(Android™) Exporting and Importing Accumulated Entropy

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.

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);

    // All Chilkat classes can be unlocked at once at the beginning of a program
    // by calling UnlockBundle.  It requires a Bundle unlock code.
    CkGlobal chilkatGlob = new CkGlobal();
    boolean success = chilkatGlob.UnlockBundle("Anything for 30-day trial.");
    if (success != true) {
        Log.i(TAG, chilkatGlob.lastErrorText());
        return;
        }

    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."
  }
}