Review: Write ChangeCipherSpec

[hannestschofenig]

This issue it to track the review and potential rework of the client- and server-side side backwards compatibility mode in preparation for upstreaming.

This issue is focused on the review of the ChangeCipherSpec.

The relevant function is mbedtls_ssl_write_change_cipher_spec_process() on the client-side in the following states:

• MBEDTLS_SSL_CLIENT_CCS_AFTER_CLIENT_HELLO,
• MBEDTLS_SSL_CLIENT_CCS_BEFORE_2ND_CLIENT_HELLO, and
• MBEDTLS_SSL_CLIENT_CCS_AFTER_SERVER_FINISHED.

For the server-side the following states are relevant:

• MBEDTLS_SSL_SERVER_CCS_AFTER_HRR
• MBEDTLS_SSL_SERVER_CCS_AFTER_SERVER_HELLO

Things to look for:

Security
Functional correctness
Clarity of code and information flow
Adherence to Mbed TLS style

Acceptance criteria: One or more PRs improving on the above points to the satisfaction of the team, or a confirmation of review with the result that no changes were necessary.