In the case of web browsers surfing secure web sites, SSL communication starts with the web browser requesting the digital certificate from the web server. The certificate contains the hostname of the web server, an expiration date of the certificate, the public key of the web server, and is signed by a Certificate Authority. The web browser can validate all of these pieces of information except for the public key of the web server. If all of the verifiable components pass validation, the web browser will generate its own public key and send it back to the web server. When the web browser's public key is sent back to the web server as a response, it uses the web server's public key, which was contained within the certificate, to encrypt the browser's public key being sent. Now both the web server and web browser will be able to communicate with each using secure, encrypted communications because they have exchanged each of their public keys.
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