Signing

Sign messages and transaction hashes with the on-device wallet.

LifecycleUserWalletKYCRailFundMove money

The SDK exposes two signing methods. Both require a provisioned wallet, and both verify the PIN before calling the native layer when requirePin: true.

All signatures are:

• 0x-prefixed 130-character hex strings
• Recoverable ECDSA in r(32) ‖ s(32) ‖ v(1) format
• Low-s normalised (EIP-2 compliant)
• v ∈ {27, 28} — verifiable with ecrecover

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Sign a personal message (EIP-191)

signMessage prepends the \x19Ethereum Signed Message:\n{length} prefix before hashing and signing. This matches the personal_sign RPC method used by SIWE, login challenges, and most wallet connect flows.

final String signature = await BmoniEmbeddedSdk.signMessage(
  'Welcome to BMONI!',
  pin: '123456',
);
// 0x...130 chars...

When requirePin: false, omit the pin argument:

BmoniEmbeddedSdk.initialize(requirePin: false);
final String sig = await BmoniEmbeddedSdk.signMessage('Welcome to BMONI!');

---

Sign a 32-byte hash

signTransactionHash signs a pre-computed 32-byte digest directly, without adding any prefix. Use this for:

• ERC-4337 userOpHash
• EIP-712 structured-data digests
• Raw Ethereum transaction hashes
• Any other 32-byte payload you’ve already hashed yourself

final String signature = await BmoniEmbeddedSdk.signTransactionHash(
  '0x1c8aff950685c2ed4bc3174f3472287b56d9517b9c948127319a09a7a36deac8',
  pin: '123456',
);

The argument must be a 0x-prefixed 64-hex-character (32-byte) string. Anything else throws signInvalidHash.

---

Server-side verification

Signatures are ECDSA-recoverable. Your backend (or a smart contract) only needs the address from initWallet():

// Solidity
address recovered = ecrecover(hash, v, r, s);
require(recovered == expectedAddress, "invalid signature");

// ethers.js / viem
import { verifyMessage } from 'ethers';
const recovered = verifyMessage('Welcome to BMONI!', signature);
console.log(recovered === walletAddress); // true

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Gating pattern

A common pattern is to ask for the PIN only when requirePin is true, and skip the prompt otherwise:

Future<String?> _sign(String message) async {
  String? pin;

  if (BmoniEmbeddedSdk.requirePin) {
    pin = await showPinDialog(context);
    if (pin == null) return null; // user dismissed
  }

  return BmoniEmbeddedSdk.signMessage(message, pin: pin);
}

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Prerequisites checklist

Before calling either signing method you need:

1. A provisioned wallet (initWallet() succeeded or hasWallet() is true)
2. A PIN set (hasPin() is true) — only when requirePin: true

Calling a signing method without satisfying these will throw pinNotSet or result in a native error.