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Best of John Robison

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Ask the Slot Expert: Russians cheat a slot machine's RNG

8 February 2017

Question: The RNG can't be beat? Google the headline Russians Engineer a Brilliant Slot Machine Cheat and take a read.

Answer: The headline refers to a recent article in Wired.

The RNG cannot be beaten -- not without inside knowledge.

I've written about RNG cheats many times in the past. Each time a vulnerability is discovered and exploited, regulations are strengthened to eliminate the problem.

RNG cheats usually rely on the fact that RNGs aren't really RNGs. The random number generator in a slot machine is just a mathematical function and there's nothing random about addition, subtraction, multiplication and division. The function takes one or more previously generated numbers, possibly mixes in some other values like the number of milliseconds on the clock, performs some mathematical operations on the inputs and returns the result of the operations as the next number in the stream of random numbers. The truly correct name for the RNG is pseudo-random number generator (PRNG). The number stream generated by a PRNG satisfies many of the tests for randomness, but the number stream is not truly random.

The RNGs in modern slot machines constantly generate new results and a result is locked in only after the player starts the spin. The constantly running RNG regulation came about because of an RNG cheat. Some early computer-controlled slot machines locked in the result of the next spin right after the conclusion of the current spin. The machine then sat with a result locked in waiting for a player to make a bet and reveal the result.

Whenever a machine is sitting with a result locked in, it is vulnerable to being cheated.

About 30 years ago, a man named Leo Weeks had knowledge of the simple RNG function used in some slots and built a small computer in a box to simulate it. The box came to be known as a Weeks Box. You played a few spins and entered the results into the box. The computer in the box could then figure out where the RNG function was in its cycle and tell you whether the next spin was going to be a winner or a loser. The cheat bet the minimum on the losers and the maximum on the winners.

Video poker machines used to select all 10 cards that might be needed to play out a hand on the deal.

Whenever a machine is sitting with a result locked in, it is vulnerable to being cheated.

Cheats with knowledge of the RNG function were able to figure out the five cards waiting in the wings to replace the discards and cheat the machines. About two decades ago, regulations were changed to require machines to select only five cards on the deal, continue shuffling the electronic deck, and select the replacement cards only after the player hit draw.

In addition to requiring that the RNG constantly generate new results and not lock in a result until needed, other countermeasures include using unpredictable values (like milliseconds on the clock), periodically reseeding the RNG, and using multiple RNG functions.

The article in question refers to groups of Russian slot cheats. When Putin outlawed gambling in 2009, casinos sold their slot machines at fire-sale prices. Some machines went to cheats, who wanted find any vulnerabilities in the machines.

The machines they cheated are older Aristocrat machines. The cheats used cell phones to record about two dozen spins. The footage was sent to their support staff in St. Petersburg, where they analyzed the footage and figured out where the RNG was in its sequence. The staff then sent a series of timings to a custom app on the cheat's cell phone. The timings would cause the phone to vibrate at the moment the cheat should hit the spin button to catch the RNG when the result was a winning spin.

The cheats were caught, in part, because of the way they played. Instead of getting into a rhythm when hitting the spin button, they were seen with their hands hovering over the spin buttons, seemingly waiting for some external signal. The vibration of their cell phones was that signal.

It's very difficult to stop a cheat with detailed knowledge of how the RNG in a slot machine operates. That's the main reason why manufacturers release few details about the RNGs in their machines.

James Maida, President of Gaming Labs International, once said in a slot security seminar:

If man created it, man can cheat it.


Question: I play a lot of high limit video poker and I read an article you wrote about changes IGT made to the way they hold cards in their newer machines. I have noticed the change in the way machines play and unscientifically find it much more difficult to win on the newer machines with far fewer royals and 4-of-a-kinds.

I wonder if you have checked what constantly shuffling the hold cards does to winning percentages?

Answer: I've written about the requirement that video poker machines select cards five-on-the-deal-and-five-on-the-draw instead of 10-on-the-deal many times in the past -- including above. This change occurred about 20 years ago. Machines have been operating this way for a long time.

The constant shuffling of the deck before selecting the replacement cards for the draw has absolutely no effect on the probabilities of achieving any particular winning hand.

Consider this: Shuffle a fair deck of 52 cards. Deal five cards. Think of a card that is not one of the five dealt. What are the chances that that card is on top of the deck? One out of 47. Shuffle the deck. What are the chances? One out of 47. Shuffle the deck again and the chances remain one out of 47.

Why don't you make your unscientific observation scientific? If you play high-limit video poker, I assume you're using a strategy. You might be able to find the frequency of 4-of-a-kinds for your strategy. Now, keep track of the number of hands you play and the number of quads you hit. With luck, you can use points earned in the slot club to determine the number of hands you played. You'll have to keep track of quads on your own. See if the frequency with which you hit quads gets closer and closer to the frequency calculated for the strategy as you play more and more hands.


John Robison

John Robison is an expert on slot machines and how to play them. John is a slot and video poker columnist and has written for many of gaming’s leading publications. He holds a master's degree in computer science from the prestigious Stevens Institute of Technology.

You may hear John give his slot and video poker tips live on The Good Times Show, hosted by Rudi Schiffer and Mike Schiffer, which is broadcast from Memphis on KXIQ 1180AM Friday afternoon from from 2PM to 5PM Central Time. John is on the show from 4:30 to 5. You can listen to archives of the show on the web anytime.

Books by John Robison:

The Slot Expert's Guide to Playing Slots
John Robison
John Robison is an expert on slot machines and how to play them. John is a slot and video poker columnist and has written for many of gaming’s leading publications. He holds a master's degree in computer science from the prestigious Stevens Institute of Technology.

You may hear John give his slot and video poker tips live on The Good Times Show, hosted by Rudi Schiffer and Mike Schiffer, which is broadcast from Memphis on KXIQ 1180AM Friday afternoon from from 2PM to 5PM Central Time. John is on the show from 4:30 to 5. You can listen to archives of the show on the web anytime.

Books by John Robison:

The Slot Expert's Guide to Playing Slots