[matthew9219]

Somebody has got to check the logs and report violations. Chrome does, so CT works mostly for the world wide web, because all websites want to work in Chrome.

For a device like a router, if the router doesn't check the logs itself, and a global adversary compromises the TLS update channel for the router, and starts distributing malicious firmware... If the router itself doesn't report the violation, for how long might such a compromise go undetected? Is there any reason to think it'd ever be detected?

CT has a bit of an implicit dependency on heterogeneous configurations - that at least some clients report violations, and that attackers cannot easily distinguish reporting clients from non-reporting clients. For homogenous configurations (like the implementations of AWS, Azure, or GCM, or the deployment of routers, IOT devices, or gaming systems), it seems like a competent global adversary would simply figure out how to go unreported for that configuration, and nobody would particularly check.

[woodruffw]

> CT has a bit of an implicit dependency on heterogeneous configurations - that at least some clients report violations, and that attackers cannot easily distinguish reporting clients from non-reporting clients.

Unless I'm misunderstanding you, this isn't really how CT works: the expectation for clients under a PKI with CT is that the presented certificate is already present in one or more logs, meaning that it's never (or more accurately, never has to be) the client actually doing the reporting. Reporting is left to separate monitoring parties.

In other words: a global adversary cannot surreptitiously use a novel CA against a particular configuration; they must first make themselves visible to one or more CT logs. Failing to do so means that their CA will be rejected outright by the client (or accepted by the client if the client doesn't do CT, but still with a loss of stealth).

[matthew9219]

The client has to get the CT log from somewhere, like an update channel (typically TLS). An attacker would compromise both the target and the process by which the client gets CT log updates.

Such an attack would be detected if some clients reported which certs they actually saw the next time they connected to an uncompromised network (as Chrome does) but if no clients report, such an attack could go undetected.

[tptacek]

That's not how any of this works. Clients don't download the CT logs.

[matthew9219]

Clients get pre certificates (which are portions of the log) as claims in certificates. It's correct that they never download the whole log - I'm simplifying for clarity, not out of lack of understanding.

The fact remains - an adversary with a CA private key that can mitm all of the internet connections for a device can forge a fake CT log and go undetected, if that clients never uses a non-mitm network again.

[mjg59]

No, because the presented certificate won't have any SCT entries (which are embedded in the certificate), and you can't fake the SCT entries without having access to the CT private keys.

[matthew9219]

> you can't fake the SCT entries without having access to the CT private keys.

So... Governments like the US and China can fake the entries by using their police forces to seize the private keys?

SCT has the same set of problems as TLS - any log will do, not just logs from countries you trust.

[agwa]

You are almost correct. The claims are called SCTs (precertificates are something else). The SCT is a promise by the log that they have (or will soon) publish the certificate. The promise could be lie. To detect that, the client is supposed to audit the SCT. If the audit fails, the client reports it so the log can be distrusted.

So yes, it's true that if an adversary can permanently silo off a client, it can prevent log misbehavior from being detected, either by blocking the reporting of audit failures, or by presenting a completely different view of the log to to the client. However, in many cases it would be impractical for an adversary to keep up such an attack forever, so CT still has value and I'm a huge fan of it. But it's true it can't stop literally all attacks.

Source: I run the CT monitor which has detected misbehavior in multiple CT logs.

[tptacek]

I don't think you're providing clarity at all, because that attack doesn't work.

[matthew9219]

If you wanted to use your words to explain why you think that, that might be more constructive. I could well respond "yes, it does work" but that wouldn't be useful. I know you're an expert in the space and might appreciate learning from you, but this is not that.

[nickf]

Clients do not get pre-certs. Those are generated by the CA, and submitted to the log in return for an SCT. Forging a ‘fake CT log’ isn’t possible, either. Nor do clients talk to CT logs, at all.

[matthew9219]

> Forging a ‘fake CT log’ isn’t possible, either

Why do you think this isn't possible?

[akerl_]

I'm not sure why you're so fixated on this idea of client systems monitoring CT logs.

Operators who host infrastructure can and should be monitoring issuance in CT logs for domains they operate, which will allow them to identify and react to any unexpected issuance for those domains.

[matthew9219]

In the attack DNSSec prevents, a client is compromised by a cert that doesn't appear in the CT logs, so infrastructure monitoring is irrelevant.

[mjg59]

Most browsers will reject such a certificate. See https://googlechrome.github.io/CertificateTransparency/ct_po... for the policy Chrome imposes - my understanding is that Safari is broadly similar. Right now I don't think Firefox performs this validation, so this is possible if you know in advance that your target runs Firefox.

[matthew9219]

That page explains that Chrome (which is best in class here - most IOT devices don't do any of this stuff) fails open:

> If the installed version of Chrome has not applied security updates and has been unable to obtain an updated CT log list from the Component Updater for 70 days or more, then CT enforcement will be disabled.

That means a global adversary need merely block the update channel to targeted devices and wait. How will a Smart TV behave?

[mjg59]

You're moving the goalposts to a ridiculous degree. Chrome will be incredibly angry at you if you haven't updated in 70 days. How will a smart TV behave? I've no idea. It's probably not paying any attention to dnssec (otherwise pihole and co wouldn't work), so I don't think you're presenting a credible alternative.

[matthew9219]

That's just not what's happening. Reread the conversation. I started from here:

> Certificate transparency is cool, but it's not clear it really works for many classes of devices

Smart TVs aren't some gotcha I'm throwing in at the end. It's literally the first thing I said about CT. CT works ok for mobile phones, laptops, and other devices where you can make certain assumptions about multiple networks and frequent updates. If you want a technology that doesn't require these assumptions, you want DNSSec.

[tzs]

I don't think firmware updates for routers is a good example. That seems more like the kind of situation where you should actually be using your own PKI.