[EM] Dave Ketchum: Handcounts

Kristofer Munsterhjelm km_elmet at lavabit.com
Mon Apr 30 14:31:26 PDT 2012

On 04/29/2012 04:48 AM, Dave Ketchum wrote:
> Computers do well at performing the tasks they are properly told to
> perform - better than humans given the same directions. Thus it would
> make sense to direct the computers and expect them to do what is needed
> accurately.
> Still, we sometimes wonder exactly what the computers have been told to do.
> I keep remembering a case where it took me some studying to sort out how
> a computer program seemed to perform magic:
> Got a diskette and was told it could not be copied - because the owner
> of the program wanted to get paid for each new diskette the program was
> copied to.
> Curious, though not intending theft, I copied all the records to another
> diskette and tried running it.
> New diskette knew it was not an original, and refused to run. Looked at
> records on the diskette - there were no instructions in the copied
> records to do such a test - anyway, presumably the records had been
> copied accurately, so their content could not indicate this was a copy.
> Look more carefully - there was an odd collection of odd words. Turns
> out that, when it was time to check, this collection became instructions
> to look at the noise next to a particular record on the diskette and see
> if the "password" was there, as it would be on the original. After
> making the test, put those instructions back into their hidden form.

Ah, copy protection. They've come a long way since then: the most 
difficult programs to reverse engineer now uses virtual machines with 
purpose-made machine code for "processors" that don't exist outside of 
the VM. The people who craft protection schemes know that it ultimately 
comes down to "jump here if you've been good / if you've been bad", so 
they try to bury that under layers and layers of obfuscation.

I digress :-) But this is something I've become familiar with, so I just 
thought I should say it.

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