What is Public Key Infrastructure? 

The Public Key Infrastructure (PKI) security method has seen a major upswing in popularity and is used for everything from enabling Internet of Things (IoT) communication security to digital document signing. 

Martin Furuhed, PKI expert at Nexus Group, explains the importance of PKI, stating: “PKI enables trusted electronic identities for people, services, and things, which make it possible to implement strong authentication, data encryption, and digital signatures.”  

This guide will walk you through what PKI is, how it works, and what it’s used for.  

What is Public Key Infrastructure (PKI)? 

A typical PKI consists of policies, standards, hardware, and software that manage the creation, distribution, revocation, and administration of digital certificates. These digital certificates are used to verify the identity of individuals, devices, and services and encrypt data, ensuring secure online communication and transactions. A digital certificate contains a public and private key to make up the electronic identity.  

Think of PKI as the digital equivalent of a passport system. Just as a passport system involves rules, procedures, and various offices and personnel to issue, renew, or revoke passports, PKI comprises policies, standards, hardware, and software for similar functions in the digital world. 

Digital certificates in PKI are like passports in this analogy. A passport confirms your identity and nationality, which then allows you to travel across borders securely. In a similar way, digital certificates validate the identity of individuals, devices, and services, facilitating secure communication and transactions in the online world. Like how a passport ensures your identity is verified, digital certificates serve as proof of identity and provide a means to encrypt data, ensuring that digital interactions are both authenticated and confidential.  

Here we break down PKI encryption, public and private keys, and digital certificates and the certifying authorities:  

PKI and encryption 

Public and private keys are used in the encryption process in PKI. PKI employs two types of encryptions: symmetric and asymmetric. 

Symmetric encryption is where the same key is used for both encrypting and decrypting the data. It's like having a single key that both locks and unlocks a safe. 

When large amounts of data are to be encrypted and decrypted, symmetric cryptography has to be used, since asymmetric cryptography is too slow. Since the same key is used for both encryption and decryption in symmetric cryptography, the key first has to be shared between the two communicating parties. One of the parties generates the key and sends it to the other party using asymmetric cryptography, which is enabled by the PKI.  

Asymmetric encryption uses a public key and a private key. This method is akin to having a mailbox with a public address (the public key) where anyone can drop a message, but only the person with the unique private key can open the mailbox and read the messages. In PKI, the public key is used to encrypt data and the corresponding private key is used for decryption. 

Public and private keys explained 

Foundational elements of PKI are public and private keys. Public and private keys are generated in pairs that are mathematically linked, and they are used in asymmetric cryptography, also known as public key cryptography. 

A public key is a piece of cryptographic code that is available to the public. It is used to encrypt data or verify a digital signature. For example, when someone sends a message or data, they encrypt it using the recipient’s public key.  

The private key is also a piece of cryptographic code that is kept secure by the individual it belongs to. This key is used to decrypt data that has been encrypted with its corresponding public key or create a digital signature.  

This dual-key system of both public and private keys ensures that digital communications are secure and that only authorised parties can access sensitive information. This approach to encryption and decryption is what makes PKI an effective tool for maintaining the confidentiality and integrity of digital data. 

Digital certificates and certifying authorities 

Digital certificates are electronic documents that link a public key with an entity’s identity. A digital certificate is a file containing a range of information, such as identifying information, a serial number, and expiration dates. It also includes the digital signature of the certificate authority (CA) that has issued the certificate, which is what gives validity to the certificate, as well as the certificate holder’s public key.  

Certificate authorities (CAs) are trusted entities responsible for issuing and managing these digital certificates. The process usually involves verifying the credentials of the entity requesting a certificate. Once the CA is satisfied with the verification, it issues a digital certificate that contains the entity's public key and other identification information.  

This is the purpose of PKI: to reliably tie a public key to a person, service, or thing. Nearly all digital certificates used within PKI are based on the X.509 standard, and support for these certificates are built into a lot of software, such as email clients and servers, web servers, and operating systems.  

PKI and electronic identity 

Electronic identity in PKI is made up of a digital certificate, which is publicly available and includes a public key, and a private key. It validates and secures the identity of a person, device, or service on the internet. Electronic identity ensures the legitimacy and security of online transactions and communications.  

How PKI works 

PKI works by establishing a chain of trust between entities, whether those entities are individuals, devices, or services. The trust is created with the use of digital certificates, each acting as a verified identity marker. 

When an entity presents its digital certificate as part of a communication or transaction, the recipient of the certificate undertakes a verification process. It involves checking the digital certificate to ensure it is authentic, has not been altered or tampered with, and is still within its validity period. The authenticity of the certificate is confirmed by tracing it back to the issuing CA.  

The CA's own certificate is also verified, ensuring a secure chain of trust. This layered approach of verification is essential for ensuring that each participant in a digital interaction can confidently trust the identity and the security of the entities they are communicating with.  

What is Public Key Infrastructure used for? 

PKI has diverse applications in today's digital world. These security mechanisms are used to grant secure access to physical and digital resources; secure communication between people, services, and things; and enable digital signing of documents and transactions. 

Secure communication 

PKI is essential for securing communications over the internet. It encrypts emails, instant messages, and other forms of digital communication. This encryption ensures that only the intended recipient can access the message content, protecting against unauthorised access. 

Authentication 

Authentication is another critical use of PKI. It involves verifying the identity of a user, device, or service. PKI enables strong authentication by using digital certificates and key pairs. This ensures that only authorised entities can access sensitive systems and data. 

When, for example, a user tries to authenticate their identity to a server, the server generates random data and sends it to the user. The user encrypts the data with their private key and sends it back to the server. The server decrypts the data with the public key in the user’s digital certificate, and if the decrypted data is the same as the sent data, the server knows that the user is who they claim to be. 

Data integrity 

PKI plays a vital role in maintaining data integrity. It ensures that data has not been altered or tampered with during transmission. Digital signatures, created using PKI, verify that the data received is exactly as it was sent, providing assurance of its authenticity and integrity. 

Securing web transactions 

In e-commerce and online banking, PKI secures web transactions. It encrypts sensitive information like credit card numbers and personal data, ensuring safe and secure online transactions. This not only protects the users' data but also builds trust in online platforms. 

FAQs about PKI 

What is the difference between private and public key infrastructure? 

The main difference lies in the management of keys. In private key infrastructure, a single key is used for both encryption and decryption. In public key infrastructure, two keys are used: a public key for encryption and a private key for decryption. This distinction makes PKI more secure and suitable for a wide range of applications. 

What are PKI certificates used for? 

PKI certificates are used for encrypting data, securing online transactions, authenticating users, devices, and services, and ensuring data integrity through digital signatures.  

How is PKI used in business? 

In business, PKI is used to protect sensitive data, authenticate employees and clients, secure email communications, and safeguard online transactions. It is also vital for regulatory compliance in many industries, providing a secure framework for handling confidential information.