Beefy Boxes and Bandwidth Generously Provided by pair Networks
Just another Perl shrine
 
PerlMonks  

Re^3: Get the order of HTTP request headers

by WizardOfUz (Friar)
on May 06, 2010 at 10:37 UTC ( #838678=note: print w/ replies, xml ) Need Help??


in reply to Re^2: Get the order of HTTP request headers
in thread Get the order of HTTP request headers

First, the order of values returned by hashes isn't random, at least not in any formal sense of the word. There's definitely no attempt to make them random as implied by saying the order is "randomized".

I always assumed that the order is indeed "randomized". Well, I guess I misunderstood perlsec and perlrun. Or maybe not.


Comment on Re^3: Get the order of HTTP request headers
Re^4: Get the order of HTTP request headers
by ikegami (Pope) on May 06, 2010 at 17:00 UTC

    The perlsec passage to which you linked says the order shouldn't be assumed to be random. The perlrun passage to which you link doesn't explicitly mention changing key orderings at all. It is not the purpose of the feature to alter the key ordering, much less making it random.

    Two problems with your assumption:

    • The feature in question only kicks in when needed.

    • Even if you could reliably trigger the feature, it's purpose is to salt the hashing algorithm (i.e. perturb bucket selection). While this has the side effect of affecting key ordering, it hasn't been shown to produce random key orderings.

    But enough theory. Let's look at a practical example:

    for (1..100) { my %h; $h{$_} = 1 for 'a'..'z'; print(join('', keys(%h)), "\n"); }
    wraxdjyukhgftienvmslcpqbzo wraxdjyukhgftienvmslpcqbzo wraxdjyukhgftienvmslpcqbzo wraxdjyukhgftienvmslpcqbzo wraxdjyukhgftienvmslpcqbzo wraxdjyukhgftienvmslpcqbzo ... wraxdjyukhgftienvmslpcqbzo wraxdjyukhgftienvmslpcqbzo

    And the same if I run the program again.

      A simple question: Are the results of your example predictable for any combination of hash keys without knowing the hash seed?

        Yes.

        Without knowing the seed or the hashing algorithm, I correctly predicted 99% of the results the first time I ran the program, and I correctly predicted 100% results the second time I ran the program.

        The hash was never reseeded since the pathological case was never encountered.

        In a more casual sense, the order is unpredictable. It just hasn't been shown to be random. Even if the hash had been reseeded.

Log In?
Username:
Password:

What's my password?
Create A New User
Node Status?
node history
Node Type: note [id://838678]
help
Chatterbox?
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others lurking in the Monastery: (7)
As of 2014-10-25 20:45 GMT
Sections?
Information?
Find Nodes?
Leftovers?
    Voting Booth?

    For retirement, I am banking on:










    Results (148 votes), past polls