The "X.509 standard":/article/137/x509-certificate defines extensions for Version 3. One of them is the SKI extension. From the spec: bq. "The subject key identifier extension provides a means of identifying certificates that contain a particular public key. To facilitate certification path construction, this extension MUST appear in all conforming CA certificates." The authority key identifier of a certificate contains the value of the subject key identifier of the CA (certificate authority) as a reference to that CA.