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Thread: Unhashing possible?

  1. #1
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    Default Unhashing possible?

    Alright guys, I want to unhash some names. But I need code to unhash it, I'm NOT begging you for giving me that code, I just want to know if it is possible to unhash this. I would have no idea how, so that's also why I'm asking you

    Would it be possible to reverse the name.charAt(j) code? Because that's the hardest part.

    	public int getHash(String name) {
    		int hash = 0;
    		name = name.toUpperCase();
    		for (int j = 0; j < name.length(); j++)
    			hash = (hash * 61 + name.charAt(j)) - 32;
    		return hash;
    	}

    Thanks, Endian.
    Last edited by Endian; January 24th, 2012 at 01:35 PM.

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    Super Moderator pbrockway2's Avatar
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    Default Re: Unhashing possible?

    Would it be possible to reverse the name.charAt(j) code?
    Given a string and a value of j, charAt(j) tells you the j-th character of the string. The "reverse" process (I am guessing, you don't say) is to set the j-th position to some given character. Strings are immutable, but you have the StringBuilder class which does allow you to set the character at any given position. So to reverse the hash you might try builing up a StringBuilder instance, setting the characters as you find them, then, at the end, forming a string from the string builder.

    Because that's the hardest part.
    But there are more strings than Java ints! I suspect that reversing charAt() is the easiest part of unhashing, much like using a funnel to pour the contents into a glued together eggshell is the easiest part of unscambling an egg.

    But, certainly it should be reasonably easy to come up with some string that hashes to a given value.

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    Default Re: Unhashing possible?

    Hashing is not like encryption, where all the information of the original value is captured and can be recaptured. Your hash is built upon the values of a String, and given you don't know those values when unshashing you don't have an easy way to decipher the hash. If you want reversibility, use an encryption algorithm or store the hash and String in a Map.

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    Super Moderator Norm's Avatar
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    Default Re: Unhashing possible?

    Hashing is a technique to map long numbers into shorter numbers. By definition there can be collisions, which means one hashcode can have many numbers that it was computer from.

    By numbers I mean any string of bytes in a computer.

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    Default Re: Unhashing possible?

    Thanks all! That really helped.