(Android™) 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 Android™ Downloads Android™ Java Libraries Android C/C++ Libraries

// Important: Don't forget to include the call to System.loadLibrary
// as shown at the bottom of this code sample.
package com.test;

import android.app.Activity;
import com.chilkatsoft.*;

import android.widget.TextView;
import android.os.Bundle;

public class SimpleActivity extends Activity {

  private static final String TAG = "Chilkat";

  // Called when the activity is first created.
  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    CkCert cert = new CkCert();

    boolean success;

    // Load a digital certificate.
    success = cert.LoadFromFile("digitalCert.cer");
    if (success != true) {
        Log.i(TAG, 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).
    Log.i(TAG, "Public Key from Certificate:");
    Log.i(TAG, 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) {
        Log.i(TAG, 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...).
    Log.i(TAG, "Private Key from DER:");
    Log.i(TAG, privKey.getXml());


  }

  static {
      System.loadLibrary("chilkat");

      // Note: If the incorrect library name is passed to System.loadLibrary,
      // then you will see the following error message at application startup:
      //"The application <your-application-name> has stopped unexpectedly. Please try again."
  }
}