Hashing string into integer in Java applet - how does it work?

Question

I'm now working on a challenge from website http://www.net-force.nl/challenges/ and I stand before an interesting problem I can't solve. I'm not asking for the whole result (as it would be breaking the rules), but I need help with the programming theory of hash function.

Basically, it's based on Java applet with one textfield, where user has to enter the right password. When I decompile the .class file, one of the methods I get is this hash method.

string s contains entered password, immediately given to the method:

private int hash(string s)
{
   int i = 0;
   for(int j = 0; j < s.length(); j++)
   i += s.charAt(j);

   return i;
}

The problem is that the method returns integer as the "hash", but how can characters be converted to integer at all? I got an idea that maybe the password is a number, but it doesn't lead anywhere at all. Another idea talks about ASCII, but still nothing.

Thanks for any help or tips.

"Hashing string into integer in Java applet - how does it work?" It works in an applet, exactly the same way it does in a frame, on the command line, or in a servlet. Or to put that a different way, this question has nothing to do with applets. — Andrew Thompson
– Andrew Thompson, Commented Feb 19, 2012 at 18:38

Jon Skeet · Accepted Answer · 2012-02-19 18:28:50Z

4

The trick is that it's converting each character into an integer. Each character (char) in Java is a UTF-16 code unit. For the most part¹, you can just think of that as each character is mapped to a number between 0 and 65535 inclusive, in a scheme called Unicode. For example, 65 is the number for 'A', and if you'd typed in the Euro symbol, that would map to Unicode U+20AC (8364).

Your hashing function basically adds together the numbers for each character in the string. It's a very poor hash (in particular it gives the same results for the same characters regardless of ordering), but hopefully you'll get the idea.

¹ Things get trickier when you need to bear in mind surrogate pairs, where a single Unicode character is actually made up of two UTF-16 code units - that's for characters with a Unicode number of more than 65535. Let's stick to the basics for the moment though :)

edited Feb 19, 2012 at 18:28

answered Feb 19, 2012 at 18:17

Jon Skeet

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1 Comment

Miroslav Mares Over a year ago

OK, say that we have this character. fileformat.info/info/unicode/char/1c0/charset_support.htm (\u01C0). Does it mean, that when I convert it to integer it returns 448, as is written in decimal representation on that website?

Cratylus · Accepted Answer · 2012-02-19 18:37:59Z

0

The hash function you present is the simplest hashing function you could possibly right for a string.

It is easy to implement and really fast in its computation.

It is problematic though since it doesn't distributes the input well.

Assuming ASCII chars the hash can take values from 0 to 1016 since an ASCII char is between 0 - 127.
I.e. each character in the string is "treated" as its ASCII equivalent (For more advance analysis check @John's answer).

Anyway you should note that strings containing the same characters but in different order map to the same hash value with this function.Perhaps this is of interest to you in the challenge you are trying to attack (??)

answered Feb 19, 2012 at 18:37

Cratylus

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Miroslav Mares Over a year ago

Well, as far as I understand the applet, the purpose is that I have to find the right string (password) to write it into GET php request (like ?pass=something). And the way of recognizing the right password seems to be comparing its hash (as explained above) with another hash there. Well, if I'm right, the challenge is fairly more complicated than I thought. And - some characters are in Unicode, so it seems I work with UTF-16 representation.

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