PRNG in VB

17 Dec 2003

      I am answering the courteous inquiry from Thomas Womack about my request for 
help with a Visual Basic-only PRNG.  The premise is that those who will not or
can not afford hardware-based RNG's need something relatively secure in the
face of nothing at all or at best a lesser implementation.

I was reluctant to post code at first but rather asked for an interpretation 
of 10 runs of 9999 characters each.  I want peer review of source code but 
the threads in CypherPunks have been of a shall we say "low signal to noise" 
ratio of late.  Even more so than usual.  I just didn't want to feed the 
beast at this time.

That said, here's the outline of what I'm doing.  A splash screen loads first
displaying a random tip'o'the day about good key management.  I make use of 
the getcurrenttime() Win API a lot.  Said to have 50 millicent increments.

' Load splash screen first
Sub Form_Load ()

  ' first randomize using number of milliseconds since Windows was launched
  Randomize getcurrenttime()

  ' then rotate the tip'o'the day (30 currently, adding unknown some delay)
  For j = 1 To Int(n * Rnd + 1)
    Select Case j
      Case 1
        hint.Caption = "You passphrase is SUPPOSED to look like gibberish."
      ...
      Case n
        hint.Caption = "Change your passphrases often."
       End Select
  Next j

End Sub

' user clicks an OK button and up comes 2nd screen
Sub Form_Load ()

 ' mix things up
  Randomize getcurrenttime()

End Sub

' main screen's OK button
Sub Command1_Click ()

  Screen.MousePointer = 11
  scramble = ""
  keyLen = Val(keyLength.Text)

  ' repeat for the number of characters in the desired key
  For i = 1 To keyLen
    ' character set is ASCII 33 to ASCII 127
    scramble = scramble & Chr$(Int(94 * Rnd + 33))
    ' reseed
    Randomize getcurrenttime()
    ' now make it wobble to throw off any regularity in the loop
    ' this loop works because as the 7 increases, so does execution time
    For j = 1 To Int(7 * Rnd + 1)
      'idle
    Next j
  Next

  Screen.MousePointer = 0
  secondaryForm.keyLabel.Caption = scramble

End Sub

I'd be happy with an analysis of just how random this is.  My working 
assumption is that over a >8 character key, it beats trying to dream one 
up in one's head.  Besides the getcurrenttime API, I don't know what else 
to sample without an external DLL or hardware.

I started out attempting a keyboard timing routine but had second thoughts.
My software company has been doing bar code printing tools for years. Now 
we're moving from simple encoding to encryption.

After playing with RSA Secure briefly, I realized that one way to spoof a 
request to type willy nilly to initialize anything is to use a bar code 
scanner.  The common type is sometimes called a wedge because you plug your 
keyboard into it, and it into the keyboard port.  It's wedged between the 
keyboard and the CPU.

So when asked to type (obstensibly to input randomly timed events) I scan 
a very large block of bar coded material.  I fill the keyboard buffer at a 
fixed rate; the throughput of my scanner and PC.  If I scan a large bar 
code, yes, I'll fill the keyboard buffer as fast as possible.  Little 
entropy in my eyes.

Oh yeah, with common symbologies like Code 39 and Code 128, I can recreate 
the whole lower ASCII 128 including tabs, LF/CR, etc.  So I can tab to 
activate buttons in some UI scenarios. Or do macros any any combination that 
may include control characters.

So gang, what about bar code scanners being used to thwart random typing 
requests??

Jerry Whiting
jwhiting@azalea.com