How to get a private key given a certificate file with Java

Question

I want to determine what a public key is given a certificate file(something.cer file), this is my Java code:

public class X509Read {
private static String hex(String  binStr) {
    String newStr = new String();
        try {
            String hexStr = "0123456789ABCDEF";
            byte [] p = binStr.getBytes();
            for(int k=0; k < p.length; k++ ){
                int j = ( p[k] >> 4 )&0xF;
                newStr = newStr + hexStr.charAt( j );
                j = p[k]&0xF;
                newStr = newStr + hexStr.charAt( j ) + " ";
            }   
        } catch (Exception e) {
            System.out.println("Failed to convert into hex values: " + e);
        } 
        return newStr;
}
public static void main(String[] args) {
    String CA_Data[]=new String [15];
    String field;
    try{
        InputStream inStream = new FileInputStream("C:\\123.cer");
        CertificateFactory cf = CertificateFactory.getInstance("X.509");
        X509Certificate cert = 
            (X509Certificate)cf.generateCertificate(inStream);
        field=cert.getType().toString();
        CA_Data[0]=field;
        System.out.println("Type : "+field);
        field=Integer.toString(cert.getVersion());
        CA_Data[1]=field;
        System.out.println("Version : "+field);
        field=cert.getSubjectX500Principal().toString();
        CA_Data[2]=field;
        System.out.println("Name : "+field);
        field=cert.getSerialNumber().toString(16);
        CA_Data[3]=field;
        System.out.println("SerialNumber : "+field);
        field=cert.getSubjectAlternativeNames().toString();
        CA_Data[4]=field;
        System.out.println("SubjectAlternativeNames : "+field);
        field=cert.getNotBefore().toString();
        CA_Data[5]=field;
        System.out.println("NotBefore : "+field);
        field=cert.getNotAfter().toString();
        CA_Data[6]=field;
        System.out.println("NotAfter : "+field); 
        field=cert.getIssuerX500Principal().toString();
        CA_Data[7]=field;
        System.out.println("IssuerDN : "+field);
        field=cert.getSigAlgName().toString();
        CA_Data[8]=field;
        System.out.println("SigAlgName : "+field);
        byte [] tempPub = null;
        String sPub = null;
        RSAPublicKey pubkey = (RSAPublicKey) cert.getPublicKey();
        tempPub = pubkey.getEncoded();
        sPub = new String( tempPub );
        field=cert.getPublicKey().getAlgorithm();
        CA_Data[9]=field;
        System.out.println("Public Key Algorithm : " + field);
        field=hex(sPub);
        CA_Data[10]=field;
        System.out.println("Public key : \n" + field );        
        inStream.close();   
        }catch(Exception exception){ 
            exception.printStackTrace();
        }
    }
}

And it get this data: 30 EF BF BD 01 22 30 0D 06 09 2A EF BF BD 48 EF BF BD EF BF BD 0D 01 01 01 05 00 03 EF BF BD 01 0F 00 30 EF BF BD 01 0A 02 EF BF BD 01 01 00 CA BF 19 EF BF BD 24 EF BF BD 2F EF BF BD 6A EF BF BD EF BF BD 4C EF BF BD 2C EF BF BD 30 EF BF BD EF BF BD EF BF BD EF BF BD 7D EF BF BD 11 EF BF BD EF BF BD 7E EF BF BD 32 EF BF BD EF BF BD 42 73 EF BF BD EF BF BD 07 EF BF BD 15 EF BF BD 1D EF BF BD 00 39 33 66 4D 56 EF BF BD 67 EF BF BD EF BF BD EF BF BD 19 EF BF BD 17 64 EF BF BD 24 19 EF BF BD EF BF BD EF BF BD 15 EF BF BD EF BF BD C3 8C 3A EF BF BD EF BF BD EF BF BD 05 2D 3D C5 81 EF BF BD EF BF BD 48 71 EF BF BD 43 EF BF BD EF BF BD EF BF BD DE AE 20 EF BF BD EF BF BD 2B EF BF BD 1D D4 AE EF BF BD EF BF BD 25 EF BF BD 39 7B EF BF BD 70 0D 76 EF BF BD EF BF BD 7F 25 18 EF BF BD 22 EF BF BD EF BF BD EF BF BD 27 EF BF BD 0F EF BF BD EF BF BD EF BF BD DA 93 EF BF BD 0A 2C 03 3B EF BF BD 79 30 53 EF BF BD EF BF BD 26 30 EF BF BD 1D 3C 69 4C 20 EF BF BD 36 EF BF BD EF BF BD 41 7F EF BF BD 60 6B 58 EF BF BD EF BF BD EF BF BD 18 EF BF BD EF BF BD 01 6E 64 7F EF BF BD 5F 3B EF BF BD 7B 25 C9 83 43 15 EF BF BD 6F 78 EF BF BD 21 EF BF BD EF BF BD 38 59 EF BF BD EF BF BD EF BF BD EF BF BD 0B EF BF BD EF BF BD DE 9B 37 43 22 6A EF BF BD 15 28 EF BF BD 39 12 54 2B 38 4B EF BF BD 0A 43 67 0B 48 EF BF BD 59 02 60 57 EF BF BD 54 EF BF BD 6A 55 CE 91 EF BF BD 5A EF BF BD 78 76 66 13 EF BF BD 72 27 0A EF BF BD EF BF BD EF BF BD EF BF BD EF BF BD 0B 3E EF BF BD D2 9F EF BF BD 0E 4F EF BF BD EE B6 A5 5B EF BF BD 11 02 03 01 00 01

But the correct public key is (Double-click the *.cer file)

30 82 01 0a 02 82 01 01 00 ca bf 19 98 24 af 2f 8c 6a 94 e0 4c 9e 2c 86 30 9a 93 94 c5 7d f7 11 d8 e5 7e 8e 32 9d 90 42 73 99 ef 07 ed 15 a6 1d ff 00 39 33 66 4d 56 ee 67 9d 93 fd 19 f9 17 64 d9 24 19 de e3 d5 15 8a aa c3 8c 3a a7 d6 ec 05 2d 3d c5 81 da d0 48 71 d9 43 f1 e5 82 fc de ae 20 cc e3 2b c5 1d d4 ae eb a3 e3 93 25 e6 39 7b 8d 70 0d 76 b8 c0 7f 25 18 9d 22 b6 d8 ff 27 9e 0f f6 fe c4 da 93 9d 0a 2c 03 3b 80 79 30 53 c9 d0 26 30 9c 1d 3c 69 4c 20 80 36 a8 99 41 7f 99 60 6b 58 b0 84 a3 18 8b b8 01 6e 64 7f bc 5f 3b ce 7b 25 c9 83 43 15 af 6f 78 fa 21 c0 b7 38 59 90 b9 82 e0 0b f1 f6 de 9b 37 43 22 6a fb 15 28 da 39 12 54 2b 38 4b b8 0a 43 67 0b 48 9f 59 02 60 57 e6 89 54 dd 6a 55 ce 91 ad 5a f2 78 76 66 13 86 72 27 0a 94 82 a5 81 be 0b 3e fc d2 9f e0 0e 4f ba ee b6 a5 5b f9 11 02 03 01 00 01

I know EF BF BD is a replacement character when I transform the Binary to Hex.

1. How do I get the correct public key?

2. How to get the private key?

Answer 1

To extend Uwe's answer, the reason you see different values is your strange handling of the public key data:

RSAPublicKey pubkey = (RSAPublicKey) cert.getPublicKey();
tempPub = pubkey.getEncoded();
sPub = new String( tempPub );
field=hex(sPub);
System.out.println("Public key : \n" + field );

Here you are creating a string from a stream of bytes (never a good idea) and then hex-encoding the result. No wonder it looks weird! In fact, your hex method looks very strange and probably should be thrown away.

Instead, just try this:

RSAPublicKey pubkey = (RSAPublicKey) cert.getPublicKey();
field = DatatypeConverter.printHexBinary(pubkey.getEncoded());
System.out.println("Public key : \n" + field );

If you are using Java 5 or below, Google for a decent byte-to-hex function as DatatypeConverter is not available.

Answer 2

To get the public key simply use the methd getPublicKey() from X509Certificate.

The private key cannot be retrieved from the certificate as it is not included.

Answer 3

The private key is stored on authentication server, which should not be read by clients.