TLS handshake
TLS is an encryption and authentication protocol designed to secure Internet communications. A TLS handshake is the process that kicks off a communication session that uses TLS. During a TLS handshake, the two communicating sides exchange messages to acknowledge each other, verify each other, establish the cryptographic algorithms they will use, and agree on session keys. TLS handshakes are a foundational part of how HTTPS works.
A TLS handshake takes place whenever a user navigates to a website over HTTPS and the browser first begins to query the website’s origin server. A TLS handshake also happens whenever any other communications use HTTPS, including API calls and DNS over HTTPS queries.
TLS handshakes occur after a TCP connection has been opened via a TCP handshake.
TLS handshakes are a series of datagrams, or messages, exchanged by a client and a server. A TLS handshake involves multiple steps, as the client and server exchange the information necessary for completing the handshake and making further conversation possible.
The exact steps within a TLS handshake will vary depending upon the kind of key exchange algorithm used and the cipher suites supported by both sides. The RSA key exchange algorithm, while now considered not secure, was used in versions of TLS before 1.3. It goes roughly as follows:
- The ‘client hello’ message: The client initiates the handshake by sending a “hello” message to the server. The message will include which TLS version the client supports, the cipher suites supported, and a string of random bytes known as the “client random.”
- The ‘server hello’ message: In reply to the client hello message, the server sends a message containing the server’s SSL certificate, the server’s chosen cipher suite, and the “server random,” another random string of bytes that’s generated by the server.
- Authentication: The client verifies the server’s SSL certificate with the certificate authority that issued it. This confirms that the server is who it says it is, and that the client is interacting with the actual owner of the domain.
- The premaster secret: The client sends one more random string of bytes, the “premaster secret.” The premaster secret is encrypted with the public key and can only be decrypted with the private key by the server. (The client gets the public key from the server’s SSL certificate.)
- Private key used: The server decrypts the premaster secret.
- Session keys created: Both client and server generate session keys from the client random, the server random, and the premaster secret. They should arrive at the same results.
- Client is ready: The client sends a “finished” message that is encrypted with a session key.
- Server is ready: The server sends a “finished” message encrypted with a session key.
- Secure symmetric encryption achieved: The handshake is completed, and communication continues using the session keys.
TLS 1.3 does not support RSA, nor other cipher suites and parameters that are vulnerable to attack. It also shortens the TLS handshake, making a TLS 1.3 handshake both faster and more secure.
The basic steps of a TLS 1.3 handshake are:
- Client hello: The client sends a client hello message with the protocol version, the client random, and a list of cipher suites. Because support for insecure cipher suites has been removed from TLS 1.3, the number of possible cipher suites is vastly reduced. The client hello also includes the parameters that will be used for calculating the premaster secret. Essentially, the client is assuming that it knows the server’s preferred key exchange method (which, due to the simplified list of cipher suites, it probably does). This cuts down the overall length of the handshake — one of the important differences between TLS 1.3 handshakes and TLS 1.0, 1.1, and 1.2 handshakes.
- Server generates master secret: At this point, the server has received the client random and the client’s parameters and cipher suites. It already has the server random, since it can generate that on its own. Therefore, the server can create the master secret.
- Server hello and “Finished”: The server hello includes the server’s certificate, digital signature, server random, and chosen cipher suite. Because it already has the master secret, it also sends a “Finished” message.
- Final steps and client “Finished”: Client verifies signature and certificate, generates master secret, and sends “Finished” message.
- Secure symmetric encryption achieved
References
#protocol #internet #layer #encryption #handshake #computer_science #security #web #transport #tls