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What Are Certificate Transparency Logs? Why Are CT Logs Important?

What are Certificate Transparency Logs and Why are CT Logs Important?
14 Aug 2023

What Are Certificate Transparency Logs (CT Logs)?

Certificate Transparency (CT) logs are publicly accessible and append-only logs that record information about SSL/TLS certificates issued by certificate authorities (CAs). The purpose of CT logs is to enhance the security and trustworthiness of the SSL/TLS ecosystem by providing transparency and accountability in the issuance of certificates.

Why Certificate Transparency Logs (CT Logs) Were Created

Certificate Transparency (CT) is a relatively new concept in the grand scheme of things and was first proposed by Google in 2012 as an open framework to address security and trust issues in the SSL/TLS certificate system. The goal was to create a FULLY-transparent and accountable ecosystem for cert issuance and validation.

In 2012, Google published the initial proposal for Certificate Transparency, outlining the need for a public log infrastructure to detect certificate mis issuance and improve the overall security of SSL/TLS. Google started developing an open-source CT log server and began collaborating with industry stakeholders. Less than a year later in 2013, Google launched a pilot project, deploying CT logs for certain Google services. They collaborated with a few certificate authorities (CAs) to log all certificates they issued. The pilot aimed to test and refine the CT protocol, identify challenges, and gather feedback from the industry. Over time, the CT ecosystem grew, with the establishment of multiple public and private CT log servers. More CAs started logging certificates, and CT log transparency became an industry norm to promote security and accountability.

Why Are Certificate Transparency Logs Important?

Today, Certificate Transparency is widely accepted and adopted within the industry, improving security, trustworthiness, and accountability in the SSL/TLS infrastructure. CT logs play a crucial role in detecting and mitigating security incidents related to certificate issuance, benefiting both end-users and organizations relying on secure communication.

CT logs help with:

  1. - SSL Certificate expiry notification.

  2. - Similar domain being used for phishing.

  3. - Monitor for new subdomains created.

  4. - Alert on domains containing your domain. (This one is common on phishing campaigns where the attacker might use your domain as a subdomain. For example,

  5. - Invalid certificate installed.

  6. - CT Logs also helped us find over [30k vulnerable domains](!

  7. - Creating a [CAA (Certificate Authority Authorization Mismatch)](

  8. - And more!

Certificate Transparency logs have become an integral part of the SSL/TLS ecosystem, promoting accountability, early detection of security incidents, and trustworthiness in certificate issuance. Browser vendors and industry standards now often require or encourage CAs to log all newly issued certificates in CT logs to ensure greater transparency and security.

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