How do SSL certificates work?

What is an SSL Certificate?

An SSL certificate is a digital security certificate that establishes a secure connection between a website and a user’s browser. It encrypts the data transmitted between the two parties, protecting it from eavesdropping, tampering, and man-in-the-middle attacks. SSL certificates are essential for safeguarding sensitive information exchanged online, such as credit card numbers, personal details, and passwords.

SSL Certificate Components

An SSL certificate consists of two main components:

1. Public Key: The public key is a unique identifier that is publicly available and used to encrypt data sent from the user’s browser to the website.
2. Private Key: The private key is a secret key that is held securely by the website and is used to decrypt the data received from the user’s browser.

These keys are generated using a mathematical algorithm called asymmetric cryptography, which ensures that only the intended recipient can decrypt the data.

Establishing a Secure Connection

When a user visits a website that has an SSL certificate installed, the browser initiates a handshake process to establish a secure connection. During this process, the following steps occur:

1. Client Hello: The user’s browser sends a message to the website, requesting a secure connection and specifying the supported SSL ciphers (encryption algorithms).
2. Server Hello: The website responds with a message, confirming the support for the chosen cipher and providing its public key.
3. Client Encryption: The user’s browser encrypts a random session key using the server’s public key and sends it to the website.
4. Server Encryption: The website decrypts the session key using its private key and sends back an encrypted message containing its own certificate and a signature.
5. Certificate Verification: The user’s browser verifies the authenticity of the website’s certificate by checking its validity and chain of trust with trusted Certificate Authorities (CAs).
6. Session Key Exchange: The user’s browser and the website exchange the encrypted session key, allowing them to encrypt and decrypt subsequent communication between them.

Ciphers and Encryption Strength

SSL certificates use various encryption algorithms, known as ciphers, to secure the communication channel. Ciphers are rated based on their strength, typically measured by the length of their encryption keys. Stronger ciphers provide greater protection against brute-force attacks, where hackers attempt to guess the encryption key by trying various combinations.

Common SSL cipher suites include:

• AES-128-GCM: A modern cipher with a 128-bit key length, providing a balance between performance and security.
• AES-256-GCM: An even stronger cipher with a 256-bit key length, offering enhanced protection against brute-force attacks.
• ECDHE-ECDSA: A cipher that uses elliptical curve cryptography, providing efficient encryption and key exchange with smaller key sizes.

The choice of ciphers for an SSL certificate is determined by factors such as browser support, server capabilities, and the desired level of security.

Validating SSL Certificates

To ensure the authenticity of an SSL certificate, web browsers rely on Certificate Authorities (CAs) to verify the identity of website owners. CAs are trusted organizations that issue and manage SSL certificates. They perform thorough checks to confirm the identity of website owners and the validity of their domain names.

When a user’s browser encounters an SSL certificate, it verifies the certificate’s validity by checking its chain of trust, which links the certificate back to a trusted CA’s root certificate. If the certificate is valid, the browser displays a green padlock icon in the address bar, indicating a secure connection.