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aaron
2022-06-20 21:43:53 +02:00
parent 3365ce2c4e
commit 0a15570340
2 changed files with 28 additions and 23 deletions
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2
README.md
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@@ -37,4 +37,4 @@ Hack the box december [CTF](https://ctf.hackthebox.com/ctf/249)
## Writeups ## Writeups
- Each challenge subfolder contains a small README.md - Each challenge subfolder contains a README.md

33
crypto/XMASSpirit/decrypt.py
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@@ -6,45 +6,50 @@ from math import gcd
# the modulus for the lcg prng # the modulus for the lcg prng
n = 256 n = 256
# the header of the pdf v1.5 file # the header of the pdf v1.5 file
header = [ 0x25, 0x50, 0x44, 0x46, 0x2d, 0x31, 0x2e ] header = [0x25, 0x50, 0x44, 0x46, 0x2D, 0x31, 0x2E]
def get_factors(ct: bytes, n: int = 256) -> (int, int): def get_factors(ct: bytes, n: int = 256) -> (int, int):
''' find a and b for n and ct ''' """find a and b for n and ct"""
# first generate a list of all numbers without common divisor with 256 # first generate a list of all numbers without common divisor with 256
nogcds = [x for x in range(1, n) if gcd(x, n) == 1] nogcds = [x for x in range(1, n) if gcd(x, n) == 1]
for b in range(1, n): for b in range(1, n):
for a in nogcds: for a in nogcds:
for i, enc in enumerate(header): for i, enc in enumerate(header):
candidate = (a * enc + b) % n candidate = (a * enc + b) % n
if candidate != ct[i]: break if candidate != ct[i]:
if(i+1 == len(header)): break
if i + 1 == len(header):
return (a, b) return (a, b)
print('[-] No solution found.') print("[-] No solution found.")
return (0, 0) return (0, 0)
def generate_lookuptable(a: int, b: int) -> dict[int, int]: def generate_lookuptable(a: int, b: int) -> dict[int, int]:
''' generate a lookuptable for the translation of the enc file''' """generate a lookuptable for the translation of the enc file"""
out = {} out = {}
for i in range(0, 256): for i in range(0, 256):
lt = (i * a + b) % 256 lt = (i * a + b) % 256
out[lt] = i out[lt] = i
return out return out
def decrypt(ct: bytes, lut: dict) -> bytes: def decrypt(ct: bytes, lut: dict) -> bytes:
''' decrypt the file using the lookup table ''' """decrypt the file using the lookup table"""
res = b'' res = b""
for byte in ct: for byte in ct:
dec = lut[byte] dec = lut[byte]
res += bytes([dec]) res += bytes([dec])
return res return res
if __name__ == '__main__':
ct = open('encrypted.bin', 'rb').read() if __name__ == "__main__":
ct = open("encrypted.bin", "rb").read()
a, b = get_factors(ct) a, b = get_factors(ct)
print(f'[+] Factors solved: a={a}, b={b}') print(f"[+] Factors solved: a={a}, b={b}")
lut = generate_lookuptable(a, b) lut = generate_lookuptable(a, b)
print(f'[+] Lookuptable generated: \nlookup = {lut}') print(f"[+] Lookuptable generated: \nlookup = {lut}")
dec = decrypt(ct, lut) dec = decrypt(ct, lut)
print(f'[+] Encrypted file decrypted') print(f"[+] Encrypted file decrypted")
with open('letter.pdf', 'wb') as f: with open("letter.pdf", "wb") as f:
f.write(dec) f.write(dec)