Tools / Cyber Security Interview Questions
Explain the internal working of the TLS Handshake?
sequenceDiagram
participant C as Client
participant S as Server
C->>S: ClientHello (supported ciphers, random value)
S->>C: ServerHello + Certificate
C->>C: Verify certificate against trusted CA
C->>S: Key exchange material
Note over C,S: Both derive the same session key
C->>S: Finished (encrypted)
S->>C: Finished (encrypted)
The TLS 1.3 handshake establishes a secure, authenticated channel in as few round trips as possible.
- The client sends a ClientHello listing supported cipher suites and a random value
- The server responds with a ServerHello, its digital certificate, and its own random value
- The client verifies the certificate against a trusted Certificate Authority chain
- Both sides use a Diffie-Hellman key exchange to independently derive the same shared session key without ever transmitting it
- Each side sends a Finished message, encrypted with the new session key, confirming the handshake succeeded
From that point on, all application data is encrypted with the derived symmetric session key. TLS 1.3 cut this down to a single round trip in most cases, compared to two in TLS 1.2, reducing connection latency while dropping support for weaker legacy cipher suites.
How is the shared session key established during the TLS handshake?
How many round trips does TLS 1.3 typically need compared to TLS 1.2?
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