What is PKI in Simple Terms?

By John King, CISSP, PMP, CISM •  Updated: 12/29/22 •  3 min read

Public Key Infrastructure (PKI) is used to exchange digital information securely. It provides a secure and convenient way for people to communicate and transact online.

At the heart of PKI are two keys: a public key and a private key. The public key is a string of characters shared with others, while the private key is kept secret. The keys are used together to encrypt and decrypt messages.

To use PKI, a user first generates a pair of keys and then sends their public key to a trusted third party, known as a Certificate Authority (CA). The CA verifies the user’s identity and then issues a digital certificate, which is a document that contains the user’s public key and other identifying information.

The digital certificate serves as an “ID card” for the user, providing a way for others to verify the user’s identity and encrypt messages only the user can decrypt. PKI is often used to secure online transactions, such as online banking and e-commerce, and protect sensitive information, such as medical records and government documents.

Here’s a more detailed explanation of how PKI works:

  1. Key generation: The first step in using PKI is to generate a pair of keys. This can be done using specialized software or a hardware device, like a smart card or a USB token. The keys are mathematically related. They are used together to encrypt and decrypt messages.
  2. Certificate request: Once the keys have been generated; the user sends a certificate request to a CA. The request includes the user’s public key and other identifying information, such as the user’s name and email address.
  3. Identity verification: The CA verifies the user’s identity by checking government-issued identification documents or other forms of documentation. This helps to ensure that only legitimate users are issued certificates.
  4. Certificate issuance: The CA issues a digital certificate if the user’s identity has been successfully verified. The certificate contains the user’s public key, other identifying information, and the CA’s signature. The certificate serves as an “ID card” for the user, allowing others to verify the user’s identity.
  5. Certificate distribution: The CA distributes the certificate to the user and may send copies to other CAs or a central repository known as a certificate store. This allows others to access the certificate and verify the user’s identity.
  6. Encryption and decryption: When two users want to exchange encrypted messages, they can use each other’s public keys to do so. The sender uses the recipient’s public key to encrypt the message. The recipient uses their own private key to decrypt the message. By doing so, only the intended recipient can read the message, even if an attacker intercepts it.
  7. Certificate revocation: If a user’s private key is compromised or if the user’s identity can no longer be verified, the CA can revoke the user’s certificate. This prevents the certificate from being used to authenticate the user or to encrypt messages. CAs maintain lists of revoked certificates, known as certificate revocation lists (CRLs), which are made available to users and other CAs.

Limitations of PKI

PKI is a powerful tool for securely exchanging digital information, but it has limitations. One challenge is the need for CAs to be highly trustworthy and secure, as they hold a central role in the PKI system. If a CA’s security is compromised, it could issue fraudulent certificates or revoke legitimate ones. To address this risk, CAs must follow strict security protocols and are subject to regular audits.

John King, CISSP, PMP, CISM

John King currently works in the greater Los Angeles area as a ISSO (Information Systems Security Officer). John has a passion for learning and developing his cyber security skills through education, hands on work, and studying for IT certifications.