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TLS keys to AES key in STM32F4

Question asked by robison.don on Nov 22, 2013
I'm having trouble bridging the two pools of information I read on the subject of TLS handshakes and the AES cryptographic processor.

TLS is an exchange of a server certificate, a client premaster secret, and a public key encapsulated in the server certificate.  The bit lengths are 2048 usually for the secrets and keys.

The AES on the STM32F4 is a cryptographic hash processor that in order to work, in my case, needs a 256bit AES key.

I have the impression I want to use TLS_RSA_WITH_AES256_CBC_SHA.  If this isn't proper I welcome a better suggestion. 

How to derive the proper 256bit value to put in the cryptographic STM32f4 registers from all the TLS objects?  I even took the time to read on the key exchange algorithms, but I have a hard time identifying which object(public key, private key, premaster secret) is which variable in the algorithm.

Thanks in advance.