The service-provider may create an authorization web-service, which supports the
OAuth 2.0 ([D. Hardt, Ed., 2012](https://datatracker.ietf.org/doc/html/rfc6749)) Client Credentials grant type.
BOG will use an HTTP POST request method for communication with the authorization endpoint.

### Header Parameters

- `Content-Type` — application/x-www-form-urlencoded, required

### Body Parameters

- `grant_type` — client_credentials, required

  The value is a static text: `client_credentials`

- `client_id` — <client_id>, required

- `client_secret` — <client_secret>, required

**CURL**

```bash
curl --location --request POST 'http://localhost:8080/ess-api/authenticate' \
--header 'Content-Type: application/x-www-form-urlencoded' \
--data-urlencode 'grant_type=client_credentials' \
--data-urlencode 'client_id=client_id' \
--data-urlencode 'client_secret=client_secret'
```

### Response
The service-provider must return the following JSON response:

- `access_token` — string

  The Token received from the authorization server.

- `created_at` — string

  UNIX timestamp – the amount of milliseconds that have passed since January 1st, 1970 at UTC .

- `expires_in` — number

  The number of seconds while the token stays active.

- `token_type` — number

  Token type (A static value - `Bearer`).

**RESPONSE**

```json
{
  "access_token": "xx82194x21412xxx219124x678x888x2xxx1900xx",
  "created_at": 1693236686855,
  "expires_in": 300,
  "token_type": "Bearer"
}
```

For any subsequent calls to the API, when the token is valid, BOG will use the HTTP header of the form:

- `Authorization` — Bearer <token>, required

  „Bearer „ + „&#60;token&#62;“ (e.g.: „Bearer xx82194x21412xxx219124x678x888x2xxx1900xx“),
  where `token` is the value returned from the service-provider's authorization endpoint.