Add references to zaps.

[mmaker]

Witness indistinguishably is a notion slightly weaker of zero knowledge that can still provide a lot of useful applications (e.g. anonymous credentials). While no protocols can be proven simultaneously knowledge sound and zero-knowledge in the standard model (no trusted setup), there exist protocols that are simultaneously knowledge sound and witness indistinguishable.

Non-interactive zaps are non-interactive witness indistinguishable proof system where the prover can send a proof to the verifier in one shot: no trusted setup, no pre-shared public keys, just a single message.