ofb

Auteur : zTeeed

23 Mar 2018

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download encrypt.py
download flag.png.enc
download solve.py

encrypt.py

import struct

def lcg(m, a, c, x):
	return (a*x + c) % m

m = pow(2, 32)

with open('lcg') as f:
	a = int(f.readline())
	c = int(f.readline())
	x = int(f.readline())

d = open('flag.png').read()
d += '\x00' * (-len(d) % 4)
d = [d[i:i+4] for i in range(0, len(d), 4)]

e = ''
for i in range(len(d)):
	e += struct.pack('>I', x ^ struct.unpack('>I', d[i])[0])
	x = lcg(m, a, c, x)

with open('flag.png.enc', 'w') as f:
	f.write(e)
	f.close()

solve.py

#! /usr/bin/env python3
import struct


def lcg(m, a, c, x):
    return (a*x + c) % m


f = open("flag.png.enc", "rb")
ciphertext = f.read()
f.close()

# D'après les headers de PNG...
cleartext = b"\x89\x50\x4E\x47\x0D\x0A\x1A\x0A\x00\x00\x00\x0D\x49\x48\x44\x52"

print("m: {}".format(pow(2, 32)))
for i in range(len(cleartext)//4):
    block = struct.unpack(">I", ciphertext[i*4:(i+1)*4])[0]
    key = block ^ struct.unpack(">I", cleartext[i*4:(i+1)*4])[0]

    print("key {}: {}".format(i, key))

# On a maintenant les 4 premières clés, on peut résoudre ez le système
# d'équations:
# https://www.wolframalpha.com/input/?i=(2445943554*x%2By)%252%5E32+%3D+2225636917,+(2225636917*x%2By)%252%5E32+%3D+1320590709,+(1320590709*x%2By)%252%5E32+%3D4196912501

# et hop on decipher

a = 3204287424
c = 1460809397
m = pow(2, 32)

block = struct.unpack(">I", ciphertext[:4])[0]
x = block ^ struct.unpack(">I", cleartext[:4])[0]

cleartext = b""

for i in range(len(ciphertext)//4):
    cipherchunk = struct.unpack(">I", ciphertext[i*4:(i+1)*4])[0]
    cleartext += struct.pack(">I", x ^ cipherchunk)
    x = lcg(m, a, c, x)

f = open("flag.png", "wb")
f.write(cleartext)
f.close()