What is Public Key Infrastructure?
Also known as: PKI
Public Key Infrastructure (PKI) is a system of policies, procedures, hardware, and software for creating, distributing, managing, storing, and revoking digital certificates used in public-key cryptography.
Public Key Infrastructure (PKI) is the framework that binds public keys to identities through digital certificates. It encompasses the roles, policies, and technical components needed to issue, manage, distribute, and revoke these certificates. PKI solves the problem of authenticating parties in an untrusted network by providing a trusted third party, the Certificate Authority (CA), to vouch for the binding between a public key and an entity.
At the core of PKI is the Certificate Authority, which signs certificates using its own private key. A Registration Authority (RA) may verify the identity of certificate requestors before the CA issues a certificate. Certificates follow the X.509 standard (RFC 5280), which defines fields such as issuer, subject, validity period, and extensions. To handle revocation, PKI uses Certificate Revocation Lists (CRLs) or the Online Certificate Status Protocol (OCSP). These mechanisms allow relying parties to check whether a certificate is still valid. The entire system depends on a chain of trust: each certificate is signed by the CA above it, ending at a self-signed root certificate that is trusted by default.
PKI underpins most secure internet communications. It is the foundation of TLS/SSL for HTTPS, S/MIME for email signing and encryption, code signing, and document signing. Without PKI, there would be no scalable way to verify the identity of websites or software publishers. Organizations may operate their own internal PKI for enterprise applications, while public CAs like Let's Encrypt, DigiCert, and GlobalSign serve the broader internet. Proper PKI management includes key generation, secure storage, certificate lifecycle management, and periodic audits of the CA's practices.
Key facts
- Relies on a hierarchical trust model rooted in one or more Certificate Authorities (CAs).
- Digital certificates conform to the X.509 standard, defined in RFC 5280.
- Certificate Revocation Lists (CRLs) and OCSP provide revocation checking.
- PKI enables secure communications over untrusted networks like the internet.
- Used in TLS/SSL, S/MIME, code signing, and document signing.
How it works in practice
Related terms
References
- RFC 5280: Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile
- RFC 3647: Internet X.509 Public Key Infrastructure Certificate Policy and Certification Practices Framework
- NIST SP 800-32: Introduction to Public Key Technology and the Federal PKI Infrastructure
More in TLS & PKI
ACME Protocol
ACME (Automated Certificate Management Environment) is a protocol that automates the issuance, renewal, and revocation of TLS certificates, defined in RFC 8555.
Certificate Authority
A Certificate Authority (CA) is a trusted entity that issues digital certificates after verifying that the requester controls the domain or identity named in the certificate.
Certificate Chain
A certificate chain is an ordered list of certificates, starting with the server certificate and ending with a root CA, that a client validates to establish trust in the server's identity.
Certificate Pinning
A security technique where an application trusts only a specific, pre-selected certificate or public key for a given server, bypassing the standard chain of trust.
Cipher Suite
A cipher suite is a named set of cryptographic algorithms negotiated during a TLS handshake, specifying key exchange, authentication, encryption, and integrity protection for secure communications.
Domain Validation
Domain Validation (DV) is the lowest level of certificate validation used in TLS/SSL, where the certificate authority verifies only that the applicant controls the domain name, typically via an HTTP or DNS challenge.
ECH
Encrypted Client Hello (ECH) is a TLS extension that encrypts the Client Hello message, including the Server Name Indication (SNI), to prevent on-path observers from learning the target hostname during the handshake.
Extended Validation
Extended Validation (EV) is the highest level of TLS certificate assurance, requiring the certificate authority to perform rigorous, human-verified checks on the legal identity and operational existence of the requesting organization before issuance.
HSTS
HTTP Strict Transport Security (HSTS) is a web security policy mechanism that forces browsers to interact with a website only over HTTPS, preventing downgrade attacks and cookie hijacking.
Intermediate Certificate
An intermediate certificate is a subordinate CA certificate signed by a root CA, used to sign end-entity certificates and enable path validation while the root remains offline.