> ## Documentation Index
> Fetch the complete documentation index at: https://docs.crossmint.com/llms.txt
> Use this file to discover all available pages before exploring further.

# CrossmintApiKeySigner

> Flutter Final Class

**Final Class**

A signer that delegates all signing to the Crossmint backend via API key authentication.

```dart theme={null}
final class CrossmintApiKeySigner implements CrossmintWalletApprovalSigner
```

This signer never performs local signing.  When a wallet uses an API key
signer, the Crossmint backend automatically approves transactions and
signatures on behalf of the user.

Calling `signMessage` or `signTransaction` directly will throw; signing
is handled server-side during the transaction/signature creation API call.

## Constructors

### CrossmintApiKeySigner

```dart theme={null}
const CrossmintApiKeySigner({String? locator})
```

Creates an API key signer.

## Properties

### type

```dart theme={null}
String get type
```

### locator

```dart theme={null}
final String locator
```

## Methods

### signMessage

```dart theme={null}
Future<Object> signMessage(String message)
```

Always throws — API key signers do not support local message signing.

### signTransaction

```dart theme={null}
Future<Object> signTransaction(String transaction)
```

Always throws — API key signers do not support local transaction signing.