Some number of years ago, I was lucky enough to get a Sinclair ZX-81 for Christmas. There was much wonderment and joy to be had amid the frustration of RAM-pack wobble, the agonising waits for software to load from tape, and the never-ending search for a replacement keyboard that wasn’t as bad (or worse) than the original.

The best thing about the computers of olde was the built-in interpreter, usually for BASIC – here was an item of consumer electronics that wouldn’t do a single thing unless you told it to, an unthinkable concept to the marketeers of today. Putting in a tape and typing LOAD “” was the easy way out of this predicament; however, the real solution to the inert nature of your newly purchased box of future was to open the manual and learn how to code.

So learn we did. One day, my father proudly showed me a program he’d written – a version of the card game “snap”, with graphics and everything. After whiling away a good part of the afternoon playing, I looked over the source code. Showing an early leaning towards white-box pentesting, it didn’t take long to find a simple flaw. By simply keeping your finger pressed on your “snap” key (regardless of the two cards on the top of the deck) you could beat even the quickest opponent when a true “snap” finally came around. If both players were aware of this exploit (or “expliot” as I’d almost certainly have spelled it at the time) you had to make sure that you were player 2 since the subroutine that checked which key was pressed during a “snap” condition checked for player 2’s “snap” key before player 1’s.

Easily exploitable vulnerabilities? Some things never change, huh? In terms of Incident Detection, prime Indicators of Compromise included my little sister complaining to our parents that Daddy’s game was no fun because Alec always won.

To restore the game back to a test of speedy reactions my father rolled out some countermeasures in the form of a patch. The next time we played, my tactic resulted in the computer labelling me a cheat and docking me five cards every time I pressed my snap key when it wasn’t snap. To further add injury to insult, the losing player was crushed by a one-ton weight falling from the ceiling. I’m sure you can imagine what a terrifying visual experience this must have been, especially if you remember the graphics capabilities of the ZX-81…

To sample the full glory of this dance of measure and countermeasure, here’s the actual source code as submitted to ZX Computing magazine nearly thirty years ago. Lines 570 and 580 show the horrifying corpses of the losing players, squashed flat by Newtonian Justice From Above. Enjoy 🙂



Alec Waters is responsible for all things security at Dataline Software, and can be emailed at alec.waters@dataline.co.uk



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