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Re: Challenge "Lunchtime Attack on FHE"  

  By: karthik24 on Oct. 9, 2011, 4:38 a.m.

Hi,

I have just started this challenge. I was going through the challenge description and the link given for the oracle cgi script is not working.

I am getting a 500 internal server.

http://www.mysterytwisterc3.org/cgi-bin ... orphic.cgi

Any idea on where I can get the script?

Re: Challenge "Lunchtime Attack on FHE"  

  By: Veselovský on Oct. 9, 2011, 2:13 p.m.

Hi,
it works, but you have to append ?c=N , where N is some number.
For example like this:
http://www.mysterytwisterc3.org/cgi-bin ... cgi?c=1234

Viktor

Re: Challenge "Lunchtime Attack on FHE"  

  By: karthik24 on Oct. 9, 2011, 3:30 p.m.

Hi,
it works, but you have to append ?c=N , where N is some number.
For example like this:
http://www.mysterytwisterc3.org/cgi-bin ... cgi?c=1234

Viktor

Thanks Viktor, it is working. I guess this is what happens when you don't read the instructions and just click on a link. [HTML_REMOVED]

Re: Challenge "Lunchtime Attack on FHE"  

  By: Luy on Nov. 1, 2011, 11:45 p.m.

While the described lunchtime attack is pretty interesting, you can solve this problem much easier by exploiting the homomorphic property of the encryption scheme since the oracle can be used after you know the ciphertext (which is not the case in a "lunchtime setting" where you first may use the oracle and then get the challenge ciphertext). However, this was fun :)

Re: Challenge "Lunchtime Attack on FHE"  

  By: Witten on Dec. 30, 2011, 6:36 p.m.

Nice nice nice!!!

Re: Challenge  

  By: cruachan on Oct. 22, 2013, 7:11 p.m.

Could someone explain how to read the .txt-files that have been provided with this challenge? If I understand everything correctly, there should only be one d, but the text in public.txt contains a few symbols different than numbers (,{}:%). And can't really understand how I should read cipher.txt as well.

Re: Challenge  

  By: ariobarzan on Oct. 27, 2013, 12:39 a.m.

format "n: {c6,c5,c4,c3,c2,c1,c0}% "
"n:" -> "index of character"
each index have seven cipher number that can be decrypted to 7-bit ASCII Code character .
so every cipher number decrypt to a bit of character.

Re: Challenge  

  By: cruachan on Oct. 27, 2013, 4:17 p.m.

Thanks. Now it all makes sense. If im correct, every bit is encoded with a different key, and public.txt has the same format as cipher.txt.

Re: Challenge  

  By: cruachan on Oct. 28, 2013, 4:59 p.m.

Ok, I figured out that my last post was nonsense. There is just one key, but the file public.txt contains the entire ciphertext as well as the key. Deleting anything starting with the string 'cipher' leaves only the public key.

Re: Challenge  

  By: madness on Feb. 10, 2020, 11:12 p.m.

Could anyone here verify that I have the correct w?
I ask because my decryption looks like jibberish.
TIA

Re: Challenge  

  By: madness on Feb. 10, 2020, 11:26 p.m.

Nevermind. After a few tries I figured out what
"reduction of z modulo d on the interval …" meant.
Thanks anyway.

Re: Challenge  

  By: code000 on Sept. 30, 2020, 1:33 a.m.

Could anyone here verify that I have the correct w?
I ask because my decryption looks like jibberish.
TIA

I have reached the same stage as this. Hahahahah

Re: Challenge  

  By: code000 on Sept. 30, 2020, 1:34 a.m.

Nevermind. After a few tries I figured out what
"reduction of z modulo d on the interval …" meant.
Thanks anyway.

Will try figuring this out now.

Re: Challenge  

  By: code000 on Sept. 30, 2020, 11:39 a.m.

Nevermind. After a few tries I figured out what
"reduction of z modulo d on the interval …" meant.
Thanks anyway.

Will try figuring this out now.

Hahahaha got it!!
It was fun… was doing a small mistake in rounding off.
Nice challenge.

Re: Challenge  

  By: madness on Sept. 30, 2020, 6:16 p.m.

Glad you figured it out. Your messages arrived during the night time where I live, so I didn't respond sooner.


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