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Signal Protocol cryptography library for Java
There is a newer version: 2.8.1
/**
 * Copyright (C) 2014-2016 Open Whisper Systems
 *
 * Licensed according to the LICENSE file in this repository.
 */
package org.whispersystems.libsignal.util;

import org.whispersystems.libsignal.IdentityKey;
import org.whispersystems.libsignal.IdentityKeyPair;
import org.whispersystems.libsignal.InvalidKeyException;
import org.whispersystems.libsignal.ecc.Curve;
import org.whispersystems.libsignal.ecc.ECKeyPair;
import org.whispersystems.libsignal.state.PreKeyRecord;
import org.whispersystems.libsignal.state.SignedPreKeyRecord;

import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;
import java.util.LinkedList;
import java.util.List;

/**
 * Helper class for generating keys of different types.
 *
 * @author Moxie Marlinspike
 */
public class KeyHelper {

  private KeyHelper() {}

  /**
   * Generate an identity key pair.  Clients should only do this once,
   * at install time.
   *
   * @return the generated IdentityKeyPair.
   */
  public static IdentityKeyPair generateIdentityKeyPair() {
    ECKeyPair   keyPair   = Curve.generateKeyPair();
    IdentityKey publicKey = new IdentityKey(keyPair.getPublicKey());
    return new IdentityKeyPair(publicKey, keyPair.getPrivateKey());
  }

  /**
   * Generate a registration ID.  Clients should only do this once,
   * at install time.
   *
   * @param extendedRange By default (false), the generated registration
   *                      ID is sized to require the minimal possible protobuf
   *                      encoding overhead. Specify true if the caller needs
   *                      the full range of MAX_INT at the cost of slightly
   *                      higher encoding overhead.
   * @return the generated registration ID.
   */
  public static int generateRegistrationId(boolean extendedRange) {
    try {
      SecureRandom secureRandom = SecureRandom.getInstance("SHA1PRNG");
      if (extendedRange) return secureRandom.nextInt(Integer.MAX_VALUE - 1) + 1;
      else               return secureRandom.nextInt(16380) + 1;
    } catch (NoSuchAlgorithmException e) {
      throw new AssertionError(e);
    }
  }

  public static int getRandomSequence(int max) {
    try {
      return SecureRandom.getInstance("SHA1PRNG").nextInt(max);
    } catch (NoSuchAlgorithmException e) {
      throw new AssertionError(e);
    }
  }

  /**
   * Generate a list of PreKeys.  Clients should do this at install time, and
   * subsequently any time the list of PreKeys stored on the server runs low.
   * 
   * PreKey IDs are shorts, so they will eventually be repeated.  Clients should
   * store PreKeys in a circular buffer, so that they are repeated as infrequently
   * as possible.
   *
   * @param start The starting PreKey ID, inclusive.
   * @param count The number of PreKeys to generate.
   * @return the list of generated PreKeyRecords.
   */
  public static List generatePreKeys(int start, int count) {
    List results = new LinkedList<>();

    start--;

    for (int i=0;i