From 807248f9343a4cabb48c3be1a512b27f6377e871 Mon Sep 17 00:00:00 2001
From: Julien Voisin
Date: Tue, 3 Mar 2015 15:58:59 +0100
Subject: First commit!

---
 modules/entropy.py | 56 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 56 insertions(+)
 create mode 100644 modules/entropy.py

(limited to 'modules/entropy.py')

diff --git a/modules/entropy.py b/modules/entropy.py
new file mode 100644
index 0000000..48b2924
--- /dev/null
+++ b/modules/entropy.py
@@ -0,0 +1,56 @@
+''' This module uses shannon's Entropy to detect packed malwares
+'''
+import os
+import math
+import logging
+logging.basicConfig(level=logging.DEBUG)
+
+import scanmodule
+
+def main():
+    return Entropy()
+
+class Entropy(scanmodule.ScanModule):
+    name = 'entropy'
+    def populate(self, path):
+        pass
+    def load(self, path):
+        pass
+    def save(self, path):
+        pass
+
+    def __compute_score(self, path):
+        return (self.__entropy(path) - 5) * 100
+
+    def is_malware(self, path):
+        score = self.__compute_score(path)
+        logging.info('Entropy score for ' + path + ' : ' + str(score))
+        return  score > 75
+
+    def evaluate(self, path):
+        ''' Computes an arbitraty score for the given path
+        @ret A sorted list of the form [name, match_in_percent_superior_to_zero]
+        '''
+        score = self.__compute_score(path)
+        if score > 0:
+            return [['MALWARE', score],]
+        return None
+
+    def __entropy(self, path):
+        ''' Computes shannon's entropy for the given file
+        @param path Path to the file
+        '''
+        # Computes the frequency of each byte in the file
+        fsize = max(float(os.path.getsize(path)), 1.0)
+
+        freq = [0] * 256
+        with open(path, 'rb') as f:
+            for c in f.read():
+                freq[ord(c)] += 1
+
+        entropy = 0.0
+        for f in freq:
+            if f:
+                f /= fsize
+                entropy += f * math.log(f, 2)
+        return -entropy
-- 
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