In VB.NET, writing AES key and encrypted data to same MemoryStream not working

Question

I'm attempting to use symmetric encryption to encrypt some data, and store the key necessary to decrypt with the data in a MemoryStream. (While I know that this alone is really dumb security-wise, I'm going to use RSA to encrypt the symmetric key. Right now, though, I'm trying to just get this part to work.)

I'm using the FileHelpers library to parse the data for my delimiter (the semicolon, as I don't believe I'll ever have a semicolon in the data). Unfortunately, in my decryption function, when it's parsing, it only returns back one record. And, if I show the entire string of encrypted data created at the end of this function, it doesn't appear to be using multiple records.

I'm wondering if when I create the new cryptostream, it's defaulting to the beginning of the memory stream, so when I write my encrypted data, it overwrites the data I had just written to the memory stream. Do you think that's right?

Thanks for your help!

Private Function Encrypt(ByVal data As String) As String
    Dim utf8 As New UTF8Encoding
    ' Convert string data to byte array
    Dim inBytes() As Byte = utf8.GetBytes(data)
    ' Create memory stream for storing the data we've manipulated
    Dim ms As New MemoryStream()

    Dim aes As New RijndaelManaged()
    aes.KeySize = 256

    Dim cs As New CryptoStream(ms, aes.CreateEncryptor(), CryptoStreamMode.Write)

    ' Write key to beginning of memory stream
    ms.Write(aes.Key, 0, aes.Key.Length)
    ' Add semicolon delimiter to memory stream
    ms.Write(utf8.GetBytes(";"), 0, utf8.GetBytes(";").Length)
    ' Write IV to memory stream
    ms.Write(aes.IV, 0, aes.IV.Length)
    ' Write semicolon delimiter to memory stream
    ms.Write(utf8.GetBytes(";"), 0, utf8.GetBytes(";").Length)
    ' Ensure that the data we've just written is in the memory stream, before
    ' we add the encrypted data
    ms.Flush()

    ' Write the encrypted data
    cs.Write(inBytes, 0, inBytes.Length) ' encrypt
    cs.FlushFinalBlock()

    ' Return encrypted data as string
    Return Convert.ToBase64String(ms.GetBuffer(), 0, ms.Length)
End Function

Answer 1

Here is a quick sample of how you can achieve this. I have included both the Encrypt and Decrypt because it is obviously important that you follow the same pattern on both sides. I will leave the relevant error checking etc. to you :)

Imports System.Security.Cryptography
Imports System.IO
Imports System.Text

Module Module1

  Sub Main()
    Dim encrypted As String = Encrypt("Here is the data")
    Dim decrypted As String = Decrypt(encrypted)
    Console.WriteLine(decrypted)
    Console.ReadKey()
  End Sub

  Private Function Encrypt(clearText As String) As String
    Dim aes As New RijndaelManaged
    aes.KeySize = 256

    Using ms As New MemoryStream
      ms.WriteByte(aes.Key.Length)
      ms.Write(aes.Key, 0, aes.Key.Length)
      ms.WriteByte(aes.IV.Length)
      ms.Write(aes.IV, 0, aes.IV.Length)

      Using cs As New CryptoStream(ms, aes.CreateEncryptor(), CryptoStreamMode.Write)
        Dim bytes() As Byte = Encoding.UTF8.GetBytes(clearText)
        cs.Write(bytes, 0, bytes.Length)
      End Using

      Return Convert.ToBase64String(ms.ToArray())
    End Using
  End Function

  Private Function Decrypt(cipherText As String) As String
    Dim ms As New MemoryStream(Convert.FromBase64String(cipherText))

    Dim keyLength As Byte = ms.ReadByte()
    Dim key(keyLength - 1) As Byte
    ms.Read(key, 0, keyLength)

    Dim ivLength As Byte = ms.ReadByte()
    Dim iv(ivLength - 1) As Byte
    ms.Read(iv, 0, ivLength)

    Dim dataOffset As Integer = ms.Position

    Dim aes As New RijndaelManaged()
    aes.Key = key
    aes.IV = iv

    Using cs As New CryptoStream(ms, aes.CreateDecryptor(), CryptoStreamMode.Read)
      Using sr As New StreamReader(cs, Encoding.UTF8)
        Return sr.ReadToEnd()
      End Using
    End Using
  End Function
End Module