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affine.py
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affine.py
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#!/usr/bin/env python3
# pip3 install pycryptodome
from Crypto.Util.number import inverse
lookup = {
'A': 0,
'B': 1,
'C': 2,
'D': 3,
'E': 4,
'F': 5,
'G': 6,
'H': 7,
'I': 8,
'J': 9,
'K': 10,
'L': 11,
'M': 12,
'N': 13,
'O': 14,
'P': 15,
'Q': 16,
'R': 17,
'S': 18,
'T': 19,
'U': 20,
'V': 21,
'W': 22,
'X': 23,
'Y': 24,
'Z': 25
}
def egcd(a, b):
if a == 0:
return (b, 0, 1)
else:
g, y, x = egcd(b % a, a)
return (g, x - (b // a) * y, y)
def encrypt(text, a, b):
# y = E(x) = (ax + b) mod 26
enc = ''
for x in text:
if x in lookup:
enc += get_key( (a*lookup[x] + b) % 26 )
else:
enc += x
return enc
def decrypt(text, a, b):
# x = movinv(a, 26) * (y-b) % 26
dec = ''
for y in text:
if y in lookup:
dec += get_key( inverse(a, 26)*(lookup[y] - b) % 26 )
else:
dec += y
return dec
def test():
m = 'FLAG{TESTING_AFFINE}'
c = encrypt(m, 5, 8)
print(c)
print(decrypt(c, 5, 8))
# Brute-force all possible solutions
def affine_BF(c):
for a in range(26):
for b in range(26):
print(f"[a:{a}, b:{b}]> {decrypt(c, a, b)}")
if __name__ == '__main__':
# test()
c = 'HLIM{ZCUZWVM_IHHWVC}'
c = c.upper()
affine_BF(c)