Question

I’m trying out the Rijndael to generate an encrypted license string to use for our new software, so we know that our customers are using the same amount of apps that they paid for. I’m doing two things:

1. Getting the users computer name.

2. Adding a random number between 100 and 1000000000

I then combine the two, and use that as the license number(This probably will change in the final version, but I’m just doing something simple for demonstration purposes).

Here is some sample codez:

 Private Sub Main_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
    Dim generator As New Random
    Dim randomValue As Integer
    randomValue = generator.Next(100, 1000000000)


    ' Create a new Rijndael object to generate a key
    ' and initialization vector (IV).
    Dim RijndaelAlg As Rijndael = Rijndael.Create

    ' Create a string to encrypt.
    Dim sData As String = My.Computer.Name.ToString + randomValue.ToString
    Dim FileName As String = "C:\key.txt"

    ' Encrypt text to a file using the file name, key, and IV.
    EncryptTextToFile(sData, FileName, RijndaelAlg.Key, RijndaelAlg.IV)

    ' Decrypt the text from a file using the file name, key, and IV.
    Dim Final As String = DecryptTextFromFile(FileName, RijndaelAlg.Key, RijndaelAlg.IV)

    txtDecrypted.Text = Final


End Sub

That’s my load event, but here is where the magic happens:

Sub EncryptTextToFile(ByVal Data As String, ByVal FileName As String, ByVal Key() As Byte, ByVal IV() As Byte)

    Dim fStream As FileStream = File.Open(FileName, FileMode.OpenOrCreate)
    Dim RijndaelAlg As Rijndael = Rijndael.Create
    Dim cStream As New CryptoStream(fStream, _
                                   RijndaelAlg.CreateEncryptor(Key, IV), _
                                   CryptoStreamMode.Write)
    Dim sWriter As New StreamWriter(cStream)

    sWriter.WriteLine(Data)
    sWriter.Close()
    cStream.Close()
    fStream.Close()



End Sub

There is a couple things I don’t understand. What if someone reads the text file and recognizes that it is Rijndael, and writes a VB or C# app that decrypts it? I don’t really understand all of this code, so if you guys can help me out I will love you all forever.

Thanks in advance

Answer 1

You have stumbled on a very basic problem: It’s easy enough to encrypt some data, but how do you transfer the key?

You can take a look at asymmetric encryption (RSA), you could add a signature similar to that of signed assemblies.

A very top-level description:

1. You generate a key-pair (public and private key) 1 time. Keep the private key secret.
2. You get UserName, Hash it and encrypt the hash with the Private key.
3. You send encrypted hash (signature) to the customer as part of your license file.
4. Your program uses (embedded) public key to decode signature
5. Your program hashes local Username and compares with decoded version.
6. Your program decides if it licensed or not and acts accordingly.

The hashing is used to limit the number of bytes to encrypt. For Username you can substitute any data you need (ie Username + EndDate).