1
out[i] = CONST[i] XOR license[i mod 10]   for i = 0..45

1
# !/usr/bin/env python3
2
import re
3

4
CONST = bytes.fromhex(
5
    "dbbc3342678166ae9a08e0c6154e46ac7fb9c245aa873868"
6
    "14a07fa0984ead83547d7bb8598ac30ffa87542611a8"
7
)
8

9
PREFIX = b"BITSCTF{"
10

11
def decode_with_key10(key10: bytes) -> bytes:
12
    return bytes(CONST[i] ^ key10[i % 10] for i in range(len(CONST)))
13

14
def main():
15
    key = [None] * 10
16
    for i, ch in enumerate(PREFIX):
17
        key[i] = CONST[i] ^ ch
18

19
    pattern = re.compile(r"^BITSCTF\{[A-Za-z0-9_]+\}$")
20

21
    for k8 in range(256):
22
        for k9 in range(256):
23
            key[8] = k8
24
            key[9] = k9
25
            key10 = bytes(key)
26
            pt = decode_with_key10(key10)
27

28
            try:
29
                s = pt.decode("ascii")
30
            except UnicodeDecodeError:
31
                continue
32

33
            if pattern.fullmatch(s):
34
                print(f"key10_hex={key10.hex()}  flag={s}")
35

36
if __name__ == "__main__":
37
    main()

1
key10_hex=99f5671124d520d5f63c
2
flag=BITSCTF{l4y3r_by_l4y3r_y0u_unr4v3l_my_53cr375}