Use subject CN as DNS name

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

@@ -218,23 +218,39 @@ public class EntityCertificate {
         List<String> result = new ArrayList<>();
 
         Collection<List<?>> altNames = certificate.getSubjectAlternativeNames();
-        if (altNames == null)
-            return result;
-
-        for (List altName : altNames)
-            try {
-                if (altName.get(0).equals(GeneralName.dNSName))
-                    result.add((String) altName.get(1));
-                else if (altName.get(0).equals(GeneralName.iPAddress))
-                    if (altName.get(1) instanceof String)
+        if (altNames != null)
+            for (List altName : altNames)
+                try {
+                    if (altName.get(0).equals(GeneralName.dNSName))
                         result.add((String) altName.get(1));
-                    else {
-                        Object val = altName.get(1);
-                        Log.e("GeneralName.iPAddress type=" + (val == null ? null : val.getClass()));
+                    else if (altName.get(0).equals(GeneralName.iPAddress))
+                        if (altName.get(1) instanceof String)
+                            result.add((String) altName.get(1));
+                        else {
+                            Object val = altName.get(1);
+                            Log.e("GeneralName.iPAddress type=" + (val == null ? null : val.getClass()));
+                        }
+                } catch (Throwable ex) {
+                    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);
-            }
+        } catch (Throwable ex) {
+            Log.e(ex);
+        }
 
         return result;
     }