TODO

- check padding
- implement TLS 1.2 extension to choose hash function other than SHA256

- improve BigInt multiply and pow performance

TLS 1.2:
- fix session reuse


In case we should ever support the selection of a certificate according
to the server name indication, we should send an empty server name indication
in return to signal that we have used that information (see RFC 6066):

   A server that receives a client hello containing the "server_name"
   extension MAY use the information contained in the extension to guide
   its selection of an appropriate certificate to return to the client,
   and/or other aspects of security policy.  In this event, the server
   SHALL include an extension of type "server_name" in the (extended)
   server hello.  The "extension_data" field of this extension SHALL be
   empty.