[hmac,dv] Save and Restore improvements

[martin-velay]

Description

This issue follows that one https://github.com/lowRISC/opentitan/issues/21708 where the main parts of the feature has been tested via this PR https://github.com/lowRISC/opentitan/pull/23116. Nevertheless, this PR also indicates that there are still some missing things to be added:

• [ ] Verify context B with multi-streamed (= different context switching)
• [ ] Remove msg_length check from the hmac_base_vseq as should be part of the SCB - [ ] Verify intermediate digest when switching crossed out as this will be covered by #23240

FYI @gdessouky @andreaskurth @ballifatih : and if you feel that anything else as to be tested, please rise it here.

[vogelpi]

Thanks for creating the issue @martin-velay .

Verify context B with multi-streamed (= different context switching)

Right now, only context A is verified meaning we just check that context A can be saved and restored which is fine I believe.

Verify intermediate digest when switching

This would allow detecting when exactly things get out of sync in case of issues. It would ease debugging but I think as long as the design passes verification, it's not a very high priority to allow more fine grained debugging.

So for me, both these items are not too important but rather nice to have. It's fine to defer this to M5 in my view.

[gdessouky]

[ ] Verify context B with multi-streamed (= different context switching)

And for sufficient assurance for M4, we cover this in the driver-based TLT for now.

[andreaskurth]

I think we would do this as part of HMAC V3 work (M7) -- @martin-velay WDYT?

[martin-velay]

@andreaskurth: OK

[martin-velay]

This is covered at top level but it would be better to also cover this at block level as part of M7