# Public Key Cryptography

Public-key cryptography solves the logistical problem of key distribution by using both a public key and a private key. The public key can be sent openly through the network while the private key is kept private by one of the communicating parties. The public and the private keys are cryptographic inverses of each other; what one key encrypts, the other key will decrypt.

Assume that Bob wants to send a secret message to Alice using public-key cryptography. Alice has both a public key and a private key, so she keeps her private key in a safe place and sends her public key to Bob. Bob encrypts the secret message to Alice using Alice's public key. Alice can later decrypt the message with her private key.

If Alice encrypts a message using her private key and sends the encrypted message to Bob, then Bob can be sure that the data he receives comes from Alice; if Bob can decrypt the data with Alice's public key, the message must have been encrypted by Alice with her private key, and only Alice has Alice's private key. The problem is that anybody else can read the message as well because Alice's public key is public. Although this scenario does not allow for secure data communication, it does provide the basis for digital signatures. A digital signature is one of the components of a public key certificate, and is used in TLS to authenticate a client or a server. See Public Key Certificates and Digital Signatures.

Public-key cryptography is also called asymmetric cryptography because different keys are used to encrypt and decrypt the data. A well-known public key cryptographic algorithm often used with TLS is the Rivest Shamir Adleman (RSA) algorithm. Another public key algorithm used with TLS that is designed specifically for secret key exchange is the Diffie-Hellman (DH) algorithm. Public-key cryptography requires extensive computations, making it very slow. It is therefore typically used only for encrypting small pieces of data, such as secret keys, rather than for the bulk of encrypted data communications.