Certificate Transparency (CT)
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Certificate Transparency (CT) is an open standard (RFC 9162) that requires all publicly trusted [[ssl-tls|SSL/TLS certificates]] to be logged in publicly auditable, append-only ledgers called CT logs. Browsers enforce CT by requiring that every certificate include signed proofs (SCTs) from recognized CT logs before it is trusted. CT enables domain owners to monitor for unauthorized certificates issued for their domains and allows security researchers to detect misissuance by certificate authorities. Services like crt.sh and Google's Transparency Report provide searchable access to CT log data.
Example
A company monitoring CT logs via crt.sh discovers a certificate was issued for 'secure.theircompany.com' by an unauthorized CA — indicating a potential phishing setup or CA compromise.