Hackvent_2018/Day 11/decode.py

41 lines
971 B
Python
Raw Normal View History

2018-12-11 14:15:19 +01:00
#!/usr/bin/python
import math
def linear_diophantine_equation(a, b):
if b > a:
return linear_diophantine_equation(b, a)
if b == 0:
return a, 1, 0
d, x, y = linear_diophantine_equation(b, a % b)
return (d, y, x - (a // b) * y)
# c = (a * b) % p
c = 0x7E65D68F84862CEA3FCC15B966767CCAED530B87FC4061517A1497A03D2
p = 0xDD8E05FF296C792D2855DB6B5331AF9D112876B41D43F73CEF3AC7425F9
b = 0x7BBE3A50F28B2BA511A860A0A32AD71D4B5B93A8AE295E83350E68B57E5
def test_solution(a):
return (a * b) % p == c
d, x, y = linear_diophantine_equation(b, p)
a = y * c
a += abs(a // p) * p
solution = hex(a).replace("-", "")[2:]
2018-12-13 13:54:10 +01:00
while True:
2018-12-11 14:15:19 +01:00
a += p
solution = hex(a).replace("-", "")[2:]
if not test_solution(a):
print("Not a valid solution")
exit()
ascii = ''.join([chr(int(solution[i:i+2],16)) for i in range(0, len(solution), 2)])
if ascii[0:4] == "HV18":
print(hex(a))
print(ascii)
break