KB Hybrid Card × Stablecoin Payment — Flow Map (reference)

Scope: flow map only (jay, 2026-07-17) — a reference diagram of how a TradFi card rail (ISO 8583) combines with on-chain settlement (Avalanche subnet + stablecoin), modeled on KB국민카드's reported design. Not a dev item — no PoC ladder; kept for architecture reference and future tie-ins.

Source: analysis note pasted in session 2026-07-17 (no URL — this doc is the canonical copy).

The four layers at a glance

flowchart TB
    subgraph OFF["① Off-chain — TradFi card rail (1–2s)"]
        U[User swipes card] --> VAN[VAN / PG]
        VAN -->|ISO 8583 auth request| KB[Card-issuer server]
    end

    subgraph MID["② Middleware — bridge"]
        KB --> CHOICE{Funding source?}
        CHOICE -->|Credit| LEGACY[Legacy credit path<br/>unchanged]
        CHOICE -->|Stablecoin| CUST[Custody / WaaS<br/>MPC wallet infra, e.g. OpenAsset]
        CUST -->|balance check + hold| AUTH[✅ Authorize<br/>off-chain, asset frozen]
    end

    subgraph ON["③ On-chain — Avalanche subnet (minutes–daily batch)"]
        AUTH -.->|batched, later| BURN[Burn / lock stablecoin<br/>ERC-20 + ERC-2612 permit]
        ORACLE[Oracle: Chainlink / Pyth<br/>USDC↔KRW rate] --> BURN
        AA[ERC-4337 AA wallet<br/>card# ↔ address 1:1<br/>Paymaster pays gas] -.-> BURN
    end

    subgraph SETTLE["④ Merchant settlement"]
        BURN --> POOL[KRW liquidity pool<br/>issuer's fiat reserves]
        POOL -->|KRW payout| M[Merchant<br/>receives won, never coins]
    end

Timing split — the core trick (authorize now, settle later)

sequenceDiagram
    participant U as User/Card
    participant KB as Issuer server
    participant C as Custody (MPC)
    participant AVAX as Avalanche subnet
    participant M as Merchant

    Note over U,KB: t = 0 (must finish in 1–2s)
    U->>KB: ISO 8583 auth request
    KB->>C: balance check
    C-->>KB: OK + hold (freeze) stablecoin
    KB-->>U: APPROVED ✅ (nothing on-chain yet)

    Note over KB,AVAX: t = minutes ~ T+1 (batch)
    KB->>AVAX: batch settle (burn/lock held coins)
    AVAX->>AVAX: oracle-fixed USDC↔KRW rate
    KB->>M: KRW payout from liquidity pool

Card auth must return in 1–2s; chain finality can't. So authorization is purely off-chain (hold on the custody balance) and the on-chain movement is deferred batch settlement — the same authorize/settle split card networks already use, with the settle leg on-chain.

Why each piece (one line each)

• Avalanche subnet (Evergreen) — an issuer-dedicated lane: public-chain security with gas-free config and KYC'd validators; Solidity carries over unchanged.
• ERC-2612 permit + ERC-4337/Paymaster — end users never see gas or seed phrases; card number ↔ wallet address is 1:1, issuer sponsors gas.
• MPC custody — no single private key to leak (same pillar as dsrv-portal.md ①).
• Oracle (Chainlink/Pyth) — fixes the USDC↔KRW rate per settlement batch to bound FX drift.
• KRW liquidity pool — merchants want won, not coins; the pool fronts fiat while the stablecoin leg settles.

Tie-ins (future, not scheduled)

• AP2/x402 track (ap2-test.md): the source note's closing idea — agent earns stablecoin → spends it via a real card — is exactly the aiaas wallet with this flow map as its off-ramp.
• agentic-aa.md: pillar 2 (paymaster) and pillar 1 (scoped keys) are the same building blocks this design assumes.
• dsrv-portal.md: MPC custody + compliance pipeline are shared pillars.

Status

Reference only — flow map, not a dev item.