Seeds of Time

Flag: HQX{c126bb454bbe38c397a5daf0729fbd08}

Approach (Step by Step)

1. We are given a Python encryption script and a ciphertext.
2. The script encrypts a secret FLAG using a keystream generated from Python’s random module, seeded with the current Unix timestamp.
3. The goal is to recover the original FLAG.
4. Given Encryption Code:

import time
import random
t = time.time()
seed = int(t)
random.seed(seed)
FLAG = "REDACTED"
keystream = bytearray(
    int(random.random() * 256) for _ in range(len(FLAG))
)
cipher = bytes([f ^ k for f, k in zip(FLAG.encode(), keystream)])
print(cipher.hex())

And ciphertext:

f60d1ef6307bc56ed4f3f8fe41eacd9883b571ae87dc9dd018b0083b601303b923ebba81ca

1. The encryption process works as follows:
   • The random number generator is seeded using int(time.time())
   • A keystream of the same length as the flag is generated
   • Each byte of the flag is XORed with the keystream
   • The result is output as hex

   Mathematically: cipher = FLAG ⊕ keystream

   Since XOR is reversible: FLAG = cipher ⊕ keystream

2. The critical weaknesses are:
   • Predictable Seed: seed = int(time.time()) Unix timestamps change once per second. The seed space is very small (≈ 86,400 per day).
   • Non-Cryptographic PRNG: random.random() Unlike other PRNGs, Python’s random module is deterministic. Same seed → same keystream.
3. Instead of brute-forcing the flag, I brute-forced the seed.
4. Ciphertext length tells us the flag length.
5. Seed is likely close to the current time.
6. Reproducing the keystream allows full decryption.
7. Attack Plan:
   • Convert ciphertext from hex to bytes.
   • Loop over plausible timestamp values.
   • Regenerate the keystream for each seed.
   • XOR keystream with ciphertext.
   • Check if output looks like a valid flag.

Script and Output:

import time
import random

ciphertext_hex = "f60d1ef6307bc56ed4f3f8fe41eacd9883b571ae87dc9dd018b0083b601303b923ebba81ca"
ciphertext = bytes.fromhex(ciphertext_hex)

# Assuming the challenge was generated recently, we check times around now
current_time = int(time.time())

# We know the flag starts with HQX{
for seed in range(current_time - 31536000, current_time + 3600):
    random.seed(seed)
    keystream = bytearray(int(random.random() * 256) for _ in range(len(ciphertext)))
    decoded = bytes([c ^ k for c, k in zip(ciphertext, keystream)])
    
    if decoded.startswith(b"HQX{"):
        print(f"Found seed: {seed}")
        print(f"Flag: {decoded.decode('utf-8', errors='ignore')}")
        break