Add a new judge to support message reporting in an interoperable E2EE messaging setting!

[davidbeechey]

This is for my 4th year Honours Project at the University of Edinburgh, where I am exploring enabling user message reporting in interoperable E2EE messaging :) Opening as a PR in case I change something before it's "done".

A summary of the changes to the code/AMF scheme:

• Adds a new judge, SP (sender provider) and renames J to RP (recipient provider)
• New clauses added for my requirements from the modified scheme:
  • Two which are essentially mirrors of the original clauses to enable the second moderator to judge and the recipient to verify it
  • Another two which give the judges the ability to verify that the other judge will be able to judge it too
• judge algorithm is consequently split into rp_judge and sp_judge
• New forging algorithms are required:
  • j_forge becomes rp_forge, and it needs its mirror sp_forge
  • rp_r_forge and sp_r_forge are created to handle deniability in the case where we have one service provider and the recipient's secret keys
  • rp_sp_forge for when both judge's secret keys (and recipient's) are exposed
• The original frank, verify, and forge algorithms are updated to support the new additions to the scheme