How To Encrypt Files That My VB App Can Decrypt

Try adding the following 3 functions to your program.  It will encrypt a string of characters that you can write to a file.  The pwd is what you will lock and unlock your code with.  I often you my app name, but you may want to use something a bit harder to decode.  Hope this helps.





Public Function encrypt(ByVal pwd As String, CryptKey As Integer) As String

  Dim ctr As Integer
  Dim newpwd As String
  Dim cryptcode As Integer

  ctr = 1
  Do Until ctr = Len(pwd) + 1
    cryptcode = Asc(Mid(pwd, ctr, 1))
    If ctr Mod 2 = 0 Then
      cryptcode = cryptcode - CryptKey
    Else
      cryptcode = cryptcode + CryptKey
    End If
    cryptcode = (cryptcode Xor CryptKey) - 9
    newpwd = newpwd & Chr$(cryptcode)
    ctr = ctr + 1
  Loop
  encrypt = newpwd

End Function


Public Function decrypt(pwd As String, CryptKey As Integer) As String

  Dim ctr As Integer
  Dim newpwd As String
  Dim cryptcode As Integer

  ctr = 1
  Do Until ctr = Len(pwd) + 1
    cryptcode = Asc(Mid(pwd, ctr, 1)) + 9 Xor CryptKey
    If ctr Mod 2 = 0 Then
      cryptcode = cryptcode + CryptKey
    Else
      cryptcode = cryptcode - CryptKey
    End If
    newpwd = newpwd & Chr$(cryptcode)
    ctr = ctr + 1
  Loop
  decrypt = newpwd

End Function

Public Function GetCryptKey(UserName As String) As Integer

  Dim WorkingKey As Long
  Dim ctr As Integer
  Dim KeyString As String

  For ctr = 1 To Len(UserName)
    WorkingKey = WorkingKey + Asc(Mid$(UserName, ctr, 1))
  Next ctr

  Do Until (WorkingKey >= 1) And (WorkingKey <= 9)
    KeyString = CStr(WorkingKey)
    WorkingKey = 0
    For ctr = 1 To Len(KeyString)
      WorkingKey = WorkingKey + CInt(Mid$(KeyString, ctr, 1))
    Next ctr
  Loop

  GetCryptKey = CInt(WorkingKey)

End Function

The user name in the getCryptKey will probally be changed to something more consistant.  I use it to crypt my passwords in a database based of the users name.  That way it is harder to break to encryption.

A more secure version would use a longer key.  One that might work for you is to use the App.Major, App.Minor, and App.Revision properties to build your key:

Private AppKey as Variant

Function GetKey() as Variant

Dim TK(0 to 15) as Byte
Dim I as Integer

' Generate a 16 byte key

TK(0) = Rnd(App.Major^2 * App.Minor ^5 * App.Revision * -1) * 255  ' set seed for sequence

For I = 1 To 15
  TK(I) = Rnd
Next I
GetKey = Array(TK(0),TK(1),TK(2),TK(3),TK(4),TK(5),TK(6),TK(7),TK(8),TK(9),TK(10),TK(11),TK(12),TK(13),TK(14),TK(15))
End Function

Function CryptString (Src as String) as String
'  Passing a plaintext string encrypts it
'  Passing an encrypted string decrypts it

Dim vKey as Variant
Dim sTmp as String
Dim I as Integer

sTmp = ""
If Len(Src) > 0 Then
  vKey = GetKey()
  For I = 1 to Len(Src)
    sTmp = sTmp & Chr(Asc(Mid(Src,I,1)) XOR vKey(I Mod 16))
  Next I
End If
CryptString = sTmp
End Function


NOTE:  XOR encryption, though fast and easy, is not especially secure (to be precise, it's considered a joke in cryptography circles).  You might consider using another encryption method.  Look around for an OCX or other control that implements more secure encryption.

Good suggestions I suppose, but the missing consideration is that I want to be able to encrypt already existing files (txt, exe, mp3 etc.) that are in a directory and have only my Program be able to run, see, play them.
??

Thanks for the points! Glad I could help!


Cheers!®©