add xmasspirit

crypto/XMASSpirit/challenge.py

@@ -0,0 +1,25 @@
+#!/usr/bin/python3
+
+import random
+from math import gcd
+
+def encrypt(dt):
+	mod = 256
+	while True:
+		a = random.randint(1,mod)
+		if gcd(a, mod) == 1: break
+	b = random.randint(1,mod)
+
+	res = b''
+	for byte in dt:
+		enc = (a*byte + b) % mod
+		res += bytes([enc])
+	return res
+
+dt = open('letter.pdf', 'rb').read()
+
+res = encrypt(dt)
+
+f = open('encrypted.bin', 'wb')
+f.write(res)
+f.close()