slug: crypto_santas_weak_seed
title: "Santa's Weak Seed"
description: "North Pole Presents Vault token service was supposed to generate short one-time tokens for maintenance tasks. But elfs seeded the PRNG with the system time (seconds) and published a log of a few recent tokens with a rough time window. The attacker intercepted three tokens created in sequence. If you can recover the epoch seed used, you can predict the next token — and that token unlocks presents. Show that predictable seeds and weak PRNG choices can be fatal."
category: "crypto"
points: 700
is_visible: true
complexity: 7
zip: "local"
flag_plaintext: "CTF{prng_s33d_1s_n0t_s0_s3cr3t}"
hints:
  - text: "The page gives you a couple of tokens and a numeric epoch window. Try reproducing tokens for different seeds."
    penalty_points: 10
  - text: "Check seeds in the inclusive window and compare tokens in sequence."
    penalty_points: 20
  - text: "If found, compute token(s+K) for the next token and request /flag?token=<token>"
    penalty_points: 30