Transport Layer Security: Encyclopedia II - Transport Layer Security - How it works

Transport Layer Security - How it works

The SSL protocol exchanges records; each record can be optionally compressed, encrypted and packed with a message authentication code (MAC). Each record has a content_type field that specifies which upper level protocol is being used.

When the connection starts, the record level encapsulates another protocol, the handshake protocol, which has content_type 22.

The client sends and receives several handshake structures:

• It sends a ClientHello message specifying the list of cipher suites, compression methods and the highest protocol version it supports. It also sends random bytes which will be used later.

• Then it receives a ServerHello, in which the server chooses the connection parameters from the choices offered by the client earlier.

• When the connection parameters are known, client and server exchange certificates (depending on the selected public key cipher). These certificates are currently X.509, but there's also a draft specifying the use of OpenPGP based certificates.

• The server can request a certificate from the client, so that the connection can be mutually authenticated.

• Client and server negotiate a common secret called "master secret", possibly using the result of a Diffie-Hellman exchange, or simply encrypting a secret with a public key that is decrypted with the peer's private key. All other key data is derived from this "master secret" (and the client- and server-generated random values), which is passed through a carefully designed "Pseudo Random Function".

TLS/SSL have a variety of security measures:

• Numbering all the records and using the sequence number in the MACs.
• Using a message digest enhanced with a key (so only with the key can you check the MAC). This is specified in RFC 2104).
• Protection against several known attacks (including man in the middle attacks), like those involving a downgrade of the protocol to previous (less secure) versions, or weaker cipher suites.
• The message that ends the handshake ("Finished") sends a hash of all the exchanged data seen by both parties.
• The pseudo random function splits the input data in 2 halves and processes them with different hashing algorithms (MD5 and SHA), then XORs them together. This way it protects itself in the event that one of these algorithms is found vulnerable.

Adapted from the Wikipedia article "How it works", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki