[hadcomplained]

Although I do not deny the necessity for certificate authorities for convenience, I do not understand why using CAs is the only option. Why does the TLS protocol not allow for using a key pair which is agreed-upon between a server and a client beforehand, like in SSH connections where a public key to be used in a connection is placed in a server prior to the connection?

There are many CAs out there and in the event of China or Russia hacking into one of them, it would enable them to perform man-in-the-middle attacks. I'd like to eliminate such a possibility, but the TLS protocol requires me to trust a certificate authority. I might just be a conspiracy theorist, but I am suspecting why it's impossible to use TLS without trusting a third-party called certifcate authority is exactly because someone needed to leave a way to do MITM attacks.

[bmm6o]

I don't think TLS cares why the certificate is trusted, validating certificates is higher on the stack, and you've probably seen the browser UX for accepting an invalid certificate. I know I've added a non-CA cert to my trusted list (on Windows) and gotten the result you are describing.

[1vuio0pswjnm7]

https://en.wikipedia.org/wiki/Trust_on_first_use#First_known...

[mmalone]

Firefox is the only browser that ships with a built-in trust store. Other browsers use your operating system’s trust store and you can add & remove trusted CA certs from that trust store using OS-specific utilities.

The big three root store programs are run by Apple, Microsoft, and Mozilla/NSS. Most (all?) Linux distros are based on the Mozilla store. Until recently, Google also used Mozilla’s store for things like ChromeOS and Android. They just recently announced that they’re gonna start running their own program though.

FWIW the browsers seem to do a pretty good job of policing CAs. Probably better than most end-users would do.

[corty]

Browsers do a crappy job in general for any CA usecase that isn't https on public websites. Guidelines for the WebPKI are, of course, very web-centric. E.g. short lifetimes may be acceptable there. Automated frequent reissuance may be an option. But if you e.g. look at email and auth certs, possibly stored on a smartcard, things are quite different. Lifetime should be long and can be long, because the key is used rarely (compared to webservers) and maybe stored in dedicated smartcards. Verification is another thing thats totally different for an email address or a person's name.

[1vuio0pswjnm7]

Can users remove CA certs from ChromeOS and Android?

[mmalone]

Good question. I know you can add a root CA cert.

I’m on iOS and don’t have an Android or ChromeOS device handy. I don’t see a way to remove a cert from Apple’s iOS trust store (settings just tells me what store version I’m running). There may be a way to do it using mobile device management (MDM) tools.

[SAI_Peregrinus]

On Android (Galaxy Note9 in my case), you can disable any CA certificate from being used. There's a "view security certificates" screen with the ability to disable each individually.

[im3w1l]

It would be nice to use a hybrid: (only) Trust CA on first use. But I guess in practice some random company is much more likely to misplace their keys than you are to be mitmed by CA's.

[mmalone]

Also, the Web PKI model has no real granular authorization when it comes to which CA can issue for which domain. A trusted CA can issue for any domain. So if you TOFU in my CA to connect to my website you’re also allowing me to issue for google.com.

Obviously this is all addressable in theory, but now you’d need some kinda policy system baked in pretty much everywhere.

[im3w1l]

No I meant like this:

Your website hands me a cert. I have never seen it before so I make sure CA says it's legit. From then on I keep using that same cert to connect to you, and CA no longer matters.

[jlgaddis]

I haven't checked/verified recently but from your comment I'm guessing that the major browsers still don't support (i.e., enforce) the Name Constraints extension?

[corty]

There are CAA records in DNS, but those are far too weak. The CAs are supposed to check them at issue-time. To be useful, the clients would have to check them at acceptance-time.

[jlgaddis]

That wouldn't quite work the way you think it would...

The CAA record is useful only at the time a certificate is issued (signed) by a CA.

A client has no way to know what the CAA record was at the time the certificate was issued -- a browser cannot ("at acceptance-time") use the current value of the CAA record to determine whether a certificate was properly issued or not.

[kjaftaedi]

You don't have to use a third party, you just have to specify an authority.

If you say the self-signed certificate is the authority, that's no problem.

In my private lab environment I have one certificate (for everything).

The certificate is installed on all servers, and even the clients use the same certificate to authenticate, and everyone trusts the cert as an authority.

You'd never run a production setup like this, but there is nothing stopping you.

[nsgi]

Because the SSH trust model relies on verifying the fingerprint out of band, which isn't practical with a website, and even if it was non-technical users and even most technical users wouldn't do it. Certificate Transparency is a good mitigation for the risk of a CA being compromised

[tssva]

TLS-SRP allows using an agreed to password alone or in conjunction with a certificate. Gnutls, openssl, curl and a few other libraries contain implementations. It just has not found widespread use and none of the major browsers support it.

[mmalone]

See my response to superkuh elsewhere in this thread. This would introduce new problems and, overall, would probably make Web PKI worse.

[tinus_hn]

It does, you just have to wrap a certificate around it.

[tashian]

TLS does have a mode that uses a pre-shared key, but in TLSv1.3 I believe it's only used for session resumption.

Edit: Also, I recently learned about DNS Certification Authority Authorization (CAA) records. You can specify which of the public certificate authorities a browser is allowed to respect, for your domain. I don't think it's verified by all browsers yet, but it's a step.

[tialaramex]

Other people responded to your CAA mistake (CAA absolutely shouldn't be enforced by anybody else except a CA, even researchers monitoring it for other sites is dubious, though as a research project it isn't inherently dangerous like enforcement would be)

But let's talk about PSKs (pre-shared keys).

TLS 1.3 itself doesn't care why this PSK is used. It's true that today your Web Browser will only use it for resumption, because it offers a significant speed-up in some scenarios on second and subsequent visits.

But for IoT applications it is envisioned that a group of devices might share one or more PSKs out of band. Maybe they're a factory setup thing, maybe the devices are to be set up in close physical proximity using low bandwidth Bluetooth, then they'll build themselves a WiFi network when deployed using PSKs to secure TLS.

Browsers could do that, but all the vendors are clear there's no way they'd actually want to do that. What would the UX look like? "Please enter the hexadecimal PSK for the site in this box?". So today they only use PSKs for resumption.

The reason this one feature (PSKs) has two very different purposes is to narrow the security proof work. Mathematicians worked really hard to prove some important things about TLS 1.3 and the more extra different features it has, the less focus can be put on any particular feature.

Even as it is they missed the fact that PSKs are symmetric. If Alice and Bob have a single PSK to "authenticate" them and are both capable of serving, then Mallory can trick Alice into thinking she's talking to Bob when she's actually talking to herself. It's a small problem, but the proofs didn't cover it, and so it was not spelled out in the TLS 1.3 document that you should worry about this.

[mmalone]

Pretty sure CAA is supposed to be enforced by CAs, not by browsers. So, for instance, Let's Encrypt should refuse to issue a cert for your domain if you have CAA setup for digicert.

[JonathonW]

CAA records (which specify which CAs are allowed to issue certificates for a given domain) are intended to be enforced by CAs, not by browsers. This prevents unintentional misissuance of a certificate, but not deliberate MITM attempts if the CA is actively involved.

Their counterpart for browsers are TLSA records, which associate specific keys or certificates with a domain name. This is the part that actually prevents MITM attacks on the client side (assuming the client's getting a complete and accurate DNS response, which is a whole other issue), since it'll cause a compliant client to reject any other keys or certificates. (No idea how widespread the implementation of this is on the client side, though.)

[mmalone]

Neat. I had never heard of TLSA.

I’ll also add that certificate transparency (CT) is another mechanism designed to mitigate malicious cert issuance by a CA. A CT log is an public, append-only data structure. It doesn’t actively prevent anything, but it does ensure that a malicious issuance is easily detectable. In practice it seems to be a pretty effective deterrent against nation-state attacks: they won’t go undetected for long.