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Scanning is the heartbeat of an inbox, and doing it server-side enables push notifications and instant loads. The safe way to do this is a view-only client: it holds the viewing private key and the spending public key, so it can detect every incoming payment but can never move funds.

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The key split

CSAP derives two keys from the wallet signature: the viewing key v (scanning) and the spending key s (spending). A scanner needs only v and the spending public key S (CSAP §2.8 watch-only delegation). Give your server v and S — never the spending key.
A server that holds the viewing key can link every incoming payment to the user. That is an inherent privacy tradeoff of server-side scanning, separate from any end-to-end encryption of labels or contacts. A breached scanning server can read the inbox; it can never spend.

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View-only client

import { OpaqueClient } from "@opaquecash/opaque";

// On the server: only the viewing private key + spending public key are provisioned.
const viewer = await OpaqueClient.createViewOnly(
  {
    chainId: 11155111,
    rpcUrl,
    ethereumAddress: userAddress,
    wasmModuleSpecifier: "/pkg/cryptography.js",
    solana: { cluster: "devnet" },
  },
  { viewingKey, spendPublicKey },   // 0x hex or bytes
);

viewer.isViewOnly; // true

const inbox = await viewer.scan({ chains: ["ethereum", "solana"], includeCrossChain: true });
const tokenBalances = await viewer.getTokenBalancesForOutputs(inbox, { ethereum: ["0xUSDC..."] });
Spending is hard-disabled — sweep, getStealthSignerPrivateKey, and reputation key reconstruction throw, because the server has no spending key: 
await viewer.sweep({ output: inbox[0], chain: "ethereum", destination });
// throws: "view-only client has no spending key ..."
The user’s app constructs a full client with OpaqueClient.create / fromWallet (holding the spending key) to sweep or spend.

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Where the keys come from

Derive both keys client-side from the wallet signature, then send only the viewing key and spending public key to your server over an authenticated channel: 
import { deriveKeysFromSignature, keysToStealthMetaAddress } from "@opaquecash/opaque";

const { viewingKey, spendingKey } = deriveKeysFromSignature(walletSignature);
const { S: spendPublicKey } = keysToStealthMetaAddress(viewingKey, spendingKey);
// Upload { viewingKey, spendPublicKey } only. Keep spendingKey on the device.

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Native scanner service

Server-side you can also run the native Rust scanner (opaque-scanner on crates.io) instead of the WASM build — same algorithm, no WASM runtime needed. Either way, scanning needs only the viewing key, so the view-only model holds regardless of which implementation you run.