We say that a certificate supports Certificate Transparency if it comes with CT information that demonstrates it has been logged in several CT logs. This CT information must comply with the Certificate Transparency in Chrome policy. We sometimes refer to a site that “supports” CT as using a certificate that is “CT qualified” or “disclosed via CT.”
In general, a site operator does not need to take special action to support Certificate Transparency. This is because RFC 6962 defines three ways of providing the necessary information for CT: within the certificate, within a stapled OCSP response, or directly by the TLS server. Nearly every CA supports CT through the first method, meaning that when you get a certificate, it will already support CT and require no further configuration. If you are using a cloud provider to terminate your TLS connections, the cloud provider may also support CT via TLS, requiring no further action on your part.
Supporting CT within the certificate itself is the preferred and recommended way to enable CT support. If you obtain a certificate from your CA and it does not support CT, then that generally indicates that your CA is not following industry best practice, and you should probably look for another CA to provide certificates for your sites.
Configuring support for CT via the TLS extension is not recommended for most site operators. This is because supporting CT via this method requires constant monitoring of the CT ecosystem, such as for changes in the list of trusted logs or testing compatibility with various CT-supporting clients. This method works well for organizations with the ability to dedicate resources to that, such as hosting and cloud providers. If you are hosting your own website, you should try to ensure that your certificates support CT, and avoid supporting CT via the TLS extension. Supporting CT via the TLS extension may require rapid changes to your configuration, and thus may be riskier for organizations without staff dedicated to this.
If you are getting longer-lived certificates (for example, 1 year), it’s possible that changes in the CT ecosystem may mean that the CT information may expire before the certificate expires. If your CA also supports delivering CT via OCSP responses, then supporting OCSP stapling on your server may allow fresh CT information to be provided without having to replace the certificate. Alternatively, if your server does not support OCSP stapling, or your CA does not support CT in their OCSP responses, you may need to replace your certificate.
These policies only apply to publicly-trusted CAs - that is, CAs that your browser or device trust without any additional configuration. For organizations using their own CAs, or for locally installed CAs, see Certificate Transparency for Enterprises.
Supporting CT by disclosing the certificate to a CT Log means that the full contents of the certificate will be publicly accessible and viewable. In particular, this means that the domains a certificate are for will be included in the Certificate Transparency log, as well as the organization they are affiliated with, if they are validated to a level higher than Domain Validation or issued from an organization-specific CA.
For most certificates, this is no different than what is already available. Publicly-trusted certificates have been subject to aggregation for public analysis for some time, such as through products and tools such as Censys or scans.io. While Certificate Transparency provides an interoperable protocol for exchanging these datasets, in many cases, the certificate details and domains were already publicly detectable.
Requiring that the full certificate be disclosed if it was issued by a publicly-trusted CA is an important part of the security goals of Certificate Transparency. Permitting some of the information to be hidden from certificates allows for both attackers and untrustworthy CAs to hide certificates that could be used to compromise users. Certificate Transparency has detected issues at a large number of CAs, many that the CAs themselves were not even aware of, and so public disclosure is critical to keeping all users safe.
While proposals for hiding domain names were presented during the development of Certificate Transparency, none of them were able to balance the needs of site operators that did not need to hide their domains, those that did, and the security risks that users would face.
Because of this, Chrome does not support any method for hiding domain names or other information within publicly-trusted certificates, nor are there any plans to support such mechanisms. Domain operators that wish to hide their certificates, enabling security risks and attacks, have two options:
*as the most specific DNS label (for example,
*.internal.example.comis valid for
wiki.internal.example.com, but not for
two.levels.internal.example.com). Wildcard certificates require greater care by the site operator to protect their private key, but also can have their issuance controlled via technologies such as CAA (RFC 6844). This still requires the certificate be disclosed, but can limit how much of the domain is disclosed.
As noted in Chrome Policies, all certificates issued after 30 April 2018 are expected to be disclosed via Certificate Transparency in a way that is compliant with the Certificate Transparency in Chrome policy. Virtually all publicly-trusted CAs have committed to supporting CT for their customers by default by this date, meaning that site operators should not have to do anything special and can continue getting certificates that just work on 1 May 2018.
However, there’s still a chance that a CA may not have adopted Certificate Transparency, may have an infrastructure issue, or may not have communicated to their partners, such as resellers or subordinate CAs, to ensure that the transition would be as smooth as possible for their customers.
If you’re receiving a
message, the best thing to do is to contact your CA’s support or sales team
to diagnose the error with them. They will most likely need to replace your
certificate with a new one that properly supports CT.