Ending CAPTCHAs: Introducing Cryptographic Attestation of Personhood

Also, what is the cost of a single one of these keys? If it's relatively high (say, $50 each), then this would keep out a large portion of the 4 billion people that cloudflare claims to be seeking to help. If relatively low, then it would enable a farm to be run quite cheaply, even with aggressive rate limiting on each key.

So this does not at all prove personhood, it proves access to money. In that sense, it is nearly identical to a proof of work system. The parallels are actually quite amusing. Recall the slogan "one computer, one vote", which was originally applied to bitcoin, until someone noticed that custom hardware could compute hashes order of magnitude faster than a pc could. I can't see how this system will proceed any differently.

>With our current set of trusted manufacturers, this would be slower than the solving rate of professional CAPTCHA-solving services, while allowing legitimate users to pass through with certainty.

They are only considering speed, not price. Here is a captcha system for you: the site sends you a token. You wait K seconds. The token becomes valid. K is an adjustable parameter, so it can be made longer than whatever the time it takes for captcha solving services to work.

>The very idea that we’re all wasting 500 years per day on the Internet — that nobody had revisited the fundamental assumptions of CAPTCHAs since the turn of the century — seemed absurd to us.

We aren't, and someone has. The majority of people don't fill out any captchas, ever. Google, in its great benevolence and wisdom, monitors their browsing habits. If it determines them to be reflective of a human, then when they click the recaptcha button, it will let them through without a hitch. A very small minority of users behave in ways that are suspect, such as by rejecting cookies, resetting their browsing history, or using tor. These are the users that face frequent captchas. Since they are a heavy minority of users, even if they solve ten captchas a day, it doesn't add up to anything near 500 years per day of captchas.

First, I don't like the idea of having Cloudflare as the sole judge of the trusted key set; a more open model like the CA-Browser forum would be much easier to trust, as it helps reduce perverse incentives (for example, forcing token manufacturers to implement/ignore features, block certain platforms, give kickbacks, etc.)

Second, it's hard to support a proposal where a full deployment would require every person on the internet to buy AT LEAST a new security token (from an Approved™ vendor), which may not even be compatible with their platform, or may be impossible to acquire (because of export restrictions, poverty, someone inventing a cryptocurrency that requires a hardware token to mine...)

CloudFlare -> Browser: challenge{...}

Browser -> CloudFlare: {sha256((device_public_key)^root_signature), (challenge)__device_priv_signature}

CloudFlare -> CA: "get signed public key by hash"

CA -> CloudFlare: {(device_public_key)^root_signature}

CloudFlare -> self: "verifies hash, decrypts returned challenge"

CloudFlare -> Browser: "ok"

Where "CA" is some source of device public keys, "^" means signed, and "__" means encrypted.

I find Alice and Bob stories impenetrable. This is just an HN comment and not a thought through design, but to do the verification, it needs that challenge to bind to the private key on the device, which uniquely identifies it. They need something with more diversification than what I've described to provide privacy and anonymous attestation.

The way they would need to do this with any degree of anonymity would be to use a symmetric key on the device that was both a) derived from a vendor/manufacturer key, and b) the response was diversified by both a KDF on the device, and the challenge. I missed this part in their description.

The other piece is that I don't know how we verify that FIDO token vendors aggregate their batch keys into cohorts large enough that they provide trustworthy anonymity. Maybe I'm into the weeds without details, but what this post talks about is a non-trivial problem.

So you must keep the checkbox unchecked[0]

[0]: https://i.imgur.com/2ORwgEs.png

[0] https://cloudflarechallenge.com/

[1] https://d.pr/i/HEGY85/Uz3jgdqXXZ

I don't care about assurances that they don't do that.

The other issue is more practical: few people have such a key (especially when they won't support what is built into computers), and I for one am more likely to solve a captcha, or close the page, than I am to take of my headphones, go to another room and find my jacket to get my fido token.

The idea isn't bad, but we need to get to a place where bots aren't bad and that websites can deal with them.

Is it true that most of these FIDO keys try to prevent automation by, for example, requiring someone to activate a button on the device?

[0] https://en.wikipedia.org/wiki/Hashcash

If you're a business owner selling office supplies, do you really care if an order was placed by a biological human or by an automatic reorder script? No. So long as somebody is going to pay for the order, it doesn't matter.

Conversely, if someone is trying to brute force a PIN, does it matter if it's a human typing in combinations or a script entering them at the same rate? Nope.

The concern about bots is that they can do things repeatedly and quickly. Placing one order is great, placing 10,000 orders is problematic. Brute forcing a 4 digit PIN is tedious for a human, trivial for a machine. But just because a script can potentially be used to abuse a service does not mean there are no legitimate use cases, and just because it's harder for a human to do these things doesn't mean they won't.

When people want to stop bots, what they really want is to limit the rates and attempts of certain actions. CAPTCHAs somewhat achieve this goal - if the adversary doesn't employ an effective CAPTCHA solver then its effectively an attempt limiter, whereas with such a solver it's a rate limiter. However, since the goal of a CAPTCHA is not to effectively limit rates or attempts, it is not a very good limiter - the captcha might kick in on the first attempt, blocking perfectly legitimate scripts, and with a solver the rate limitation may be trivially small, and remain constant. A properly designed system would only kick in after some suspicious activity had been detected, and would progressively become more burdensome as the user strayed further from expected behavior.

This new system may exceed CAPTCHAs at identifying humans, but that is not what we want. An automatic button presser is much easier to implement than a CAPTCHA solver, and whereas the CAPTCHA solver might be broken by a mild change to the CAPTCHA on the backend, the button presser will work so long as the button its pressing does. Thus it is much worse as an attempt limiter. As a rate limiter, they claim it has a longer interval than current solvers, but again more advanced CAPTCHAs are easily possible, and it still suffers from the same limitation as CAPTCHA in that it must remain unobtrusive for legitimate users, so it can not make its rate limitation too large. In short, this is just a more easily defeated and more difficult to improve version of CAPTCHA.

There may be some niche applications where guaranteeing a human user is actually the desired outcome rather than a proxy, and in those cases this key might be a good alternative to CAPTCHAs for the further small minority who have disabilities that make CAPTCHAs difficult to use, yet for whatever reason have no problem accessing and using a physical device. Still you're talking about an edge case of an edge case of an edge case.

Also, the potential to just use automated button-pushing devices was mentioned. It would be quite a lot harder to automate scanning of my fingerprint.

So ultimately servers should deal with bots on their end. You can rate limit, provide API for bots to pull data etc.

Yea - And they do this since Cloudflare decided to move from a no-click CAPTCHA to one that requires people to click half a dozen images.

So don't you act all innocent Cloudflare - You were the ones who instigated this!