What is the public key infrastructure scheme? How does it provide security?

Think of digital certificates like virtual ID cards issued in the digital realm. They certify the ownership of a public key in the Public Key Infrastructure (PKI) system, making the link between an individual or entity's identity and their public key. This linkage is critical because it means if you trust the certificate, you can trust that a public key belongs to the entity named in the certificate.

When you send an email or visit a secure website, digital certificates work silently behind the scenes to verify identities. The process of issuing a digital certificate involves key pairs (private and public keys) and a Certificate Authority (CA). Once issued, these certificates can be used to ensure secure communications over the internet, assuring the identity of the remote side you're communicating with.

Encryption is like a secret code that protects information from prying eyes. In the context of Public Key Infrastructure, encryption involves complex algorithms to transform readable data, known as 'plaintext', into a scrambled, unreadable format, known as 'ciphertext'. This safeguarding occurs using a pair of keys – a public key and a private key.

The public key, as the name suggests, is publicly available and is used to encrypt the data. Its counterpart, the private key, remains confidential and is used to decrypt the data. This means that only someone with the correct private key can decipher the secured information. The brilliance of this method is in the way it enables secure exchanges over insecure channels like the internet, ensuring that no matter who intercepts the encrypted data, without the private key, it remains indecipherable.

The Certificate Authority, or CA for short, serves as a digital notary public in the PKI universe, attesting to the identity of entities and linking them with a public key by issuing digital certificates. You could compare a CA to a passport office, which verifies your identity before giving you a passport.

A CA is a trusted third party that performs several critical functions: it validates the identity of a person or organization requesting a certificate, it issues the certificate to them, and it maintains a directory of issued certificates and their statuses, including whether any certificates have been revoked. The role of CAs is foundational in PKI, as trust in these certificates hinges on the trustworthiness of the issuing CA.

Authentication within PKI is the act of confirming 'you are who you say you are'. It's the foundation of trust online. When you access a secure site or establish a secure connection, authentication protocols verify the digital certificate tied to the site or entity with which you're interacting.

This ensures that the public key you're using to, say, encrypt a message, genuinely belongs to the intended recipient, rather than an imposter. Successful authentication relies on thorough verification processes often performed by a Registration Authority (RA), which works hand-in-hand with a CA. Authentication is a cornerstone of online security, preventing impersonation and fraud in digital communications.