> For the complete documentation index, see [llms.txt](https://docs.payments.thalescloud.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.payments.thalescloud.io/merchant-tokenization/mastercard-taf/implement-taf/authenticate-a-transaction.md).

# Authenticate a transaction

Use the Thales SDK to authenticate a transaction with Mastercard Token Authentication Framework (TAF).

This call generates a Mastercard payload. Use it to retrieve a TAF eligible transaction cryptogram.

## Authentication flow

<figure><img src="/files/MHfrYIQsyOIS6LFgJU4x" alt=""><figcaption><p>Mastercard TAF authentication flow at checkout.</p></figcaption></figure>

## SDK integration

{% tabs %}
{% tab title="Android" %}
Call `MastercardTAFHelper.authenticateTransaction`.

Start device authentication in `onDeviceAuthentication`.

You can customize the biometric UI strings.

Handle the final result in `onSuccess` or `onError`.

```java
public void mastercard_AuthenticateTransaction(@NonNull final String tokenID, @NonNull TransactionContext transactionContext, @NonNull MastercardTAFHelper masterCardTAFHelper, @NonNull FragmentActivity fragmentActivity) {
    masterCardTAFHelper.authenticateTransaction(tokenID, transactionContext, new TransactionListener() {
        @Override
        public void onDeviceAuthentication(DeviceAuthentication deviceAuthentication) {
            CharSequence title = "Title";
            CharSequence subTitle = "Subtitle";
            CharSequence description = "Description";
            CharSequence negativeButtonText = "Negative Button";
            deviceAuthentication.startAuthentication(fragmentActivity,
                    title,
                    subTitle,
                    description,
                    negativeButtonText);
        }

        @Override
        public void onError(TMGClientException exception) {
            // Check error
            int errorCode = exception.getErrorCode();
            String errorMessage = exception.getMessage();
        }

        @Override
        public void onSuccess(String payload) {
            // Decode the Mastercard Payload and verify the Transaction assertion
        }
    });
}
```

`onSuccess` returns the Mastercard payload.

`onError` returns a `TMGClientException` with an error code and message.
{% endtab %}

{% tab title="iOS" %}
Call `authenticateTransaction` with a `TransactionContext`.

Start device authentication in `deviceAuthenticationHandler`.

```swift
let tokenID = ""
let transContext = MastercardTAFHelper.TransactionContext(merchantName: "ABC", amount: "100", currencyCode: "USD")
// 1. Start authenticate transaction
mastercardTAFHelper.authenticateTransaction(forTokenID: tokenID, context: transContext,
    deviceAuthenticationHandler: { auth in
        // 2. Start user authentication
        let customMessage = "" // Pass in the custom message. e.g: "Authenticate with Face ID"
        auth.startAuthentication(withMessage: customMessage)
    },
    completionHandler: { (mastercardPayload, error) in
        // 3. Success in generating payload
        if error == nil {
        
        } else {
            // Handle error
        }
    }
)
```

{% endtab %}
{% endtabs %}


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