In public key encryption, we solve this problem: Google, for example, wants to receive confidential data from users, such as passwords and credit card numbers, but they don't have a secure communication channel; what they have is the public internet, and any data being sent might be eavesdropped upon by any number of attackers. Thus, there's no way to deliver a shared secret key, and symmetric encryption algorithms, such as AES, cannot solve this problem. That's where public key encryption comes in.

Google creates a key pair. They keep the private key secret and don't tell anyone, and they publish public key so anyone can know it. Everyone who wants to send secrets to Google can encrypt them with the public key and send them over an insecure channel because the only one who can decrypt them is Google, who has the private key. Mailboxes work...