Android™

Exporting and Importing Accumulated Entropy

See more PRNG Examples

When a system starts, a good established source of entropy may be lacking. The Fortuna algorithm keeps 32 internal entropy pools. When AddEntropy is called, it is added to these internal pools. (The details of the Fortuna algorithm can be read elsewhere.)

This example demonstrates how the entropy that has accumulated within the internal pools can be exported (such as when an application exits) and then imported when an application starts. (It is the application's job to persist the exported entropy to a database, file, etc.)

Importing entropy helps ensure an application is beginning with an adequate source of entropy for random number generation.

Android™

// 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);

    boolean success = false;

    // This example assumes the Chilkat API to have been previously unlocked.
    // See Global Unlock Sample for sample code.

    success = false;

    CkPrng fortuna = new CkPrng();

    // Generate random data and periodically add entropy...
    success = fortuna.AddEntropy(fortuna.getEntropy(32,"hex"),"hex");
    Log.i(TAG, fortuna.genRandom(16,"hex"));
    Log.i(TAG, fortuna.genRandom(16,"hex"));
    Log.i(TAG, fortuna.genRandom(16,"hex"));
    success = fortuna.AddEntropy(fortuna.getEntropy(16,"hex"),"hex");
    Log.i(TAG, fortuna.genRandom(16,"hex"));
    Log.i(TAG, fortuna.genRandom(16,"hex"));
    Log.i(TAG, fortuna.genRandom(16,"hex"));
    success = fortuna.AddEntropy(fortuna.getEntropy(16,"hex"),"hex");
    Log.i(TAG, fortuna.genRandom(16,"hex"));
    Log.i(TAG, fortuna.genRandom(16,"hex"));
    Log.i(TAG, fortuna.genRandom(16,"hex"));
    success = fortuna.AddEntropy(fortuna.getEntropy(16,"hex"),"hex");
    Log.i(TAG, fortuna.genRandom(16,"hex"));
    Log.i(TAG, fortuna.genRandom(16,"hex"));
    Log.i(TAG, fortuna.genRandom(16,"hex"));

    String exportedEntropy = fortuna.exportEntropy();
    // The application would persist the exported entropy in some way,
    // such as to a file, or to a database, etc.

    // Create a new PRNG object and initialize the internal entropy pools
    // with the previously exported entropy:
    CkPrng fortuna2 = new CkPrng();
    success = fortuna2.ImportEntropy(exportedEntropy);

    // Generate more random data, continuing to add entropy as we go along..
    Log.i(TAG, fortuna2.genRandom(16,"hex"));
    Log.i(TAG, fortuna2.genRandom(16,"hex"));
    Log.i(TAG, fortuna2.genRandom(16,"hex"));
    success = fortuna2.AddEntropy(fortuna2.getEntropy(16,"hex"),"hex");
    Log.i(TAG, fortuna2.genRandom(16,"hex"));
    Log.i(TAG, fortuna2.genRandom(16,"hex"));
    Log.i(TAG, fortuna2.genRandom(16,"hex"));
    success = fortuna2.AddEntropy(fortuna2.getEntropy(16,"hex"),"hex");
    // ..

  }

  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."
  }
}