A TLS full handshake
No. Client Server
1

Send: - ClientHello

 <wait for ClientHello>
2 <wait for ServerHelloDone>

Send:

  • ServerHello
  • Certificate*
  • ServerKeyExchange*
  • CertificateRequest*
  • ServerHelloDone
3

Send:

  • Certificate*
  • ClientKeyExchange
  • CertificateVerify*
  • [ChangeCipherSpec]
  • Finished
 <wait for client’s Finished>
4 <wait for server’s Finished>

Send:

  • [ChangeCipherSpec]
  • Finished

Hello Messages

  • ClientHello and ServerHello establish:
    • Protocol version
    • Session ID
    • Cipher suite
    • Compression method
  • And generate:
    • ClientHello.random
    • ServerHello.random
  • When the client sends a ClientHello to server, it can either respond with a ServerHello, or ignore it, leading to a fatal error and the closing of the connection.

2.Server:

  • Certificate:
    • If it is to be authenticated
  • ServerKeyExchange:
    • If server doesn’t have a certificate, or
    • if server’s certificate is for signing only
  • CertificateRequest:
    • If:
      • server is authenticated, and
      • it is appropriate to the cipher suite selected
  • ServerHelloDone:
    • To indicate that the hello-message phase of the handshake is complete

3.Client:

  • Certificate:
    • If the server sent a CertificateRequest
  • ClientKeyExchange:
    • The content of this message depends on the public key algorithm selected between ClientHello and ServerHello
  • CertificateVerify:
    • If the client certificate sent is with signing ability
  • Digitally signed
    • Verifies the possession of private key in certificate
  • ChangeCipherSpec:
    • Send this and copy the pending cipher spec into the current cipher spec
  • Finished
    • Sent under the new algorithms, keys, and secrets

4.Server:

  • ChangeCipherSpec:
    • Send this and copy the pending cipher spec into the current cipher spec
  • Finished
    • Sent under the new cipher spec

Session Resumption:

No. Client Server
1

Send:

  • ClientHello
 <wait for ClientHello>
2 <wait for Server’s Finished>

Check session cache for a match.

  • If the session ID is not found:
    • Generate a new session ID and perform a full handshake
  • If the session ID is found:
    • Is willing to re-establish the connection under the specified session state:
      • If No:
        • Generate a new session ID and perform a full handshake
      • If Yes, proceed to 3.
3 <wait for server’s Finished>

Send:

  • ServerHello
  • [ChangeCipherSpec]
  • Finished
4

Send:

  • [ChangeCipherSpec]
  • Finished
 <wait for client’s Finished>

1.Client:

  • ClientHello:
    • Sent using the session ID of the session to be resumed.

3.Server:

  • ServerHello:
    • Sent with the same Session ID value (as in the ClientHello message).

Server as a state machine:

Input Current State Next State Output
ClientHello IDLE CHECK_SESSION_CACHE –
id_found_somehow CHECK_SESSION_CACHE WAIT_RESUME

(ServerHello, [ChangeCipherSpec], Finished)
id_not_found_somehow CHECK_SESSION_CACHE WAIT

(ServerHello, Certificate*, ServerKeyExchange*, CertificateRequest*, ServerHelloDone)
Finished (from Client) WAIT APP_DATA

([ChangeCipherSpec], Finished)
Finished (from Client) WAIT_RESUME APP_DATA –
ClientHello APP_DATA APP_DATA Alert(no_renegotiation)

Client as a state machine:

Input Current State Next State Output
– IDLE WAIT_1 ClientHello
ServerHelloDone WAIT_1 WAIT_2

(Certificate*, ClientKeyExchange, CertificateVerify*, [ChangeCipherSpec], Finished)
Finished (from Server) WAIT_1 APP_DATA

([ChangeCipherSpec], Finished)
Finished (from Server) WAIT_2 APP_DATA –
HelloRequest APP_DATA APP_DATA Alert(no_renegotiation)

* Indicates optional or situation-dependent messages that are not always sent.

Note: To help avoid pipeline stalls, ChangeCipherSpec is an independent TLS protocol content type, and is not actually a TLS handshake message.

Common states for both state machines:

Input Current State Next State Output
Alert(close_notify) APP_DATA SHUTDOWN (Alert(close_notify), close_callback(False), indicate_EOF_to_the_application_somehow)
Session.alert(close_notify) APP_DATA HOST_INITIATED_CLOSING Alert(close_notify)
Alert(close_notify) HOST_INITIATED_CLOSING SHUTDOWN (close_callback(True), indicate_EOF_to_the_application_somehow)
Session.write_data APP_DATA APP_DATA write_callback()