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Update: *sigh* Apparently, the READMORE tag has some issues. My apologies to all for the long post.

Well, this is the first obfu from Erudil that I was able to decipher, so I thought I'd break it down and write the spoiler.


#!/usr/bin/perl -w # how to (ab)use substr use strict; my $pi='3.14159210535152623346475240375062163750446240333543375062'; substr ($^X,0)= substr ($pi,-6);map{ substr ($^X,$.++,1)=chr( substr($pi,21,2)+ substr($pi,$_,2))}(12,28,-18,-6,-10,14);map{$^O=$"x( substr ($pi,-5,2)); substr ($^O,sin(++$a/8)*32+ substr ($pi,-2)/2+1,1)=$_; substr ($^O,sin($a/4)*( substr ($pi,2,2))+ substr ($pi,-7,-5)-1,1)=$_;print"$^O$/";eval($^X.('$b,'x3). substr ($pi,-3,1).'.'. substr ($pi,9,2));}(map{chr($_+ substr ($pi,21,2))}( substr ($pi,8)x3)=~/../g);


Let's clean up the formatting a bit first:

substr($^X,0)=substr($pi,-6); map{ substr($^X,$.++,1)=chr( substr($pi,21,2)+ substr($pi,$_,2)) }(12,28,-18,-6,-10,14); map{ $^O=$"x(substr ($pi,-5,2)); substr($^O,sin(++$a/8)*32+ substr($pi,-2)/2+1,1)=$_; substr($^O,sin($a/4)*( substr($pi,2,2))+ substr($pi,-7,-5)-1,1)=$_; print"$^O$/"; eval($^X.('$b,'x3). substr($pi,-3,1).'.'. substr($pi,9,2)); }(map{ chr($_+ substr($pi,21,2)) }(substr($pi,8)x3)=~/../g);


Let's remove some of the pesky maps

substr($^X,0)=substr($pi,-6); foreach (12,28,-18,-6,-10,14) { substr($^X,$.++,1)=chr( substr($pi,21,2)+ substr($pi,$_,2)); } # We need this for the moment to get # rid of the second map below. # Don't worry, it goes away. my @tempa; foreach (substr($pi,8)x3)=~/../g) { push(@tempa,chr($_+substr($pi,21,2))); } map{ $^O=$"x(substr ($pi,-5,2)); substr($^O,sin(++$a/8)*32+ substr($pi,-2)/2+1,1)=$_; substr($^O,sin($a/4)*( substr($pi,2,2))+ substr($pi,-7,-5)-1,1)=$_; print"$^O$/"; eval($^X.('$b,'x3). substr($pi,-3,1).'.'. substr($pi,9,2)); }(@tempa);


Now, most of the substr($pi,....) statements return the same value each time. Let's just replace those statements with their values.

substr($^X,0)=375062; foreach (12,28,-18,-6,-10,14) { substr($^X,$.++,1)=chr( 64+ substr($pi,$_,2)); } # We need this for the moment to get # rid of the second map below. # Don't worry, it goes away. my @tempa; foreach (substr($pi,8)x3)=~/../g) { push(@tempa,chr($_+64)); } map{ $^O=$"x(75); substr($^O,sin(++$a/8)*32+ 62/2+1,1)=$_; substr($^O,sin($a/4)*( 14)+ 33-1,1)=$_; print"$^O$/"; eval($^X.('$b,'x3). 0.'.'. 05); }(@tempa);


If we look at the first foreach, we can see that we are replacing the numerical values of $^X with character values, one by one. The resulting value of $^X is 'select', so we can eliminate the first foreach entirely (and do some more tidying).

$^X="select"; # We need this for the moment to get # rid of the second map below. # Don't worry, it goes away. my @tempa; foreach (substr($pi,8)x3)=~/../g) { push(@tempa,chr($_+64)); } map{ $^O=$"x75; substr($^O,sin(++$a/8)*32+32,1)=$_; substr($^O,sin($a/4)*14+32,1)=$_; print"$^O$/"; eval($^X.('$b,'x3).'0.05'); }(@tempa);


Now, we can finally get rid of that last pesky map statement.

$^X="select"; foreach (substr($pi,8)x3)=~/../g) { $_ = chr($_+64); $^O=$"x75; substr($^O,sin(++$a/8)*32+32,1)=$_; substr($^O,sin($a/4)*14+32,1)=$_; print"$^O$/"; eval($^X.('$b,'x3).'0.05'); }


Just for the sake of simplification, let's remove that eval statement as well, since it's really a select.

foreach (substr($pi,8)x3)=~/../g) { $_ = chr($_+64); $^O=$"x75; substr($^O,sin(++$a/8)*32+32,1)=$_; substr($^O,sin($a/4)*14+32,1)=$_; print"$^O$/"; select $b,$b,$b,0.05; }


Now it's fairly simple to tell what's going on. To make it even easier to see, let's replace the expression in the foreach. The combination of substr($pi,8)x3=~/../g and the $_ = chr($_+64) results in the string "Just~another~Perl~hacker~" repeated three times. Let's just add that to the foreach and clean up a bit more.

foreach (("Just~another~Perl~hacker~"x3)=~/./g) { $^O=$"x75; substr($^O,sin(++$a/8)*32+32,1)=$_; substr($^O,sin($a/4)*14+32,1)=$_; print"$^O$/"; select $b,$b,$b,0.05; }


That's about all we can do. From here, it should be fairly simple to divine what's happening. $^O gets set to 75 spaces. Then, the position of the next characters are determined by the sin function and placed into $^O with the substr. $^O is then printed followed by the local line-break character. The script then sleeps for 5 ms before printing the next line.

I hope I gave a good explanation. I know I skipped over several steps, but it should be apparent from step to step what I've changed. Anything that didn't change I didn't include from the original script. Yes $pi was tampered with, but I'd only expect PM users to notice that it's only good to the first 5 digits. :)

Guildenstern
Negaterd character class uber alles!

In reply to SPOILER! (Guildenstern)Re: How to (ab)use substr by Guildenstern
in thread How to (ab)use substr by Erudil

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