What is HTTPS and how does it differ from HTTP? What happens during a TLS handshake and what does the browser check in the certificate?

HTTPS (HyperText Transfer Protocol Secure) is an extension of HTTP that uses encryption to protect data transmitted between the browser and the server. The main difference between HTTPS and HTTP is the use of the TLS (Transport Layer Security) protocol, which ensures confidentiality, integrity, and authentication.

What happens during a TLS handshake:

1. The client (browser) sends the server a list of supported TLS versions and cipher suites.
2. The server selects encryption parameters and sends its digital certificate to the client.
3. The client verifies the certificate (see below for details).
4. The client and server exchange keys to establish a shared secret key.
5. After a successful exchange, an encrypted connection begins.

What the browser checks in the certificate:

• Authenticity of the certificate: whether it is signed by a trusted certification authority (CA).
• Match of the domain name in the certificate with the website address.
• Validity period of the certificate (whether it has expired).
• The chain of certificates up to the root CA.

If the verification is successful, the browser establishes a secure connection and displays a lock in the address bar.