ECC keygen for PR #528 ("Attestation driver and proxy")

[Nicolai Stange <nstange@suse.de>]

Hi all,

I managed to carve out and cleanup the first batch ([1]) from my work on
an encrypted FS by now. The FS cleanup itself is still WIP, but the
crypto parts should be in a usable state.

As mentioned on last week's svsm-devel call, it might help with
addressing those stack size related issues with ECC keygen in the
context of PR #528 ([2]).

I prepared some example code for generating an ECC key with NIST P-521,
to be found at [3].

From some lax experiments in userspace, peak stack usage is at about 2.3kB.
(Which is still way above what I would have expected, given that no
 buffers are stored on the stack. I'm currently investigating that).

I'm not sure whether merely generating the key is all you need -- FWIW
there's also support for ecdh, ecdsa and ecschnorr, in case you're
wondering. I'd be happy to come up with some example code for these as
well.

Please let me know if you have any questions, either here or in today's
call.

Thanks!

Nicolai

[1] https://github.com/nicstange/cocoon-tpm
[2] https://github.com/coconut-svsm/svsm/pull/528
[3] https://github.com/nicstange/cocoon-tpm-crypto-ec-key-gen-demo

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