Wired reports that a Russian group designed a brilliant slot machine cheat that they’ve used to bilk casinos out of millions of dollars. The article is sketchy on technical details, but there’s enough information there for me to speculate how it was done.

Understand, I don’t *know* anything about the internals of the software running on these machines, but I know enough about pseudorandom number generators, their use and misuse, to offer a plausible explanation of the vulnerability and how it’s exploited. I also know a few people in the industry. What I describe below is possible, and from my experience quite likely to have happened. Whether it’s exactly what happened, or if it’s even close to the mark, I have no way of knowing.

First, a little background.

In modern computer controlled slot machines (say, anything built in the last 30 years), the machine uses random numbers to determine the results of a spin. In concept, this is like rolling dice, but the number of possibilities is huge: on the order of about four billion. *In theory*, every one of those four billion outcomes is equally likely every time you roll the dice. That would be true in practice if the computer were using truly random numbers.

But computers are deterministic; they don’t do random. Instead, they use algorithms that simulate randomness. As a group, these algorithms are called pseudorandom number generators, or PRNGs. You can probably guess that PRNGs differ in how well they simulate true randomness. They also differ in ease of implementation, speed, and something called “period.” You see, a PRNG is just a mathematical way to define a deterministic, finite sequence. Given a starting state (called the seed), the PRNG will generate a finite set of values before it “wraps around” to the beginning and starts generating the same sequence all over again. Period is the number of values generated before wrapping. If you know what PRNG is being used and you know the initial state (seed), then you know the sequence of numbers that will be generated.

The machines in question were purchased on the secondary market sometime before 2009. It’s probably safe to say that the machines were manufactured sometime between 1995 and 2005. During that era, the machines were almost certainly running 32 bit processors, and likely were generating 32 bit random numbers using PRNGs that maintained 32 bits of state. That means that there are 2^32 (four billion and change) possible starting states, each of which has a maximum period of 2^32. Overall, there are 2^64 possible states for the machine to be in. That’s a huge number, but it’s possible to compute and store every possible sequence so that, if somebody gave you a few dozen random numbers in sequence, you could find out where they came from and predict the next number. It’d take a few days and a few terabytes of disk storage to pre-compute all that, but you’d only have to do it once.

It’s likely that the PRNG used in these machines is a linear congruential generator which, although potentially good if implemented well, is easy to reverse-engineer. That is, given a relatively small sequence of generated numbers, it’s possible to compute the seed value and predict the next values in the sequence. All this can be done without knowing exactly which specific LCG algorithm is being used.

The hackers did have the source code of the program (or they disassembled the ROM), but they didn’t have access to the raw numbers as they were generated. Instead, they had to deduce the random number based on the outcome of the spin. But again, that just takes a little (okay, more than just a little, but not too much) computation time to create a table that maps reel positions to the random sequence.

My understanding is that slot machines continually generate random numbers on a schedule, even when the machine is idle. Every few milliseconds, a new number is generated. If it’s not used, then it’s discarded and the next number is generated. So if you know where you are in the sequence at a given time, then you can predict the number that will be generated at any time in the future. Assuming, of course, that your clock is synchronized with the slot machine’s clock.

If you refer back to the article, you’ll see that the agent who was working the machine would record the results of several spins, then walk away and consult his phone for a while before coming back to play. That phone consultation was almost certainly uploading the recorded information to the central site, which would crunch the numbers to determine where the machine was in the random sequence. The system knows which numbers in the sequence correspond to high payouts, so it can tell the phone app when to expect them. The agent then goes back to the machine and watches his phone while hovering his finger over the spin button. When the phone says spin, he hits the button.

The system isn’t perfect. With perhaps up to 200 random numbers being generated every second, and human reaction time being somewhat variable, no player will hit the big payout every time. But he’s increased his odds tremendously. Imagine somebody throwing one gold coin into a bucket of a million other coins, and another gold coin into a bucket of 200 other coins. You’re given the choice to blindly choose from one of the two buckets. Which would you choose from?

That might all sound complicated, but it’s really pretty simple in concept. All they did was create a map of the possibilities and devise a way to locate themselves on the map. Once you know where you are on the map, then the rest is a simple matter of counting your steps. Creating the map and the location algorithm likely took some doing, but conceptually it’s very simple.

The above explanation is overly broad, I’ll admit, and I wave my hand over a number of technical details, but people at work with whom I’ve discussed this generally agree that this is, at least in broad strokes, how the hackers did it. Understand, I work at a company that develops slot machine games for mobile devices, and several of the programmers here used to work for companies that make real slot machines. They know how these machines work.

When I originally saw the article, I assumed that some programmer had made a mistake in coding or using the PRNG. But after thinking about it more critically, I believe that these machines are representative of the state of the art in that era (1995-2005). I don’t think there was a design or implementation failure here. The only failure would be that of not imagining that in a few years it would be possible for somebody who didn’t have a supercomputer to compute the current machine state in a few minutes and exploit that knowledge. This isn’t a story about incompetence on the part of the game programmers, but rather a story about the cleverness of the crooks who pulled it off. I can admire the technical prowess it took to achieve the hack while still condemning the act itself and the people who perpetrated it.