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

submitted 11 months ago by [email protected] to c/[email protected]

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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[–] [email protected] 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.

[–] [email protected] 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!

[–] [email protected] 1 points 11 months ago (1 children)

Possibly. I'm not a big crypto guy, but it's my understanding that any kind of transaction has a chance of being repeated. If there were a bad actor, and that bad actor used a VPN to swap identities, he could narrow this down considerably and weaken encryption. My code is as dumb as it gets, willing to consider 1 as a valid encryption key, but smarter code would be a lot more efficient.

On top of that, you wanted this minimum code to run on A's computer. If you do not trust A, then you've given a potential bad actor a program that could be decompiled to unencrypt your keys.

It sounds to me like in your current state, you need to trust A before you do this operation, and if you do, you can just share an unencrypted B.

[–] [email protected] 0 points 11 months ago

Alright thank you for your reply, I'll think about it :) maybe having a vérification that can be done in any computer and any amount of time is just not possible for my use case

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