This commit adds declarations and dummy implementations for
the restructured incoming client key exchange handling that
will replace the previous ssl_parse_client_key_exchange().
The entry point for the CliKeyExchange handling that is called
from the handshake state machine is
`ssl_process_client_key_exchange()`,
splitting the processing into the following steps:
- Fetching: Read next message from the messaging layer
and check that it has the correct type.
The ClientKeyExchange message is never
omitted, so there is no ambiguity in what
to expect, and hence no dedicated preparation
step as for other handshake states.
- Parsing: Parse the ClientKeyExchange message and
use the information in it to derive keying
material such as the shared (EC)DHE secret.
- Postprocessing:
Compute the session keys from the available
keying material. This splits in two steps:
(1) Build the PreMasterSecret (PMS) from the
available keying material, e.g. concatenate
the (EC)DHE secret with a PSK, if used.
(2) Extract the MasterSecret and Session Keys
from the PreMasterSecret.
The subsequent commits will scatter the code from the previous
monolithic function ssl_parse_client_key_exchange() among those
dedicated functions, commenting out each part of
ssl_parse_client_key_exchange() that has already been dealt with.
This gradual progression is meant to ease reviewing. Once all
code has been moved and all changes explained,
ssl_parse_client_key_exchange() will be removed.
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