How to reverse a hashing function.

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Java

<pre>
public int hashKey(String str){
		int sum = 0;
		int exp = 1;
		for(int i = 0; i < str.length(); i++){
			int temp = str.charAt(i) * exp;
			sum += temp;
			exp *= 26;
		}
		
		return sum % 10;
}

What I have tried:

I have the code above which gives me the hash key for a word.
<<< I will assume that the ascii code for a=1 , b=2 , c=3 >>>

for example if I inserted the word "abc" the output will be 214%10 = 4

How can I get the first letter of this word "abc" without changing anything in the function

Posted 11-Apr-20 19:59pm

AbdelrahmanNassar

Updated 11-Apr-20 22:01pm

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Comments

Richard MacCutchan 12-Apr-20 4:32am

It should be obvious that given the number 4, you cannot guess that it is the hash of "abc". It could equally be the hash of many more strings.

2 solutions

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OriginalGriff · Answer 1 · 2020-04-11T20:24:00

You can't reverse hashing functions: they are not a form of encryption.
That's why hashes are always the same length regardless of the input length: they throw away information in order to generate a (hopefully) unique value from the input.

Encryption doesn't do that: so the size of encrypted data is both variable and often larger than the input data - it is not possible to create a fixed size output from an encryption function without padding it out to a specified length - and even then if the input exceeds that length, it will still require more space for the encrypted version.

Think about it: suppose your hash is "add them all together and take the remainder when divided by ten" Since there are only 10 possible results - 0 to 9 inclusive - you cannot tell from the result of 7 if the input was 7, 17, [1, 6], [2, 5], Or a massive set of other possible values.

[edit]
I thought I'd written up something about hash reversing ... and I had: Decrypting MD5 and SHA: Why You Can't Do It[^] - clearly, it's based around more complex hashes than yours, but it still applies and covers the "why" better than my comments above.
[/edit]

Patrice T · Answer 2 · 2020-04-11T22:02:00

Quote:
How to reverse a hashing function.

Short answer: you can't.
By design, a hash function can not be reversed.
See your code, for any string as input, there is only 10 different output.

Quote:
<<< I will assume that the ascii code for a=1 , b=2 , c=3 >>>

By the way, your code is wrong because a=97 , b=98 , c=99.

Quote:
How can I get the first letter of this word "abc" without changing anything in the function

Java

hashKey("a") == hashKey("k") == hashKey("u") == 7

from 7, how to choose between 'a', 'k' or 'u' ?

How to reverse a hashing function.

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How to reverse a hashing function.

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