How Does SSL Work with Public Keys?

It’s important to understand that the client and server do not share the same key directly. Instead, they use the public key to create a shared secret key. The client generates this secret key and encrypts it with the server’s public key. When the server receives it, it decrypts the secret key with its private key. So, they effectively communicate using this shared secret key, not the public key itself.

SSL uses a public/private key system for secure communication. The public key is shared for encryption, while the private key, which only the server keeps secret, is used for decryption. During the handshake process, the client verifies the server's public key (usually through a trusted certificate) and then creates a random symmetric key for fast data encryption. This symmetric key is encrypted with the server's public key and sent to the server, which can decrypt it using its private key. This way, neither the symmetric key nor the private key is shared openly, maintaining security.

The public key is for others to encrypt messages sent to the server, while the private key is what the server uses to decrypt those messages. This means that while you can share your public key widely, the private key stays with you. Thus, only you can read messages encrypted with your public key.

SSL relies on asymmetric cryptography to establish an initial secure channel. The public keys don't just pop up randomly; they relate to a trusted certificate authority (CA) that guarantees their authenticity. The client must trust the CA to ensure the server's identity. Once verified, they can securely exchange the symmetric session key that’s used for the actual data transfer.

The distinction between public and private keys is crucial. The keys can encrypt and decrypt messages, but the processes have unique purposes. You encrypt something with the public key, which anyone can access, but only the holder of the private key can decrypt it. This ensures that communications can be secured without ever revealing the keys themselves.