> ## Documentation Index
> Fetch the complete documentation index at: https://bkey.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.
# Signing
> Sign messages and transaction hashes with the on-device wallet.
The SDK exposes two signing methods. Both require a provisioned wallet, and both verify the PIN before calling the native layer when `requirePin` is `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`
***
## 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 WalletConnect flows.
```ts theme={null}
const signature = await BmoniEmbeddedSdk.signMessage(
'Welcome to BMONI!',
'123456'
);
// 0x...130 chars...
```
When `requirePin` is `false`, omit the PIN:
```ts theme={null}
BmoniEmbeddedSdk.initialize({ requirePin: false });
const 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 it for:
* ERC-4337 `userOpHash`
* EIP-712 structured-data digests
* Raw Ethereum transaction hashes
* Any other 32-byte payload you have already hashed yourself
```ts theme={null}
const signature = await BmoniEmbeddedSdk.signTransactionHash(
'0x1c8aff950685c2ed4bc3174f3472287b56d9517b9c948127319a09a7a36deac8',
'123456'
);
```
The argument must decode to exactly 32 bytes; the `0x` prefix is optional. Anything else throws `signInvalidHash`.
***
## Server-side verification
Signatures are ECDSA-recoverable. Your backend (or a smart contract) only needs the address from `initWallet()`:
```solidity theme={null}
// Solidity
address recovered = ecrecover(hash, v, r, s);
require(recovered == expectedAddress, "invalid signature");
```
```typescript theme={null}
// ethers.js
import { verifyMessage } from 'ethers';
const recovered = verifyMessage('Welcome to BMONI!', signature);
console.log(recovered === walletAddress); // true
```
***
## Gating pattern
A common pattern is to ask for the PIN only when `requirePin` is `true`, and skip the prompt otherwise:
```ts theme={null}
async function sign(message: string): Promise {
let pin: string | undefined;
if (BmoniEmbeddedSdk.requirePin) {
const entered = await showPinPrompt();
if (entered === null) return null; // user dismissed
pin = entered;
}
return BmoniEmbeddedSdk.signMessage(message, pin);
}
```
Because `pin` is `undefined` when the gate is off, the same call site works in both modes.
***
## 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` is `true`
Calling a signing method without satisfying these throws `pinNotSet`, or `pinInvalid` if you omit the PIN entirely while the gate is on.