51 lines
1.5 KiB
Python
Executable File
51 lines
1.5 KiB
Python
Executable File
#!/bin/env python
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import random
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from math import gcd
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# the modulus for the lcg prng
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n = 256
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# the header of the pdf v1.5 file
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header = [ 0x25, 0x50, 0x44, 0x46, 0x2d, 0x31, 0x2e ]
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def get_factors(ct:bytes, n:int=256) -> (int, int):
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''' find a and b for n and ct '''
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# first generate a list of all numbers without common divisor with 256
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nogcds = [ x for x in range(1, n) if gcd(x, n) == 1 ]
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for b in range(1, n):
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for a in nogcds:
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for i, enc in enumerate(header):
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candidate = (a * enc + b) % n
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if candidate != ct[i]: break
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if(i+1 == len(header)):
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return (a,b)
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print('[-] No solution found.')
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return (0,0)
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def generate_lookuptable(a:int, b:int) -> dict[int,int]:
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''' generate a lookuptable for the translation of the enc file'''
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out = {}
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for i in range(0, 256):
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lt = (i * a + b) % 256
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out[lt] = i
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return out
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def decrypt(ct:bytes, lut:dict) -> bytes:
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''' decrypt the file using the lookup table '''
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res = b''
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for byte in ct:
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dec = lut[byte]
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res += bytes([dec])
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return res
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if __name__ == '__main__':
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ct = open('encrypted.bin', 'rb').read()
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a, b = get_factors(ct)
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print(f'[+] Factors solved: a={a}, b={b}')
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lut = generate_lookuptable(a, b)
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print(f'[+] Lookuptable generated: \nlookup = {lut}')
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dec = decrypt(ct,lut)
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print(f'[+] Encrypted file decrypted')
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with open('letter.pdf', 'wb') as f:
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f.write(dec)
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