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There is a popular high-stakes card game. Unfortunately, for whatever reason, it's impossible to get out a new pack of cards for each round. The same cards must be used throughout.

This game would of course be ruined if marking cards were possible, so how can we make it so it's impossible?

Each 'card' must fulfill all of the following conditions:

  • Each card is slim (not much more than twice as thick as a standard playing card)
  • Each card is stackable
  • Any marks that can feasibly be made by players unobtrusively during the game without special equipment have to be so unsubtle that the dealer can pick them out
  • There are markings on the card that designate what is is (J, 2, A, etc.), that are easily and unambiguously deciphered (not something along the lines of morse code, notches, etc.)
  • They all have identical backing.

Cost is no issue, in fact if it costs somewhere between a 10-pack of beer and a very cheap car to make one set, all the better.

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    $\begingroup$ Why not have AR cards? $\endgroup$ – RBarryYoung Mar 3 '18 at 20:59
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    $\begingroup$ Make the cards out of engraved titanium, and wash them in a strong solvent before each deal. $\endgroup$ – Hot Licks Mar 4 '18 at 15:21
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    $\begingroup$ Technology? If higher tech... Could the cards be ultra-thin monitors controlled by a central computer? Doesn't matter if you mark the Ace this round, next round it's something different? $\endgroup$ – WernerCD Mar 4 '18 at 18:09
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    $\begingroup$ How much of this is worldbuilding, and how much is simply ruleset design for serious gamblers? $\endgroup$ – can-ned_food Mar 4 '18 at 20:29
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    $\begingroup$ What is the setting? Current times? Middle ages? Sci-fi future? $\endgroup$ – vsz Mar 5 '18 at 7:21

12 Answers 12

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Make the cards out of gold/silver/platinum and then use something like vapor deposition to give them a nice coating of diamond. This way they are strong, non-stick and scratch resistant.

https://en.wikipedia.org/wiki/Chemical_vapor_deposition

[T]he growth of diamond directly on a substrate allows the addition of many of diamond's important qualities to other materials .... Diamond films are being grown on valve rings, cutting tools, and other objects that benefit from diamond's hardness and exceedingly low wear rate .... Diamond's very high scratch resistance and thermal conductivity, combined with a lower coefficient of thermal expansion than Pyrex glass, a coefficient of friction close to that of Teflon (polytetrafluoroethylene or PTFE) and strong lipophilicity would make it a nearly ideal non-stick coating for cookware if large substrate areas could be coated economically.

By regulating the processing parameters—especially the gases introduced, but also including the pressure the system is operated under, the temperature of the diamond, and the method of generating plasma—many different materials that can be considered diamond can be made. Single crystal diamond can be made containing various dopants.[25] Polycrystalline diamond consisting of grain sizes from several nanometers to several micrometers can be grown.[23][26] Some polycrystalline diamond grains are surrounded by thin, non-diamond carbon, while others are not. These different factors affect the diamond's hardness, smoothness, conductivity, optical properties and more.

However, this may make them very hard to shuffle as they are rigid, but you only said they had to be stack-able, not bendable. In any case you could invent a special machine to shuffle them. This machine could polish up any finger prints, remove any dirt or marker ink, and even check the cards for defects. You may want to give them a matte finish instead of a high gloss one, that way they could hide fingerprints, or some other kind of coating that evaporates fingerprints, otherwise someone could just leave an oily print on a card to mark it. Of course this depends on the nature of the game and if cards are cycled between players without being reshuffled/cleaned.

I envision a shimmering card of transparent diamond, with a intricate pattern of precious metals on the back. Maybe even do different colored diamond layers for the colors.

Something like this this (just for examples sake, I just added the link for the sake of citing the image source)

http://www.collectableplayingcards.com/bicycle-aurora-playing-cards-collectable-playing-cards-p-49478.html

enter image description here

enter image description here

Basically someone would describe them as mesmerizing and distracting to look at. Make them over the top opulent and extravagant. And who doesn't like cards of precious metal coated/plated with diamond. You could even throw in some holographic properties as well, so something like "foil cards" from some of those collectable card games like MTG or Pokemon (you know the ones).

I just wanted to go for the ridiculously fabulous and expensive answer :-p If it's a high profile high stakes game, part of the fun would be the exotic nature of the cards. So may them something special and flashy.

Preventing counterfeiting is pretty easy. It could be something as routine as the difficulty in making them and their intricate detail. Or you could make the cards worth more then the actual winnings of the game. This way it wouldn't be worth faking them ... ha ha.

  • slim - Vapor deposition can be on the order of a few atoms thick, if you have the tech for it. The thickness would depend on how strong the underlying "backer" is. You could start with something like titanium, plate that with gold, silver, copper, platinum for the designs. Then cover it with the diamond. Like a lamination.

  • stackable - check. Although they are not bendable.

  • marks - they are basically diamond so with the high gloss any scratches would show easy. They are hard, so you would need something harder then diamond to even do the scratching. I'm not sure how hard it is to write on diamond, but if you added enough layers to give it depth, it would look like any marks float above the background, because you would have a under layer of background and several layers of "clear". Similar if you made them somewhat holographic, any marks would stand out like a sore thumb.

  • card types - is a given and sort of out of the scope of the construction of the card, and more a matter of the specifics of the game.

  • identical backing - see above.

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  • $\begingroup$ The cards would all have to be exactly the same weight and surface texture. A really good eye could detect the value of a card (especially the difference between a face card) based on how it moved across the table. $\endgroup$ – Justin Thyme Mar 4 '18 at 13:33
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    $\begingroup$ Sure, so they are all the same weight. I don't see an issue with that. It shouldn't be difficult to make it so. One would assume that issue was taken care of as part of the manufacturing process. And indeed, a counterfeit card could be found by it's weight not exactly matching the specs.... If you mean because a card with a bigger symbol or say gold instead of silver will be different. Then I am sure the weight difference can be dealt with by adding material that may not even be visible, such as a negative image embedded in the layers $\endgroup$ – ArtisticPhoenix Mar 4 '18 at 13:52
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    $\begingroup$ For example you have a card with the image of a king on it (in gold and silver), so under that layer you do a negative of the king (in gold and silver), so that those layers have a constant weight for all cards. you have a set weight of gold and a set weight of silver... etc As far as the color layers go they could be only a few atoms thick, just enough to be opaque. $\endgroup$ – ArtisticPhoenix Mar 4 '18 at 13:57
  • $\begingroup$ Would the differences show up in infra-red, from the back (different heat absorption of the gold and silver) so a patron wearing glasses that could detect heat would recognize them? $\endgroup$ – Justin Thyme Mar 4 '18 at 14:24
  • $\begingroup$ That I don't know, but if you did a negative image of it as I described above it would cancel out. I doubt you can tell by just infrared if something is gold or silver. You may be able to tell by the light it reflects (but does gold reflect light through titanium), but again you can have many layers so you put a material in that reflects infrared if you need to. The nice thing about laminate layers is you can stack different materials in them for different bulk properties. So even if that was a problem (I don't think it is) it could be fixed by adding a layer of something that is opaque $\endgroup$ – ArtisticPhoenix Mar 4 '18 at 14:42
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The question boils down to: How can we make marking cards impossible?

The answer is simple, and Casinos have used this rule for a long time as well - don't let the players touch the cards directly.

If you want to go the complex route, you can even set up an interface where the players input commands or actions and a robotic hand does the actions far away.

If you set up the game to be digital, VR, or AR, this will also avoid the problem.

Don't over complicate the problem and make extra work for yourself.

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  • $\begingroup$ This is obviously the correct answer, but it takes all the fun out of playing cards! $\endgroup$ – JBH Mar 4 '18 at 2:54
  • $\begingroup$ @JBH I don't know about that, when I go to a casino and play black jack, they never let me touch the cards, but I still have fun. $\endgroup$ – Aify Mar 4 '18 at 2:55
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    $\begingroup$ @ JBH Next, they will not let you handle the dice at the craps table. Wait,some casinos actually do that now! The die are cast using a cup. $\endgroup$ – Justin Thyme Mar 4 '18 at 22:14
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    $\begingroup$ That's not always enough. In this case for example, the players were able to mark the cards without touching them (they made the dealer do so unknowingly). $\endgroup$ – GilZ Mar 5 '18 at 11:54
  • $\begingroup$ @GilZ That used edge sorting, which could be solved by simply having perfect card-backs. $\endgroup$ – Aify Mar 6 '18 at 18:22
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A card isn't a card until it's dealt.

The cards are digital. The value is shown on a screen, like a mini-LED display. They are blank, until they go through a 'dealing' machine, which uses some form of, perhaps, RF to 'transfer' the randomly-generated card value into a chip embedded on each card. When they are collected, the values are completely erased until dealt again. Sort of like the concept behind video slot machines.

Thus, the face of every 'real' physical card will be different, every time it is dealt. Marking it will be of no use. The next time it is dealt, it will change value.

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    $\begingroup$ But then you have to solve the problem of making your dealing machine unhackable. $\endgroup$ – Gregor Mar 4 '18 at 16:00
  • $\begingroup$ Same problem with video slot machines. How well do you trust them? They are random by algorithm, not random by pure chance. $\endgroup$ – Justin Thyme Mar 4 '18 at 22:04
  • $\begingroup$ @JustinThyme: are are a few true RNGs that use statistical noise to generate numbers. random.org is possibly the best known, though I can attest from personal experience that it’s a bit tricky to build your own and avoid the various pitfalls that make it not random. $\endgroup$ – Joe Bloggs Mar 6 '18 at 7:51
  • $\begingroup$ @Joe Bloggs There is no such thing as a true RNG that is powered by an algorithm. There are algorithms that produce random-like sequences of numbers, that tend to satisfy requirements for a random number, but ultimately if an algorithm can generate it, then the algorithm can predict it. If the same absolute situation will produce the exact same number, it is not a true random number. True random numbers are not reproducible, even if given exactly the same inputs. $\endgroup$ – Justin Thyme Mar 6 '18 at 14:28
  • $\begingroup$ @JustinThyme: you’re talking about Pseudorandom number generators (PRNGs). I agree, they are not truly random. There do, however, exist true random number sources. Random.org uses one such source (atmospheric noise) to generate truly random unreproducible numbers, and you can pick up a hardware TRNG for very little. In theory you can crack it if you have a hardware connection to the RNG, but at that point you basically own the system. Heck, there are chipsets you can get that implement TRNGs on the chip using thermal noise. $\endgroup$ – Joe Bloggs Mar 6 '18 at 15:51
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You need to have multiple-factor safety: I am assuming the players are not allowed to bring to the table any writing or cutting instruments, and also the dealer checks for alteration of the card shape (cutting, ripping, etc.)

If so, the cards can be made of shape-memory polymers.

They are polymeric smart materials that have the ability to return from a deformed state (temporary shape) to their original (permanent) shape induced by an external stimulus (trigger), such as temperature change.

Whatever sign is made on the card altering its shape will be nullified by bringing the deck to the trigger temperature, and this can be achieved either by the dealer holding the card in their hands, or by setting the deck in a thermostatic holder.

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    $\begingroup$ This is brilliant, and close to a perfect solution. It solves the most common problems of indentations made with fingernails, rings, or other objects. Will they return to their shape if edges are shaved (by a blade) or damaged (by the rougening along a ring-file)? If they can "fill in the gap" from damage, they're perfect. $\endgroup$ – JBH Mar 4 '18 at 2:40
  • $\begingroup$ What if I rip it? AFAIK, ripping a piece into 2 doesn't allow them to reform into a single piece when exposed to the trigger; it allows it to reform to its original "shape" persay, but retains the property of being 2 pieces. $\endgroup$ – Aify Mar 4 '18 at 8:14
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    $\begingroup$ @Aify - Just make cutting or ripping the card painfully obvious, so the dealer and other players can't miss it - e.g. make the cards hollow with a bit of some obvious material inside them (perfume, acid, something which on exposure to air generates noise / colorful smoke, hard-to-remove paint etc.) - you can make this as pleasant / unpleasant as the casino's style and clientele dictates (a high-class casino can have colorful, perfumed smoke - obvious but harmless - a mobster's casino can have strong acid...). Just make sure the 'reset trigger' doesn't hurt the material inside. $\endgroup$ – G0BLiN Mar 4 '18 at 9:23
  • $\begingroup$ @G0BLiN I'm not so sure that the inside of the card can hold that and then release it when its torn. If it's stored as a gas, there's basically nothing there - if it's compressed into a liquid, surface tension will probably keep it in place, since the card is so thin... $\endgroup$ – Aify Mar 4 '18 at 18:24
  • $\begingroup$ @Aify, there are materials (used to manufacture OLEDs) which are very sensitive to oxygen and moisture, and when exposed to normal atmosphere they release a peculiar smell of burnt. Encapsulating them into the card would result in this smell to be perceived at the minimum damage of the card surface. $\endgroup$ – L.Dutch Mar 4 '18 at 18:34
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Paint the back of the cards with a brittle, refracting white enamel.

When it's intact, it is perfectly white and very difficult to mark with a felt pen or ink without it being obvious (some kind of ultraviolet ink with suitable contact lenses, perhaps...).

The least scratch on the surface, and the enamel will crack, instantly generating a multicolored craquelure that is impossible to miss - a sort of 2D Prince Rupert'ts Drop's effect.

With the appropriate solvent and another hand of enamel, the card can be repaired cheaply.

Several enamels already exhibit this property and actually have to be doped with softening and crack-stopping agents to prevent cracking from happening. The trick would be to add enough softener as to keep the card slightly bendable, but not enough that a scratch doesn't trigger the craquelure.

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  • $\begingroup$ Even more effective if the cards have a kind of varnish that leaves traces on the fingers of the person who marked it. $\endgroup$ – kikirex Mar 5 '18 at 19:01
  • $\begingroup$ If the material was also water and lipid repellent (ie. non-stick) you wouldn't be able to draw on it either. $\endgroup$ – Draco18s Mar 5 '18 at 20:26
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Take any set of cards (paper, metal, whatever). Put them into card sleeves, of which you can produce a lot of identical sets. Only let trusted experts change the card sleeves so the cards are not damaged. Requirements fulfilled.

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    $\begingroup$ This. This is the best answer. $\endgroup$ – Renan Mar 4 '18 at 2:15
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    $\begingroup$ If it's impossible to get out a new pack of cards each round (OP requirement), what makes it possible to get out a new set of sleeves each round? It might require clarification by the OP, but it seems this doesn't fulfill his/her requirements at all. $\endgroup$ – JBH Mar 4 '18 at 2:37
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    $\begingroup$ Are the sleeves changed each round? if not, what's stopping the players from marking the sleeves? $\endgroup$ – G0BLiN Mar 4 '18 at 9:25
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    $\begingroup$ @JBH "what makes it possible to get out a new set of sleeves each round" Buy several for each round in advance. OP never said there couldn't be breaks between rounds to exchange the sleeves if needed. Cards in sleeves are slim, stackable, decipherable and have identical backside. What is your problem? $\endgroup$ – SK19 Mar 4 '18 at 14:04
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    $\begingroup$ @JBH - even with one set of sleeves, it could work ... one would de-sleeve and shuffle both cards and sleeves before re-sleeving, so even if there were marks in a sleeve the player wouldn’t know what it holds, since the mark wouldn’t match the card. $\endgroup$ – Megha Mar 4 '18 at 20:51
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Okay, so a comment sparked my imagination.

Use biology.

'Grow' the card structure so that it was self-healing. Make them 'living'.

Every time it was shuffled, make it so it grows a new skin, and sheds the old one.

Every deal, the cards would change their backing, and their 'look'.

Design it so that it screamed when cut or nicked. Maybe even bleed.

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  • $\begingroup$ What happens if you eat one, you know if you get really hungry at the table and just by accident ... chomp. Is it non-toxic... lol $\endgroup$ – ArtisticPhoenix Mar 7 '18 at 1:02
  • $\begingroup$ And if it dies, it would be from a cardiac arrest. $\endgroup$ – Justin Thyme Mar 7 '18 at 3:32
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You could have a system where any marks on the cards would just be instantly detectable. for instance both glass https://www.huffingtonpost.com/2013/09/12/worlds-thinnest-glass_n_3915085.html and something called a silver mirror https://www.youtube.com/watch?v=7I-y3I3VzM8 http://www.sciencemadness.org/talk/viewthread.php?tid=5857 can be made to be extremely thin. if you where to use thin glass sheets with silver deposited on them for a backing you could get it down to a few tens of microns thick (less than a sheet of paper, actually an entire deck would be pretty short i'd probably actually increase the thickness from the minimum to A. make it less fragile ,B.increase the intensity of colour you get on the pattern and C. make it cheaper to about the thickness of a sheet of paper)

how do you get an image on the glass? well by "staining" the glass, glass can be made to absorb different frequency of light by adding trace amounts of different metals. gold, copper and selenium make red. Iron and chromium make blue. silver, titanium and uranium make yellow. tin makes white etc... (since the glass is so thin i'd probably use a wide verity of metals per colour to ensure its as vibrant as possible. so when making red use gold and copper and selenium).

since producing thin yet strong glass is a difficult process to begin with and you presumably want specific well defined shapes i'd probably add theese metals afterwards with a small particle accelerator (which is yes quite expensive but not nearly as much as it sounds). this accelerator would add metal ions to the glass giving it colour and have its beam (or the card under the beam) moved to add them to specific places like an inkjet printer. this process would be done under vacuum but a lot processes are these days so that's nothing special.

as for appearance the card appears: from one side like a plain mirror . the other side however the mirror is seen through a small sheet of highly stained glass, giving it sort of semi-transparent markings. these markings would change in intensity depending on what angle you look at the card from (varying with sec of the angle between the perpendicular of the sheet and the incident light ray i think). looking at it straight on would make the markings appear weak like a water colour (you could probably actually just use the card as a mirror if you felt like it) and they would get more vibrant as the angle moved closer to edge on. This effect would be noticeable during a game especially by someone who's job is in part to notice it and so any markings on the glass would obviously not have it. it also helps that most things wipe off glass.

the price would be highly dependant on the technological aptitude of your civilisation but would probably fall between "ten pack of beer and small car". closer to the small car if this civilisation is more earth like and there aren't enough of these cards to warrant mass production closer to ten pack of beer if everyone has a deck and its the 2070's or so.

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    $\begingroup$ Would these cards not abrade just in the act of shuffling? Glass and mirrors show finger prints exceedingly well. A patron could put cream on their hands. Maybe the patron wears special glasses that filter out or enhance a particular frequency of light. $\endgroup$ – Justin Thyme Mar 4 '18 at 13:38
  • $\begingroup$ there is also no such thing as a one way mirror, so called one way mirrors rely on differentia lighting which cards can't have. If you can see the value of the card from one side you will be able to see it from the other. Glass is a good idea for its durability however. $\endgroup$ – John Mar 6 '18 at 17:10
  • $\begingroup$ while its true that fat and grease adhere to glass they also come off just as easily with a quick wipe and i don't think abrasion is a problem if you grind away any sharp edges on the cards, giving them a smooth bevel. as for one-way mirrors, this is not a one way mirror, it is effectively a mirror with a piece of stained glass on one side if that makes sense? $\endgroup$ – Ummdustry Mar 6 '18 at 19:22
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Don't give them anyplace to hide marks. The less a card has on it the harder it is to hide marks. most marking relies on hiding in the pattern on the card, no pattern no place to hide. Go with a flat white back or any solid color for that matter. Remember to work marks have not visible from a distance, the most subtle marks only work for the dealer becasue they require direct handling to feel.

You could even make most of the card transparant making it even harder to mark. Both make creases, marks, and scratches very obvious, clear ones also stop rubbing since they will stop being clear. There are even plastics that will change color when stressed, so even bending the card to curve it will make it turn from clear to white.

to stop counterfeiting you just use a plastic that can optically or chemically checked. works even better if you add the checking system into the shuffling machine. For more standard anti counterfeiting you can also make the face symbols complex and unique becasue marking that will not matter. A tougher plastic will also have the advantage of being difficult to mark in the first place, you cant scratch it away with your fingernail.

you could also combine this with Alfy's answer and give them completely blank unmarked transparent cards that just act as a stand in for a digitally tracked system combined with something like google glasses. enter image description here

enter image description here

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  • $\begingroup$ The reason card backs are so 'busy' is to mask any marks. Make them blend in, less able to be seen by the player. On pure white surfaces, even small marks are noticeable if you know specifically what to look for. There is nothing to mask it. It is an optical effect, an optical illusion, an artifact of the way we see. The mind unconsciously 'fills in' or 'completes' the pattern. $\endgroup$ – Justin Thyme Mar 6 '18 at 16:11
  • $\begingroup$ This also makes it easier to hide marks however. I am assuming the cards are actually inspected not just relying on incidental noticing. That is the reason I prefer the clear plastic ones, marks are easier to detect mechanically and with the right kind of plastic marking is difficult enough to make it unreliable and/or obvious. $\endgroup$ – John Mar 6 '18 at 17:06
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How about you use a transparent armor for the cards?

The cards might be irreplaceable however you can protect them with sheets similar to those used on phones!

enter image description here

What you see above is Gorila Glass! It is hard enough to stop most attempts to mark it! And in case it is scratched it can be replaced rather easily!

It might be hard to When two sheets are stuck together you can use magnetic sheets on both side and a special device to pull them apart.

I think it fills all the requirements:

  • The cards are slim.
  • You can stack the cards.
  • Markings can be easily seen.
  • They all have identical backing.
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Prevent marking from giving an advantage;

If each player and the dealer has a privacy shield large enough to keep other players from being able to see their cards, while blocking the view of the others, there is no advantage in marked cards. Dealing the cards could be either put in a box / envelope or covered so players can't tell during transfer.

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  • $\begingroup$ Except that privacy shields prevent other players from seeing you exchange cards for one 'up your sleeve'. Cards are always visible at all times by all players for a reason. $\endgroup$ – Justin Thyme Mar 6 '18 at 16:17
  • $\begingroup$ blocked from the players view is different than blocked from the dealers view however. $\endgroup$ – John Mar 6 '18 at 17:12
  • $\begingroup$ Depending on the setup for the game, there could be cameras/security or cards made in a way that it is difficult to counterfeit $\endgroup$ – M'' Mar 6 '18 at 19:41
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If your using physical cards it is simply impossible to prevent a determined cheat to get marked cards into the game or to effectively stop them from gaining some advantage by using some scheme to mark the cards during play.

Some marks are so subtle, fit so well into the cards that it can take experts days of study to figure out how it was done. It is almost impossible to confidentially secure a card game.

Electronic cards already exist, just go online nd play any card game and you will see them. They cannot be marked. However securing the identity of these cards to all but the intended player is a problem as complex and hard to solve as that of marked cards. So far in online poker everything from the shuffle with a PRNG to the packets used to deliver the cards to a player have been hit and compromised, and even with 20 years of online poker play behind us, there is no guarantee that other hacks are not coming.

Playing cards have been around for a number of centuries now, and nobody in all this time has been able to design a deck or a process that is secure. Not likely the question here has a good answer beyond it cannot be done with 100% confidence. About all one can do is increase the difficulty of cheating.

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