Automatic commit Sun Apr 22 21:46:16 EEST 2018

Author: Alamot

crypto/xorknown.py

@@ -1,7 +1,7 @@
 #!/usr/bin/env python2
 # Author: Alamot
 # This is a XOR plaintext attack tool: If we know a part of the plaintext maybe
-# we can recover the key and the whole text in successive steps.
+# we can recover the key and the whole text.
 from __future__ import print_function
 from __future__ import division
 import string, sys
@@ -13,6 +13,7 @@
 
 
 def is_printable(text, ignore_code):
+    ''' Function to check if every character in text is printable '''
     for ch in text:
         if ord(ch) == ignore_code:
             continue
@@ -26,15 +27,30 @@ def lrotate(s, d):
     return s[d:] + s[0:d]
 
 
-if len(sys.argv) < 2:
-    print("Usage: "+sys.argv[0]+"<filename> <known plaintext> [max_key_length]")
+if len(sys.argv) < 2 or sys.argv[1].strip().lower() == "--help":
+    print("Known-plaintext attack: "+sys.argv[0]+" <filename> <known plaintext> [max_key_length]")
+    print("Decrypt using known key: "+sys.argv[0]+" <filename> --key=the_known_key")
     exit()
 
 filename = sys.argv[1]
-known_plaintext = sys.argv[2]
+
+if sys.argv[2].strip().lower()[:5] == "--key":
+    known_key = sys.argv[2].strip()[6:]
+    with open(filename, "rb") as f:
+        data = f.read()
+    decrypted_text = ""
+    repeated_key = (known_key)*((len(data) // len(known_key)) + 1)
+    for x in range(len(data)):
+        decrypted_text += chr(ord(data[x]) ^ ord(repeated_key[x]))
+    print("Key length: "+str(len(known_key)), "\nPartial Key: "+known_key, "\nPlaintext: "+decrypted_text)
+    exit()
+else:
+    known_plaintext = sys.argv[2]
+
 if len(known_plaintext) > max_key_length:
     print("The length of the known plaintext is greater than max_key_length (="+str(max_key_length)+"). Please give a smaller plaintext or incrase max_key_length.")
     exit()
+    
 if len(sys.argv) > 3:
     print(len(sys.argv))
     max_key_length = int(sys.argv[3])+1
@@ -45,7 +61,7 @@ def lrotate(s, d):
 print("Searching XOR-encrypted "+filename+" for string '"+known_plaintext+"'")
 
 try:    
-    for i in range(len(data)-len(known_plaintext)):
+    for i in range(len(data)-len(known_plaintext)): # Try known plaintext in every position
         partial_key = ""
         for j in range(len(known_plaintext)):
             if known_plaintext[j] == ignore_code:
@@ -54,18 +70,18 @@ def lrotate(s, d):
                 partial_key += chr(ord(data[i+j]) ^ ord(known_plaintext[j]))
         #print("Single key: "+partial_key)
         if is_printable(partial_key, ignore_code) or not printable_key:
-            for n in range(len(partial_key), max_key_length):    #try different key lengths
-                for m in range(n): #try different partial key positions
+            for n in range(len(partial_key), max_key_length): # Try different key lengths
+                for m in range(n):                            # Try different partial key positions
                     expanded_key = lrotate(partial_key+chr(ignore_code)*(n-len(partial_key)), m)
                     #print(expanded_key, m)
                     repeated_key = (expanded_key)*((len(data) // len(expanded_key)) + 1)
                     decrypted_text = ""
-                    for x in range(len(data)):
+                    for x in range(len(data)): # Try to decrypt the encoded text
                         if ord(repeated_key[x]) == ignore_code:
                             decrypted_text += chr(ignore_code)
                         else:
                             decrypted_text += chr(ord(data[x]) ^ ord(repeated_key[x]))
-                    if is_printable(decrypted_text, ignore_code):
+                    if is_printable(decrypted_text, ignore_code): # Is the whole result printable?
                         print("Key length: "+str(len(expanded_key)), "\nPartial Key: "+expanded_key, "\nPlaintext: "+decrypted_text)
                         print("")
 except KeyboardInterrupt: