Privacy-preserving age and identity verification via anonymous credentials
blog.cryptographyengineering.com
blog.cryptographyengineering.com
Anonymous age verification isn't a technical problem to be solved, as it's already been solved, it's a societal problem in that either the companies or the politicians pushing for age verification don't want to support it.
I've tried to explain this to people, that a digital ID done well is better than the fraud-enabling 1960's hodgepodge in use that has served fraudsters better than citizens for 30 years. They set their teeth and refuse based on use of the word "digital" in the title alone.
It will take generational change for the US to get something as banal as a digital ID already in use in dozens of countries, for no other reason than mindless panic over misunderstanding everything about digital ID systems, how IDs even work, and how governments work.
States in the US in a lot of ways are more comparable to countries in the EU. It's not exactly like that but in many ways it is. So it would be like requiring an EU ID on top of a national ID.
I also don't think privacy per se is the real issue of concern, it's concern about consolidation of federalized power. Privacy is one criterion by which you judge the extent to which power has been consolidated or can be consolidated.
The question isn't "can this be federalized safely in theory", it's "is it necessary to federalize this" or "what is the worse possible outcome of this if abused?"
As we are seeing recently, whatever can be abused in terms of consolidated power will be eventually, given enough time.
I guess discussions of whether or not you can have cryptographic verification with anonymity kind of miss the point at some level. It's good to be mindful of in case we go down the dystopian surveillance route, but it ignores the bigger picture issues about freedom of speech, government control over access (cryptographic guarantees of credential verfication don't guarantee issuance of the id appropriately, nor do they guarantee that the card will be issued with that cryptographic system implemented in good faith), and so forth.
If the only thing that came out of the ID was those letters then you wouldn't need the ID, you could just type "yes" or "no" when the site asks you if you're over 18. So it's presumably not doing that, instead it's providing some kind of signature.
And then the privacy implied by "just returns a yes/no response" isn't actually there, because it's actually returning more than that. Does the response have a fixed signature which is unique to the ID, therefore able to be correlated across sites? Does the ID have a unique public keypair that it uses to sign, with the same problem? If someone extracts the key from one ID, or just hooks it up to a computer, can they now set up a service to anonymously sign for everyone in the world? If they can't anonymously sign for everyone, can't the same mechanism used to identify them also be used to identify anyone else?
"Someone attempted to do this but no one uses it" is no proof that their attempt was any good or addressed the concerns people have about doing this.
As I'm not an expert on the crypto underlying the protocol, feel free to check the eIDAS standard for more info (the documents are in English, even if the link is not): https://www.bsi.bund.de/EN/Themen/Unternehmen-und-Organisati...
https://en.wikipedia.org/wiki/Group_signature
Which allow the group manager (presumably the government, or anyone who compromises them) to identify who signed something.
If using the same card multiple times with the same site allows the site to correlate them then that obviously also allows the site to link two accounts you intended to be separate, or two sites to set themselves up as the same "vendor" and thereby correlate your accounts between them.
Trust is bad in security. It's not complicated to understand this.
Something as simple as a JWT with claims (and random uuid id) would work
1. You don't want these to be replayable (give your JWT to someone else to use) so they need to be bounded in some ways (eg intended website, time, proof it came from you and not someone else).
2. You don't want the government to know which website you're going to, nor allow the government and the website to collaborate to deanonymize you (or have the government force a website to turn over the list of tokens they got). So the government can't just hand you a uuid that the website could hand back to them to deanonymize.
The SD JWT and related specs solve for these, which is how mDL and other digital IDs can preserve privacy in this situation.
But these are the things that make it non-anonymous, because then instead of one token that says "is over 18" that you get once and keep forever, everyone constantly has to request zillions of tokens. Which opens up a timing attack, because then the issuer and site can collude to see that every time notbob69 signs into the website, Bob Smith requested a token, and over really quite a small number of logins to the site, that correlation becomes uniquely identifying.
Meanwhile we don't need to solve it this way, because the much better solution is to have the site provide a header that says "this content is only for adults" than to have the user provide the site with anything, and then let the user's device do what it will with that information, i.e. not show the content if the user is a minor.
On top of that, it's a timing attack, so all you need is the logs from both of them. Government gets breached and the logs published, all the sites learn who you are. Government becomes corrupt/authoritarian, seizes logs from sites openly or in secret (and can use the ones from e.g. Cloudflare without the site itself even knowing about it), retroactively identifies people.
Unless you're getting the device logs from the users device (in which case... All of this is moot) there is no timing attack. Six months ago you got your mobile drivers license. And then today you used it to validate your age to a website anonymously. What's the timing attack there.
The article proposes a hypothetical solution where you get some finite number of tokens at once, but then the obvious problem is, what happens when you run out? First, it brings back the timing correlation when you ask for more just before you use one, and the number of times you have to correlate in order to be unique is so small it could still be a problem. Second, there are legitimate reasons to use an arbitrarily large number of tokens (e.g. building a search index of the web, content filters that want to scan the contents of links), but "finite number of tokens" was the thing preventing someone from setting up the service to provide tokens to anyone.
I'm thinking perhaps a system where you feed it a credential, a small program runs and maintains a pool of tokens that has some reasonably finite lifespan. The server that issues the tokens restricts the number of uses of the credential. Timing attacks are impossible because your token requests are normally not associated with your uses of the tokens.
And when you use a token the site gives back a session key, further access just replays the session key (so long as it's HTTPS the key is encrypted, hard to do a replay attack) up to whatever time and rate limits the website permits.
I feel like "we should ban all search engines" is going to be pretty unpopular.
> And when you use a token the site gives back a session key
And then you have a session key, until you don't, because you signed out of that account to sign into another one, or signed into it on a different browser or device etc.
> The server that issues the tokens restricts the number of uses of the credential.
Suppose I have a device on my home or corporate network that scans email links. It's only trying to filter malware and scams, but if a link goes to an adult content barrier then it needs tokens so it can scan the contents of the link to make sure there isn't malware behind the adult content barrier.
If I only have a finite number of tokens then the malware spammer can just send messages with more links than I have tokens until I run out, then start sending links to malware that bypass the scanner because it's out of tokens.
This is what VPNs or public libraries are for.
> I dont think government should be part of the average person's threat model.
Tell that to the people in places with governments that are a threat to the average person.
"It can't happen here" is a dangerous hubris.
On top of that, do notice that there is more than one government. What happens when Salt Typhoon comes for this stuff?
People can loan their ID to someone else (ask college kids with an older sibling...)
When you use your physical ID, the government frequently can deanonymize you either through automated databases (especially when purchasing drugs) or subpoenaing for camera footage, visitor lists, etc
The idea was your id would be an autehnticator of sorts. You need to verify yourself, the website asks Aadhar if the person is genuine, the website returns binary yes no. Same for you, is gender male? Or ages above 18?
They would not return any other data.
In the end, it became just another "formality" and tool for politicians and to flex muscles.
People ended up taking photocopies of your card "just in case" and "that's the norm" even when it was said that's a bad idea.
People still do Aadhar kyc but it is in hands of politicians now and the bureaucracy.
The trick with age verification is to do it in a way that doesn't allow tracking by the service itself (i.e. returning the same token/signature every time) or from the government (shouldn't see what sites you use when). That has pretty much been solved now, though.
Anonymity from whom? Does the German government doesn't know that Gunter Shmidt has just verified his age to the site GreatBDSMPartiesInBerlinForDragQueens.com ? Even if they obtain the logs from the site?
Surely the safety of children is worth it right ?
Right now with age assurance laws and online services there has been no singular approach beside falling back to use of government ID that any country has required. Each country has just said 'here are the minimum criteria, choose what you want' and left it up to services to comply.
So what have services chosen? The least friction and cheapest existing solution to be compliant. For most services that's been using readily available facial scanning services and government IDs as fallback. Not all of them of course but it's so scattered that it makes it difficult for a person to know what they'll need for one service vs another (and perhaps even avoid use of a service if their approach doesn't align with the person's values).
Without mandating better minimum privacy criteria governments can just point to the fact they're not preventing such tech from being used and leave it at that. But solutions also need to be affordable to adopt for a wide range of sites/services and have good support (interfaces, etc) around them to catch on so it's not just entirely whether tech exists per se.
If anyone implemented this privacy preservation scheme, would all the laws flip to say "yeah we really did mean it govt id tied to your post".
Maybe wait for the next terror-attack before pushing for it, but it's an easy fix to a culture that already accepted a layer control against the user. The end user will only perceive a small difference in whether they provide full ID or just verified age information.
I want to believe that some supporters of age verification are not cynical. However, whatever good can be achieved through age verification seems such a small win, compared to the dangerous precedent it sets for the internet in general. I cannot get my head around it.
In the real world it's always people looking to suppress information or dissent that are pushing for such schemes. It always masquerades as protecting minors (protecting them from what? The one proper attempt to prove sexual materials are harmful found no evidence of said harm.) or as hunting for CSAM (and if you do implement an effective system it will get circumvented by putting relays in hostile countries.)
The key issue however is trust. The underlying protocols may support zero-knowledge proofs. But as a user I'm unlikely to be able to inspect those underlying protocols. I need to be able to see exactly what information I'm allowing the Issuer to see. Otherwise a "correct" anonymous scheme is indistinguishable from a "bad" scheme whereby the Issue sees both my full ID and details of the Resource I wish to access. Assuming a small set of centralized Issuers, they are in a position of great power if they can see exactly who is trying to access exactly what at all times. That's the question of trust - trust in the Issuer and in the implementation, not the underlying math.
For more information check the out technology behind it: https://www.eid.admin.ch/en/technology
The data is of such form that the phone then can pass challenges of type "are you of at least x years old" without giving out any other information.
And the user cannot share that data with other users because their phone will not let them.
From what I understand the issuer signs a credential and then the user on their local device generates unique proofs based on the signature each time, preventing verifiers from colluding/tracking the original signature across services. It also seems to be designed with safeguards against the issuer.
Info based on credentials can be selectively disclosed like whether you're over 18 or whether you have above a certain threshold in an account without disclosing the underlying data.
Obviously if the type of services you use need literal PII then they can still tie activity to a real-world identity but for services only requiring age assurance being able to prove you're over 18 without providing the actual age or other identifiers is better than solutions being actively used.
see this image from the article: https://blog.cryptographyengineering.com/wp-content/uploads/...
The central question the post attempts to answer is "The problem for today is: how do we live in a world with routine age-verification and human identification, without completely abandoning our privacy?"
My rephrase is an attempt to surface that, compared to the dry and academic title that will get overlooked. I think this is a very important topic these days where we are rapidly ceding are privacy to at best, confused and at worst, malicious regulations.
Just ban social media outright. Facebook, Twitter, Instagram, TikTok, dating apps, etc. They created this problem. They're destroying the fabric of our society. Sometimes the best solution is subtractive.
> These techniques are described in a great paper whose title I’ve stolen for this section.
Let me explain the simple solutions:
Don't let phone manufacturers lock the bootloader on phones. Let the device owner lock it themselves with a password if they want to. Someone will make a kid-friendly OS if there is market demand and tech-savvy parents can install that and lock the bootloader.
What about the non-tech-savvy parents?
Don't restrict people from sideloading apps. Let the user set a password-based app installation lock if they want to. It should be a toggle in the phone's settings. Someone will make kid-friendly apps if there is demand. This lets average parents control what apps get installed or uninstalled on their kid's phone.
But what about apps or online services that adults also use?
Apps and online services can add a password-protected toggle in their user account settings that enables child mode. Let the user set the password and toggle it themselves. Parents can take their child's phone and toggle this.
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Notice how easy these things are to implement? All of these features could be implemented in less than a week. But instead of doing this, they want to implement much more complicated schemes where the gov and corps control all the toggles, and you control none. Why is it like that? Surely there are no ulterior motives, right?
One must be a realist. If there is no profit motive, it won't happen. Ever.
One profit motive is "the government has regulations, I will be fined if I don't do this".
Another is "my competitors do it, and people buy their stuff because of it".
All the technical solutions are easy. And you're right, it's not about age verification, it's about profits.
The same way cars are regulated to have air bags, couches be made from non flammable materials, and so on.
Human nature has been the same forever. It will never, ever change. Ever. Profit drives all.
Ignoring this when planning and discussing won't help the end goal. It will doom one to failure.
The solve must fit the puzzle.
First you have people running around claiming that systems are "anonymous" and "privacy preserving"... meaning that they're anonymous if you trust somebody you shouldn't be trusting. Just simple snake oil. Often offered by people who know perfectly well what they're doing, too.
Then somebody like Matt Green writes something like this, or some standards committee decides to try to do zero knowledge right, or whatever. But (a) people don't understand how it's different from the snake oil, and (b) almost nobody understands how hard it is to get it right. Information wants to be free, and even if you have a perfect privacy-preserving protocol, it doesn't work if you embed it in a workflow that turns around and leaks the information you're trying to hide. So there are an infinite number of more mistakes to argue against.
But all of THAT just distracts from the fact that age verification is a bad goal. Setting up an ubiquitous, actually functional age verification tool is just handing weapons to people like, say, [Ken Paxton, Attorney General of the great state of Texas](https://thehill.com/policy/healthcare/5762893-paxton-opinion...). A system like that is an attractive nuisance that should not exist. What people like that will do with it is far worse than any problem it could possibly solve.
... and that fact gets lost in all the other stuff...