WIP supersedes #420 by replacing FHE with ZK ElGamal as the cryptographic backend for Polkadot Confidential Assets

This work will be moved out of this repository for #421 and does not need to be merged here.

TODO:

• DESIGN.md outlining architecture implementing ZK ElGamal backend for Polkadot Confidential Assets
• pallet-zkhe: implements an on-chain cryptographic backend for confidential transfers mirroring the Solana Confidential Token implementation.
• pallet-confidential-assets: implements the OpenZeppelin Confidential Contracts Standard for multiple assets.
• confidential-assets-primitives: shared types and traits
• zkhe-primitives: shared curves, types, transcripts to keep verifier and prover in sync such that only valid proofs pass verification
• zkhe-verifier: no_std verifiers used at runtime to configure pallet-zkhe::ZkVerifier
• zkhe-prover: std-friendly helpers to construct proofs that pass verification iff valid
• tests between verifier and prover in std environment (verifier pallet unit tests)
• feature: on/off ramp functionality for pallet-confidential-assets (adds verify functions ie mint/burn)
• more tests between verifier and prover in std environment for added verify_{mint, burn, etc}
• pallet-confidential-xcm-bridge
• demo via xcm-simulator
• runtimes and nodes (prove no_std path)
• update docs to latest design
• open PR against new repo containing all code + new CI running compilation + tests

Next Steps

• set up separate repo with CI, permissions, etc
• split this PR into separate repo
• send link for new repo to sub0 organizers that requested it

Precompiles

Frontier

• frontier-precompile-confidential-assets
• impl callback OnConfidentialTransferReceived for Frontier precompile without introducing reentrancy attack vector (see moonbeam randomness precompile for a frontier example)
• evm-template: configure frontier-precompile-confidential-assets
• evm-template: test frontier-precompile-confidential-assets

PolkaVM

• polkavm-precompile-confidential-assets
• impl callback OnConfidentialTransferReceived for PolkaVM precompile without introducing reentrancy attack vector (TODO find PolkaVM example of reentrancy attack vector mitigation)