TL;DR
An SSL/TLS handshake is the behind-the-scenes negotiation that establishes a secure connection between a browser and a server. Though commonly called an "SSL handshake," the process today relies on the more secure Transport Layer Security (TLS) protocol. The handshake determines encryption methods, authenticates both parties via certificates, and exchanges session keys — all in a matter of seconds. While robust, the process is not immune to attacks such as BREACH or man-in-the-middle exploits.
How the SSL/TLS handshake connects devices
A Secure Sockets Layer (SSL) handshake defines a connection between two devices, such as your browser and the server that supports the website you want to visit.
During an SSL handshake, the two devices determine:
- What security version both parties will use
- What type of encryption will protect the information
- How both parties are verified
The word "SSL" in SSL handshake is a misnomer. The secure sockets layer (SSL) protocol is old, and people rarely use it these days. Now, most devices use transport layer security (TLS).
The term "TLS handshake" is more accurate, but it's common for people to call this step a simple SSL handshake instead.
What is an SSL handshake?
Pull up a website on your browser, and you may believe the connection happened both instantly and spontaneously. In reality, the two devices need to negotiate how they'll communicate and transfer information. That negotiation happens through an SSL handshake.
As we mentioned, the SSL handshake is sometimes called the TLS handshake. Here's why.
Netscape developed the SSL protocol in 1995. Unfortunately, it was riddled with security flaws. In the early 2000s, the industry moved to the TLS protocol for the promise of better security. The handshake process remains the same despite the name change.
An SSL handshake is a process that begins a communication session. The two parties acknowledge one another, determine how they will protect information, verify one another's security protocols, and set session keys.
How does the SSL/TLS handshake process work step by step?
As we've explained, SSL handshakes are negotiations. The two parties agree on styles and protocols. The SSL handshake steps result from those agreements, and they can vary depending on what the two sides want.
In general, an SSL handshake proceeds via these steps:
Contact: A browser sends a "client hello" message to the server. The message includes critical details, such as the SSL version the client uses, cipher settings (more on that in a minute), and session-specific information.
First response: The server sends back proof of security (via certificates), the server's cipher settings, and session-specific data.
Authentication: The browser verifies the security certificate to ensure it made contact with the right authority.
Key exchange: The browser and the server exchange keys, validating the security of their exchange.
Wrap up: Both the server and the browser confirm that the work is complete, and the handshake is finished.
Despite the complexity of these steps, it takes just a few seconds for both sides to do their work. Few users notice the delay.
How secure is an SSL/TLS handshake?
Look through the SSL handshake steps, and you'll notice a mention of key exchange. That process involves encryption, and the process uses two forms.
The two forms of encryption include:
| Type | Key Used | Purpose/Notes |
|---|---|---|
| Asymmetric encryption | Public key (for encryption); private key (for decryption) | A public key, readily available via the server's security certificate, is used for description. A private key is used for decryption. |
| Symmetric encryption | Shared session key | After asymmetric encryption, the two parties set up a shared key. This single session key allows for a secure connection with lower server strain. |
The two parties also agree on a so-called "cipher suite," a set of rules about what type of authentication will be required, how they'll encrypt data, and more.
What are the known vulnerabilities of the SSL/TLS handshake?
These steps should protect data in transit. But unfortunately, attacks are still possible. The BREACH attack, for example, allows hackers to change data in transport. Hackers can also take over and perform a man-in-the-middle attack. The problem begins with data that attackers steal as it's encrypted.
Many security experts became aware of security risks with encryption in 2012, when news of vulnerable Android apps broke. Did you miss the story? Read our recap here.
Frequently asked questions
What is the difference between SSL and TLS?
SSL was developed by Netscape in 1995 but was riddled with security flaws. In the early 2000s, the industry moved to the TLS protocol for the promise of better security. The handshake process remains the same despite the name change.
What happens during an SSL/TLS handshake?
During an SSL/TLS handshake, a browser sends a "client hello" message to the server, the server responds with its certificate and cipher settings, the browser authenticates the certificate, the two parties exchange keys, and both sides confirm the handshake is complete.
Why is the SSL handshake sometimes called a TLS handshake?
The term "SSL handshake" is a misnomer. The SSL protocol is old and rarely used today. Most devices now use TLS, making "TLS handshake" the more accurate term, though the older name persists colloquially.
What types of encryption are used in an SSL/TLS handshake?
The handshake uses two forms of encryption: asymmetric encryption, which uses a public key for encryption and a private key for decryption, and symmetric encryption, in which the two parties set up a shared session key that allows for a secure connection with lower server strain.
What is a cipher suite in the context of an SSL/TLS handshake?
A cipher suite is a set of rules that both parties agree on, defining what type of authentication will be required, how they'll encrypt data, and more.
What security risks are associated with the SSL/TLS handshake?
Attacks are still possible despite the protections of the SSL/TLS handshake. The BREACH attack, for example, allows hackers to change data in transport. Hackers can also take over and perform a man-in-the-middle attack, with the problem beginning with data that attackers steal as it's encrypted.
References
What is Transport Layer Security (TLS)? (November 2018). Network World.
Inside the BREACH Attack: How to Avoid HTTPS Traffic Exploits. TechTarget.
