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🔒 UMDCTF 2023 / ✅-crypto-reduce-thyself
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Sutx BOT 04/28/2023 7:59 PM
Reduce Thyself - 500 points
Category: Crypto Description: I don't think Pokemon really like the idea of self-reducibility... RESOURCE: 10.5.1 in this textbook may prove useful. Author: gary nc 0.cloud.chals.io 33047 Files:Tags: No tags.
04/28/2023 7:59 PM
Sutx pinned a message to this channel. 04/28/2023 7:59 PM
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Sutx BOT 04/28/2023 8:44 PM
@fleming wants to collaborate 🤝
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Sutx BOT 04/28/2023 9:19 PM
@layka_ wants to collaborate 🤝
21:22
@unpickled admin bot wants to collaborate 🤝
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layka_ 04/28/2023 9:45 PM
one idea i got is generating a random u such that u.flag_enc is prime .. and decrypt it using the oracle .. depending on n ig there is a non zero probability to have such u .. the paper suggests using dlog .. but there is no checks made ig my methodo works
21:45
i cannot implement it cuz am out fir 3 days
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layka_
one idea i got is generating a random u such that u.flag_enc is prime .. and decrypt it using the oracle .. depending on n ig there is a non zero probability to have such u .. the paper suggests using dlog .. but there is no checks made ig my methodo works
unpickled admin bot 04/28/2023 9:45 PM
i tried bruting this a bit ago
21:45
maybe i was unlucky
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but
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tbf i only tried once
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so imma try a few more times ig
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sahuang 04/28/2023 9:46 PM
probability is 0.1
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unpickled admin bot 04/28/2023 9:46 PM
however, i will say the probablility is probably negligble?
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sahuang 04/28/2023 9:46 PM
so you need to try 10 times or sth ig
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unpickled admin bot 04/28/2023 9:46 PM
wait how did you get .1?
21:46
ill do it ig
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sahuang 04/28/2023 9:46 PM
in paper
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unpickled admin bot 04/28/2023 9:46 PM
oh....
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uh
21:46
yk i prob shouldnt have skimmed that lmao
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layka_ 04/28/2023 9:47 PM
ig in the paper is just an exampl3 no ?
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sahuang 04/28/2023 9:47 PM
yeah example
21:47
idk our case
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layka_ 04/28/2023 9:47 PM
another way is submitting flag_enc^2
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sahuang 04/28/2023 9:47 PM
you can only submit prime right
21:47
oh
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layka_ 04/28/2023 9:48 PM
yep .. but with %n can be prime
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sahuang 04/28/2023 9:48 PM
yes
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layka_ 04/28/2023 9:48 PM
a non zero proba also
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sahuang 04/28/2023 9:48 PM
yeah idk how likely it is
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layka_ 04/28/2023 9:48 PM
and with each iteration it has new params
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unpickled admin bot 04/28/2023 9:48 PM
also wait
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@sahuang
21:48
it doesnt take mod n before checking
21:48
so maybe just spam add n till its prime?
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sahuang 04/28/2023 9:49 PM
b'UMDCTF{s3lf_r3duc!bility_1s_n0t_ju5t_f0r_DH_97912837923}'
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i solved it
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layka_ 04/28/2023 9:49 PM
it has conn.recv(1024)
21:49
goood
21:50
how was that
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sahuang 04/28/2023 9:50 PM
just send a prime
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curr = flag_ct + n while not isPrime(curr): curr += n
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lmao
21:50
gives result in 5 seconds
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unpickled admin bot
it doesnt take mod n before checking
unpickled admin bot 04/28/2023 9:50 PM
wait so this
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oh
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lmao
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sahuang 04/28/2023 9:50 PM
ye
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right lol
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funny
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unpickled admin bot 04/28/2023 9:50 PM
sadge i should have tried that earlier :< (edited)
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sahuang
used /ctf solve
Sutx BOT 04/28/2023 9:50 PM
✅ Challenge solved.
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unpickled admin bot 04/28/2023 9:50 PM
owel
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sahuang 04/28/2023 9:50 PM
yeah i overlooked it
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unpickled admin bot 04/28/2023 9:50 PM
same ig lmao
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sahuang 04/28/2023 9:51 PM
@layka_ nice catch lmao
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