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Minimum of two values with minimum knowledge (jlai.lu)

submitted 11 months ago by payasson@jlai.lu to c/cybersecurity@sh.itjust.works

8 comments fedilink hide all child comments

Hey! I program a lot but I'm not very good with cybersecurity and stuff, although I have a basic usage of GPG and asymetrical encryption.

My problem is:

Let's imagine that Alice (A) and Bob (B) each have a file with a number written in it.

Ideally, I'd like a program that A can run on her computer that would take the B file but encrypted, and output the minimum of the two values contained in A and B files.

But without any way for A to know what the number of B is, except if B value is the minimum, obviously.

Can someone help me with that? Thank you for reading!

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[–] Khanzarate@lemmy.world 5 points 11 months ago (5 children)

If A can run this program at will and it determines the minimum value, it's O(log(n)) to determine what B is, even with perfect encryption, by using arbitrary values of A.

INT X = MAX INT PREV_X = 0 BOOL B_IS_MIN = True

While (X != PREV_X){

PREV_X = X B_IS_MIN = Encrypted_Min(X,B)

If(B_IS_MIN), X = X/2 If(!B_IS_MIN), X = X*1.5

}

Unless I've made a typo, this psuedocode will step to B in log time, and will break the while loop once it's found, even if the user has no way to know the value of B besides the minimum.

[–] payasson@jlai.lu 0 points 11 months ago (4 children)

Indeed, I didn't think of that. And would it be possible to allow only one check and destroy/make the information of B unusable after this check?

Thank you for your reply!

[–] mattreb@feddit.it 3 points 11 months ago (1 children)

With some more restrictions, you may want to look into oblivious transfer https://yewtu.be/watch?v=wE5cl8J27Is

[–] payasson@jlai.lu 0 points 11 months ago

that's interesting indeed! thank you very much!

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