2556: Turing Complete
Title text: Thanks to the ForcedEntry exploit, your company's entire tech stack can now be hosted out of a PDF you texted to someone.
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A Turing machine is a theoretical form of computer (as an idealised thought exercise) that has an infinite tape of symbols and can act upon and change these values as it moves up and down this tape according to specific deterministic rules.
This very simple machine can be shown to do every computational task that what we think of as a "computer" can do, given the right setup and enough time. Something that is Turing complete is able to act as a Turing machine, though generally with the limitation of having a finite tape, and this means it is also able to do basically every computational task.
Many pieces of hardware and software are supposed to be Turing complete (even Excel, as previously pointed out in 2453: Excel Lambda). Some other things turn out to be turing complete, even if they weren't designed for it (for instance, the tabletop game Magic: The Gathering or, at least within xkcd meta-reality, rocks in a desert). Whatever Ponytail has been referring to is not shown, but it seems to be an anecdote about how something seemingly too simple and/or specialised to exhibit such a computational equivalence has been discovered to actually be that capable.
Mario Kart is a racing video game. Running video games, such as Doom, is one common way of demonstrating the ability to run arbitrary programs on devices that were not intended as general purpose computers.
With complex processors now installed in many household items, including large kitchen appliances like dishwashers, the possibility is raised that someone has 'hacked' such a device to do the same computational work as an actual games console. Alternatively, if hackers working on behalf of a foreign government have discovered an undetected exploit in a nation's cyber-defenses, the fact that a piece of infrastructure accessible to outsiders is Turing-complete (and could thus potentially be used to execute arbitrary code) may come as a very unpleasant surprise to the nation being attacked.
The ForcedEntry exploit is a way that was developed to allow PDF files to force malware onto various devices. PDF files are normally used to present documents. The exploit uses a PDF's ability to do logic operations on pixels to implement a simple virtual CPU within one of the PDF renderer's decompression functions. Constructing a CPU in this way is similar to how a hardware CPU is made of individual logic gates. ForcedEntry was publicized a few days before this comic came out.
In the title-text it is suggested that this mechanism can be used for what might be more legal and practical purposes, although this might be up to some interpretation depending upon who has the right (and permission) to do what.
A tech stack is one shorthand way of describing the way an integrated grouping of communicating software packages provides everything from the deepest data handling (even as low-level as an operating system itself) to the user interface. All of these will normally be on a computer (or possibly many of them, whether locally or distributed worldwide) and if a sufficiently functional surrogate system is capable of emulating this (computing what the original computer(s) would do) then it can be considered to effectively be the same stack of technology and duplicate or replace the originals.
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- [Ponytail and Cueball are standing next to each other]
- Ponytail:...Now, it turns out this is actually Turing-Complete...
- [Caption below the panel:]
- This phrase either means someone spent six months getting their dishwasher to play Mario or you are under attack by a nation-state.