update

README.md

@@ -37,4 +37,4 @@ Hack the box december [CTF](https://ctf.hackthebox.com/ctf/249)
 
 ## Writeups
 
-- Each challenge subfolder contains a small README.md
+- Each challenge subfolder contains a README.md

crypto/XMASSpirit/decrypt.py

@@ -6,45 +6,50 @@ from math import gcd
 # the modulus for the lcg prng
 n = 256
 # 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):
-    ''' find a and b for n and ct '''
+
+def get_factors(ct: bytes, n: int = 256) -> (int, int):
+    """find a and b for n and ct"""
     # 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 a in nogcds:
             for i, enc in enumerate(header):
                 candidate = (a * enc + b) % n
-                if candidate != ct[i]: break
-                if(i+1 == len(header)):
-                  return (a,b)
-    print('[-] No solution found.')
-    return (0,0)
+                if candidate != ct[i]:
+                    break
+                if i + 1 == len(header):
+                    return (a, b)
+    print("[-] No solution found.")
+    return (0, 0)
 
-def generate_lookuptable(a:int, b:int) -> dict[int,int]:
-    ''' generate a lookuptable for the translation of the enc file'''
+
+def generate_lookuptable(a: int, b: int) -> dict[int, int]:
+    """generate a lookuptable for the translation of the enc file"""
     out = {}
     for i in range(0, 256):
         lt = (i * a + b) % 256
         out[lt] = i
     return out
 
-def decrypt(ct:bytes, lut:dict) -> bytes:
-    ''' decrypt the file using the lookup table '''
-    res = b''
+
+def decrypt(ct: bytes, lut: dict) -> bytes:
+    """decrypt the file using the lookup table"""
+    res = b""
     for byte in ct:
         dec = lut[byte]
         res += bytes([dec])
     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)
-    print(f'[+] Factors solved: a={a}, b={b}')
+    print(f"[+] Factors solved: a={a}, b={b}")
     lut = generate_lookuptable(a, b)
-    print(f'[+] Lookuptable generated: \nlookup = {lut}')
-    dec = decrypt(ct,lut)
-    print(f'[+] Encrypted file decrypted')
-    with open('letter.pdf', 'wb') as f:
+    print(f"[+] Lookuptable generated: \nlookup = {lut}")
+    dec = decrypt(ct, lut)
+    print(f"[+] Encrypted file decrypted")
+    with open("letter.pdf", "wb") as f:
         f.write(dec)