C#•5d ago

Hash collision checkin

I wanted to do some fun testing with SHA256 and ya how do i do collision checkin? 😛

29 Replies

Denis•5d ago

You can use a HashSet<T> or a Dictionary<TKey, TValue>. Add the created hash to these collections and when creating a new hash, check if said collections already contain it

Jimmacle•5d ago

a collision just means 2 inputs produced the same hash so to check for them, check for duplicate hashes over your inputs

mtreit•5d ago

You will never see a hash collision with SHA256 so this is kind of an irrelevant question.

Petris•5d ago

well that's not true every hash will have a collision at some point just for now there are no known ones for sha256

mtreit•5d ago

I mean in practice. He was trying to find one. It's not going to happen for him.

Yesn'tOP•4d ago

well what about j4f ikr i just dont know how im gonna do it more than a while true and feed random stuff in i tried to do so, it ate my RAM very quickly so im just looking for suggestion from people, one might have better approach than me

Jimmacle•4d ago

what is your end goal?

Yesn'tOP•4d ago

see if any collision appear, by accident

Jimmacle•4d ago

but why

Yesn'tOP•4d ago

i dont actually know

Jimmacle•4d ago

it's more likely that an asteroid will hit earth and kill you in the next second than you finding a sha256 collision

Yesn'tOP•4d ago

every hash will have a collision at some point

wanna find my luck

Jimmacle•4d ago

i think there are more productive things to do as you've already found, you don't have the resources to do this in a meaningful way

Yesn'tOP•4d ago

does it have to really be meaningful?

Jimmacle•4d ago

if you don't want to waste your time, yeah

Yesn'tOP•4d ago

i have too much time to waste just wanna invest some into a j4f project

Jimmacle•4d ago

i mean if it's fun watching nothing happen and spiking your power bill go for it not sure what you need our help for

Yesn'tOP•4d ago

i want an efficient way more than while true and fill a hashset with results

Jimmacle•4d ago

then try to find one

Yesn'tOP•4d ago

im finding one while waiting for an answer here i want different answers so i can learn something

Jimmacle•4d ago

consider how many computers are mining bitcoin, which basically depends on the uniqueness of sha256 hashes all that compute hasn't found a hash collision yet

Yesn'tOP•4d ago

so you know im doing it for no real purpose i dont have a purpose im just doing it for fun

Jimmacle•4d ago

but my point is, even for fun you can't do it you don't have the memory to store all the inputs and hashes you're creating

Yesn'tOP•4d ago

i know i cant but why dont just try it?

Jimmacle•4d ago

i can't stop you, i can only tell you there's no point if it's fun for you it's fun for you

Yesn'tOP•4d ago

i absolutely know that there's absolutely no point

Jimmacle•4d ago

well nobody will have better suggestions good luck i guess

GrabYourPitchforks•4d ago

using System;
using System.Collections.Generic;
using System.Numerics;
using System.Security.Cryptography;

BigInteger UPPER_BOUND = BigInteger.Pow(2, 256);
Dictionary<string, BigInteger> seenHashes = new();

for (BigInteger i = 0; i <= UPPER_BOUND; i++)
{
    byte[] iHashed = SHA256.HashData(i.ToByteArray());
    string iHashedAsString = BitConverter.ToString(iHashed);
    if (!seenHashes.TryAdd(iHashedAsString, i))
    {
        Console.WriteLine($"Collision seen: {seenHashes[iHashedAsString]} and {i}");
        return;
    }
}

using System;
using System.Collections.Generic;
using System.Numerics;
using System.Security.Cryptography;

BigInteger UPPER_BOUND = BigInteger.Pow(2, 256);
Dictionary<string, BigInteger> seenHashes = new();

for (BigInteger i = 0; i <= UPPER_BOUND; i++)
{
    byte[] iHashed = SHA256.HashData(i.ToByteArray());
    string iHashedAsString = BitConverter.ToString(iHashed);
    if (!seenHashes.TryAdd(iHashedAsString, i))
    {
        Console.WriteLine($"Collision seen: {seenHashes[iHashedAsString]} and {i}");
        return;
    }
}

Ignoring pesky things like OOMs and random solar flares flipping bits of your computer's memory, this is guaranteed to find a collision. It's an implementation of Denis's idea at the top of the thread. We'll ignore for now that Dictionary can't hold 2^256 elements, since you'll OOM well ahead of that point. 😉 You'll see that I put an upper bound on this loop. Since each iteration of the loop increments i, we can hopefully all agree that this program will eventually terminate. Ignoring things like OOMs, solar flares, dictionary limitations, etc., here's a question left to the reader: is this program guaranteed to hit the Console.WriteLine block before it terminates, or might it terminate without writing anything to the console (by simply exhausting the loop condition)?

mtreit•4d ago

It should be guaranteed to print a collision before the loop terminates, no?

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