What is Certificate Transparency?

Certificate Transparency (CT) is an open framework designed to monitor and audit SSL/TLS certificates. It was created to address security concerns related to fraudulent certificates by requiring certificate authorities to submit all issued certificates to public logs.

For OSINT researchers, these public logs are a goldmine of information, as they reveal domains, subdomains, and organizational relationships that might otherwise remain hidden.

How Certificate Transparency Works

When a certificate authority issues an SSL/TLS certificate, it submits the certificate to multiple CT logs. These logs are append-only databases that anyone can query. Each certificate contains information such as:

• Domain names (including subdomains)
• Organization information
• Issue and expiry dates
• Certificate authority information

OSINT Applications of Certificate Transparency

Domain and Subdomain Discovery

One of the most valuable uses of CT logs for OSINT is discovering domains and subdomains associated with a target organization. Since all SSL/TLS certificates must be logged, CT logs often reveal subdomains that aren't linked from public websites or indexed by search engines.

Organizational Structure Mapping

Certificates often contain organization names and sometimes department information. By analyzing certificates issued to an organization, you can map out its digital infrastructure and potentially its organizational structure.

Acquisition and Partnership Discovery

When companies merge or form partnerships, they often issue certificates that reflect these relationships. CT logs can reveal these connections before they're publicly announced.

Tools for Certificate Transparency Research

Censys

Censys indexes certificate data and makes it searchable. You can use queries like parsed.subject.organization:"Target Organization"to find all certificates issued to a specific organization.

crt.sh

crt.sh is a specialized search engine for CT logs. It provides a simple interface for querying certificate data by domain, organization, or certificate fingerprint.

Certificate Transparency Search

Google's Certificate Transparency Searchallows you to search for certificates by domain name.

Practical OSINT Techniques

Wildcard Searches

Use wildcard searches to find all subdomains of a target domain:

%.example.com (on crt.sh)

parsed.names:*.example.com (on Censys)

Organization Searches

Search for certificates issued to a specific organization:

parsed.subject.organization:"Target Organization" (on Censys)

Historical Analysis

Analyze certificate issuance patterns over time to identify changes in an organization's infrastructure or security practices.

Ethical Considerations

While CT logs are publicly accessible, it's important to use this information ethically:

• Respect privacy and legal boundaries
• Use the information for legitimate research purposes
• Consider responsible disclosure if you discover security issues

Conclusion

Certificate Transparency logs provide valuable insights for OSINT investigations, particularly for mapping an organization's digital footprint. By understanding how to effectively query and analyze CT data, researchers can discover domains, relationships, and infrastructure details that might not be visible through other OSINT techniques.