56 lines
1.5 KiB
Python
Executable File
56 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]:
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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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