implement a fix for CO15

The base OT protocol CO15 used in mpz is known to have a broken security proof.
In this discussion https://github.com/emp-toolkit/emp-ot/issues/74 it was suggested to use a fix from https://eprint.iacr.org/2018/499
But that fix (see APPENDIX A) introduces a selective failure attack.

We need to investigate if the selective failure attack is acceptable within the context of the TLSNotary protocol.
If it is acceptable, we need to implement the fix.

Start by reviewing the CO15 implementation in mpz and the linked emp-ot discussion and ePrint paper, especially Appendix A. Determine whether the selective failure attack is acceptable for the TLSNotary protocol before deciding on an implementation. Done means the security impact is resolved and, if appropriate, the fix is implemented with validation in the relevant protocol tests.

Written by the indexing model from the issue text.