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| ~~~
```python
#!/usr/bin/env python3
"""
LitCTF2026 — One-time pad reused for two messages (40 bytes each).
Players receive output.txt and README; they do not receive secret.py.
"""
from __future__ import annotations
import argparse
import os
from pathlib import Path
try:
from secret import M1_FLAG
except ImportError:
raise SystemExit(
"secret.py (organizer) is required to generate ciphertext; "
"players work from output.txt only."
)
# Public second message — duplicated in README for contestants.
M2_KNOWN = b"litctf2026_xor_keystream_reuse_40bytes!!"
assert len(M1_FLAG) == len(M2_KNOWN) == 40
def xor_bytes(a: bytes, b: bytes) -> bytes:
return bytes(x ^ y for x, y in zip(a, b))
def main() -> None:
parser = argparse.ArgumentParser()
parser.add_argument(
"--write",
type=Path,
help="Write hex lines to file.",
)
args = parser.parse_args()
n = len(M1_FLAG)
k = os.urandom(n)
c1 = xor_bytes(M1_FLAG, k)
c2 = xor_bytes(M2_KNOWN, k)
lines = [
f"c1 = {c1.hex()}",
f"c2 = {c2.hex()}",
f"len = {n}",
]
text = "\n".join(lines) + "\n"
print(text, end="")
if args.write:
args.write.write_text(text, encoding="utf-8")
if __name__ == "__main__":
main()
# c1 = 5f70a847ce12759e156e3cad1aa9530a119386a02ffc1c31bf14ab7a0a82ccc108f8476f75c98a28
# c2 = 5f70a847ce123cc153283ca710ae7f042b8490a238eb2228970fad6a2694f2985dc5557e69e5f474
# len = 40
```
## 解题代码
```python
c1_hex = "5f70a847ce12759e156e3cad1aa9530a119386a02ffc1c31bf14ab7a0a82ccc108f8476f75c98a28"
c2_hex = "5f70a847ce123cc153283ca710ae7f042b8490a238eb2228970fad6a2694f2985dc5557e69e5f474"
c1 = bytes.fromhex(c1_hex)
c2 = bytes.fromhex(c2_hex)
M2 = b"litctf2026_xor_keystream_reuse_40bytes!!"
M1 = bytes(a ^ b ^ c for a, b, c in zip(c1, c2, M2))
print(M1)
'''
c1=M1⊕k
c2=M2⊕k
将两式 XOR：
c1⊕c2=(M1⊕k)⊕(M2⊕k)=M1⊕M2
因此：M1=(c1⊕c2)⊕M2
'''
```
```python
```
# lit_elgamal_handshake
## 题目
~~~python
#!/usr/bin/env python3
"""
LitCTF2026 — ElGamal handshake (story)
Someone left debug logging on; the private exponent x was printed alongside ciphertext.
"""
from __future__ import annotations
import argparse
from pathlib import Path
from random import randrange
from Crypto.Util.number import bytes_to_long, getPrime, getRandomRange
try:
from secret import FLAG
except ImportError as e:
raise SystemExit("secret.py (FLAG) is required to encrypt.") from e
def generate_elgamal_keypair(bits: int = 512) -> tuple[int, int, int, int]:
p = getPrime(bits)
for _ in range(1000):
g = getRandomRange(2, min(6, p - 1))
if pow(g, (p - 1) // 2, p) != 1:
break
else:
raise RuntimeError("could not find suitable g")
x = randrange(2, p - 1)
y = pow(g, x, p)
return p, g, y, x
def main() -> None:
parser = argparse.ArgumentParser()
parser.add_argument(
"--write",
type=Path,
help="Write captured output to this file (for organizers).",
)
args = parser.parse_args()
p, g, y, x = generate_elgamal_keypair(bits=512)
k = randrange(1, p - 2)
m = bytes_to_long(FLAG)
if m >= p:
raise ValueError("flag too large for chosen p — shorten FLAG")
c1 = pow(g, k, p)
c2 = (m * pow(y, k, p)) % p
lines = [
"=== Public key (p, g, y) ===",
f"p = {p}",
f"g = {g}",
f"y = {y}",
"",
"=== Ciphertext (c1, c2) ===",
f"c1 = {c1}",
f"c2 = {c2}",
"",
"# [DEBUG] prod accidentally logged the long-term secret:",
f"x = {x}",
]
text = "\n".join(lines) + "\n"
print(text, end="")
if args.write:
args.write.write_text(text, encoding="utf-8")
if __name__ == "__main__":
main()
# === Public key (p, g, y) ===
# p = 9000784855376359808051354825193962042770028561343848432778443672755982397391267124312572697249531643069409873722736348916207732622884411596948807031140651
# g = 3
# y = 269130883529708333054320571854006406481346665463416017026083074488011546059928157925990665431751017523964760326934454181952822744463714981243407307134357
# === Ciphertext (c1, c2) ===
# c1 = 5245857426274383693193378669425243235151460522527004924092730024427525619244222247576829782077334810173274945751493387545849499010408499951268967774043627
# c2 = 6059939492718262451327758167005534191200936922719178843825888167191062504030471358635203794720371216217447404436172970111033824674731063386612549785069654
# # [DEBUG] prod accidentally logged the long-term secret:
# x = 6333662932190226841086284837534236574773242538336571410337629717617476693446496678870023479078822412461192231264928632918867512055053600497937288513718
|
