        byte[] bytes = new byte[str.Length * sizeof(char)];
        System.Buffer.BlockCopy(str.ToCharArray(), 0, bytes, 0, bytes.Length);
        return bytes;
    }

Answer 1

You are using different IVs. For your decryption method, you are using the base64 decoded ciphertext:

byte[] buffer = Convert.FromBase64String(ciphertext);
MemoryStream memoryStream = new MemoryStream(buffer);
rijndaelManaged.IV = rb(memoryStream);

For your encryption method, you are deriving it from the password using PBKDF2 (Rfc2898DeriveBytes) which is the right way to do if and only if you construct your Rfc2898DeriveBytes with a random salt. Alternatively, you can use a RNGCryptoServiceProvider to get a completely random IV.

rijndaelManaged.IV = rfc2898DeriveBytes.GetBytes(rijndaelManaged.BlockSize / 8);

From a security perspective, you should use a random salt for deriving your key / IV and prepend it to the ciphertext when encrypting. For decrypting, you should cut it off again (and use it for deriving the decryption key / IV).