Investigate Semapore Rate Limiting Nullifier with zkNSARKs

[oskarth]

See https://ethresear.ch/t/semaphore-rln-rate-limiting-nullifier-for-spam-prevention-in-anonymous-p2p-setting/5009

Understand if it is feasible and desirable as an alternative to PoW. Outline further benchmarking etc needing.

[oskarth]

Current possible blocker:

1. Prove time (~seconds)

• might be mitigated by pre-generating

2. Prover key size ~500mb

• might be easy to optimize, also right now for ~2**28 members
• starks and other forms of zkp might be relevant here, but with other trade offs (starks ~45kb proof size iirc)

[oskarth]

Initial feasibility checks done. See https://github.com/vacp2p/research/tree/master/zksnarks/semaphore

Enough information to make follow up issues for things to research and perf blocks/benchmarks needed. Writing up feasibility report, creating some follow up issues and then this one can be marked as done