> ## 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.

# CrossmintSignerAuthRequiredCallback

> Flutter Type Alias

**Type Alias**

Callback fired when a non-custodial signer needs the user to authenticate.

```dart theme={null}
typedef CrossmintSignerAuthRequiredCallback = Future<void> Function( String signerType, String signerLocator, bool needsAuth, CrossmintOtpSendCallback sendOtp, CrossmintOtpVerifyCallback verifyOtp, CrossmintOtpRejectCallback reject, )
```

The SDK passes:

* `signerType` — `"email"` or `"phone"`.
* `signerLocator` — the identifier (email or phone number in E.164 form).
* `needsAuth` — `true` when the challenge starts, `false` when it ends
  (e.g. after a successful verify).
* `sendOtp` / `verifyOtp` / `reject` — callbacks that drive the flow.

Implementers show a UI prompt, call `sendOtp` to trigger delivery, then
`verifyOtp(code)` on submission or `reject()` on cancel. The
`CrossmintWalletController.otp` controller provides a ready-made
implementation that marshalls the challenge through a
`ValueListenable` — use that unless you need bespoke wiring.