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Explain public-key infrastructure as you would to a capable student, in 3-4 sentences.

Public-key infrastructure (PKI) is a system that enables secure communication over networks using pairs of cryptographic keys: a public key that anyone can use to encrypt messages to you, and a private key that only you possess to decrypt them. Think of it like a mailbox where anyone can drop in letters (using your public key), but only you have the key to open it and read them (your private key). PKI also includes trusted organizations called Certificate Authorities that verify identities and issue digital certificates, essentially vouching that a particular public key truly belongs to who they claim to be—like a digital passport system. This infrastructure powers much of internet security, from HTTPS websites to encrypted email and digital signatures.