(Java) About RSA Public/Private Keys

This example provides some additional information for understanding public/private key pairs. In demonstrates how a private key is a superset of the public key. A public key contains the modulus and exponent. The matching private key also contains the modulus and exponent, but also contains the additional private key parts.

Chilkat Java Downloads Java Libs for Windows, MacOS, Linux, Alpine Linux, Solaris Java Libs for Android

import com.chilkatsoft.*;

public class ChilkatExample {

  static {
    try {
        System.loadLibrary("chilkat");
    } catch (UnsatisfiedLinkError e) {
      System.err.println("Native code library failed to load.\n" + e);
      System.exit(1);
    }
  }

  public static void main(String argv[])
  {
    CkCert cert = new CkCert();

    boolean success;

    // Load a digital certificate.
    success = cert.LoadFromFile("digitalCert.cer");
    if (success != true) {
        System.out.println(cert.lastErrorText());
        return;
        }

    CkPublicKey pubKey;

    // A .cer file does not contain the private key.  It should contain
    // the public key...
    pubKey = cert.ExportPublicKey();

    // Let's have a look at it (in XML format).
    System.out.println("Public Key from Certificate:");
    System.out.println(pubKey.getXml());

    // An RSA public key consists of a modulus and exponent.

    // An RSA private key includes both the modulus and exponent,
    // as well as other "big" numbers: P, Q, D, etc.

    // Let's load an RSA private key from a DER-encoded file:
    CkPrivateKey privKey = new CkPrivateKey();

    success = privKey.LoadAnyFormatFile("PrivateKey.key","");
    if (success != true) {
        System.out.println(privKey.lastErrorText());

        return;
        }

    // If this private key is the matching half to the public key from
    // the certificate, then the modulus and exponent should
    // be identical.  (Thus, a "private key" really contains both the public part as well as the private parts...).
    System.out.println("Private Key from DER:");
    System.out.println(privKey.getXml());
  }
}