# Authentication approval

If the user approves the authentication request with its **PKSA**, the latest will send an `auth_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

### auth\_ack

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

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

* **`uuid`**: the request identifier
* **`data`**: `auth_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 `auth_wait` message and ignore the `auth_ack` message if they don't match.

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

> This `auth_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 an authentication request.
>
> As the encryption key has been provided off-line to the **PKSA**, it is unknown to the **HAS,** which therefore cannot decrypt, tamper then (re-)encrypt any data exchanged between the **APP** and the **PKSA**.

The structure of the `auth_ack_data` is:

### auth\_ack\_data

```javascript
{
    expire: number
    challenge_data: object = undefined
    token: string // DEPRECATED - protocol < 1.0 only    
}
```

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

* **`expire`**: UNIX timestamp when the authentication will expire
* **`challenge_data`**: *optional* - if the **APP** provided a `challenge` object with the `auth_req` sent to the **PKSA**, it contains a `challenge_ack_data` object (see [Challenge approval](/developer-documentation/has-for-applications/challenge/challenge-approval.md)).<br>
* **`token`**: session token - ***Deprecated since protocol v1.0***
  {% endtab %}
  {% endtabs %}

The application should store the `auth_key` it used to authenticate. This `auth_key` can be used with each subsequent request. This saves the user from having to re-authenticate with that same **PKSA** until `auth_key` has expired.

It is up to the **APP** to reuse the `auth_key` for as long as it is valid or to shorten its life by reinitiating a new authentication process.

When the **auth\_key** expires, any related transaction requests will be rejected.


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