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Certificate Transparency (CT) is an experimental Internet security standard and open source framework for monitoring and auditing digital certificates. The standard creates a system of public logs that seek to eventually record all certificates issued by publicly trusted certificate authorities, allowing efficient identification of mistakenly or maliciously issued certificates.
In 2011, a reseller of the certificate authority Comodo was attacked and the certificate authority DigiNotar was compromised, emphasising existing flaws in the certificate authority ecosystem and accelerating work on various mechanisms to prevent or monitor unauthorized certificate issuance. Ben Laurie, Adam Langley and Emilia Kasper began work on an open source framework to combat these issues the same year. They submitted the first draft of the standard as a IETF Internet Draft in 2012 under the code-name "Sunlight".
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.
- Laurie, Ben; Langley, Adam; Kasper, Emilia (June 2013). Certificate Transparency. IETF. doi:10.17487/RFC6962. ISSN 2070-1721. RFC 6962. https://tools.ietf.org/html/rfc6962.
- Bright, Peter (August 30, 2011). "Another fraudulent certificate raises the same old questions about certificate authorities". Ars Technica. Retrieved 2018-02-10.
- "Known Logs - Certificate Transparency". certificate-transparency.org. Retrieved 2015-12-31.
- "DigiCert Announces Certificate Transparency Support". Dark Reading. Archived from the original on October 10, 2013. Retrieved 2013-11-12.
- Woodfield, Meggie (December 5, 2014). "Certificate Transparency Required for EV Certificates to Show Green Address Bar in Chrome". DigiCert Blog. DigiCert.
- Laurie, Ben (February 4, 2014). "Updated Certificate Transparency + Extended Validation plan". cabfpub (Mailing list). Archived from the original on 2014-03-30.
- "Symantec Certificate Transparency (CT) for certificates issued before June 1, 2016". Symantec Knowledge Center. Symantec. June 9, 2016.
- Sleevi, Ryan (October 28, 2015). "Sustaining Digital Certificate Security". Google Security Blog. Google.