Ok, so it seems the functionality is described in http://tools.ietf.org/html/rfc5705. If I understand correctly, it takes an ASCII label string, an optional context bytestring, and the length of the desired derived key. It then returns a bytestring of the given length, shared between client and server.

The low-level implementation of the ssl module is in Modules/_ssl.c. We probably want to add a method to PySSLSocket_Type there. Then in Lib/ssl.py there'll have to be a SSLSocket method to call the former method.

Tests should go in Lib/test/test_ssl.py.