open-standards/openpay/docs/00-foundation.md

OpenPay — Foundation Document

Status: Draft v0.1 Date: 2026-05-17 Toolchain: the stable Rust pinned in the repository root's rust-toolchain.toml, edition 2024 License: Apache-2.0 (matches Hyperswitch, Fineract, ISO 20022 Rust crates)

---

1. Mission

A free, open-source, universal payment-acceptance stack that runs on any modern phone, tablet, or x86/ARM box, integrates every major payment rail (cards, A2A/instant, wallets, QR), and ships a single safety-critical Rust core compiled to every platform via cxx, jni, and wasm-pack.

Not a startup. A reference implementation that any merchant, bank, fintech, or government can deploy with no per-transaction tax to OpenPay itself.

---

2. The Tax Stack We Eliminate (Quantified)

For a merchant doing $1M/yr in card volume on the current US stack:

Layer	Typical cost	Source
Interchange (issuer)	1.5–3.0%	Aeropay 2026 cost analysis
Network assessment (Visa/MC)	~0.13%	Aeropay 2026 cost analysis
Processor markup	0.3–0.5% (often 2.9%+$0.30 blended)	Aeropay 2026
POS SaaS (Toast/Square/Clover)	$69–$300/mo × 12 = $828–$3,600/yr	Vendor pricing pages
Hardware lease	$50–$150/mo per terminal	Vendor pricing pages
Chargebacks	$15–$100+ per dispute	Aeropay 2026
Total effective rate	3.0–5.0%
On $1M volume	$30K–$50K/yr

A2A / pay-by-bank rails (FedNow, RTP, PIX, UPI, SEPA Instant) charge cents per transaction, not percentages. FedNow's 2026 fee schedule increased the per-transaction limit from $1M to $10M; participation has grown to 1,500+ US financial institutions. A merchant routing 50% of volume to A2A saves ~$25K/yr on a $1M business before any SaaS savings.

---

3. Hard Constraints (Non-Negotiable Physics)

3.1 Tap to Pay on iPhone requires a PSP

Per Apple's developer documentation: "App developers who want to offer Tap to Pay on iPhone to merchants will first need to integrate with a supported payment service provider (PSP) who processes the payments read by Tap to Pay on iPhone and provides the certified terminal configurations that are loaded on the merchant's device."

Implication: OpenPay cannot remove the PSP for in-person iPhone card acceptance. It can (a) treat PSPs as pluggable, swappable drivers and (b) route to non-card rails wherever the customer permits.

3.2 PCI DSS scope is unavoidable for card data

Any code path that touches raw PAN (Primary Account Number) puts the operator in PCI scope. OpenPay's design must never see the PAN in the application layer — only tokens issued by the PSP or by the device secure element (Apple Secure Enclave, Android StrongBox, host card emulation).

3.3 ISO 20022 is the global messaging substrate

FedNow, PIX, SEPA Instant, India's NPCI (post-IMPS migration), and SWIFT all use ISO 20022 (PACS for clearing/settlement, PAIN for initiation, CAMT for cash management). The Rust crate family open-payments-iso20022-* v1.0.1 ships full type-safe serde parsers for every message type. We adopt it directly; we don't rebuild it.

3.4 We are not a money transmitter

OpenPay the project ships software, not regulated money movement. Anyone who deploys OpenPay to clear funds becomes a money transmitter / payment institution / authorized payment provider under their own jurisdiction's law and is responsible for licensing, AML/KYC, and capital requirements. The codebase enforces this with a clear separation between the op-core orchestration layer and op-rails-* adapter crates that must be configured with the operator's own credentials.

---

4. Prior Art We Build On (Not Reinvent)

Project	Role	License	Why we use it
Hyperswitch (Juspay)	Payment orchestration server	Apache 2.0	Rust, PCI-cert, 175M tx/day claimed, 100+ PSP connectors
open-payments-iso20022	ISO 20022 + FedNow parsers	Apache/MIT	Type-safe pacs/pain/camt in Rust
Apache Fineract	Core banking / ledger	Apache 2.0	Account management; we plug in via REST for ledger ops
Mojaloop	Interoperability switch	Apache 2.0	Inter-DFSP transfers, Gates Foundation reference
NPCI open source	UPI reference stack	Apache 2.0	Distributed payment processing platform
Burn (tracel-ai)	ML framework	Apache 2.0 / MIT	Cross-platform inference: native, WASM, no_std for embedded
ort (pykeio)	ONNX Runtime bindings	Apache 2.0 / MIT	Fast inference for production fraud models

OpenPay's net new contribution is the edge integration: a single safety-critical Rust core that runs on the merchant device (iPhone/Android/web/Linux kiosk), speaks to Hyperswitch or any PSP for cards, speaks directly ISO 20022 to A2A rails, and runs fraud inference on-device before any network hop.

---

5. Architecture

┌─────────────────────────────────────────────────────────────────────┐
│                    MERCHANT DEVICE (any platform)                   │
│  ┌──────────────┐  ┌──────────────┐  ┌──────────────┐  ┌─────────┐  │
│  │  iOS shell   │  │ Android      │  │ Web PWA      │  │ Kiosk   │  │
│  │  (Swift +    │  │ shell        │  │ (wasm-bindgen│  │ (Linux  │  │
│  │   cxx FFI)   │  │ (Kotlin +    │  │  + JS/TS UI) │  │  + Rust │  │
│  │              │  │  jni)        │  │              │  │  native)│  │
│  └──────┬───────┘  └──────┬───────┘  └──────┬───────┘  └────┬────┘  │
│         └─────────────────┴──────┬──────────┴───────────────┘       │
│                                  │                                  │
│                  ┌───────────────▼────────────────┐                 │
│                  │  op-core  (Rust 1.95, no_std-  │                 │
│                  │  compatible where possible)    │                 │
│                  │                                │                 │
│                  │  ┌──────────────────────────┐  │                 │
│                  │  │ Payment state machine    │  │                 │
│                  │  │ (typestate pattern)      │  │                 │
│                  │  └──────────────────────────┘  │                 │
│                  │  ┌──────────────────────────┐  │                 │
│                  │  │ op-emv: EMV TLV decode   │  │                 │
│                  │  │ op-iso20022: ISO parsers │  │                 │
│                  │  │ op-vault: tokens only,   │  │                 │
│                  │  │   never raw PAN          │  │                 │
│                  │  │ op-fraud: on-device      │  │                 │
│                  │  │   inference (Burn/ort)   │  │                 │
│                  │  └──────────────────────────┘  │                 │
│                  └────────┬───────────────────────┘                 │
└───────────────────────────┼─────────────────────────────────────────┘
                            │
        ┌───────────────────┼───────────────────┐
        │                   │                   │
┌───────▼────────┐  ┌───────▼────────┐  ┌───────▼────────┐
│ op-rails-card  │  │ op-rails-a2a   │  │ op-rails-wallet│
│                │  │                │  │                │
│ adapter trait: │  │ adapter trait: │  │ Apple Pay,     │
│ → Hyperswitch  │  │ → FedNow       │  │ Google Pay,    │
│ → Stripe       │  │ → RTP (TCH)    │  │ host card      │
│ → Adyen        │  │ → PIX/SPI      │  │ emulation      │
│ → Finix/Moov   │  │ → UPI/NPCI     │  │                │
│ (PSP swappable)│  │ → SEPA Inst.   │  │                │
│                │  │ (rail-direct)  │  │                │
└────────────────┘  └────────────────┘  └────────────────┘
        │                   │                   │
        └───────────────────┼───────────────────┘
                            │
                ┌───────────▼────────────┐
                │ op-ledger              │
                │ (optional; Fineract    │
                │  adapter or sled       │
                │  embedded DB for       │
                │  edge-only deploys)    │
                └────────────────────────┘

---

6. Crate Layout (Cargo Workspace)

openpay/
├── Cargo.toml                  # workspace root
├── crates/
│   ├── op-core/                # state machines, domain types, traits
│   ├── op-iso20022/            # re-export + helpers over open-payments-iso20022
│   ├── op-emv/                 # EMV TLV parsing, BER-TLV codec
│   ├── op-vault/               # token-only secure storage abstraction
│   ├── op-fraud/               # on-device inference (Burn primary, ort backend)
│   ├── op-rails-card/          # PSP adapter trait + Hyperswitch driver
│   ├── op-rails-a2a/           # FedNow, PIX, UPI, SEPA Instant drivers
│   ├── op-rails-wallet/        # Apple Pay / Google Pay token handling
│   ├── op-ffi-swift/           # cxx bridge for iOS / macOS
│   ├── op-ffi-jni/             # jni bridge for Android / JVM
│   └── op-wasm/                # wasm-bindgen bridge for web / PWA
├── examples/
│   ├── kiosk-linux/            # standalone binary
│   └── headless-server/        # for testing
└── tests/
    ├── conformance/            # ISO 20022 message round-trips
    ├── emv/                    # EMV test vectors
    └── property/               # proptest invariants on state machine

---

7. Core Principles (Enforced by the Type System)

1. No raw PAN in application memory. The PaymentMethod enum carries Token(VaultRef) or EmvTag(SecureBlob) variants only. Constructing a PaymentMethod::RawPan requires the pci-scope cargo feature and is gated behind #[cfg(feature = "pci-scope")].

2. Typestate payment machine. Payment<S> where S: PaymentState. Transitions are functions, not method calls on a mutable struct. Illegal transitions don't compile.

3. Money is never f64. All amounts are op_core::Money { minor_units: i64, currency: Iso4217 }.

4. Errors are sealed enums, never Box<dyn Error>. Each crate exposes exactly one Error type with thiserror. Callers exhaustive-match.

5. No unsafe outside op-ffi-* and op-emv (raw byte parsing). Enforced by #![forbid(unsafe_code)] at every other crate root.

6. Every public function is #[must_use] when it returns a non-() value that represents a state transition or side-effect.

7. Cross-platform via Rust 1.95's cfg_select! for clean target-specific bodies instead of nested cfg_if!.

---

8. What Comes Next (Sequenced)

Each phase produces verifiable artifacts; we do not move on until the prior phase's tests pass.

• Phase 1 (this commit): foundation doc, workspace skeleton, op-core domain types (Money, Currency, PaymentMethod, typestate machine scaffolding).
• Phase 2: op-iso20022 wrapper + conformance tests against PACS.008 / PAIN.001 / CAMT.054 test vectors.
• Phase 3: op-emv BER-TLV codec with EMVCo test vectors.
• Phase 4: op-rails-card trait + Hyperswitch driver, mocked against Hyperswitch's sandbox.
• Phase 5: op-rails-a2a FedNow driver against FRB test harness.
• Phase 6: op-fraud Burn model with synthetic transaction stream.
• Phase 7: FFI bridges (op-ffi-swift, op-ffi-jni, op-wasm).
• Phase 8: example kiosk + headless server + end-to-end test harness.