How to make Rijndael CBC mode work in vb.net

Question

I'm trying to make rijndael work in CBC mode. I'm not exactly sure how should I do it. I think problem in my current code is that the stream is initialized every time in the beginning of encryption, so no avalanche effect occurs (same data is encrypted twice and the output of those two encryption is the same which it should not be).

I tried to initialize the cryptostream only once but then my coded crashed because the canwrite property of cryptostream goes to false after the first write to the cryptostream.

Here is the code what I have now:

Sub Main()

        Dim rij As New RijndaelManaged
        Dim iv(15) As Byte
        Dim key(15) As Byte
        Dim secret() As Byte = {59, 60, 61}

        Dim cs As ICryptoTransform
        Dim cstream As CryptoStream

        Dim out() As Byte
        Dim NewRandom As New RNGCryptoServiceProvider()

        NewRandom.GetBytes(iv)
        NewRandom.GetBytes(key)

        rij = New RijndaelManaged()

        rij.KeySize = 128
        rij.Padding = PaddingMode.PKCS7

        rij.Mode = CipherMode.CBC

        rij.IV = iv
        rij.Key = key
        cs = rij.CreateEncryptor()

        Dim ms_in As New MemoryStream
        cstream = New CryptoStream(ms_in, cs, CryptoStreamMode.Write)


        Using cstream
            cstream.Write(secret, 0, 3)
        End Using

        out = ms_in.ToArray
        Console.WriteLine(ArrayToString(out, out.Length))
        Erase out

        ms_in = New MemoryStream
        cstream = New CryptoStream(ms_in, cs, CryptoStreamMode.Write)


        Using cstream
            cstream.Write(secret, 0, 3)
        End Using

        out = ms_in.ToArray
        Console.WriteLine(ArrayToString(out, out.Length))

    End Sub

and the conversion function to convert an array to string

 Public Function ArrayToString(ByVal bytes() As Byte, ByVal length As Integer) As String

        If bytes.Length = 0 Then Return String.Empty
        Dim sb As New System.Text.StringBuilder(length)

        Dim k As Integer = length - 1
        Dim i As Integer

        For i = 0 To k
            sb.Append(Chr(bytes(i)))
        Next


        Return sb.ToString()

    End Function

---

This is what I need:

    cs = rij.CreateEncryptor()
    Dim ms_in As New MemoryStream
    cstream = New CryptoStream(ms_in, cs, CryptoStreamMode.Write)

    Using cstream
        cstream.Write(secret, 0, 3) 'encrypt
    End Using

    out = ms_in.ToArray
    Console.WriteLine(ArrayToString(out, out.Length)) 'see the encrypted message
    Erase out

    Using cstream
        cstream.Write(secret, 0, 3) 'encrypt, this will crash here and this is the problem I'm trying to solve
    End Using

    out = ms_in.ToArray
    Console.WriteLine(ArrayToString(out, out.Length)) 'see the encrypted message this should not be the same as the first one

Answer 1

Try this:

Public Sub Run()
    Dim key() As Byte = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15}

    Dim plaintext1 As Byte() = {59, 60, 61}

    Dim plaintext2 As Byte() = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15, _
                             0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15, _
                             0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15, _
                             0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15 _
                             }

    Roundtrip(plaintext1, key)
    System.Console.WriteLine()
    Roundtrip(plaintext2, key)

End Sub


Public Sub Roundtrip(ByRef plaintext As Byte(), ByRef key As Byte())

    Dim rij As New RijndaelManaged
    Dim iv(15) As Byte

    Dim encryptor As ICryptoTransform
    Dim decryptor As ICryptoTransform

    Dim out() As Byte

    'Dim NewRandom As New RNGCryptoServiceProvider()
    'NewRandom.GetBytes(iv)
    'NewRandom.GetBytes(key)

    Console.WriteLine("Original:")
    Console.WriteLine(ArrayToString(plaintext))
    System.Console.WriteLine()

    rij = New RijndaelManaged()
    rij.KeySize = key.Length * 8  ' 16 byte key == 128 bits
    rij.Padding = PaddingMode.PKCS7
    rij.Mode = CipherMode.CBC
    rij.IV = iv
    rij.Key = key
    encryptor = rij.CreateEncryptor()

    Using msIn = New MemoryStream

        Using cstream = New CryptoStream(msIn, encryptor, CryptoStreamMode.Write)
            cstream.Write(plaintext, 0, plaintext.Length)
        End Using

        out = msIn.ToArray
        Console.WriteLine("Encrypted:")
        Console.WriteLine("{0}", ArrayToString(out))
        System.Console.WriteLine()

    End Using

    decryptor = rij.CreateDecryptor()
    Using msIn = New MemoryStream

        Using cstream = New CryptoStream(msIn, decryptor, CryptoStreamMode.Write)
            cstream.Write(out, 0, out.Length)
        End Using

        out = msIn.ToArray
        Console.WriteLine("Decrypted:  ")
        Console.WriteLine("{0}", ArrayToString(out))
        System.Console.WriteLine()

    End Using

End Sub


Public Shared Function ArrayToString(ByVal bytes As Byte()) As String

    Dim sb As New System.Text.StringBuilder()

    Dim i As Integer
    For i = 0 To bytes.Length-1
        if (i <> 0 AND i mod 16 = 0) Then
            sb.Append(Environment.NewLine)
        End If
        sb.Append(System.String.Format("{0:X2} ", bytes(i)))
    Next

    Return sb.ToString().Trim()

End Function

I made these basic changes to get it to work:

• create a Decryptor
• properly manage buffers and streams (see the Using clauses I added)

I also re-organized a little, and modified your code slightly to use a constant IV (all zeros) and use a constant key. This is so that you can get repeatable results from one run to the next. In a real app you would use a randomized IV and use a password-derived key. (See Rfc2898DeriveBytes)

---

ok, that allows the code to compile. I think you want to see the effect of chaining. That is not so easy, but maybe something like this will show you what you want to see:

        For i As Integer = 1 To 2
            Using ms = New MemoryStream
                Using cstream = New CryptoStream(ms, encryptor, CryptoStreamMode.Write)
                    For j As Integer = 1 To i
                        cstream.Write(plaintext, 0, plaintext.Length)
                    Next j
                End Using
                out = ms.ToArray
                Console.WriteLine("Encrypted (cycle {0}):", i)
                Console.WriteLine("{0}", ArrayToString(out))
                System.Console.WriteLine()

            End Using
        Next i