docs: Qash EVM migration technical plan - #304
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Plan for forking the Qash monorepo and replacing its Miden chain layer with PolyPay's EVM stack (zkVerify signer privacy + relayer + MetaMultiSigWallet on Base). Covers target architecture, identity/auth design (Para session, passkey PRF secret custody, commitment-bound Noir circuit), schema changes to the three multisig tables, Phase 1 scope (end-to-end batch bill payment on Base Sepolia), workstream breakdown, and risks.
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Technical plan for the PolyPay to Qash EVM migration.
Decision
Fork the Qash monorepo and replace its Miden chain layer with PolyPay's EVM stack — not the other way round. Qash's backend already has every business model we need (Company, TeamMember, Employee, Invoice, Bill, Payroll, Proposal); PolyPay's has none of them. Both are NestJS 11 + Prisma 7 + Postgres, so the transplant is mechanical rather than architectural.
What the plan covers
Two findings worth review
The transaction circuit's ECDSA check currently constrains nothing.
pub_key_x/pub_key_yare private inputs recovered from the prover's own signature, and nothing binds them tosecretorcommitment— the contract stores only commitments. A prover holdingsecretcan generate a throwaway keypair, sign with it, and pass. The check costs most of the proving time and buys no security. The plan fixes this by bindingcommitment = poseidon2(secret, hash(pubkey)), which makes the intended two-factor property real. This applies to PolyPay today, not only to Qash EVM.PolyPay's secret derivation cannot work with Para.
secret = keccak256(sign("noir-identity"))assumes deterministic ECDSA. Para is a 2-of-2 MPC wallet: no party holds the full private key, so RFC 6979 is mathematically unavailable and every signature differs. Hence the passkey-based custody design.Plan document: QASH-EVM-PLAN.md