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BitAuth for Decentralized Authentication (bitpay.com)
105 points by richardburton on July 1, 2014 | hide | past | web | favorite | 40 comments

OK, I'll be the obligatory critic: I don't see how this solves any problems that TLS client certificates don't. The introduction says BitPay considered client certificates, but doesn't say why they rejected them. And "easy to implement wherever TLS is implemented" sounds like more of a selling point than "easy to implement wherever the Bitcoin protocol is implemented."

Also, it seems a little questionable to claim "passwords may travel over plaintext" as a benefit of BitAuth over password authentication. My initial impression is that when not using HTTPS, BitAuth is more secure than password authentication against a passive adversary, but that both are equally vulnerable to MITM attacks.

Author here. Thanks for the healthy criticism! This is still a work in progress, and we're looking for this kind of feedback.

We had the objective of minimizing the surface area for attack, so we did not want to expand the scope to include non-ECDSA algorithms. Utilizing the same curve as Bitcoin (secp256k1) has a number of advantages, not the least of which is incentivizing the verification of its security (not to mention, the integrity of the secp256r1 curve is in doubt).

Furthermore, the path to building a clean user experience around a clean Javascript library that runs [and performs] well in all browsers is much less tenuous than getting browser support for a new algorithm — lending to faster innovation.

Can you unpack on why begin a new protocol with an algorithm in doubt? I understand implementations are well understood, but the "incentivizing the verification of its security" period will seem small comfort when the break comes, no?

For other readers, here is some discussion of secp256k1 security. (You mention r1 above, but BitAuth references k1?)

http://blog.cr.yp.to/20140323-ecdsa.html https://bitcointalk.org/index.php?topic=380482.0 http://lists.randombit.net/pipermail/cryptography/2014-Janua...

If you register for a site where the connection is safe, then go to a dangerous place, this method has an advantage over a password even during MITM attack.

The attacker can't imitate you because the site has your public key already.

But the protocol doesn't do mutual authentication, so without using HTTPS, you have no way of ensuring that you're contacting the correct server. So during the authentication process, the attacker can control your view of the site and get you to sign whatever login requests they want.

Yeah I think you're right.

And if all authentication is done client-side with javascript, the attacker could steal your private key and use it to attack other sites.

You can still use SSL, why that argument? BitAuth makes you sign-up and log-in with a public key, SSL plays along to verify the server. Win-win?

Sure, and any sane implementation of BitAuth should use SSL.

The point I'm trying to make is that if you can trust SSL to protect a BitAuth signature, you can also trust it to protect a password. So claiming "passwords may travel over plaintext" as a relative advantage of this scheme is disingenuous; in both cases, you need to provide that protection at a lower protocol level.

That's right. But it does reduce the cost of a compromised server. Since no passwords are stored in the server itself, if it's compromised, the user's passwords won't be exposed (even if they're encrypted, they're often weakly encrypted, especially as time goes by).

Passwords should usually not be encrypted. Saying a password is encrypted implies that there is a decryption function that can produce the original password in plaintext. This should not be the case. Instead, typically, a key derivation function like bcrypt, scrypt, etc. is applied. The output of these are sometimes referred to as "password hashes" because like hashes they are not reversible.

Yes, my mistake. I meant hashed.

If someone could make the UI work in a browser, client certificates will come back with a vengeance. Until then...

It's Tuesday, so I guess it's time for another "assume a public-key infrastructure" post - this time with OMG BITCOINZ sauce on top.

It still handwaves away the biggest problem: getting users to store a private key securely. If that was straightforward, we'd all be using GPG already...

For decent security, both problems need to be addressed- trusting the identity of public keys, as well as securing your private key.

Unfortunately, I can't think of a decent key-protection solution that doesn't involve custom hardware.

This seems like the HMAC authorisation in OAuth 1, moved to HTTP via JS and custom headers. If the comparison is right, it should be a generally secure and easy to implement auth protocol.

It would be great for something like this to be commonplace, but the UI definitely needs to be solid. A few questions looking at it initially:

- How does the browser know your private keys?

- How does the browser know how to send the signed header?

- How are nonces managed (especially over multiple devices, if the new nonce always needs to be higher than a previously used one)?

- Can you have more than one SIN?

- If so, what's the benefit of having multiple SINs vs multiple passwords?

On a sidenote, bitpay make some seriously kick ass tools and I love how creative they are.

Multiple SINs (read: identities) are an intended benefit of this mechanism. In fact, nonces are verified per SIN, so this provides sequence enforcement per client as we intend for each device to have its own SIN.

We have a reference implementation for key management in [our example client](https://github.com/bitpay/node-bitpay-client)[1], and will be publishing a more formal document as soon as it is complete.

[1]: I am fully aware that HN does not support proper Markdown.

Not only does HN not support markdown, but it breaks links you write using markdown :)

Really nice work. Question - if you generate the private keys client-side and use a password only to decrypt them for use (still client-side), wouldn't this make associating accounts across devices kinda difficult? As insecure as email/password auth is, it's pretty portable.

Looking forward to playing with bitauth!

Not particularly. The pattern for resolving this is already increasingly commonplace; login with your username and password, then confirm the login from one of your already authorized devices.

In fact, this even lends itself to expansion into n-factor authorization methods.

So, they've basically just reinvented SRP [1] using EC crypto. I'm a big fan of SRP and use it in production, but it has already years of refinements which makes it very secure. Designing auth mechanisms is hard, there are many subtle ways you can get it wrong. If you're looking for this kind of auth mechanism, I recommend to take a look at SRP first.

Still, kudos for their efforts. If they continue to improve this, it could take a fair amount of auth "market" some day.

[1] http://srp.stanford.edu

As usual in decentralized authentication posts, I'd like to mention Mozilla Persona[1] which is a decentralized third party authentication service over the open source BrowserID protocol.

I don't mean to diminish the author's work in any way; but if you're interested in these things and don't know about Persona, please look into it!

[1]: https://www.mozilla.org/en-US/persona/

I lost interest in Persona/BrowserID when it became apparent that hosts like Fastmail[1] were unable to act as identity providers.

[1]: https://github.com/mozilla/persona/issues/1523

The right intrepation is that Mozilla is not funding the project, but the protocl (browser id) and Persona source code are still out there.

The downside is Mozilla MIGHT sunset their Persona infrastructure one day. But that's not a real concern because Mozilla is never interested in being people's Persona verifier. They were hoping Persona would take off and people would host their own bridge, instead of relying on Mozillia's.

Persona was not discontinued. Persona lost active development and marketing. A wrong decision, but not one that affects the protocol itself.

I mean this stuff is just catnip for me. Unexpected new uses of technology from one domain to another, just serves to reinforce how seminal bitcoin was/is.

Having looked at it with my very layperson eyes, I struggle to see how it is significantly better than say client side certificates. Same idea - sign a request with local private key, only difference is using the SIN mechanism to let the server know I am to be trusted.

But it's fascinating to see the evolution of these things in real time

Digital signatures were not invented with Bitcoin.

TLS supports client certificates perfectly well, and your browser probably supports them.

This scheme also has a lot of beginner-level oversights, IMO. It doesn't delimit the URI from the body, and it doesn't protect the HTTP method itself nor any potentially important headers. This makes me suspect that no cryptographers laid their eyes on this before the announcement.

I really really really want to do a redesign of the client certificates UI for Firefox, but alas I'm not a UI person.

Client certificates certainly work, but most people wouldn't see them in their browser, let alone seek them out.

I agree. I don't see where this is significantly better. You can add data to signed certs as well, so you can get the SIN functionality that way if I'm not mistaken.

I also didn't see if this touched the blockchain at all. They made note about the SIN being public (then again, so is a Public Key), so I'm looking for the SIN to be in a blockchain somewhere.

I do need to dig into SIN for that same reason - it makes sense - I think a lot of government identity and records schemes would get vastly simpler if I put my public key online and encrypted my own records

I discussed it with a govt architect who wanted to get rid of the "bloody database at centre of every application that thinks it needs to store citizens data"

It's a powerful idea

The most important piece that needs to be handled is described in the github repo:

'There also needs to be a public place to store SIN's, preferably in a decentralized blockchain or datastore like namecoin. Key revocations could be stored here as well as reviews/feedback to build a reputation around an identity.'

Otherwise, apps that integrate will need to store SINs in their user tables. I see that this would add an unnecessary amount of work around management of SINs.

Great work, I'm looking forward to see where this goes.

So the workflow is generate asymmetric key pairs, generate SIN, sign the request with your private key to verify?

How exactly does it work for multiple devices? Each device has its own key pair and an unique SIN? How do we connect multiple SINs as one user?

Supporting multiple devices is a hard problem. We worked for a long time to enable it. Not Bitcoin protocol specific, but some words about our asymmetric key based multi-device solution here; https://www.mepin.com/lost-device/

The solution ^ still require a traditional account registration, or at least connection to the device that was used to create the account (if we go with passwordless registration). Otherwise, "must link their MePIN app to an actual user account in the target service" wouldn't be true.

How is this different/better than SSH?

Why on earth would you outsource two factor auth to a cloud service?

Talk about putting all your eggs in the same shared basket.

Funnily; the reason to outsource is exactly about not putting all your eggs in the same shared basket. You outsource the 2nd factor and keep the first factor (passwords) in-house. Implementing everything in-house is a "same shared basket".

So you're dependent on a cloud startup for basic auth and you've got no more security than any other isolated two factor auth system.

looks great, very professional... but it appears not to be free.

If an application wants to offer their users multi-factor authentication with LaunchKey it's free. If the application wants to use it internally for their employees with some of the Pro features or requires forced factors then it starts costing money. Non-profits it's free.

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