The Roles of the Public Key and Private Key

Public-key cryptography uses a matched pair of keys — a public key and a private key. The pair belongs to the recipient, Bob: he shares the public key openly and keeps the private key to himself. A message the sender, Alice, encrypts with that public key can only be decrypted by Bob, who holds the matching private key. Even if intercepted along the way, it can't be read without that private key.