The master key is a piece of data from which all other keys are derived. For the two protocols described in this document, SSL 3.0 and TLS 1.0, it is 48 bytes in length. The master key is created by performing a Diffie-Hellman key exchange.
The Client Side (Diffie-Hellman)
The Server Side (Diffie-Hellman)
Specifying the Algorithms (Diffie-Hellman)