What attribute is included in a X.509-certificate?

An X.509 certificate contains the Distinguished Name (DN) of the subject (the entity to which the public key belongs), which serves as a unique identifier for that entity within the certificate. Other critical attributes include the public key, serial number, validity period, and the issuer's DN.

While a telephone number might appear in some certificate extensions or subject alternative names, it is not a universally required or core attribute of the base X.509 certificate structure like the subject's DN.

The secret key (private key) of the issuing Certificate Authority (CA) is used by the CA to sign the certificate, but it is never included in the issued certificate itself; keeping it secret is paramount to PKI security.

An X.509 certificate contains only the public key of the certificate holder, not the entire key pair (public and private key). The private key must remain confidential to the certificate holder.