What is Public Key Infrastructure and How Does it Work?

Public key infrastructure (PKI) is a system of technologies, policies, procedures and people that enables secure digital communications and provides cryptographic security for electronic transactions. PKI establishes and governs the process of generating, distributing, managing and revoking digital certificates that link a public key to an entity such as a person, organization or device.

PKI enables trusted and private communications over insecure public networks like the internet by establishing the identity of entities involved It’s one of the most common forms of internet encryption and authentication.

In this article, we’ll explain what PKI is, why it’s important, how it works, its components, types of PKI, and more.

What is PKI?

PKI provides the basis for trusting digital communications and electronic transactions It enables two parties to securely communicate over an insecure channel by verifying identities and facilitates data encryption to ensure confidentiality and integrity

PKI establishes a link between a public key and its owner to prevent impersonation It does this through digital certificates issued and verified by a trusted third-party Certificate Authority (CA)

The core elements of a PKI include:

• Digital certificates – contain identity credentials and public key to authenticate and verify identity
• Public and private key pairs – used for encryption and decryption
• Certificate Authorities – issue and revoke digital certificates
• Registration Authorities – verify identity and credentials for certificate issuance
• Repositories and directories – allow access to certificates and certificate revocation lists
• Certificate policies and practice statements – govern PKI processes and procedures

PKI provides trust services related to confidentiality, integrity, authentication and non-repudiation. It enables secure electronic transfer of information for activities like e-commerce, online banking and confidential communication.

Why is PKI Important?

PKI is essential for securing communications and transactions where basic password authentication is insufficient. PKI establishes trust in digital identities which provides:

• Privacy – Messages remain confidential through encryption using public-private key pairs
• Authentication – Digital signatures and certificates verify identity of sender
• Integrity – Detects any changes to message contents through hashing algorithms
• Non-repudiation – Senders cannot deny sending the message as their private key is used

Without PKI, there would be no trusted way to conduct secure online transactions, communications and interactions. PKI enables business critical activities like online banking, e-commerce, confidential email and protects access to connected devices and networks.

How Does PKI Work?

PKI uses public-key cryptography to enable trust and security. Here’s an overview of how PKI works:

1. A user generates a public-private key pair and requests a digital certificate from a CA
2. The CA verifies the user’s identity through the Registration Authority (RA)
3. The CA issues a certificate binding the user’s identity to their public key
4. The user retains the private key secretly while the public key is embedded in the certificate
5. When the user wants to communicate with another party, they exchange certificates
6. The public keys in the certificates are used to encrypt and decrypt messages
7. The authenticity and integrity of messages are validated through the certificates

This establishes an encrypted channel allowing the parties to communicate securely. The private keys are only known to their owners so messages can only be decrypted by intended recipients.

Digital Certificates

Digital certificates allow the public key to be trusted. They contain information about the key owner’s identity and public key signed by the issuing CA.

Certificates adhere to X.509 standard and include fields like:

• Issuer – CA that issued the certificate
• Subject – Identity of key owner (individual, organization, device)
• Public key – Key used to encrypt messages
• Validity period – Certificate expiration date
• Signature – CA digital signature to authenticate origin

Public and Private Keys

Public key cryptography uses key pairs for encryption and decryption.

• Public key – Shared openly, used to encrypt messages
• Private key – Kept secret, used to decrypt messages

Public keys allow anyone to encrypt messages to the key owner. But only the paired private key can decrypt them. This enables confidentiality.

Private keys also create digital signatures which authenticate the sender’s identity. The signature is verified using the public key.

Certificate Authorities

CAs are trusted third parties that issue, manage and revoke digital certificates. They validate identities and issue certificates binding entities with their public key.

CAs digitally sign certificates using their own private key. Trust in the user’s certificate relies on trusting the CA’s signature.

CAs can be independent or part of private organization PKIs. Browsers and OSs come with pre-installed trusted root CA certificates.

PKI Components

The core components of a PKI include:

Certificate Authority – Issues, publishes, manages and revokes digital certificates. Acts as the trusted entity at the center of the PKI.

Registration Authority – Verifies and authenticates identity of entities requesting certificates from the CA. Can be separate from CA or part of it.

Certificate Database – Repository containing all issued certificates and revocation lists. Allows certificates to be retrieved and verified.

Certificate Policy – Defines the PKI’s practices, procedures and requirements. Allows external parties to gauge the PKI’s trustworthiness.

Types of PKI

There are several approaches and architectures for implementing PKI:

Hierarchical PKI

Most common type with a centralized CA issuing certificates under its trusted root certificate. Allows a chain of trust with every certificate verifiable via the root CA’s certificate. Used in commercial CAs and enterprise PKIs.

Mesh PKI

Decentralized model where CAs cross-certify each other as equals to create a mesh topology. Provides redundancy since compromise of one CA doesn’t impact trust in other CAs. Used in government, banking and defense.

Bridge PKI

Hybrid model combining hierarchical and mesh PKI. Root CAs cross-certify while lower-level CAs have hierarchical relationship. Provides advantages of both models. Used across organizations and industry groups.

Web-of-Trust

Decentralized model where individuals sign each other’s keys to establish authenticity in a web-of-trust. Doesn’t rely on CAs. Used in cases like PGP encryption. More vulnerable to compromise than hierarchical PKIs.

Real-World Applications

PKI is extensively used to secure internet communications and online transactions, including:

• HTTPS – PKI enables TLS/SSL encryption which secures HTTPS sites. CAs issue SSL/TLS certificates to websites.

• Code Signing – Signing software, binaries and scripts to verify authenticity and integrity. Prevents distribution of malware.

• Email Security – S/MIME protocol uses PKI for email encryption, authentication and signing.

• Document Signing – Applying digital signatures to documents like contracts and invoices to ensure authenticity, non-repudiation and integrity.

• Device Authentication – Machine identities use PKI to authenticate and authorize connected devices like IoT and medical equipment.

• VPN Access – Issuing certificates to allow employees secure remote access to enterprise networks and resources.

Benefits of PKI

PKI provides a robust framework for trust in digital environments by delivering:

• Security – Encryption, signatures and certificates prevent interception, impersonation and tampering

• Trust – Identities and transactions can be trusted by relying on credentials issued from trusted CAs

• Convenience – No physical documents or face-to-face processes required for transacting online

• Compliance – PKI facilitates meeting regulations around data protection and transaction records

• Scalability – Certificate automation allows easy rollout across users and systems

Limitations of PKI

While PKI is critical for security, it has some limitations:

• Certificate management can be complex with large user populations
• Compromise of root CA undermines all certificates issued by it
• Certificate revocation is challenging to execute instantly across systems
• Requires technical expertise and resources to operate PKIs
• External CAs represent single point of failure outside organization’s control

PKI provides the foundation for trusting identities and securing communications in the digital world. As use of online services and connected devices grows exponentially, PKI will continue playing a pivotal role in protecting privacy and transactions.

With growing cyber threats and regulations, building robust PKIs with efficient certificate lifecycle management will be crucial for enterprises and government agencies. Understanding PKI concepts allows organizations to deploy solutions that balance security, cost and user experience.