Consistently Static Psuedo Random Number Generator — broncoctf2026

Description

I made a birthday oracle recently, but I can't get it to work at all. It only gets the day of the week right 14% of the time! Here are my results from the last testing session... can you take a look at it and see what's wrong?

Two files are provided:

• results.txt — a transcript of the oracle guessing month / day-of-week / region 101 times.
• challenge.py — the source of the "broken" PRNG-backed oracle.

Goal: recover the flag, which is used directly as the PRNG entropy/seed.

The "14% right on day of week" hint is 1/7 — the RNG is not random at all, it is a fully deterministic linear recurrence whose entire internal state is the flag.

Analysis

The vulnerable source:

import random

class RNG:
    def __init__(self, entropy: bytes):
        self.nextrep = 0
        shift = random.randint(0, len(entropy))
        self.schedule = list(range(len(entropy)))
        self.schedule = self.schedule[shift:] + self.schedule[:shift]
        if random.random() > 0.5:
            self.schedule.reverse()
        self.state = list(entropy)

    def next(self):
        a = sum(self.state) % 256
        self.state[self.schedule[self.nextrep % len(self.schedule)]] = a
        self.nextrep += 1
        return a

flag = open("flag.txt","rb").read()
rand = RNG(flag)
# loop prints "born in {month}... on a {day}, in {area}"
guess = rand.next()
month = months[guess % 12]
day   = days[guess % 7]
area  = areas[guess % 5]

Key observations:

1. The flag IS the state. state is initialized to the flag bytes. There is no external randomness in next() — only random.randint/random.random in the constructor pick a rotation shift and an optional reverse of the update schedule.

2. Each output leaks a full state sum. Output a[k] = sum(state) % 256 before step k. Then step k overwrites exactly one state slot p = schedule[k] with a[k].

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