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Axolotl library for Java

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/**
 * Copyright (C) 2013 Open Whisper Systems
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see .
 */

package org.whispersystems.libaxolotl.kdf;

import java.io.ByteArrayOutputStream;
import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;

import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;

public abstract class HKDF {

  private static final int HASH_OUTPUT_SIZE  = 32;

  public static HKDF createFor(int messageVersion) {
    switch (messageVersion) {
      case 2:  return new HKDFv2();
      case 3:  return new HKDFv3();
      default: throw new AssertionError("Unknown version: " + messageVersion);
    }
  }

  public byte[] deriveSecrets(byte[] inputKeyMaterial, byte[] info, int outputLength) {
    byte[] salt = new byte[HASH_OUTPUT_SIZE];
    return deriveSecrets(inputKeyMaterial, salt, info, outputLength);
  }

  public byte[] deriveSecrets(byte[] inputKeyMaterial, byte[] salt, byte[] info, int outputLength) {
    byte[] prk = extract(salt, inputKeyMaterial);
    return expand(prk, info, outputLength);
  }

  private byte[] extract(byte[] salt, byte[] inputKeyMaterial) {
    try {
      Mac mac = Mac.getInstance("HmacSHA256");
      mac.init(new SecretKeySpec(salt, "HmacSHA256"));
      return mac.doFinal(inputKeyMaterial);
    } catch (NoSuchAlgorithmException | InvalidKeyException e) {
      throw new AssertionError(e);
    }
  }

  private byte[] expand(byte[] prk, byte[] info, int outputSize) {
    try {
      int                   iterations     = (int) Math.ceil((double) outputSize / (double) HASH_OUTPUT_SIZE);
      byte[]                mixin          = new byte[0];
      ByteArrayOutputStream results        = new ByteArrayOutputStream();
      int                   remainingBytes = outputSize;

      for (int i= getIterationStartOffset();i




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