Use subject CN as DNS name

app/src/main/java/eu/faircode/email/EntityCertificate.java

@@ -218,9 +218,7 @@ public class EntityCertificate {
         List<String> result = new ArrayList<>();
 
         Collection<List<?>> altNames = certificate.getSubjectAlternativeNames();
-        if (altNames == null)
-            return result;
-
+        if (altNames != null)
             for (List altName : altNames)
                 try {
                     if (altName.get(0).equals(GeneralName.dNSName))
@@ -236,6 +234,24 @@ public class EntityCertificate {
                     Log.e(ex);
                 }
 
+        try {
+            X500Name name = new JcaX509CertificateHolder(certificate).getSubject();
+            if (name != null)
+                for (RDN rdn : name.getRDNs(BCStyle.CN))
+                    for (AttributeTypeAndValue tv : rdn.getTypesAndValues()) {
+                        ASN1Encodable enc = tv.getValue();
+                        if (enc == null)
+                            continue;
+                        String cn = enc.toString();
+                        if (TextUtils.isEmpty(cn))
+                            continue;
+                        if (!result.contains(cn))
+                            result.add(cn);
+                    }
+        } catch (Throwable ex) {
+            Log.e(ex);
+        }
+
         return result;
     }