pre-allocated buffer to hold 48 bytes of inner secret.
Copy the 48 bytes large inner secret into the specified buffer
This function is typically used after the TLS/IA handshake has
concluded. The TLS/IA inner secret can be used as input to a PRF
to derive session keys. Do not use the inner secret directly as a
session key, because for a resumed session that does not include an
application phase, the inner secret will be identical to the inner
secret in the original session. It is important to include, for
example, the client and server randomness when deriving a sesssion
key from the inner secret.