certificateGenerator.setSignatureAlgorithm( "MD5WithRSAEncryption" );
certificateGenerator.setSerialNumber( new BigInteger( ""+SystemTime.getCurrentTime()));
X509Name issuer_dn = new X509Name(true,cert_dn);
certificateGenerator.setIssuerDN(issuer_dn);
X509Name subject_dn = new X509Name(true,cert_dn);
certificateGenerator.setSubjectDN(subject_dn);
Calendar not_after = Calendar.getInstance();