Resolve "Currency Converter API" can not get Currency Rate

app/code/Magento/Directory/Model/Currency/Import/CurrencyConverterApi.php

@@ -7,12 +7,17 @@
 
 namespace Magento\Directory\Model\Currency\Import;
 
+/**
+ * Currency rate import model (From http://free.currencyconverterapi.com/)
+ *
+ * Class \Magento\Directory\Model\Currency\Import\CurrencyConverterApi
+ */
 class CurrencyConverterApi extends AbstractImport
 {
     /**
      * @var string
      */
-    const CURRENCY_CONVERTER_URL = 'http://free.currencyconverterapi.com/api/v3/convert?q={{CURRENCY_FROM}}_{{CURRENCY_TO}}&compact=ultra'; //@codingStandardsIgnoreLine
+    const CURRENCY_CONVERTER_URL = 'http://free.currencyconverterapi.com/api/v3/convert?q={{CURRENCY_FROM}}_{{CURRENCY_TO}}&compact=ultra&apiKey={{API_KEY}}'; //@codingStandardsIgnoreLine
 
     /**
      * Http Client Factory
@@ -46,7 +51,7 @@ public function __construct(
     }
 
     /**
-     * {@inheritdoc}
+     * @inheritdoc
      */
     public function fetchRates()
     {
@@ -74,23 +79,25 @@ public function fetchRates()
      */
     private function convertBatch($data, $currencyFrom, $currenciesTo)
     {
+        $apiKey = $this->scopeConfig->getValue(
+            'currency/currencyconverterapi/api_key',
+            \Magento\Store\Model\ScopeInterface::SCOPE_STORE
+        );
+        if (!$apiKey) {
+            $this->_messages[] = __('No API Key was specified.');
+            return $data;
+        }
         foreach ($currenciesTo as $to) {
+            //phpcs:ignore Magento2.Functions.DiscouragedFunction
             set_time_limit(0);
             try {
                 $url = str_replace('{{CURRENCY_FROM}}', $currencyFrom, self::CURRENCY_CONVERTER_URL);
                 $url = str_replace('{{CURRENCY_TO}}', $to, $url);
-                $response = $this->getServiceResponse($url);
+                $url = str_replace('{{API_KEY}}', $apiKey, $url);
                 if ($currencyFrom == $to) {
                     $data[$currencyFrom][$to] = $this->_numberFormat(1);
                 } else {
-                    if (empty($response)) {
-                        $this->_messages[] = __('We can\'t retrieve a rate from %1 for %2.', $url, $to);
-                        $data[$currencyFrom][$to] = null;
-                    } else {
-                        $data[$currencyFrom][$to] = $this->_numberFormat(
-                            (double)$response[$currencyFrom . '_' . $to]
-                        );
-                    }
+                    $data[$currencyFrom][$to] = $this->getCurrencyRate($currencyFrom, $to, $url);
                 }
             } finally {
                 ini_restore('max_execution_time');
@@ -100,6 +107,36 @@ private function convertBatch($data, $currencyFrom, $currenciesTo)
         return $data;
     }
 
+    /**
+     * Get currency rate from api
+     *
+     * @param string $currencyFrom
+     * @param string $to
+     * @param string $url
+     * @return double
+     */
+    private function getCurrencyRate($currencyFrom, $to, $url)
+    {
+        $rate = null;
+        $response = $this->getServiceResponse($url);
+        if (empty($response)) {
+            $this->_messages[] = __('We can\'t retrieve a rate from %1 for %2.', $url, $to);
+            $rate = null;
+        } else {
+            if (isset($response['error']) && $response['error']) {
+                if (!in_array($response['error'], $this->_messages)) {
+                    $this->_messages[] = $response['error'];
+                }
+                $rate = null;
+            } else {
+                $rate = $this->_numberFormat(
+                    (double)$response[$currencyFrom . '_' . $to]
+                );
+            }
+        }
+        return $rate;
+    }
+
     /**
      * Get Fixer.io service response
      *
@@ -137,7 +174,7 @@ private function getServiceResponse($url, $retry = 0)
     }
 
     /**
-     * {@inheritdoc}
+     * @inheritdoc
      */
     protected function _convert($currencyFrom, $currencyTo)
     {

app/code/Magento/Directory/etc/adminhtml/system.xml

@@ -47,7 +47,12 @@
             </group>
             <group id="currencyconverterapi" translate="label" sortOrder="45" showInDefault="1" showInWebsite="0" showInStore="0">
                 <label>Currency Converter API</label>
-                <field id="timeout" translate="label" type="text" sortOrder="0" showInDefault="1" showInWebsite="0" showInStore="0">
+                <field id="api_key" translate="label" type="obscure" sortOrder="0" showInDefault="1" showInWebsite="0" showInStore="0">
+                    <label>API Key</label>
+                    <config_path>currency/currencyconverterapi/api_key</config_path>
+                    <backend_model>Magento\Config\Model\Config\Backend\Encrypted</backend_model>
+                </field>
+                <field id="timeout" translate="label" type="text" sortOrder="10" showInDefault="1" showInWebsite="0" showInStore="0">
                     <label>Connection Timeout in Seconds</label>
                 </field>
             </group>

app/code/Magento/Directory/etc/config.xml

@@ -24,6 +24,7 @@
             </fixerio>
             <currencyconverterapi>
                 <timeout>100</timeout>
+                <api_key backend_model="Magento\Config\Model\Config\Backend\Encrypted" />
             </currencyconverterapi>
             <import>
                 <enabled>0</enabled>

app/code/Magento/Directory/i18n/en_US.csv

@@ -52,3 +52,4 @@ Service,Service
 "The """%1"" is not allowed as base currency for your subscription plan.","The """%1"" is not allowed as base currency for your subscription plan."
 "An invalid base currency has been entered.","An invalid base currency has been entered."
 "Currency rates can't be retrieved.","Currency rates can't be retrieved."
+"No API Key was specified.","No API Key was specified."