Public-key cryptography refers to a cryptographic system requiring two separate keys, one to lock or encrypt the plaintext, and one to unlock or decrypt the cyphertext. Neither key will do both functions. One of these keys is published or public and the other is kept private. If the lock/encryption key is the one published then the system enables private communication from the public to the unlocking key's owner. If the unlock/decryption key is the one published then the system serves as a signature verifier of documents locked by the owner of the private key. Although in this latter case, since encrypting the entire message is relatively expensive computationally, in practice just a hash of the message is encrypted for signature verification purposes.

Because this cryptographic approach uses asymmetric key algorithms such as RSA its more general name is "asymmetric key cryptography." Some of these algorithms have the public key/private key property—that is, neither key is derivable from knowledge of the other—but not all asymmetric key algorithms do. Those without derivable keys are particularly useful and have been widely deployed; hence, they are the often the meaning behind the name.
Although unrelated, the two parts of the key pair are mathematically linked. The public key is used to transform a message into an unreadable form, decryptable only by using the (different but matching) private key. By publishing the public key, the key producer empowers anyone who gets a copy of the public key to produce messages only s/he can read—because only the key producer has a copy of the private key (required for decryption). When someone wants to send a secure message to the creator of those keys, the sender encrypts it (i.e., transforms it into an unreadable form) using the intended recipient's public key; to decrypt the message, the recipient uses the private key. No one else, including the sender, can do so.