CODEWITHSUNDEEP
c#.nethash
Jishan
Jishan

Reputation: 1684

Finding Collisions In The Hash

So for a certain task I was asked to find collisions for the first n-bits (n will be 4, 8 ...) of a SHA256 digest between 2 random 64 byte sequence, which I do viz.

SHA256Managed hashstring = new SHA256Managed();
byte[] hash_1 = hashstring.ComputeHash(a);
...

I compute the string value for the hashes viz.

string hashString = string.Empty;
foreach (byte x in hash)
{
    hashString += String.Format("{0:x2}", x);
}
return hashString;

And I use the string matching function to first extract, say the first 8 bits:

string first8Char_hash1 = new string(hashDisplay(hash_1).Take(8).ToArray());

check if the first say, 8 bits match or not, viz.

if (first8Char_hash1.Equals(first8Char_hash2))
{
    //Do something
}

Now, if I do, say the first 4 bits, I find a collision within say an average 12,000 iterations, however, for the first 8 bits, well I am unable to find a collision even after 1,00,00 tries.

What's the problem? Thanks.

Upvotes: 0

Views: 271

Answers (1)

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 726987

When you print a number as hex, each 4 bits become one hex character. If you would like to find collisions among the first 4 bits, you compare one character, not four. Four characters correspond to 16-bit collisions.

For eight bits, compare the first two characters of the hex string.

Upvotes: 2

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