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Careful Evaluation

by iamcal (Friar)
on Jun 14, 2001 at 18:47 UTC ( #88437=obfuscated: print w/replies, xml ) Need Help??

A bit of a silly one. I dare somebody to deconstruct it :)
$$=q{$_=unpack'u',q|L)%\]<2$D7STG<')I;G0@(DIU<W0@86YO=&AE<B! 097)L(&AA8VME<B(G(3L`|;};$}=q{$_=unpack'u',q|<)%\])R1?/7$A*" 1?*3TH)"0]?B\N*B]G*2$G.P``|;};$$=~s;\n;;g;$;=q;$_=$};;$_='$_ =$;;';$}=~s;\n;;g; eval;eval;eval;eval;eval;eval;eval;eval;eval;eval;

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Re: Careful Evaluation
by VSarkiss (Monsignor) on Jun 15, 2001 at 00:49 UTC
    All right, nobody else has, so I'll take the bait. ;-)

    For those who may not have seen them before (*cough* young whipper-snappers), uuencode and uudecode are encoding and decoding mechanisms that were originally developed to allow transmitting binary files via uucp over lines that were not always eight-bit-clean. Just like MIME.

    Now, on to the real decode. I've separated the lines to make them a little easier to read.

    # # Set $$ -- usually the PID of the running Perl, but not a read-only # variable -- to a strange string: first part looks like code, # but latter half looks like junk. If you uudecode the junk (minus # the embedded newline) you get: # # $_=q!$_='print "Just another Perl hacker"'!; # $$ = q{$_=unpack'u',q|L)%\]<2$D7STG<')I;G0@(DIU<W0@86YO=&AE<B! 097)L(&AA8VME<B(G(3L`|;}; # # Set $} -- another strange variable -- to another strange string. # This time the uuencoded part (again minus \n) is # # $_='$_=q!($_)=($$=~/.*/g)!'; # $} = q{$_=unpack'u',q|<)%\])R1?/7$A*" 1?*3TH)"0]?B\N*B]G*2$G.P``|;}; # # The next few lines remove embedded newlines in $$ and $} (first # and fourth lines), and set these variables to these values: # $; $_=$}; (second line) # $_ $_\n=$;; (third line, including embedded newline) # $$=~s;\n;;g; $;=q;$_=$};; $_='$_ =$;;'; $}=~s;\n;;g; # # Each of these evals executes the code in $_. Before the first, # $_ contains "$_\n=$;;" as shown above. Here are the values of $_ # after each eval completes. # eval; # $_=$}; eval; # $_=unpack'u',q|<)%\\])R1?/7$A*"1?*3TH)"0]?B\\N*B]G*2$G.P``|; eval; # $_='$_=q!($_)=($$=~/.*/g)!'; eval; # ($_)=($$=~/.*/g) # BTW, the parens and "=~ /.*/g" do nothing. # Executing this is the same as "$_ = $$". eval; # $_=unpack'u',q|L)%\\]<2$D7STG<\')I;G0@(DIU<W0@86YO=&AE<B!097)L(& +AA8VME<B(G(3L`|; eval; # $_=q!$_='print "Just another Perl hacker"'!; eval; # $_='print "Just another Perl hacker"' eval; # print "Just another Perl hacker" eval; # The final eval just executes the contents of $_ seen above. eval;
    BTW, if you try to execute this in the debugger, beware: setting $$ causes it to fork. Fiendishly clever!
Re: Careful Evaluation
by Brovnik (Hermit) on Jun 15, 2001 at 01:07 UTC
    I dare somebody to deconstruct it :)

    OK. First, tidy up syntax and delimiters and rename some variables.

    $$ = q{$_=unpack'u',q|L)%\]<2$D7STG<')I;G0@(DIU<W0@86YO=&AE<B!097)L(&A +A8VME<B(G(3L`|;}; $b = q{$_=unpack'u',q|<)%\])R1?/7$A*"1?*3TH)"0]?B\N*B]G*2$G.P``|;}; #$$ =~ s/\n//g; $a = '$_=$b'; $_ = '$_=$a'; #$} =~ s/\n//g; eval; eval; eval; eval; eval; eval; eval; eval; eval; eval;
    The commented lines aren't needed if the q|| are on one line.

    So, (this is easier to read starting from the bottom of the tidied code) the first 3 evals set $_ to $a and then $b then eval the unpack inside $b.

    The unpack produces $_='$_=q!($_)=($$=~/.*/g)!';, which, after 3 more iterations sets $_ to $$ (the unpack on the first line).
    This unpack sets $_=q!$_='print "Just another Perl hacker"'!;, which, after another 2 iterations gives us the familiar
    print "Just another Perl hacker" which duly evals.

    Hope that is sufficient explanation.

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