Expect-CT

The Expect-CT header lets sites opt in to reporting and/or enforcement of Certificate Transparency requirements, to prevent the use of misissued certificates for that site from going unnoticed.

CT requirements can be satisfied via any one of the following mechanisms:

• X.509v3 certificate extension to allow embedding of signed certificate timestamps issued by individual logs
• A TLS extension of type signed_certificate_timestamp sent during the handshake
• Supporting OCSP stapling (that is, the status_request TLS extension) and providing a SignedCertificateTimestampList

Expect-CT: report-uri="<uri>",
           enforce,
           max-age=<age>

max-age

The number of seconds after reception of the Expect-CT header field during which the user agent should regard the host of the received message as a known Expect-CT host.

If a cache receives a value greater than it can represent, or if any of its subsequent calculations overflows, the cache will consider this value to be either 2,147,483,648 (2^31) or the greatest positive integer it can represent.

report-uri="<uri>" Optional

The URI where the user agent should report Expect-CT failures.

When present with the enforce directive, the configuration is referred to as an "enforce-and-report" configuration, signalling to the user agent both that compliance to the Certificate Transparency policy should be enforced and that violations should be reported.

enforce Optional

Signals to the user agent that compliance with the Certificate Transparency policy should be enforced (rather than only reporting compliance) and that the user agent should refuse future connections that violate its Certificate Transparency policy.

When both the enforce directive and the report-uri directive are present, the configuration is referred to as an "enforce-and-report" configuration, signalling to the user agent both that compliance to the Certificate Transparency policy should be enforced and that violations should be reported.

The following example specifies enforcement of Certificate Transparency for 24 hours and reports violations to foo.example.

Expect-CT: max-age=86400, enforce, report-uri="https://foo.example/report"

Root CAs manually added to the trust store override and suppress Expect-CT reports/enforcement.

Browsers will not remember an Expect-CT policy, unless the site has 'proven' it can serve a certificate satisfying the certificate transparency requirements. Browsers implement their own trust model regarding which CT logs are considered trusted for the certificate to have been logged to.

Builds of Chrome are designed to stop enforcing the Expect-CT policy 10 weeks after the installation's build date.