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''' 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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