How does SSL/TLS work - part one


The Secure Sockets Layer (SSL) is a cryptographic protocol designed to secure communications over TCP/IP networks. SSL was developed by Netscape during the early 1990's, but various security flaws meant that it wasn't until SSL 3.0 was released in 1996 that SSL became popular.

It was also during this time that an open source implementation of SSL called SSLeay was made available by Eric Young, which helped ensure its widespread adoption on the Internet. The Apache web server was also gaining in popularity, and Ben Laurie of Apache fame used SSLeay to produce Apache-SSL, one of the first freely available secure web servers.

SSL became Transport Layer Security (TLS) with the publication of the TLS 1.0 standard in 1999, followed by TLS 1.1 and TLS 1.2, the most recent version. All versions of TLS are in widespread use, and it is only recently that support for SSL 3.0 has been discontinued in response to the POODLE vulnerability. For simplicity, we'll refer to SSL/TLS as TLS for the remainder of this article.


TLS is intended to provide secure connections between a client (e.g. a web browser), and a server (e.g. a web server) by encrypting all data that is passed between them.

Ordinary network connections are not encrypted, so anyone with access to the network can sniff packets, reading user names, passwords, credit card details and other confidential data sent across the network - an obviously unacceptable situation for any kind of Internet-based e-commerce.

Previous articles in this series have discussed how encryption works, including public key encryption and certificates. TLS uses public key encryption to verify the parties in the encrypted session, and to provide a way for client and server to agree on a shared symmetric encryption key.

The details of how this is achieved are discussed in the next post.

Posted by John Faulds in