Public Key Infrastructure (PKI)

Public Key Infrastructure (PKI) is a framework of policies, technologies, and procedures used to create, manage, distribute, use, store, and revoke digital certificates. PKI provides the necessary infrastructure to secure electronic communications and transactions through public key cryptography.

Key Components of PKI:

1. Certificate Authority (CA):
   • A trusted entity that issues, verifies, and revokes digital certificates. The CA acts as the root of trust for validating identities.
   • Root CA: The top-level CA in a hierarchy, whose certificate is self-signed.
   • Intermediate CA: Subordinate CAs that are certified by the Root CA, providing a scalable trust model.
2. Registration Authority (RA):
   • An entity responsible for accepting requests for digital certificates and authenticating the identity of the certificate requester before forwarding the request to the CA.
3. Certificate Repository:
   • A publicly accessible database or directory service where certificates and certificate revocation lists (CRLs) are stored and can be retrieved by users and systems.
4. Certificate Revocation List (CRL):
   • A list of certificates that have been revoked by the CA before their expiration date. The CRL is periodically published by the CA.
5. Online Certificate Status Protocol (OCSP):
   • An alternative to CRLs, OCSP allows users to query the CA about the revocation status of a specific certificate in real-time.
6. Key Management Services:
   • Services that handle the generation, distribution, storage, backup, recovery, and destruction of cryptographic keys used in PKI.

PKI Functions:

1. Key Pair Generation:
   • Generating a pair of cryptographic keys (public and private) for use in public key cryptography. This can be done by the user or by the CA/RA.
2. Certificate Issuance:
   • After verifying the identity of the requester, the CA issues a digital certificate that binds the public key to the requester’s identity.
3. Certificate Distribution:
   • Distributing the issued digital certificates to the certificate holder and making them available in the certificate repository.
4. Certificate Validation:
   • Checking the validity of a certificate, including its signature, expiration date, and revocation status, to ensure it can be trusted.
5. Certificate Revocation:
   • Revoking certificates that are no longer trustworthy due to reasons such as compromise, expiration, or change in the holder’s information.

PKI Trust Models:

1. Single CA:
   • A single CA is responsible for issuing all certificates. Simple but not scalable and presents a single point of failure.
2. Hierarchical Trust Model:
   • A Root CA at the top issues certificates to Intermediate CAs, which in turn issue certificates to end entities. Provides scalability and allows for distribution of trust.
3. Mesh or Web of Trust:
   • Used in decentralized environments (e.g., PGP). Each participant acts as a CA and signs each other’s certificates based on trust.

Applications of PKI:

1. Secure Email (S/MIME):
   • PKI secures email communications through encryption and digital signatures.
2. Secure Web Browsing (SSL/TLS):
   • PKI underpins SSL/TLS protocols used to secure HTTPS connections between web browsers and servers.
3. Virtual Private Networks (VPNs):
   • PKI is used to authenticate and secure connections in VPNs.
4. Code Signing:
   • PKI ensures the integrity and authenticity of software by signing code with a developer’s digital certificate.
5. Document Signing:
   • PKI provides digital signatures for documents, ensuring their integrity and authenticity.

Public Key Infrastructure (PKI) is essential for secure digital communications and transactions. It relies on a combination of cryptographic technologies, trusted entities, and standardized procedures to manage digital certificates and public keys. PKI enables the authentication of identities, the encryption of data, and the integrity of transactions, making it a cornerstone of modern cybersecurity practices.