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Challenge "Lunchtime Attack on the Fully Homomorphic Encryption Scheme"  

  By: admin on Jan. 14, 2011, 2:19 p.m.

The Fully Homomorphic Encryption Scheme by Gentry and Halevi turns out to be vulnerable to so called lunchtime attacks. In this challenge you have to perform such an attack.
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 Last edited by: admin on Oct. 31, 2021, 2:54 a.m., edited 1 time in total.

Re: Challenge "Lunchtime Attack on FHE"  

  By: DarkFibre on July 12, 2011, 11:53 p.m.

That one was fun! Thanks!

Re: Challenge "Lunchtime Attack on FHE"  

  By: fretty on July 13, 2011, 1:02 p.m.

I am quite limited to what I can program and I only really know how to use maple. How am I supposed to get my computer to automatically access the oracle?

Also, if I can get my computer to do this then why couldnt I just get my computer to decrypt the message without the secret key, just using the oracle?

Re: Challenge "Lunchtime Attack on FHE"  

  By: cfwramaekers on July 15, 2011, 12:59 a.m.

I am quite limited to what I can program and I only really know how to use maple. How am I supposed to get my computer to automatically access the oracle?

Also, if I can get my computer to do this then why couldnt I just get my computer to decrypt the message without the secret key, just using the oracle?

Hi,

I'm not too familiar with Maple, but it appears to have an HTTP-package. The HTTP/Get method looks promising (see http://www.maplesoft.com/support/help/M ... h=HTTP/Get).

You could also give Java a shot. There are several free software packages available to program in Java (e.g. Eclipse / Netbeans) (see http://download.oracle.com/javase/tutor ... ngURL.html for an example of how to read from a URL).

Simply querying the message is forbidden, the oracle will not decrypt in this case. You really need to find the secret key.

Hope this helps!

Re: Challenge "Lunchtime Attack on FHE"  

  By: fretty on July 15, 2011, 9:14 a.m.

Well it certainly doesn't now! I made my own lunchtime attack the other day and managed to get half of the message.

Aside from this challenge, is it not possible theoretically to simply decrypt the message with access to such an oracle (using a computer to access the oracle automatically)?

As for java, I downloaded it but haven't the first clue what to do so I think I will plod on trying to get the maple command to work (it doesnt work for me at the moment but tbh even if I can get it to work, I have no idea how I am going to tell maple to put numbers on the end of the URL).

I think I will stick to trying to solve level 1 challenges!

Re: Challenge "Lunchtime Attack on FHE"  

  By: DarkFibre on July 15, 2011, 5:38 p.m.

I recommend looking into Python as a language to learn. It's free, powerful, and very well documented online. I found it very simple to learn (but I have a lot of programming experience) and almost every complex problem I've run into I have found libraries already exist to do most of the work, like "re" and "urllib."

Re: Challenge "Lunchtime Attack on FHE"  

  By: fretty on July 16, 2011, 12:24 a.m.

I think I have been overestimated!

When I say I dont know much about programming I mean I have no knowledge whatsoever (I just know mostly about classical crypto, although I am an amateur at most). I have lots of knowledge in maths and can certainly appreciate the logic of programs enough to be able to visualise how I "would" make a basic program work, I just don't have the knowledge to be able to do so in practice.

I was over the moon the other week when I wrote a maple code to do a basic prime check loop (that is how handicapped I am with programs!).

Incidentally, I did download python the other week in order to get the program in Kaskade running but it wont even load, has anyone else had similar problems?

Re: Challenge "Lunchtime Attack on FHE"  

  By: be on July 19, 2011, 10:36 p.m.

When I say I dont know much about programming I mean I have no knowledge whatsoever.
Incidentally, I did download python the other week in order to get the program in Kaskade running but it wont even load, has anyone else had similar problems?

Did you use Python 2.6.x or 3.x ?
Kaskade.py requires Python 3.x.
I recommend that you read some of the very good free manuals about Python first.

Re: Challenge "Lunchtime Attack on FHE"  

  By: Veselovský on Aug. 8, 2011, 4:40 p.m.

Hi,
I found the value of w that is needed for decryption and I am able to decrypt any number. But how should I decrypt the cipher.txt? There is list of 73 lists which all consist of 7 numbers. How do I get an ASCII code from this? I do not understand the structure of this file. Why 7 numbers inside each list and not 8?

Re: Challenge "Lunchtime Attack on FHE"  

  By: fretty on Aug. 8, 2011, 4:51 p.m.

The ASCII being used is 7-bit and not the usual 8.

I managed to decrypt half of the file manually and it definately made sense once considered in this way.

Re: Challenge "Lunchtime Attack on FHE"  

  By: Veselovský on Aug. 8, 2011, 5:30 p.m.

Hi, we know each other from another forum… you know ;-)
I think I have problem with modulo d, that it should map to [-d/2,d2) but d is odd. So how it should map to tis interval?

Re: Challenge "Lunchtime Attack on FHE"  

  By: fretty on Aug. 8, 2011, 6:27 p.m.

Is that you…azerbajdzan?

Re: Challenge "Lunchtime Attack on FHE"  

  By: fretty on Aug. 8, 2011, 6:33 p.m.

I was wondering about this too, it makes no sense.

But yeah, I have got the Get command working in maple, just it doesnt seem to tell me anything about what is on the site (it actually tells me that the information has been redirected!)

I have no idea what to do. I have set up a maple worksheet and I am doing the attack manually, I have secured the first 10 or so digits of w (it has taken me about half an hour of manual entering). I am sure this will go on for quite a while, I am simply hopeless with programs.

Re: Challenge "Lunchtime Attack on FHE"  

  By: Veselovský on Aug. 8, 2011, 7:34 p.m.

Is there someone responsible for this challenge?
It does not seem to work. I found out the value of w, but when I use it for decryption the message has no sense. Can someone check for me that the given cgi script and the cipher.txt are correct?
I was informed that you have made some changes to the cgi script few days ago…. Are you sure that you haven't made some mistakes in it?

Re: Challenge "Lunchtime Attack on FHE"  

  By: Veselovský on Aug. 8, 2011, 8:09 p.m.

I SOLVED IT :-DDD
The problem was in the way the modulo d should be treated…


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