README + comments

README.md

@@ -1,2 +1,14 @@
 # IN603
 
+attack against Content Scrambling System cipher
+
+
+### execution
+
+```
+python main.py
+```
++ 17 bits lfsr check
++ 0xffffffffff encryption and decryption test
++ testing attack in 2^16 with 6 output bytes provided against CSS n times (by default 5), with keys randomly generated
++ testing safer attack (same as above but use a 7th byte in case of a collision between 2 keys with 6 output bytes)

main.py

@@ -5,8 +5,11 @@ import multiprocessing
 
 class lfsr(object):
     def __init__(self, state, taps):
-        self.state = state
-        self.taps = taps
+        if max(taps) > len(state):
+            raise ValueError("inconsistent taps and initial state")
+        else:
+            self.state = state
+            self.taps = taps
 
     # getters and setters (private attributes)
     def get_state(self):
@@ -104,25 +107,30 @@ def attack_worker(start, end, Bytes, result_queue, stop_event):
     taps17 = [0, 14]
     taps25 = [0, 3, 4, 12]
     for i in range(start, end):
-        if stop_event.is_set():
+        if stop_event.is_set(): # stop key search as soon as one worker finds a solution
             return
         lfsr17_init = [int(bit) for bit in bin(i)[2:].zfill(16)]+[1]
         lfsr17 = lfsr(lfsr17_init, taps17)
         x = []
+        # generate first 3 output bytes from lfsr 17
         for _ in range(3):
             x_bin = ""
             for _ in range(8):
                 x_bin += str(lfsr17.shift())
             x.append(int(x_bin[::-1], 2))
+        # calculate corresponding output bytes from lfsr 25
         y = [(Bytes[0]-x[0])%256]
         c=1 if x[0]+y[0]>255 else 0
         y.append((Bytes[1]-(x[1]+c))%256)
         c=1 if x[1]+y[1]>255 else 0
         y.append((Bytes[2]-(x[2]+c))%256)
+
         lfsr25_init = [int(bit) for bit in (bin(y[0])[2:].zfill(8)[::-1] + bin(y[1])[2:].zfill(8)[::-1] + bin(y[2])[2:].zfill(8)[::-1] ) ]+[1]
         lfsr25 = lfsr(lfsr25_init, taps25)
+        # first 3 bytes from lfsr 25 are already known
         for _ in range(24):
             lfsr25.shift()
+        # generate next 3 bytes from lfsr 25 and lfsr 17
         for _ in range(3):
             x_bin = ""
             y_bin = ""
@@ -131,13 +139,14 @@ def attack_worker(start, end, Bytes, result_queue, stop_event):
                 y_bin += str(lfsr25.shift())
             x.append(int(x_bin[::-1], 2))
             y.append(int(y_bin[::-1], 2))
+        # check if we can generate the next 3 output bytes z4,z5,z6 correctly
         c=1 if x[2]+y[2]>255 else 0
         z4 = (x[3]+y[3]+c)%256
         c=1 if x[3]+y[3]>255 else 0
         z5 = (x[4]+y[4]+c)%256
         c=1 if x[4]+y[4]>255 else 0
         z6 = (x[5]+y[5]+c)%256
-        if z4 == Bytes[3] and z5 == Bytes[4] and z6 == Bytes[5]:
+        if z4 == Bytes[3] and z5 == Bytes[4] and z6 == Bytes[5]:    # if these bytes are correct the key found is most likely the correct one
             key = bin(x[0])[2:].zfill(8)[::-1] + bin(x[1])[2:].zfill(8)[::-1] + bin(y[0])[2:].zfill(8)[::-1] + bin(y[1])[2:].zfill(8)[::-1] + bin(y[2])[2:].zfill(8)[::-1]
             result_queue.put(key)
             stop_event.set()
@@ -387,7 +396,7 @@ print("\n")
 # time to test attack with 100 keys
 # CPU: 6 cores at 3.8GHz
 # ~65 seconds
-test_attack(10)
+test_attack(5)
 print("\n")
 test_fail()
 print("\n")