# Challenge approval

If the user approves the challenge request with its **PKSA**, the latest will send a `challenge_ack` message to the **HAS** with an access token and its expiration.

When the **HAS** will receive the user approval from its **PKSA**, it will forward it to the **APP** which will receive the following message:

### challenge\_ack

```
{
    cmd: "auth_ack",
    uuid: string,
    data: string
}
```

{% tabs %}
{% tab title="Properties" %}

* **`uuid`**: the request identifier
* **`data`**: `challenge_ack_data` encrypted with the `auth_key` and converted to Base64
  {% endtab %}
  {% endtabs %}

The **APP** must validate the received `uuid` against the request ID (`uuid`) that it received from the **HAS** in the `challenge_wait` message and ignore the `challenge_ack` message if they don't match.

The **APP** must decrypt the `challenge_ack_data` object using the encryption key previously shared with the **PKSA** (`auth_key`). If the **APP** cannot decrypt and parse `challenge_ack_data`, the **APP** must ignore the `challenge_ack` message.

> This `challenge_ack_data` encryption/decryption process has been put in place to ensure that a malicious actor operating a **HAS** cannot bypass the **PKSA** to approve a challenge request.
>
> As the encryption key has been provided offline to the **PKSA**, the **HAS** has no access to it and is therefore unable to decrypt, tamper then (re-)encrypt any data exchanged between the **APP** and the **PKSA**.

The structure of the `challenge_ack_data` is:

### challenge\_ack\_data

```
{
    pubkey: string,
    challenge: string
}
```

{% tabs %}
{% tab title="Properties" %}

* **`pubkey`**: the account public key
* **`challenge`**: the signed challenge
  {% endtab %}
  {% endtabs %}


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