Fix issue with Nginx not recognizing the SCEP client certificate (#105)

Co-authored-by: Pahansith <pahansith@entgra.io>
Reviewed-on: community/device-mgt-core#105
Co-authored-by: Pahansith Gunathilake <pahansith@entgra.io>
Co-committed-by: Pahansith Gunathilake <pahansith@entgra.io>

components/certificate-mgt/org.wso2.carbon.certificate.mgt.core/src/main/java/org/wso2/carbon/certificate/mgt/core/impl/CertificateGenerator.java

@@ -97,10 +97,7 @@ import java.security.cert.CertificateNotYetValidException;
 import java.security.cert.X509Certificate;
 import java.security.spec.InvalidKeySpecException;
 import java.security.spec.X509EncodedKeySpec;
-import java.util.ArrayList;
-import java.util.Collection;
-import java.util.Date;
-import java.util.List;
+import java.util.*;
 import java.util.concurrent.TimeUnit;
 
 public class CertificateGenerator {
@@ -798,8 +795,16 @@ public class CertificateGenerator {
 
             BigInteger serialNumber = BigInteger.valueOf(System.currentTimeMillis());
 
-            X500Name issuerName = new X500Name(certCA.getSubjectDN().getName());
-
+            //Reversing the order of components of the subject DN due to Nginx not verifying the client certificate
+            //generated by Java using this subject DN.
+            //Ref: https://stackoverflow.com/questions/33769978 & engineering mail SCEP implementation for Android
+            String[] dnParts = certCA.getSubjectDN().getName().split(",");
+            StringJoiner joiner = new StringJoiner(",");
+            for (int i = (dnParts.length - 1); i >= 0; i--) {
+                joiner.add(dnParts[i]);
+            }
+            String subjectDn = joiner.toString();
+            X500Name issuerName = new X500Name(subjectDn);
             String commonName = certificationRequest.getSubject().getRDNs(BCStyle.CN)[0].getFirst()
                     .getValue().toString();
             X500Name subjectName = new X500Name("O=" + commonName + "O=AndroidDevice,CN=" +