BSidesSF CTF 2021 Author writeup: Hangman Battle Royale, where you defeat 1023 AI players!

Hi Everybody!

This is going to be a challenge-author writeup for the Hangman Battle Royale challenge from BSides San Francisco 2021.

This is actually a reasonable simple challenge, overall. I got the idea of using a bad mt19937 implementation (the Mersenne Twister PRNG used by Ruby and Python) from SANS Holiday Hack Challenge 2020 (which is still online if you want to play!), and wanted to build a challenge around it. I had the idea of Battleship originally, but ended up deciding on Hangman for reasons I no longer remember, but that I'm sure made sense at the time.

The game

When you run the game, it prompts for the number of rounds:

$ ruby ./hangman.rb
Welcome to Hangman Battle Royale!

           MAIN MENU

How many rounds do you want to play? (2 - 16)

If you play at least 8 rounds, you win the special prize!

When you choose a round count, it picks a bunch of CPU names to build brackets:

         ROUND 1!

This game's match-ups are:

Meirina Tresvalles    -vs-  Gelbert Chhangte
Kebede Boehmer        -vs-  Karthic Cassity
Mairtin Piedrahita    -vs-  Winston Pawlowski
Brendaliz Lumbang     -vs-  Josipa Perlow
Unusual Ballenger     -vs-  Carmellia Agregado
Jinnie Khalif         -vs-  Jeegar Madela
Vjeran Saltarelli     -vs-  Rachella Newfield

And finally...

YOU                  -vs-  Patience Saravana!

The vulnerability

The actual code powering the list of players uses Ruby's built-in PRNG, which uses a predictable Mersenne Twister to generate random numbers. I don't love how the name-choosing code was a little bit contrived, but it can leak enough state to predict future random numbers:

def get_opponents(count)
  return 0.upto(count-1).map do ||
    i = rand(0xFFFFFFFF)
    "#{ FIRST_NAMES[i & 0xFFFF] } #{ LAST_NAMES[i >> 16] }"

Each pair of names is a single 32-bit integer from the Mersenne Twister PRNG. It turns out, if you can leak 624 32-bit outputs, you can recover the full state! That means if you play at least 10 rounds, you end up with 210-1 names, or 1023 32-bit numbers (because you're the 1024th player).

Once you've gotten the state of the PRNG, you can predict everything else that's going to happen!

The exploit

My exploit is super quick and dirty. It can parse the output from the game and grab the seed using mt19937predict:

predictor = MT19937Predictor()
for _ in range(511):
    (a, b) = read_names(i)
    predictor.setrandbits(a, 32)
    predictor.setrandbits(b, 32)

(and yes, this is probably the first time I've ever written a Python solution!)

Then does a final validation on your opponent's name to make sure the solution is working:

(_, actual) = read_names(i)
first_actual = FIRST_NAMES[actual & 0x0000FFFF]
last_actual = LAST_NAMES[actual >> 16]
final_name_actual = "%s %s" % (first_actual, last_actual)

print(" -> Final name (predicted):", final_name_predicted)
print(" -> Final name (actual):   ", final_name_actual)
assert(final_name_predicted == final_name_actual)

And prints out the 10 words that will be chosen:

for i in range(10, 0, -1):
    word = predictor.getrandbits(32)
    print("Round %d: %s" % (10 - i + 1, WORDS[word & 0xFFFF]))

    # Waste RNG cycles
    for _ in range(1, (2**i) >> 1):

To use it, I just connect to the game and tee the output it into a file:

$ ruby hangman.rb | tee /tmp/hangman.txt
Welcome to Hangman Battle Royale!

           MAIN MENU

How many rounds do you want to play? (2 - 16)

If you play at least 8 rounds, you win the special prize!

> 10

         ROUND 1!

This game's match-ups are:

Carleen Murnaghan     -vs-  Willyanto Wahono
Aliecia Knutsen       -vs-  Hareth Christophersen

[... so many names ...]

Sheneen Kottwitz      -vs-  Jitlada Carrick
Janine Sellers        -vs-  Graydon Racuya
Hemali Gettinger      -vs-  Adrianna Chenna

And finally...

YOU                  -vs-  Gifted Adedamola!


          PLAYER TURN

Enter a letter to guess a letter, or a word to guess the whole thing:

    _ _ _ _ _ _ _ _ _ _ _ _

Your guess -->

Then in another window, I read the output from the tee'd file right into the python solution:

$ python ./ < /tmp/hangman.txt
Start a game with TEN (10) rounds, and paste the output (including player map-ups) here!
 -> Final name (predicted): Gifted Adedamola
 -> Final name (actual):    Gifted Adedamola
Final names check out!

Here are the words, be sure to enter them without guesses (guesses consume random cycles):
Round 1: hypocoristic
Round 2: utricularia
Round 3: trapanned
Round 4: pseudobranchiate
Round 5: twice
Round 6: misuse
Round 7: lancepesade
Round 8: stroppier
Round 9: musicalizing
Round 10: avowing

Then from there, you can just copy and paste all the words! Just don't make a wrong guess, if the CPU guesses that'll consume some RNG state and throw off the predictions:

Enter a letter to guess a letter, or a word to guess the whole thing:

    _ _ _ _ _ _ _ _ _ _ _ _

Your guess --> hypocoristic

    h y p o c o r i s t i c

Congratulations, you beat Gifted Adedamola and won this round! Let's see how the others did!


Press enter to continue

You win this round!
Wow, that was a MEGA victory!
Flag: CTF{hooray_mt19937}


I was really hoping this would be a fairly easy coding challenge, I was surprised that not a ton of teams solved it!

Hopefully this writeup, and knowing that mt19937predict is a thing will mean that more folks are able to solve this type of challenge in the future!

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