How does SSL/TLS encryption work?

SSL (Secure Sockets Layer) and TLS (Transport Layer Security) are protocols designed to provide secure communication over a network. TLS is the successor to SSL and is more secure; in practice, when people refer to SSL nowadays, they often mean TLS. Here’s a high-level overview of how SSL/TLS encryption works:

Handshake Process

The SSL/TLS handshake is the initial process where the client and server establish a secure connection. It involves several steps:

• Client Hello: The client sends a message to the server indicating that it wants to establish a secure connection. This message includes the TLS version, cipher suites (encryption algorithms), and other settings.

• Server Hello: The server responds with its own message, choosing a cipher suite from the list provided by the client, and sending its digital certificate. The certificate contains the server's public key and is issued by a trusted Certificate Authority (CA).

• Certificate Exchange: The server sends its digital certificate to the client. The client verifies this certificate against a list of trusted CAs to ensure that the server is legitimate.

• Key Exchange: Depending on the chosen cipher suite, there are different methods for exchanging keys. One common method is the Diffie-Hellman key exchange, which allows both parties to securely agree on a shared secret key.

• Client Finished: The client generates a pre-master secret, which is then used to derive the session keys. It sends a message encrypted with the server's public key to the server, indicating that it has completed the key exchange process.

• Server Finished: The server decrypts the client's message, completes its own key generation, and sends a final message to the client, confirming that the server is ready to start secure communication.

Session Encryption

Once the handshake is complete, both parties have derived the same session keys. These keys are symmetric keys used to encrypt and decrypt the data transmitted between the client and server. The encryption process typically involves:

• Symmetric Encryption: This is used to encrypt the actual data exchanged between the client and server. Symmetric encryption is fast and efficient, using the session keys that both parties share.

• Message Integrity: SSL/TLS includes mechanisms to ensure that data has not been altered in transit. This is done using cryptographic hash functions and message authentication codes (MACs).

Data Transmission

• Encryption: As data is sent between the client and server, it is encrypted using the session keys. This ensures that even if intercepted, the data cannot be read by unauthorized parties.

• Decryption: Upon receiving the encrypted data, the recipient uses the session keys to decrypt and read the data.

• Integrity Checks: Each message includes a hash value (MAC) to verify that the data has not been tampered with during transmission.

Session Termination

When the session is complete, the connection can be closed securely, ensuring that both parties agree to end the secure communication.