question: why isn't a byte of a hash more uniform? how could I improve my code to cure that?

László Sándor SandorL at gmail.com
Fri Aug 7 20:48:16 CEST 2009


Thank you, Tim. My comments are below.

On 2009-08-07 13:19:47 -0400, Tim Chase <python.list at tim.thechases.com> said:

>> After I have written a short Python script that hashes my textfile line by
>> line and collects the numbers next to the original, I checked what I got.
>> Instead of getting around 25% in each treatment, the range is 17.8%-31.3%.
> 
> That sounds suspiciously like 25% with a +/- 7% fluctuation one might 
> expect to see from non-random source data.

Could you help me where this range comes from? (If all my lines were 
"42", I wouldn't hit this range. So it cannot be a rule. Right?)

> 
> Remember that your outputs are driven purely by your inputs in a 
> deterministic fashion -- if your inputs are purely random, then your 
> outputs should more closely match your expected bin'ing.  If your 
> inputs aren't random, you get a taste of your own medicine ("my file 
> has just the number 42 on every line...why isn't my output random?").  
> And randomness-of-hash-output is a red herring since hashing is *not* 
> random.

Thanks, I tried to be correct with "pseudo-random". I understand that 
everything is dependent on input. I want it to be the case. However, I 
hoped that good hashes produce random-looking output from input with 
very little variation. It would be strange if I could not get more than 
18% of lines with a remainder of 3 (after division by 4), whatever hash 
I try just because these are names of people.

> 
> Your input is also finite -- an aspect which leaves you a far cry from 
> the full hash-space.  If an md5 has 32 bytes (256 bits) of data, your 
> input would have to cover 2**256 possible inputs to see the full 
> profile of your outputs.  That's a lot of input :)
> 
> -tkc

OK, I understand. Could anyone suggest a better way to do this, then?

(Recap: random-looking, close-to uniform assignment of one number out 
of four possibilities to strings.)

Thanks, everyone.

Laszlo




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