Once a person is trusted with a classified document, there is really no way of stopping them from using the information however they want, unless you catch it being passed on somehow. They could copy it or just memorize it. What is the value of the document to bad players, since they probably couldn't verify its authenticity? We have stories of the National Archives having no idea about these documents' existence; how would China know if it is truly real or a fake? Other than it being part of National Security law, is there any real purpose to having documents that are valuable to bad players?
The title Q is kind of nonsensical. The whole point (i.e. definition) of classified/secret info is that access to it is restricted to a limited set of people who (in theory) are trusted to not misuse it and "need to know" it. Yeah, occasionally such people defect and "spill the beans" to some other/opposing side or just make it public, but both vetting those people beforehand and punishing defectors [in the aftermath] when possible reduces the likelihood of that happening. A single person being in possession of mountains of such info (cough, Snowden, for instance) makes the damage much more extensive than what someone could merely remember etc.
there is really no way of stopping them using the information however they want unless you catch it being passed on somehow.
Well yeah, that's why it's all closely monitored, so you can catch people when they try to pass it on.
They could copy it
They could, but that would almost certainly be treason and they'll go to jail.
or just memorize it
All jokes of old people in positions of power aside, these documents are often dozens of pages or longer, you're not going to be able to memorize much of them.
What is the value of the document to bad players since they probably couldn't verify its autheticity?
Bad players have entire departments dedicated to verifying the authenticity of documents, they will figure it out. This is basically just police/detective work, and people are pretty good at it these days.
Other than it is part of National Security law, is there real purpose to having documents that are valuable to bad players?
Unless your alternative is "purge all this information from existence", yes it's useful to have these documents. They just contain all sorts of details of secret government operations and plans, sensitive things they learned about other countries, information on their own internal workings & structure, and all kinds of other things that you need to keep a country going and that can be abused by other countries.
A simple example of a document could be a list of spies operating in another country; you need that information because you need to know who your spies are, if the document falls into that countries' hands they'll be able to know who your spies are, but it's likely dozens of pages of details on their operations so someone allowed to read it is unlikely to memorize more than maybe a few names and then it's hard to know if it's authentic. But if you have the entire document, all those spies are going to be in danger.
Information is real currency in high-level diplomacy.
While it is true that bad actors cannot fully verify the authenticity of a paper document due to potential forgery, the off-chance that they might be real is still valuable information.
For example, if China obtains documents about US-EU negotiations on EV trade, they might know that US is planning to make their own EV and EU is looking for new buyer. With this knowledge, they know they have an upper hand when it comes to opening their own EV market to EU, all because US could not keep their own documents safe.
The reverbrating impact of document leak is unrpedictable depending on who obtains them. So there is good reason to try to contain their spread.
Keeping a secret is difficult and it can fail on a whole range of factors. For example any bit of information can:
- be intercepted
- fall into the wrong hands
- be compromised
- reveal relations
- compromise the exchange channel
and many more. And on top of the content of the message you've got metadata, that is data about data. So idk if two countries are at war and you get awareness of an unauthorized phone call going out from a government agency in your country to one in the other country, then regardless of the content of that call you might already expect treason. So the call itself is the information, the content is just bonus.
Similarly if you know how official documents look like, that makes it easier to forge them or might help break ciphers. Like for example if you know that each morning a routine message is broadcasted to all troops (easy to intercept), that always starts with the same formula for greeting and farewell, then you know what the original must have looked liked and what the cipher looks like and so you can possibly deduce the nature of the cipher and maybe even the code itself. The thing is while there are theoretically unbreakable ciphers, you'd still need to transfer the code to decipher the message (which can be intercepted), so either both sides need a code book or cipher machine (from the start, i.e. difficult to update), which would be physical objects that would be VEEEEEEEEERY dangerous in the wrong hands, or you'd need to use a weaker cipher, that is more versatile, but could be vulnerable to such known plaintext attacks. There are also asymmetric encryptions that allow sender and receiver to use different codes, however they might be vulnerable to other methods such as guessing and quantum computers that are fast guessers.
So it's not just the information that is of interest but also whom your communicating with (affairs, treason, business relations or relations in general, aso...), when you communicate with them (might map your schedule out of that), what the structure and style of that message looks like (in case of emulation, forgery, manipulation or verification) and many more.
Also verification is another difficult topic, because as information are subject to forgery and manipulation you're kind of in a predicament, because on the one hand you want the receiver to verify it is you who sent the message and not someone else (in case of for example orders), while depending on the content and context of the message you might also want to have plausible deniability (don't know what shady business you're plotting or what is not yet meant to be publicly known). So anything from the strength/durability of the paper, to the ink of the printer and slight imperfections in the font type or whatnot anything can serve as an identifier for better or worse. Not to mention specific dirt or even trace amounts of DNA and fingerprints. And even if the current document is worthless, the next one with the same markings might now be identified because of that. Kind of like an adverse 2-factor identification.
So essentially the more someone tries to make something a secret the more valuable next to ANY information that you can get from them and about them, can be to you, in the quest to compromise that secrecy.
Usually that's too much of an effort in most cases but when it comes to head of states, megacorp leaders and whatnot, it suddenly becomes viable in terms of the cost/risk/reward ratio.
PS: Also if you restrict information, then the leak of information might also be an information. Like if the list of people who got access to that kind of information is rather short you already know that among those there's either a mole or another breach/imperfection of security protocol.
OK, besides the other answers, let's take 2 assumptions from your Q which are flawed:
They can memorize/copy.
They can, sure. But what if it is not so much them, as their maid, which has been planted as a spy (wouldn't work with POTUS or anyone high enough to warrant secret service protection, but might lower down) and now snaps pics of carelessly kept documents in their home office? People have been planted in embassies in the past to do that.
they probably couldn't verify its authenticity
Maybe they don't have to. If you are the North Korean secret service and you see a list of informants passing on NK state secrets to the US, you can just put those people under extra surveillance and you will catch them offguard eventually. Although NK may take more coercive shortcuts to figuring things out.
Bottom line is documents with actual secrets in them (not overzealous classification) have good reasons to be carefully secured.
In reverse order,
Is there any real purpose to having documents that are valuable to bad players?
The answer to this question is simple: The documents are valuable to the holder. It would be impossible to make a nuclear weapon, a stealthy aircraft, a spy satellite, an encryption algorithm, whatever, without very detailed descriptions of those devices or concepts. That they are valuable to the holder of course also makes them valuable to the bad players. That the information is potentially valuable to the bad players is why the information is protected.
What is the value of the document to bad players, since they probably couldn't verify its authenticity?
That has been a problem and has been a part of the spy versus spy game for millennia. Countries (and companies) sometimes intentionally leak false information. They sometimes intentionally feed false information to people suspected of being a spy or a mole or an informant. The solution is to vet the received information.
What's the point of controlling the physical possession of classified documents?
Nowadays, it's not just physical possession. It's also electronic possession. The point is that the information is valuable.
For a complex system, it's fairly easy to get to the point of having invested a person-millennium in the development of that system. That's the equivalent of one person working on something for a thousand years, which is not possible. However, it is possible for one hundred people to work on something for a decade, or five hundred people to work on something for a couple of years. A "bad player" can shortcut that person-millennium of research and development time by stealing the information. So organizations protect it, screen individuals who have access to it, and limit how the information is accessed.