Numpay is a client-heavy system. The browser app owns the user’s key and signs every payment. The backend is a directory and history service: it maps identities to wallets, stores payments and requests, and holds keys only for unclaimed (claimable) wallets. Money never passes through the backend; it moves directly between smart accounts on-chain.
What actually moves on-chain. The product pages say “crypto”. Technically, every Numpay payment is sent as USDC: sends, paid requests, NEAR deposits and claimable-wallet sweeps are all ERC-20 transfer calls of native Circle USDC on Base mainnet (0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913, 6 decimals). On the Elysium testnet the same calls move bridged USDC (0x7d29d8047b905000459c0e80c34a26ceedcb47b2). Balances shown in the app are the smart account’s USDC balanceOf, displayed 1:1 in dollars.

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High-level diagram

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Components

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Design principles

Non-custodial by default

User keys never reach the backend. The backend can’t move a registered user’s funds.

Identity from verified tokens

The server reads phone and email only from Web3Auth-signed ID tokens, never from request bodies.

Deterministic addresses

A wallet address is a pure function of the user’s key, computed identically in the app and the backend.

Chain-agnostic stack

The same Kernel account gets the same address on every EVM chain, so Numpay can run on Base and Elysium with one wallet design.

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Where to go next

Identity and auth

Smart accounts

Claimable wallets