HTTPS certificate validation options

New: Enable HTTPS certificate validation by default
New: Option to disable certificate validation for all or only local addresses

src/NzbDrone.Core/Security/X509CertificateValidationPolicy.cs

@@ -0,0 +1,72 @@
+using System.Linq;
+using System.Net;
+using System.Net.Security;
+using System.Security.Cryptography.X509Certificates;
+using NLog;
+using NzbDrone.Common.Extensions;
+using NzbDrone.Core.Configuration;
+
+namespace NzbDrone.Core.Security
+{
+    public interface IX509CertificateValidationPolicy
+    {
+        void Register();
+    }
+
+    public class X509CertificateValidationPolicy : IX509CertificateValidationPolicy
+    {
+        private readonly IConfigService _configService;
+        private readonly Logger _logger;
+
+        public X509CertificateValidationPolicy(IConfigService configService, Logger logger)
+        {
+            _configService = configService;
+            _logger = logger;
+        }
+
+        public void Register()
+        {
+            ServicePointManager.ServerCertificateValidationCallback = ShouldByPassValidationError;
+        }
+
+        private bool ShouldByPassValidationError(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
+        {
+            var request = sender as HttpWebRequest;
+
+            if (request == null)
+            {
+                return true;
+            }
+
+            var req = sender as HttpWebRequest;
+            var cert2 = certificate as X509Certificate2;
+            if (cert2 != null && req != null && cert2.SignatureAlgorithm.FriendlyName == "md5RSA")
+            {
+                _logger.Error("https://{0} uses the obsolete md5 hash in it's https certificate, if that is your certificate, please (re)create certificate with better algorithm as soon as possible.", req.RequestUri.Authority);
+            }
+
+            if (sslPolicyErrors == SslPolicyErrors.None)
+            {
+                return true;
+            }
+
+            var host = Dns.GetHostEntry(req.Host);
+            var certificateValidation = _configService.CertificateValidation;
+
+            if (certificateValidation == CertificateValidationType.Disabled)
+            {
+                return true;
+            }
+
+            if (certificateValidation == CertificateValidationType.DisabledForLocalAddresses && host.AddressList.All(i => i.IsLocalAddress()))
+            {
+                return true;
+            }
+
+
+            _logger.Error("Certificate validation for {0} failed. {1}", request.Address, sslPolicyErrors);
+
+            return false;
+        }
+    }
+}