from io import StringIO
import string
template = """#!/usr/bin/env python3
import binascii
key = "{enckey}"
def mystery(s):
r = ""
for i, c in enumerate(s):
r += chr(ord(c) ^ ((i * ord(key[i % len(key)])) % 256))
return binascii.hexlify(bytes(r, "utf-8"))
"""
flag_prefix = "char_by_char"
def get_problem(random):
enckey = "".join([random.choice(string.ascii_letters) for i in range(8)])
salt = "".join([random.choice(string.hexdigits) for i in range(6)])
fmtflag = "{}_{}".format(flag_prefix, salt)
return enckey, fmtflag
def generate(random):
enckey, fmtflag = get_problem(random)
contents = template.format(enckey=enckey)
v = dict()
exec(contents, v, v)
enc = v["mystery"]("easyctf{%s}" % fmtflag).decode("utf-8")
return dict(files={
"mystery.py": StringIO(contents)
}, variables={
"enc": enc
})
def grade(random, key):
enckey, fmtflag = get_problem(random)
if(key.find(fmtflag) != -1):
return True, "Good job!"
return False, "Try again."
