Retrieve Subject alternative names of X.509 certificate in java

Question

I have tried using the solution provided in this link.

I am getting following error when i tried reading subject alternative names of X.509 Certificate

Answer 1

Many examples use hard-coded integers. For readability, I much prefer to use:

• GeneralName.dNSName = 2
• GeneralName.iPAddress = 7
• ... etc

The code:

public static String[] parseHostNames(X509Certificate cert) {
    List<String> hostNameList = new ArrayList<>();
    try {
        Collection<List<?>> altNames = cert.getSubjectAlternativeNames();
        if (altNames != null) {
            for(List<?> altName : altNames) {
                if(altName.size()< 2) continue;
                switch((Integer)altName.get(0)) {
                    case GeneralName.dNSName:
                    case GeneralName.iPAddress:
                        Object data = altName.get(1);
                        if (data instanceof String) {
                            hostNameList.add(((String)data));
                        }
                        break;
                    default:
                }
            }
        }
        System.out.println("Parsed hostNames: " + String.join(", ", hostNameList));
    } catch(CertificateParsingException | IOException e) {
        System.err.println("Can't parse hostNames from this cert.");
        e.printStackTrace();
    }
    return hostNameList.toArray(new String[hostNameList.size()]);
}

Note: The accepted answer checks for byte[], but won't compile on my system. I found some other examples using byte[] by calling new ASN1InputStream((byte[])data).readObject();, but I have no certificate to test it with, so I've removed it from my example.

Answer 2

I tried with your code for me it is working, I tested with a certificate exported from internet explorer

Internet Explorer -> Tools -> Internet Options -> Content -> Certificates -> Untrusted Publishers -> www.google.com

I exported this as ".cer", I made few changes to your code

public static List<String> getSubjectAlternativeNames(X509Certificate certificate) {
        List<String> identities = new ArrayList<String>();
        try {
            Collection<List<?>> altNames = certificate.getSubjectAlternativeNames();
            if (altNames == null)
                return Collections.emptyList();
            for (List item : altNames) {
                Integer type = (Integer) item.get(0);
                if (type == 0 || type == 2){
                    try {
                        ASN1InputStream decoder=null;
                        if(item.toArray()[1] instanceof byte[])
                            decoder = new ASN1InputStream((byte[]) item.toArray()[1]);
                        else if(item.toArray()[1] instanceof String)
                            identities.add( (String) item.toArray()[1] );
                        if(decoder==null) continue;
                        DEREncodable encoded = decoder.readObject();
                        encoded = ((DERSequence) encoded).getObjectAt(1);
                        encoded = ((DERTaggedObject) encoded).getObject();
                        encoded = ((DERTaggedObject) encoded).getObject();
                        String identity = ((DERUTF8String) encoded).getString();
                        identities.add(identity);
                    }
                    catch (UnsupportedEncodingException e) {
                        log.error("Error decoding subjectAltName" + e.getLocalizedMessage(),e);
                    }
                    catch (Exception e) {
                        log.error("Error decoding subjectAltName" + e.getLocalizedMessage(),e);
                    }
                }else{
                    log.warn("SubjectAltName of invalid type found: " + certificate);
                }
            }
        }
        catch (CertificateParsingException e) {
            log.error("Error parsing SubjectAltName in certificate: " + certificate + "\r\nerror:" + e.getLocalizedMessage(),e);
        }
        return identities;
    }

I saved the file to c:\aa1.cer

X509Certificate cert=null;
        FileInputStream fis = new FileInputStream("c:\\aa1.cer");
        BufferedInputStream bis = new BufferedInputStream(fis);

        CertificateFactory cf = CertificateFactory.getInstance("X.509");
        if (bis.available() > 0)
            try{
               cert = (X509Certificate)cf.generateCertificate(bis);
            }
            catch (CertificateException e) {
                e.printStackTrace();
            }
        System.out.println(CertificateInfo.getSubjectAlternativeNames(cert));

I got the output as [www.google.com, google.com]

Please check your certificate, I think the problem is your certificate