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A J2ME compatible Axolotl impolementation
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/**
 * Copyright (C) 2014 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.ratchet;


import org.bouncycastle.crypto.digests.SHA256Digest;
import org.bouncycastle.crypto.macs.HMac;
import org.bouncycastle.crypto.params.KeyParameter;
import org.whispersystems.libaxolotl.kdf.DerivedMessageSecrets;
import org.whispersystems.libaxolotl.kdf.HKDF;

public class ChainKey {

  private static final byte[] MESSAGE_KEY_SEED = {0x01};
  private static final byte[] CHAIN_KEY_SEED   = {0x02};

  private final HKDF   kdf;
  private final byte[] key;
  private final int    index;

  public ChainKey(HKDF kdf, byte[] key, int index) {
    this.kdf   = kdf;
    this.key   = key;
    this.index = index;
  }

  public byte[] getKey() {
    return key;
  }

  public int getIndex() {
    return index;
  }

  public ChainKey getNextChainKey() {
    byte[] nextKey = getBaseMaterial(CHAIN_KEY_SEED);
    return new ChainKey(kdf, nextKey, index + 1);
  }

  public MessageKeys getMessageKeys() {
    byte[]                inputKeyMaterial = getBaseMaterial(MESSAGE_KEY_SEED);
    byte[]                keyMaterialBytes = kdf.deriveSecrets(inputKeyMaterial, "WhisperMessageKeys".getBytes(), DerivedMessageSecrets.SIZE);
    DerivedMessageSecrets keyMaterial      = new DerivedMessageSecrets(keyMaterialBytes);

    return new MessageKeys(keyMaterial.getCipherKey(), keyMaterial.getMacKey(), keyMaterial.getIv(), index);
  }

  private byte[] getBaseMaterial(byte[] seed) {
    HMac   mac    = new HMac(new SHA256Digest());
    byte[] output = new byte[mac.getMacSize()];

    mac.init(new KeyParameter(key, 0, key.length));
    mac.update(seed, 0, seed.length);
    mac.doFinal(output, 0);

    return output;
  }
}