Why is this hash, created from a constant list, "in different order" each time the program is run?
hash, perl
Solution
The behaviour is documented in perlsec's Algorithmic Complexity Attacks.
A hash is an array of linked lists. A hashing function converts the key into a number which is used as the index of the array element ("bucket") into which to store the value. More than one key can hash to the same index ("collision"), a situation handled by the linked lists.
If a malicious user knew the hashing algorithm, he could devise values that would hash to the same index, causing the hash to degenerate into a linked list. This can lead to huge performance drops in some applications, and thus can be used as part of a denial of service (DoS) attack.
Two measures are taken to avoid that. One is to salt the hashing algorithm to randomize the order in which elements are stored, and the other makes it harder to detect the salt by perturbing the order in which the iterator visits the hash elements.
$ perl -E'
my @k = "a".."z";
for (1..3) {
my %h = map { $_ => 1 } @k;
say keys %h;
}
'
iocmbygdkranwxfejuqpzvltsh
bmcoigdywrankujfxezpqlvths
juexfwarnkgdybmcoihstlvzpq
Problem
Below is the small script in Perl. Every time I run this code I'm getting different output. Can anyone help me to understand the basics of storage of hash variables, that is how indexing is done for the key value pairs of Perl's hash variable. ``` #!/usr/bin/perl %data = ('John Paul' => 45, 'Lisa' => 30, 'Kumar' => 40); @names = keys %data; print "$names[0]\n"; print "$names[1]\n"; print "$names[2]\n"; ```