RSA in practice

Initializing SSL/TLS with

In this example the youtube server is authenticated via it’s certificate and an encrypted communication session established. Taking a packet capture of the process enables simple identification of the TLSv1.1 handshake (as described:

Packet capture download: http://mchost/sourcecode/security_notes/youtube_TLSv1.1_handshake_filtered.pcap

The packet capture starts with the TCP three-way handshake – Frames 1-3

With a TCP connection established the TLS handshake begins, Negotiation phase:

  1. ClientHello – Frame 4 – A random number[90:fd:91:2e:d8:c5:e7:f7:85:3c:dd:f7:6d:f7:80:68:ae:2b:05:8e:03:44:f0:e8:15:22:69:b7], Cipher suites, compression methods and session ticket (if reconnecting session).
  2. ServerHello – Frame 6 – chosen protocol version [TLS 1.1], random number [1b:97:2e:f3:58:70:d1:70:d1:de:d9:b6:c3:30:94:e0:10:1a:48:1c:cc:d7:4d:a4:b5:f3:f8:78], CipherSuite [TLS_ECDHE_ECDSA_WITH_RC4_128_SHA], Compression method [null], SessionTicket [null]
  3. Server send certificate message (depending on cipher suite)
  4. Server sends ServerHelloDone
  5. Client responds with ClientKeyExchange containing PreMasterSecret, public key or nothing. (depending on cipher suite) – PreMasterSecret is encrypted using the server public key
  6. Client and server use the random numbers and PreMsterSecret to compute a common secret – master secret
  7. Client sends ChangeCipherSpec record
  8. Client sends authenticated and encrypted Finished – contains a hash and MAC of previous handshake message
  9. Server decrypts the hash and MAC to verify
  10. Server sends ChangeCipherSpec
  11. Server sends Finished – with hash and MAC for verification
  12. Application phase – the handshake is now complete, application protocol enable with content type 23

client random: 90:fd:91:2e:d8:c5:e7:f7:85:3c:dd:f7:6d:f7:80:68:ae:2b:05:8e:03:44:f0:e8:15:22:69:b7 = 10447666340000000000

server random: 1b:97:2e:f3:58:70:d1:70:d1:de:d9:b6:c3:30:94:e0:10:1a:48:1c:cc:d7:4d:a4:b5:f3:f8:78 = 1988109383203082608

Interestingly the negotiation with and chromium browser resulted in Elliptic Curve Cryptography (ECC) Cipher Suitesfor Transport Layer Security (TLS) as the chosen cipher suite.

Note that there is no step mention here for the client to verify then certificate. In the past most browsers would query a certificate revocation list (CRL), though browsers such as chrome now maintain either ignore CRL functionality or use certificate pinning.

Chrome will instead rely on its automatic update mechanism to maintain a list of certificates that have been revoked for security reasons. Langley called on certificate authorities to provide a list of revoked certificates that Google bots can automatically fetch. The time frame for the Chrome changes to go into effect are “on the order of months,” a Google spokesman said. – source: