The underlying hardware security modules (HSMs) in publick key infrastructure (PKI) are the root of trust which protect PKI from being breached.
PKI authentication is based on digital certificates and uses encryption and decryption to verify machine and user identities to prove the integrity of digital transactions. With the constant threat of cyber breaches, the ever-growing number of machines used to conduct those transactions must be assured that the information is protected from attacks and can be trusted.
You can read more about HSMs in PKI here.