Finding a String with the same hashcode() value as given String

Question

I'm trying to find a string that shares the same hash result as a given string.

For Example:

If I have the String 'den' and use Java's hashcode() method the hash result = 99,341

String s = "den";
System.out.println(s.hashCode());
Result = 99,341

Knowing that the hashcode() method computes

s[0]*31^(n-1) + s[1]*31^(n-2) + ... + 2[n-1]

How would I go about taking this result (99,341) and finding a string (of the same length) as 'den' with the equivalent hash value?

Create a dictionary of String words and their associated hashcode. You can't reverse the hash formula because you don't know the word length. — Gilbert Le Blanc
– Gilbert Le Blanc, Commented Aug 29, 2021 at 23:13
The word length is the same as the given String, so 3 in this case — Wyrmbeard
– Wyrmbeard, Commented Aug 29, 2021 at 23:24

Jonatan Ivanov · Accepted Answer · 2021-08-29 23:29:05Z

2

You need to solve this equation:

(x * 31^(3-1)) + (y * 31^(3-2)) + (z * 31^(3-3)) = 99341

It's a plane, every point in that plane is good for you as long as x,y,z are integers and between 0 and 255 (if you are only talking about ASCII).

One possible solution is x=100, y=101, z=110. To simply find another one, you can just change the order of two of them and see what would be the third one, e.g.:

x=101
y=???
z=110

(101 * 31^(3-1)) + (y * 31^(3-2)) + (110 * 31^(3-3)) = 99341

where y=70 (F) so eFn should have the same hashCode as den using the function in your question.

edited Aug 29, 2021 at 23:29

answered Aug 29, 2021 at 23:27

Jonatan Ivanov

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