public function authenticate(array $request, $secure, $remote_address, $secret, $allowed) {
    if (!$secure) {
        return $this->failure(
            'must be a secure connection',
            'HTTPS required',
            $request,
            $remote_address
        );
    }

    if (!isset($request['email']) || !is_string($request['email']) || $request['email'] === '') {
        return $this->failure(
            'email must not be blank',
            'Missing or malformed email',
            $request,
            $remote_address
        );
    }

    if (!isset($request['password']) || !is_string($request['password']) || $request['password'] === '') {
        return $this->failure(
            'Password must not be blank',
            'Missing or malformed password',
            $request,
            $remote_address
        );
    }

    $user = get_user_by('email', $request['email']);

    if (
        !$user instanceof WP_User ||
        !wp_check_password($request['password'], $user->user_pass, $user->ID)
    ) {
        return $this->failure(
            'User Credentials Incorrect',
            'Credentials rejected',
            $request,
            $remote_address
        );
    }

    $buyer_number = $this->field(
        get_user_meta($user->ID, 'permanent_buyers_number', true)
    );

    return implode('|', array(
        $buyer_number,
        $this->user_field($user->ID, 'first_name', 'billing_first_name'),
        $this->user_field($user->ID, 'last_name', 'billing_last_name'),
        $this->user_field($user->ID, 'billing_city', 'city'),
        $this->user_field($user->ID, 'billing_postcode', 'postcode'),
    ));
}