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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.

> 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.

This is why multiple SCT entries are required. Could you subvert all of this with enough effort? Yes, and the same is true of dnssec. Any form of cryptographic validation is only as secure as the control of its private keys, but modern TLS is designed to require you to compromise at least 3 independent parties in the TLS ecosystem to provide a plausible fake. And at that point why not just get the browser vendor to push a fake update to the user that disables TLS validation entirely? You're either doing all validation yourself, or you're trusting someone. Modern TLS avoids you needing to place trust in any one organisation from the TLS perspective, which makes attacks on other infrastructure components much more appealing.

agwa 1109 days ago

This is not an accurate description of how CT provides security.

To begin with, CAs are allowed to operate logs themselves, so the minimum number of independent parties is 2, not 3.

Second, CT log private keys are not particularly well-protected. They are not stored in HSMs. One log's private key has been presumed compromised since 2020 because the server running the log was pwned via a vulnerability in the Salt configuration management system. SCTs from this log are still accepted by Apple (and by Chrome, until earlier this year) for satisfying the minimum SCT requirement.

Rather, CT provides security by using a Merkle Tree so that SCTs can be audited. In theory, clients can demand proof that an SCT is really included in a log, and gossip tree heads with monitors to ensure that monitors have the same view of the log as them. If an SCT fails auditing, or a log is found to have presented inconsistent views of its Merkle Tree, this can be detected and the log can be distrusted. In practice, this is quite challenging. Apple currently doesn't audit SCTs at all; Chrome probabilistically audits a subset of SCTs.

The crucial difference is who decides which cryptographic entities to trust. With TLS and CT, the browser defines the list of entities and if 3 or more of those entities live in a hostile country, you can be compromised. With DNSSec and CAA, the site operator gets to define the list of entities.

Ok. That's an argument you can make. Now go back and make that in the first place rather than diverting everyone down an incredibly tedious demonstration of you making assertive statements that are just factually wrong. There's space to talk about what tradeoffs are reasonable here (eg, if this is something you're concerned about, you can pick a CA that's not in a hostile country, and you can enable certificate stapling at the cost of increased fragility and depending on some level of TOFU), but when you start the conversation by just being clearly wrong and then double down on that when multiple people suggest that you're wrong and point at resources indicating that you're wrong, you're not doing a great job of making that space available.

It's the argument I made at the top:

> The fundamental difference is that with TLS you have to trust ALL certificate issuers, but with DNSSec you only have to trust your TLD and your certificate issuer.

It's probably fair to say I've been a bit over assertive about CT, but it's all in the margin to me. No amount of technical complexity can turn community trust into direct trust. TLS is a community trust model and DNSSec is a direct trust model. The fact that CT is a pretty good community trust model and that browsers have (so far) done a pretty good job at keeping the CT community small is interesting, but it doesn't turn community trust into direct trust. So while I'd agree it's an incredibly tedious tangent, I'd say it's tedious moreso because the pro-CT folks are missing the forest for the trees than for any technical details about CT.

Edit: because community trust fundamentally relies on the notion of the community taking action against bad actors. Whether it's a CA or a CT log provider, and whether it's malicious action or a bug or just ceasing to do business, community trust has a time axis where membership in the community changes and notions of keeping devices up to date and political struggles ("too big to fail") and the like that simply aren't needed in direct trust.

agwa 1109 days ago

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 1109 days ago

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

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.

tptacek 1109 days ago

I'm sorry, but I'm declining to do that, because I recognize the message board conversational pattern where all the information you have about the subject is coming from comments rebutting you (this is apparent from things you've said already that fundamentally misapprehend how CT works), and, rather than acknowledging any of that information, you just use it as vocabulary for new misapprehensions.

nickf 1109 days ago

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.

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

Why do you think this isn't possible?

nickf 1109 days ago

Aside from what mjg59 said, it's clear you don't quite understand how CT works. Logs are stood up and then go through a fairly rigorous acceptance process by Google (and Apple) before finally being used. 'Used' in that a CA can then submit pre-certs to it and include the resulting SCTs in signed certificates, making them functional on Chrome/Apple platforms. Even the CA using the log generally takes some communication with the log operator to ensure the right set of roots are trusted for submitted pre-certs.

CT logs are used by CAs, not clients. A 'fake' log isn't a thing.

michaelt 1109 days ago

If you are able to compromise a CA and generate a bogus certificate, why couldn't you also compromise a certificate transparency log provider and generate a bogus signed merkle hash?

(Not that I'm a DNSSEC user myself, my feet aren't bulletproof)

nickf 1109 days ago

Not sure what you really mean here - CAs are required to get SCTs from multiple, independently-operated logs. Even then, I think what you're implying here is mathematically impossible, and easily and immediately detectable. Bear in mind on at least 2 occasions these logs have detected and been decommissioned based on cosmic-ray induced bit-flips, not discovered by the actual log operators. CT is a pretty robust system.

matthew9219 1108 days ago

Fwiw (tangent), I don't necessarily believe either of those instances were cosmic-ray induced bit-flips. I'd have to dig up the study, but I read a study once that more or less concluded "cosmic-rays are more common in memory unsafe languages and on overclocked PCs". Or more accurately, engineers frequently misattribute memory corrupt and operating outside specification to cosmic rays.

Particularly when the software in question is running on somebody else computer, proprietary software and OS (or OS modules), unknown patch versions, etc.

Because they don't have access to the private key material owned by the CT logs that the browsers trust when validating SCT data in the certificate.