Certificate Transparency


Certificate Transparency (CT) is a protocol designed to fix several structural flaws in the SSL/TLS certificate ecosystem. Described by RFC 6962 and the ongoing work in RFC 6962-bis, it provides a means of providing a public, append-only data structure that can log certificates issued by certificate authorities (CAs). By logging these certificates, it becomes possible for site operators to detect when a certificate may have been issued for their domain without their approval, and allows browsers and the wider ecosystem to verify that CAs are following their expected and disclosed practices.

Certificate Transparency Basics

Broadly speaking, the goal of supporting Certificate Transparency is to ensure that certificates an application trusts will be publicly disclosed in a way sufficient for site operators and application developers to ensure that nothing is wrong.

At the most basic level, it's possible to simply introduce Certificate Transparency logs as trusted third parties, much like CAs are trusted third parties. If the logs are operated by CAs, this may not be much of a security improvement, but if the logs are operated by non-CA entities, this might serve as a sufficient counter-balance to the risks.

However, with more work, it‘s possible to minimize the trust afforded to Certificate Transparency logs, and to automatically and cryptographically verify they’re complying with their stated policies. This can provide even greater assurance to application developers, site operators, and their users, that the security expected from certificates is actually being provided.

For a more thorough threat analysis, see https://datatracker.ietf.org/doc/draft-ietf-trans-threat-analysis/ that discusses the different risks in Certificate Transparency, and how the protocol addresses them.

Certificate Transparency in //net

A goal of //net is to try to ensure that code is ‘safe by default’ when used. As part of serving that goal, in order to make a TLS or QUIC connection using code in //net, it‘s necessary for the //net embedder to make a decision about Certificate Transparency, much like it is necessary to provide a CertVerifier that describes how to verify the server’s certificate.

Because this is necessary to make a TLS or QUIC connection, this requirement surfaces upwards through each layer in the stack - applying to things like HttpNetworkSession and upwards to URLRequestContext.

This requirement is expressed by requiring two separate, but related, objects to be supplied: CTVerifier and CTPolicyEnforcer, which together can be used to express an application's policies with respect to Certificate Transparency.

As part of the goal of ensuring ‘safe by default’, //net also has various policies related to certificates issued by particular CAs whose past actions have created unnecessary security risk for TLS connections, and as a consequence, are required to have their certificates disclosed using Certificate Transparency in order to ensure that the security provided by these CAs matches the level of security and assurance that other CAs provide. These policies are implemented in TransportSecurityState, via the ShouldRequireCT method.


CTVerifier is the core interface for parsing and validating the structures defined in RFC6962 (or future versions), and for providing basic information about the SignedCertificateTimestamps present within the connection.


CTPolicyEnforcer is the core class for expressing an application's policies around how it expects Certificate Transparency to be used by the certificates it trusts and the CAs that issue these certificates.

CTPolicyEnforcer currently expresses two policies:

  • How to treat Extended Validation certificates (those for which a CertVerifier returned CERT_STATUS_IS_EV).
  • How to treat all certificates, regardless of EV status.


The TransportSecurityState::ShouldRequireCT method implements the core logic for determining whether or not a connection attempt should be rejected if it does not comply with an application's Certificate Transparency policy.

The implementation in //net provides a default implementation that tries to ensure maximum security, by failing connections that do not abide by an application's Certificate Transparency policy and are from CAs known to have security issues in the past.

Embedders can customize or override this by providing a TransportSecurityState::RequireCTDelegate implementation, which allows applications to inspect the connection information and determine whether Certificate Transparency should be required, should not be required, or whether the default logic in //net should be used.

Certificate Transparency in Chromium

As part of the open-source implementation of Chrome, the policies related to how Chromium code treats Certificate Transparency are documented at https://www.chromium.org/Home/chromium-security/certificate-transparency . This page includes the policies for how Chromium determines an acceptable set of Certificate Transparency logs and what Certificate Transparency-related information is expected to accompany certificates, both for EV and non-EV.

The implementation of these policies lives within //net/cert, and includes:

Certificate Transparency for //net Consumers

This section is intended for code that is open-sourced as part of the Chromium projects, intended to be included within Google Chrome, and which uses the //net APIs for purposes other than loading and rendering web content. Particularly, consumers of //net APIs that are communicating with a limited or defined set of endpoints and which don't use certificates issued by CAs. This may also include testing tools and utilities, as these are not generally shipped to users as part of Chrome.

Not every TLS connection may need the security assurances that Certificate Transparency aims to provide. For example, some consumers of //net APIs in Chromium use mutual authentication with self-signed certificates and which are authenticated out-of-band. For these connections, Certificate Transparency is not relevant, and it's not necessary to parse or enforce Certificate Transparency related information.

For these cases, the approach is:

  • do_nothing_ct_verifier.h: A no-op CTVerifier that does not parse or verify Certificate Transparency-related information.

  • A derived CTPolicyEnforcer implementation that indicates all certificates comply with its policies.

    TODO(rsleevi): Provide a DoNothingCTPolicyEnforcer

As documented in these classes, care should be taken before using these, as they provide much weaker security guarantees. In general, emailing net-dev@chromium.org or discussing it during a security review is the right answer, and documenting at the instantiation points why it is safe and acceptable to use these classes.

Certificate Transparency for //net Embedders

This section is intended for code that is used in other open-source Chromium based projects, but are not included in Google Chrome or related. This includes projects based on //net, such as //components/cronet or other //content embedders.

For projects and third party products that embed //net, the policies that are included as part of the open-source repository may not be appropriate. This is because the implementations may rely implicitly or explicitly on several key guarantees that come from Google-branded distributions and products, and may not be appropriate for other cases.

These key expectations are:

  • A release cycle aligned with Chrome releases; that is, every six weeks, and on the same versions as Chrome releases.
  • Widespread support for automatic updates.
  • That base::GetBuildTime() will reflect, to some degree, when the tree was branched and/or released, and will not be re-generated on recompilation. That is, this implies is_official_build for binaries released to end-users, but is not enforced in code so that developers can accurately test release behavior.
  • Support for dynamic base::FieldTrial configurations.

For projects that don‘t support automatic updates, or which measure ‘stable’ on the order of months to years, or which don’t have tools suitable to respond to changes in the Certificate Authority and Certificate Transparency ecosystem, it may not be appropriate to enable Certificate Transparency support yet.

These issues are not unique or particular to Certificate Transparency - in many ways, they‘re similar to issues already faced with determining which CAs are trusted and how to successfully validate a TLS server’s certificate. However, as the Certificate Transparency ecosystem is still growing, it may be suitable to disable support until some of the solutions to these challenges stablize.

To opt-out of enforcing Certificate Transparency, using the DoNothing variants discussed above provides a suitable implementation that will opt to ‘fail open’ instead. This may provide less security, but provides greater stability, and minimizes the risk that these //net embedding clients might cause to the Certificate Transparency ecosystem or receive from enabling Certificate Transparency.