in reply to Brute Force Algorithm with Perl reg exprs?

You mentioned you wanted to recursion. Recursion (or more specifically, a stack) is the tool of choice to use if you want to do a depth-first search. However, a depth-first search is not optimal here, since people usually choose passwords much smaller then the maximum length. For example, 

# Depth-first search
# Max length = 20
1234
1234a
1234aa
1234aaa
1234aaaa
...
1234aaaaaaaaaaaaaaaa
1234b
1234bb
...

[download]

A breadth-first search would be more appropriate. For example, 

# Breadth-first search
1234
1234a
1234b
1234c
...
12348
12349
1234aa
1234ab
1234ac
...

[download]

Breadth-first search uses a queue, not a stack, so recursion is not appropriate. Regexps are also inappropriate because they strength is in backtracking, which is done using stacks. So let's look at a breadth-first solution: 

use strict;
use warnings;

sub crack {
   my $hint       = shift;
   my $target     = shift;
   my $salt       = substr($target, 0, 2);
   my @chars      = @_;
   my @to_process = ( '' );
   my $passwd;

   while (@to_process) {
      my $pad = shift(@to_process);

      push(@to_process, $pad . $_)
         foreach @chars;
   
      for my $pos (0..length($pad)) {
         $passwd = $pad;
         substr($passwd, $pos, 0, $hint);

         return $passwd
            if crypt($passwd, $salt) eq $target;
      }
   }
}

my @chars = ('a'..'z', 'A'..'Z', '0'..'9');

print crack('12',   crypt('d12e',    'AA'), @chars), "\n";
print crack('1234', crypt('aa12345', 'AA'), @chars), "\n";

[download]

Unfortunately, the above takes way too much memory. We need to find an interator which generates the next item to process on the fly. Since the to-process list is a simple incrementing sequence, it's not only possible to generate the next item on the fly, it's rather simple. The following code performs a breadth-first search using virtually no stack or heap memory:

Working solution withheld for a week, since you said this was for class.

Replies are listed 'Best First'.
Re^2: Brute Force Algorithm with Perl reg exprs?
by ikegami (Patriarch) on May 10, 2006 at 20:26 UTC
    Oops, it's been more than a week. Here's the promised code 
    use strict;
use warnings;

sub crack {
   my $hint   = shift;
   my $target = shift;
   my $salt   = substr($target, 0, 2);
   my @chars  = @_;
   my $base   = @chars;
   my $passwd;

   use integer;

   for (my $num_chars=0; ; $num_chars++) {
      for (my $enc=0; ; $enc++) {
         my $i = $enc;
         my $pad = '';
         for (1..$num_chars) {
            my $idx = $i % $base;
            $i = $i / $base;
            $pad .= $chars[$idx];
         }

         # Go add a character.
         last if $i;

        #$pad = reverse $pad;

         for my $pos (0..$num_chars) {
            $passwd = $pad;
            substr($passwd, $pos, 0, $hint);

            return $passwd
               if crypt($passwd, $salt) eq $target;
         }
      }
   }
}

my @chars = ('a'..'z', 'A'..'Z', '0'..'9');

print crack('12',   crypt('d12e',    'AA'), @chars), "\n";
print crack('1234', crypt('aa12345', 'AA'), @chars), "\n";

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