The TLS Handshake

What is TLS?

Transport Layer Security (TLS) creates a secure channel over an insecure network.

You’re on coffee shop WiFi. You visit your bank.

Without TLS:

• Anyone on that WiFi can read your traffic
• Attackers can modify data in transit
• You can’t verify you’re talking to the real bank

With TLS:

• Confidentiality: Traffic is encrypted
• Integrity: Tampering is detected
• Authentication: Server proves its identity

SSL vs TLS

SSL (Secure Sockets Layer): The original, made by Netscape in the 90s.

TLS (Transport Layer Security): The successor, standardized by IETF.

Everyone says “SSL” but means TLS. The protocol is TLS. The name SSL stuck.

The Handshake

Before any HTTP data flows, client and server perform a handshake.

The goal: securely agree on encryption keys.

Step 1: Client Hello

The client initiates:

• TLS versions it supports
• Cipher suites it supports (e.g., TLS_AES_256_GCM_SHA384)
• Client random: 32 bytes of random data

“Here’s what I can do. Let’s talk.”

Step 2: Server Hello

The server responds:

• TLS version to use
• Cipher suite chosen from client’s list
• Server random: 32 bytes of random data
• Server’s certificate

“Let’s use this. Here’s my certificate to prove who I am.”

Step 3: Client Verifies Certificate

The client checks:

• Is the certificate valid (not expired)?
• Is it signed by a trusted CA?
• Does the domain match?

If any check fails, connection aborts.

This is where the chain of trust matters. No valid certificate, no connection.

Step 4: Key Exchange

The client generates a pre-master secret (random bytes).

Encrypts it with the server’s public key from the certificate.

Sends it to the server.

Now both sides have:

• Client random
• Server random
• Pre-master secret

Both derive the same session keys from these three values.

The pre-master secret traveled encrypted. Only the server can decrypt it.

Step 5: Finished

Both sides send a “Finished” message, encrypted with the new session keys.

If decryption works on both ends, the handshake succeeded.

The secure channel is established.

After the Handshake

All traffic is now encrypted with symmetric session keys.

Same hybrid approach as PGP: asymmetric to exchange keys, symmetric for bulk data.

Why This Complexity?

Problem: Symmetric encryption is fast, but requires a shared key.

Problem: How do you share a key over an insecure network?

Solution: Use asymmetric encryption once to securely exchange a symmetric key.

The handshake does exactly this:

1. Server proves identity with certificate
2. Client sends secret encrypted with server’s public key
3. Both derive symmetric keys
4. Switch to fast symmetric encryption

The handshake is slow. But it only happens once per connection.

TLS 1.3 Improvements

TLS 1.3 made the handshake faster and more secure:

• Fewer round trips: Handshake completes in 1 round trip (was 2)
• Removed old crypto: No more RSA key exchange, only forward-secure options
• Encrypted earlier: More of the handshake is encrypted

TLS 1.3 is what you should be using today.