How to make Rijndael CBC mode work in vb.net - 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