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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 <http://www.gnu.org/licenses/>.
*/
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<iterations + getIterationStartOffset();i++) {
Mac mac = Mac.getInstance("HmacSHA256");
mac.init(new SecretKeySpec(prk, "HmacSHA256"));
mac.update(mixin);
if (info != null) {
mac.update(info);
}
mac.update((byte)i);
byte[] stepResult = mac.doFinal();
int stepSize = Math.min(remainingBytes, stepResult.length);
results.write(stepResult, 0, stepSize);
mixin = stepResult;
remainingBytes -= stepSize;
}
return results.toByteArray();
} catch (NoSuchAlgorithmException | InvalidKeyException e) {
throw new AssertionError(e);
}
}
protected abstract int getIterationStartOffset();
}
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