---
title: "Authenticate on iOS"
description: "Log merchants in with OAuth or native login, inspect the active account, and log out."
image: "https://developer.sumup.com/og/index.png"
---

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

# Authenticate on iOS

Authenticate a merchant after [setting up the SDK](/terminal-payments/sdks/ios/setup/). Use the OAuth 2.0 authorization code flow for new integrations.

## Implementing authentication with OAuth 2.0

Use the [OAuth 2.0 authorization code flow](/tools/authorization/oauth/#authorization-code-flow) to obtain an access token for the merchant. The SDK does not support tokens from the client credentials flow.

## Log in with an OAuth access token

Pass the user-scoped access token obtained through your OAuth authorization code flow to the SDK. Client credentials tokens are not supported.

### Swift

```swift
SumUpSDK.login(withToken: accessToken) { success, error in
    guard success, error == nil else { return }
    // The merchant is logged in. You can now start a checkout.
}
```

### Objective-C

```objc
[SMPSumUpSDK loginWithToken:accessToken completion:^(BOOL success, NSError *error) {
    if (!success || error != nil) { return; }
    // The merchant is logged in. You can now start a checkout.
}];
```

## Implementing authentication with view controller

The SDK can also display its own login screen. This is the native login used in the quickstart. Use OAuth for new production integrations.

### Logging the user in

:::caution
Native login uses the user's default merchant account and does not offer an account selector. For users with access to multiple merchant accounts, we recommend using a managed user account created in the intended merchant. Alternatively, set the default merchant account before logging in. See [merchant account setup for native SDK login](/terminal-payments/quickstart/#users-with-multiple-merchant-accounts).
:::

Present the login screen from a visible `UIViewController`. Only continue when `success` is true. The merchant can cancel without an error, and the SDK also returns `success = false` if a merchant is already logged in.

![Login screen](</img/guides/ios-sdk_login.png>)

### Swift

```swift
SumUpSDK.presentLogin(from: self, animated: true) { success, error in
    guard success, error == nil else {
        print(error?.localizedDescription ?? "Login was cancelled or a merchant is already logged in")
        return
    }
    print("Merchant logged in")
}
```

### Objective-C

```objc
[SMPSumUpSDK presentLoginFromViewController:self animated:YES
                          completionBlock:^(BOOL success, NSError *error) {
    if (!success || error) {
        NSLog(@"%@", error.localizedDescription ?: @"Login was cancelled or a merchant is already logged in");
        return;
    }
    NSLog(@"Merchant logged in");
}];
```

### Logging the user out

Log out before switching merchant accounts:

### Swift

```swift
SumUpSDK.logout { success, error in
    guard success, error == nil else {
        print(error?.localizedDescription ?? "Logout did not succeed")
        return
    }
    print("Merchant logged out")
}
```

### Objective-C

```objc
[SMPSumUpSDK logoutWithCompletionBlock:^(BOOL success, NSError *error) {
    if (!success || error) {
        NSLog(@"%@", error.localizedDescription ?: @"Logout did not succeed");
        return;
    }
    NSLog(@"Merchant logged out");
}];
```

## Verify the account

Check `SumUpSDK.currentMerchant` and its `currencyCode` before creating a checkout. For the native login flow, verify that you are using the intended sandbox or live merchant.

## Next step

[Connect and manage a card reader](/terminal-payments/sdks/ios/readers/), or [activate Tap to Pay on iPhone](/terminal-payments/sdks/ios/tap-to-pay/).

Source: https://developer.sumup.com/terminal-payments/sdks/ios/authentication/index.mdx