What Is a TLS Handshake?

Q: What happens during a TLS handshake?

The client sends a ClientHello with supported TLS versions and cipher suites. The server responds with a ServerHello selecting the version and cipher suite, then sends its certificate. The client verifies the certificate, both sides perform key exchange, and they derive session keys. The handshake completes and encrypted communication begins.

Q: What is the difference between TLS 1.2 and TLS 1.3 handshakes?

TLS 1.3 reduces the handshake from 2 round trips to 1 round trip, removing support for insecure algorithms, and mandating forward secrecy. TLS 1.3 also supports 0-RTT resumption for returning clients, though with some security trade-offs. The result is faster, more secure connections.

Q: Why do TLS handshake failures occur?

Common causes include: no overlapping cipher suites between client and server, expired or invalid SSL certificates, TLS version mismatch (e.g., client requires TLS 1.2 but server only supports 1.0), certificate name mismatch, or incomplete certificate chain. Firewall or proxy interference can also cause handshake failures.

A TLS handshake is the process by which a client and server negotiate encryption parameters, authenticate each other, and establish a secure connection. It occurs at the start of every HTTPS session and every SMTP STARTTLS connection.

How the TLS Handshake Works

1. ClientHello: The client sends its supported TLS versions, cipher suites, and a random value. 2. ServerHello: The server selects a TLS version and cipher suite, sends its own random value and SSL certificate. 3. Certificate Verification: The client verifies the server's certificate chain. 4. Key Exchange: Both parties perform a key exchange (ECDHE for forward secrecy) to derive shared session keys. 5. Finished: Both sides confirm the handshake and begin encrypted communication.

TLS 1.2 vs. TLS 1.3 Handshake

TLS 1.2 requires 2 round trips (4 messages) to complete the handshake. TLS 1.3 reduces this to 1 round trip by combining steps and removing unnecessary messages. TLS 1.3 also mandates forward secrecy, removes support for weak algorithms (RSA key exchange, CBC mode, SHA-1), and supports 0-RTT resumption for returning clients.

TLS Handshake and Email

Email servers perform TLS handshakes when using STARTTLS on SMTP connections. The SMTP client issues the STARTTLS command, and both servers perform a TLS handshake before exchanging email data. MTA-STS enforces that this handshake must succeed — if TLS negotiation fails, the email is not sent in plaintext.

Frequently Asked Questions

What happens during a TLS handshake?

The client and server exchange supported algorithms, the server presents its certificate, the client verifies it, both perform key exchange to derive session keys, and encrypted communication begins.

What is the difference between TLS 1.2 and TLS 1.3 handshakes?

TLS 1.3 completes in 1 round trip instead of 2, mandates forward secrecy, removes weak algorithms, and supports 0-RTT resumption. The result is faster, more secure connections.

Why do TLS handshake failures occur?

Common causes include no overlapping cipher suites, expired or invalid certificates, TLS version mismatches, certificate name mismatches, incomplete certificate chains, or firewall interference.

How the TLS Handshake Works

TLS 1.2 vs. TLS 1.3 Handshake

TLS Handshake and Email

Frequently Asked Questions

What happens during a TLS handshake?

The client and server exchange supported algorithms, the server presents its certificate, the client verifies it, both perform key exchange to derive session keys, and encrypted communication begins.

What is the difference between TLS 1.2 and TLS 1.3 handshakes?

TLS 1.3 completes in 1 round trip instead of 2, mandates forward secrecy, removes weak algorithms, and supports 0-RTT resumption. The result is faster, more secure connections.

Why do TLS handshake failures occur?

Common causes include no overlapping cipher suites, expired or invalid certificates, TLS version mismatches, certificate name mismatches, incomplete certificate chains, or firewall interference.