https://www.pcisecuritystandards.org/pci_security/maintaining_payment_security Build and Maintain a Secure Network
1. Install and maintain a firewall configuration to protect cardholder data.
2. Do not use vendor-supplied defaults for system passwords and other security parameters.
Protect Cardholder Data
3. Protect stored cardholder data.
4. Encrypt transmission of cardholder data across open, public networks.
Maintain a Vulnerability Management Program
5. Use and regularly update anti-virus software or programs.
6. Develop and maintain secure systems and applications.
Implement Strong Access Control Measures
7. Restrict access to cardholder data by business need-to-know.
8. Assign a unique ID to each person with computer access.
9. Restrict physical access to cardholder data.
Regularly Monitor and Test Networks
10. Track and monitor all access to network resources and cardholder data.
11. Regularly test security systems and processes.
Maintain an Information Security Policy
12. Maintain a policy that addresses information security for employees and contractors.

PKI uses public-key cryptography, which is widely used on the Internet to encrypt messages or authenticate message senders. In public-key cryptography, a CA generates public and private keys with the same algorithm simultaneously. The private key is held only by the subject (user, company, or system) mentioned in the certificate, while the public key is made publicly available in a directory that all parties can access. The subject keeps the private key secret and uses it to decrypt the text encrypted by someone else using the corresponding public key (available in a public directory). Thus, others encrypt messages for the user with the user's public key, and the user decrypts it with his/her private key.