services: verify ddns domain key as Base64; closes #1035

Geez, PHP won't really tell if decode failed so we go full circle.

src/www/services_dhcp.php

@@ -284,6 +284,9 @@ if ($_SERVER['REQUEST_METHOD'] === 'GET') {
         if ((!empty($pconfig['ddnsdomain']) && !is_ipaddrv4($pconfig['ddnsdomainprimary']))) {
             $input_errors[] = gettext("A valid primary domain name server IP address must be specified for the dynamic domain name.");
         }
+        if (!empty($pconfig['ddnsdomainkey']) && base64_encode(base64_decode($pconfig['ddnsdomainkey'], true)) !== $pconfig['ddnsdomainkey']) {
+            $input_errors[] = gettext('You must specify a Base64-encoded domain key.');
+        }
         if ((!empty($pconfig['ddnsdomainkey']) && empty($pconfig['ddnsdomainkeyname'])) ||
             (!empty($pconfig['ddnsdomainkeyname']) && empty($pconfig['ddnsdomainkey']))
             ) {

src/www/services_dhcpv6.php

@@ -176,6 +176,9 @@ if ($_SERVER['REQUEST_METHOD'] === 'GET') {
         if (!empty($pconfig['ddnsdomain']) && !is_ipaddrv4($pconfig['ddnsdomainprimary'])) {
             $input_errors[] = gettext("A valid primary domain name server IPv4 address must be specified for the dynamic domain name.");
         }
+        if (!empty($pconfig['ddnsdomainkey']) && base64_encode(base64_decode($pconfig['ddnsdomainkey'], true)) !== $pconfig['ddnsdomainkey']) {
+            $input_errors[] = gettext('You must specify a Base64-encoded domain key.');
+        }
         if ((!empty($pconfig['ddnsdomainkey']) && empty($pconfig['ddnsdomainkeyname'])) ||
           (!empty($pconfig['ddnsdomainkeyname']) && empty($pconfig['ddnsdomainkey']))) {
             $input_errors[] = gettext("You must specify both a valid domain key and key name.");