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<?php
/**
* @copyright Copyright (c) 2017 Matthias Held <matthias.held@uni-konstanz.de>
* @author Matthias Held <matthias.held@uni-konstanz.de>
* @license GNU AGPL version 3 or any later version
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU Affero 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 Affero General Public License for more details.
*
* You should have received a copy of the GNU Affero General Public License
* along with this program. If not, see <https://www.gnu.org/licenses/>.
*/
namespace OCA\RansomwareDetection\Analyzer;
use OCA\RansomwareDetection\AppInfo\Application;
use OCA\RansomwareDetection\Entropy\Entropy;
use OCP\Files\File;
use OCP\Files\IRootFolder;
use OCP\Files\Storage\IStorage;
use OCP\Files\NotFoundException;
use OCP\ILogger;
class EntropyAnalyzer
{
/**
* Entropy cut-off point between normal and compressed or encrypted files.
*
* @var float
*/
const ENTROPY_CUT_OFF = 7.69;
/**
* Size of the data blocks in bytes.
*
* @var int
*/
const BLOCK_SIZE = 256;
/**
* Standard deviation cut-off point between compressed and encrypted files.
*
* @var float
*/
const SD_CUT_OFF = 0.06;
/** @var ILogger */
protected $logger;
/** @var IRootFolder */
protected $rootFolder;
/** @var Entropy */
protected $entropy;
/** @var string */
protected $userId;
/**
* @param ILogger $logger
* @param IRootFolder $rootFolder
* @param Entropy $entropy
* @param int $userId
*/
public function __construct(
ILogger $logger,
IRootFolder $rootFolder,
Entropy $entropy,
$userId
) {
$this->logger = $logger;
$this->rootFolder = $rootFolder;
$this->entropy = $entropy;
$this->userId = $userId;
}
/**
* Classifies a file using entropy measurements. It first calculates the
* native entropy of the file to decide wether it's a normal file with
* low entropy or a compressed or encrypted file with high entropy.
*
* If the file is identified as class B, it measures the
* standard deviation of the entropy of all blocks with a size of 256 bytes.
* To decide if the file is compressed or encrypted.
*
* The results classifies the file in the following three classes:
* ENCRYPTED
* COMPRESSED
* NORMAL
*
* @param File $node
*
* @return EntropyResult
*/
public function analyze($node)
{
$entropy = $this->calculateEntropyOfFile($node);
if ($entropy > self::ENTROPY_CUT_OFF) {
$standardDeviation = $this->calculateStandardDeviationOfEntropy($node, self::BLOCK_SIZE);
if ($standardDeviation > self::SD_CUT_OFF) {
return new EntropyResult(EntropyResult::COMPRESSED, $entropy, $standardDeviation);
}
return new EntropyResult(EntropyResult::ENCRYPTED, $entropy, $standardDeviation);
}
return new EntropyResult(EntropyResult::NORMAL, $entropy, 0.0);
}
/**
* Creates an array with the entropy of the data blocks.
*
* @param File $node
* @param int $blockSize
*
* @return float
*/
protected function calculateStandardDeviationOfEntropy($node, $blockSize)
{
$sum = 0.0;
$standardDeviation = 0.0;
$mean = 1;
$step = 1;
$handle = $node->fopen('r');
if (!$handle) {
$this->logger->debug('createEntropyArrayFromFile: Getting file handle failed.', array('app' => Application::APP_ID));
return 0.0;
}
while (!feof($handle)) {
$data = fread($handle, $blockSize);
$step = $step + 1;
if (strlen($data) === $blockSize) {
$entropy = $this->entropy->calculateEntropy($data);
$sum = $sum + pow($entropy, 2);
$mean = $this->entropy->streamMean($mean, $entropy, $step);
$standardDeviation = $this->entropy->streamStandardDeviation($step, $sum, $mean);
}
}
fclose($handle);
return $standardDeviation;
}
/**
* Calculates the entropy of a file.
*
* @param File $node
*
* @return float
*/
protected function calculateEntropyOfFile($node)
{
$handle = $node->fopen('r');
if (!$handle) {
$this->logger->debug('calculateEntropyOfFile: Getting data failed.', array('app' => Application::APP_ID));
return 0.0;
}
$entropy = 0.0;
$total = 0;
while (!feof($handle)) {
$data = fread($handle, 1024);
$total = $total + 1;
if (strlen($data) === 1024) {
$entropy = $entropy + $this->entropy->calculateEntropy($data);
}
}
fclose($handle);
$entropy = $entropy / $total;
if ($entropy >= 0) {
return $entropy;
} else {
return -$entropy;
}
}
}
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