is this built on top of HashCash[1]?
mCaptcha – Proof of work based, privacy respecting CAPTCHA system
51–60 of 100 posts
Re: mCaptcha – Proof of work based, privacy respecting CAPTCHA system
#52Hello, I'm the author of mCaptcha. I'll be happy to answer any questions that you might have :)
I just want to say - people critique every service out there that slows spam and bots. Those critiques are valid from the "it won't stop everything" view, but it clearly stops a proportion, and the wider variety of products out there the less likely a spammer will have a canned answer for a particular site.
Agreed, it's been an interesting discussion so far with lots of interesting ideas. mCaptcha is a very niche software, that will only work some use cases, but that's okay as long as whoever's deploying it is aware of its drawbacks. :)
Re: mCaptcha – Proof of work based, privacy respecting CAPTCHA system
#53From what I understand, this would allow somebody with reasonable resources to easily crack captchas with a high-end GPU. You could increase the complexity of the PoW, but ultimately you then just end up in a resource race.
Ideally you want an attacker to spend as long as possible using up computation resources, whilst your normal clients spend on average little. I believe you would want a multi-stage approach that involves some human interaction, which is statistically failed more often by computers, and keeps them busy for a long time doing pointless math.
The reason it would have to be multi-staged is that you don't want the attacker doing the computing to realize they are solving a pointless list of problems with no hope of getting a token. A normal user might on average do a -> b -> c, whereas you would make an attacking machine do a -> b -> c -> d -> e -> f, and the not get a token for example. (It would have to be statistically setup as not to encourage the attacker to bail early.)
I think I would make it a slightly RAM heavy problem that limits how many of these problems could be solved in parallel. You would obviously setup the problem in such a way that networking is minimal.
For example, you could send a variation of a Langton's Ant map pre-seeded with random patches for a significantly large map, and then tell the client to run for X time steps, then give you the result in some specific area.
Re: mCaptcha – Proof of work based, privacy respecting CAPTCHA system
#54I think the Lightning Network will be useful for this role someday. The only proof of work that ever needs to be necessarily run are on the Bitcoin network, and you can transfer Bitcoin or satoshis or fractions of a satoshi as the representative of work, instead of the doing the work directly.
Re: mCaptcha – Proof of work based, privacy respecting CAPTCHA system
#55I looked into doing something like this once and decided it wasn't going to be very effective, for a few different reasons. JS engines (or even WASM) aren't going to be as fast at this kind of work as native machine code would be. Especially when you consider that libraries like OpenSSL have heavily tuned implementations of the SHA algorithms. Any bot solving a SHA-based challenge would be able to extract the challen…
Yep, I have also done this and come to a similar conclusion. The best performance I got was with the WebCrypto API, where I got, IIRC, 100,000 SHA512 hashes a second, with a very optimized routine for iterating the hashcash string. SHA512 probably makes the most sense since AFAIK it's the most intensive hash supported by WebCrypto, and the WebCrypto digest API is async, so a whole lot of time is going to be spent awaiting no matter what you do.
I think WebCrypto is available in workers, so you could probably use numerous workers to get a better hashrate. Still, I don't suspect that would jump it past a million, which seems pretty bad for a desktop computer, and it would be a lot worse on a mobile phone.
It might still be a meaningful impediment when combined with other measures, but with the low bar you'd have to set for preimage bits for mobile devices, it's a little questionable.
Re: mCaptcha – Proof of work based, privacy respecting CAPTCHA system
#56I looked into doing something like this once and decided it wasn't going to be very effective, for a few different reasons. JS engines (or even WASM) aren't going to be as fast at this kind of work as native machine code would be. Especially when you consider that libraries like OpenSSL have heavily tuned implementations of the SHA algorithms. Any bot solving a SHA-based challenge would be able to extract the challen…
Thank you for your detailed response, you raise some very interesting and valid points! > JS engines (or even WASM) aren't going to be as fast at this kind of work as native machine code would be You are right. mCaptcha has a WASM and a JS polyfill implementations. Native code will definitely be faster than WASM but in an experiment I ran for fun[0], I discovered that the WASM was roughly 2s slower than native implem…
> mCaptcha is basically a rate-limiter
This is a much better explanation of what it does than captcha where I expect "proof-of-human". A PoW based rate-limiter is a really interesting idea! Usually, the challenge with unauthenticated endpoints (ex. signups) is that the server has to do more work (db queries) than the client (make an http request) so it is really easy for the client to bring the server down. With PoW, we're essentially flipping that model where the client has to do more work than the server. Good work!
Re: mCaptcha – Proof of work based, privacy respecting CAPTCHA system
#57Nice! I had written a little algorithm that one could use to implement something like this (maybe interesting if you want to understand how it could work): https://github.com/fabiospampinato/crypto-puzzle
I think there's something to this, it costs you next to nothing to generate these puzzles and you get a guaranteed, tunable, slowdown factor on attackers (or cost increase for them I guess).
Re: mCaptcha – Proof of work based, privacy respecting CAPTCHA system
#58I looked into doing something like this once and decided it wasn't going to be very effective, for a few different reasons. JS engines (or even WASM) aren't going to be as fast at this kind of work as native machine code would be. Especially when you consider that libraries like OpenSSL have heavily tuned implementations of the SHA algorithms. Any bot solving a SHA-based challenge would be able to extract the challen…
Thank you for your detailed response, you raise some very interesting and valid points! > JS engines (or even WASM) aren't going to be as fast at this kind of work as native machine code would be You are right. mCaptcha has a WASM and a JS polyfill implementations. Native code will definitely be faster than WASM but in an experiment I ran for fun[0], I discovered that the WASM was roughly 2s slower than native implem…
> mCaptcha is basically a rate-limiter.
Hmm, is it a better rate limiter than others? I know that nginx, for example, makes it pretty easy to rate limit based on IP address with the `limit_req` and `limit_req_zone` directives.
In essence, ngix's rate limiter also works by making each request consume a resource, but it makes the resource consumed an IP address (or range) rather than compute resources. It seems intuitive that a malicious actor would have an easier time scaling compute than IP addresses, while a legitimate user will _always_ have an IP address but might be on a machine with 1/100000th the compute resources of the malicious actor.
Re: mCaptcha – Proof of work based, privacy respecting CAPTCHA system
#59Wouldn't a spammer almost always be more able and more willing to dedicate resources than an actual user?
Let's say it takes a legitimate user 60 seconds of PoW on their phone. That's a dealbreaker. Now a spammer that takes them 15 seconds on their server, they will still spam.
I guess it could rate limit against high throughput of spam (more like DoS prevention than antispam really), but an IP rate limit would probably work as well if not better. An attacker with a large pool of IPs will probably have a large pool of CPUs anyway.
Re: mCaptcha – Proof of work based, privacy respecting CAPTCHA system
#60As a spammer I wouldn't care much about this, they run thousands of requests in parallel and don't care if they take 1 second or 10 seconds to finish. After all, their machines run all day long.
Much of the more professional spammers are running on hacked botnet machines anyways so they don't really care about maxing the CPUs.
Compute is cheap and the only loser here is the end user with a low end device.