How do I encrypt a file?

[ianpaisley]

Aaron,

I'm curious to know what you mean by 'Very Simple'? Do you mean any gobdah at all, with little knowledge of VB, could decrypt the file? How? By trial and error? Or would it take a considerably experienced hacker to do it? Just curious.....

Thanks!

[davidck]

I am building an application that will transfer files over the internet. I want to be able to secure these files by encrypting them or locking them so that no one can see or use them at least I want to try and do that. Does anyone have any suggestions?

[Aaron Young]

All you really need to do is jumble the contents of the file around in such a way that you know how to unjumble it at the other end, eg. You could switch pairs of bytes before transfer, then switch them back after.

How complex and secure the encryption is depends on the method you use.

Here's an example of a Very Simple method of Encryption, which generates a Key to Encrypt and Decrypt a Text File.

Private Sub Command1_Click()
    'Open Encrypted File
    Dim sText As String
    Dim iFile As Integer
    
    On Error GoTo Cancelled
    With CommonDialog1
        .CancelError = True
        .Filter = "AY Encrypted|*.aye"
        .ShowOpen
        iFile = FreeFile
        Open .FileName For Input As iFile
        sText = Input(LOF(iFile), iFile)
        Close iFile
    End With
    If Left(sText, 12) <> "AY Encrypted" Then
        MsgBox "Not an AY Encrypted File Format"
    Else
        Text1 = DecryptText(sText)
    End If
    
Cancelled:
End Sub

Private Sub Command2_Click()
    'Save Encrypted File
    On Error GoTo Cancelled
    With CommonDialog1
        .CancelError = True
        .Filter = "AY Encrypted|*.aye"
        .ShowSave
        iFile = FreeFile
        Open .FileName For Output As iFile
            Print #iFile, EncryptText(Text1);
        Close iFile
    End With
    MsgBox "Saved"
Cancelled:
End Sub

Function EncryptText(ByVal sText As String) As String
    Dim sKey As String
    Dim iKey As Integer
    Dim lPos As Long
    Dim iChar As Integer
    Dim iEx As Integer
    
    Randomize Timer
    sKey = Right("000" & Hex(Int(Rnd(1) * 4095)), 3)
    For lPos = 1 To Len(sText)
        iChar = Asc(Mid$(sText, lPos, 1))
        iEx = iChar + Val("&H" & Mid(sKey, iKey + 1, 1))
        Mid$(sText, lPos, 1) = Chr(iEx)
        iKey = (iKey + 1) Mod 3
    Next
    EncryptText = "AY Encrypted" & sText & Right(Space(20) & (Val("&H" & sKey) + Len(sText)), 20)
    
End Function

Function DecryptText(ByVal sText As String) As String
    Dim sKey As String
    Dim iKey As Integer
    Dim lPos As Long
    Dim iChar As Integer
    Dim iEx As Integer
    
    sText = Mid(sText, 13)
    sKey = Right("000" & Hex(Val(Right(sText, 20)) - (Len(sText) - 20)), 3)
    For lPos = 1 To Len(sText) - 20
        iChar = Asc(Mid$(sText, lPos, 1))
        iEx = iChar - Val("&H" & Mid(sKey, iKey + 1, 1))
        Mid$(sText, lPos, 1) = Chr(iEx)
        iKey = (iKey + 1) Mod 3
    Next
    DecryptText = Left(sText, Len(sText) - 20)

End Function

------------------
Aaron Young
Analyst Programmer
[email protected]
[email protected]