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If a CA is behaving in a bad way why would they include a bad cert they issued in certificate transparency logs? I don't understand what CT accomplishes since

If a CA is behaving in a bad way why would they include a bad cert they issued in certificate transparency logs? I don't understand what CT accomplishes since it places the onus of reporting on the party that CT is supposed to monitor.

ivanr 4y ago
CAs don't have to report their certificates to CT logs, and actually there is no reporting in a variety of use cases. However, modern browsers no longer trust certificates for which there is no (cryptographic) evidence of logging to CT. The net result is that CT gives us visibility into all web certificates (and more).
- witrak 4y ago
  "The net result is that CT gives us visibility into all web certificates [...]" including not always the desirable effect of publishing information on hosts in the non-public part of the institution network...
  - tialaramex 4y ago
    It's true that sometimes somewhere up in management tier people are annoyed to discover that non-Corp employees can find out about the DNS name product-we-bought.internal.corp.example and thus might intuit that Corp bought Product-we-bought. Or even, less often, that big-city.branches.corp.example exists even though they haven't officially launched the new Big City branch of Corp. And it's true that CT is one of the various ways people might discover that although it's probably not even the easiest. But honestly did you consider calling that purpose-of-software.internal.corp.example instead? And maybe at least use code names for projects that are actually secret? Instead of normandy-invasion.corp.example at least call it overlord.corp.example and now it's not obvious where you're landing and I won't have lined up all the tanks and guns ready.
tinus_hn 4y ago
Anyone who gets presented the certificate can check if it is in the log, and if it is not the authority is exposed and can be held accountable by the parties that manage the root certificate stores (browser and OS vendors).
charcircuit 4y ago
>why would they include a bad cert they issued in certificate transparency logs? Because secure browsers like Chrome require certificate transparency for it to trust the cert. Unfortunately there still exist insecure browsers like Firefox which leaves a percentage of the population vulnerable to bad certs that aren't logged.
- zinekeller 4y ago
  > Because secure browsers like Chrome require certificate transparency for it to trust the cert. And if it was found out to be fraudulent? "Secure" Chrome would do f***all unless you're PayPal while "insecure" Firefox always checks if the certificate is still valid.
  - agsnu 4y ago
    Since OCSP fails open and response stapling is still only deployed on a minority of servers, it doesn’t provide much of a guarantee…
- sweetbitter 4y ago
  How can Chrome be considered secure if its "security" entails the equivalent of giving a shell on the PC of all users to a shady corporation like Google?
tialaramex 4y ago
Others have responded at a high level, but I'll describe both a very low level and even higher level answer. Low level first. CT logs produce a cryptographic "receipt" for the logged item. This receipt is a promise to report the item to anybody who asks within 24 hours. Web browsers, including Chrome, Edge and Safari, want to see the receipt. They may collect some receipts and check the logs did indeed publish these items. So, technically to make your certificate work in those browsers you need to log the certificate and the log you use needs to publish it. Very high level. Logging and obtaining receipts, then providing the receipt to a browser is extra work. So, if you sell certificates it makes sense to do this work for all your customers by default and just bake the receipts into the data they have. It's extra trouble to build a mechanism to sidestep this, and so there's often no reason to do so.

ivanr 4y ago

CAs don't have to report their certificates to CT logs, and actually there is no reporting in a variety of use cases. However, modern browsers no longer trust certificates for which there is no (cryptographic) evidence of logging to CT. The net result is that CT gives us visibility into all web certificates (and more).

witrak 4y ago
"The net result is that CT gives us visibility into all web certificates [...]" including not always the desirable effect of publishing information on hosts in the non-public part of the institution network...
- tialaramex 4y ago
  It's true that sometimes somewhere up in management tier people are annoyed to discover that non-Corp employees can find out about the DNS name product-we-bought.internal.corp.example and thus might intuit that Corp bought Product-we-bought. Or even, less often, that big-city.branches.corp.example exists even though they haven't officially launched the new Big City branch of Corp. And it's true that CT is one of the various ways people might discover that although it's probably not even the easiest. But honestly did you consider calling that purpose-of-software.internal.corp.example instead? And maybe at least use code names for projects that are actually secret? Instead of normandy-invasion.corp.example at least call it overlord.corp.example and now it's not obvious where you're landing and I won't have lined up all the tanks and guns ready.

tinus_hn 4y ago

Anyone who gets presented the certificate can check if it is in the log, and if it is not the authority is exposed and can be held accountable by the parties that manage the root certificate stores (browser and OS vendors).

charcircuit 4y ago

>why would they include a bad cert they issued in certificate transparency logs? Because secure browsers like Chrome require certificate transparency for it to trust the cert. Unfortunately there still exist insecure browsers like Firefox which leaves a percentage of the population vulnerable to bad certs that aren't logged.

zinekeller 4y ago
> Because secure browsers like Chrome require certificate transparency for it to trust the cert. And if it was found out to be fraudulent? "Secure" Chrome would do f***all unless you're PayPal while "insecure" Firefox always checks if the certificate is still valid.
- agsnu 4y ago
  Since OCSP fails open and response stapling is still only deployed on a minority of servers, it doesn’t provide much of a guarantee…
sweetbitter 4y ago
How can Chrome be considered secure if its "security" entails the equivalent of giving a shell on the PC of all users to a shady corporation like Google?

tialaramex 4y ago

Others have responded at a high level, but I'll describe both a very low level and even higher level answer. Low level first. CT logs produce a cryptographic "receipt" for the logged item. This receipt is a promise to report the item to anybody who asks within 24 hours. Web browsers, including Chrome, Edge and Safari, want to see the receipt. They may collect some receipts and check the logs did indeed publish these items. So, technically to make your certificate work in those browsers you need to log the certificate and the log you use needs to publish it. Very high level. Logging and obtaining receipts, then providing the receipt to a browser is extra work. So, if you sell certificates it makes sense to do this work for all your customers by default and just bake the receipts into the data they have. It's extra trouble to build a mechanism to sidestep this, and so there's often no reason to do so.