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Certificate Transparency (CT) is an experimental IETF open standard and open source framework for monitoring and auditing digital certificates. Through a system of certificate logs, monitors, and auditors, certificate transparency allows website users and domain owners to identify mistakenly or maliciously issued certificates and to identify certificate authorities (CAs) that have gone rogue.
Flaws in the current system of digital certificate management were made evident in high-profile security and privacy breaches caused by fraudulent certificates.
One of the problems with digital certificate management is that fraudulent certificates take a long time to be spotted, reported and revoked by the browser vendors. Certificate Transparency would help by making it impossible for a certificate to be issued for a domain without the domain owner knowing.
Certificate Transparency does not require side channel communication to validate certificates as do some competing technologies such as Online Certificate Status Protocol (OCSP) and Convergence. Certificate Transparency also operates without the need to trust a third party.
Certificate Transparency logs
Certificate Transparency depends on verifiable Certificate Transparency logs. A log appends new certificates to an ever-growing Merkle hash tree.:Section 3 To be seen as behaving correctly, a log must:
- Verify that each submitted certificate or precertificate has a valid signature chain leading back to a trusted root certificate authority certificate.
- Refuse to publish certificates without this valid signature chain.
- Store the entire verification chain from the newly accepted certificate back to the root certificate.
- Present this chain for auditing upon request.
A log may accept certificates that are not yet fully valid and certificates that have expired.
Certificate Transparency monitors
Monitors act as clients to the log servers. Monitors check logs to make sure they are behaving correctly. An inconsistency is used to prove that a log has not behaved correctly, and the signatures on the log's data structure (the Merkle tree) prevent the log from denying that misbehavior.
Certificate Transparency auditors
Auditors also act as clients to the log servers. Certificate Transparency auditors use partial information about a log to verify the log against other partial information they have.:Section 5.4
Google Chrome began requiring Certificate Transparency for newly issued Extended Validation Certificates in 2015. It began requiring Certificate Transparency for all certificates newly issued by Symantec from June 1, 2016, after they were found to have issued 187 certificates without the domain owners' knowledge.
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