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/**
* Copyright (C) 2014-2015 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 <http://www.gnu.org/licenses/>.
*/
package org.whispersystems.libaxolotl.groups.ratchet;
import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;
import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;
/**
* Each SenderKey is a "chain" of keys, each derived from the previous.
*
* At any given point in time, the state of a SenderKey can be represented
* as the current chain key value, along with its iteration count. From there,
* subsequent iterations can be derived, as well as individual message keys from
* each chain key.
*
* @author Moxie Marlinspike
*/
public class SenderChainKey {
private static final byte[] MESSAGE_KEY_SEED = {0x01};
private static final byte[] CHAIN_KEY_SEED = {0x02};
private final int iteration;
private final byte[] chainKey;
public SenderChainKey(int iteration, byte[] chainKey) {
this.iteration = iteration;
this.chainKey = chainKey;
}
public int getIteration() {
return iteration;
}
public SenderMessageKey getSenderMessageKey() {
return new SenderMessageKey(iteration, getDerivative(MESSAGE_KEY_SEED, chainKey));
}
public SenderChainKey getNext() {
return new SenderChainKey(iteration + 1, getDerivative(CHAIN_KEY_SEED, chainKey));
}
public byte[] getSeed() {
return chainKey;
}
private byte[] getDerivative(byte[] seed, byte[] key) {
try {
Mac mac = Mac.getInstance("HmacSHA256");
mac.init(new SecretKeySpec(key, "HmacSHA256"));
return mac.doFinal(seed);
} catch (NoSuchAlgorithmException | InvalidKeyException e) {
throw new AssertionError(e);
}
}
}
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