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Japhs autopsies (2)

thibaultduponchelle profile image Tib ・3 min read

Con de MIME

This is a french joke 😃

Barrez vous, cons de MIMES

This Japh is from Reynold Scem

use MIME::Base64;(eval decode_base64('am9pbignJywobWFwe2NocigpfShncmVwL1xTLyxzcGxpdCgvKC4uLikvLCcwNzQxMTcxMTUxMTYwMzIwOTcxMTAxMTExMTYxMDQxMDExMTQwMzIwODAxMDExMTQxMDgwMzIxMDQwOTcwOTkxMDcxMDExMTQwNDQnKSkpKQ=='))=~/.*/;print$&
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This one... We can't guess what will happen when we eval this encoded string...

It uses actually 2 steps to produce the Japh, first base64 encoding, that we can check with base64 -d :

echo "am9pbignJywobWFwe2NocigpfShncmVwL1xTLyxzcGxpdCgvKC4uLikvLCcwNzQxMTcxMTUxMTYwMzIwOTcxMTAxMTExMTYxMDQxMDExMTQwMzIwODAxMDExMTQxMDgwMzIxMDQwOTcwOTkxMDcxMDExMTQwNDQnKSkpKQ==" | base64 -d
join('',(map{chr()}(grep/\S/,split(/(...)/,'074117115116032097110111116104101114032080101114108032104097099107101114044'))))
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From decoding I got the actual Perl code to evaluate and I feel a bit better now 😁

The code Japh string creation code looks like this :

my $str = eval { join('', ( map {chr()} ( grep/\S/, split(/(...)/, '074117115116032097110111116104101114032080101114108032104097099107101114044')))) };
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Then in the perl code there is map, chr (convert value to char), grep \S (match all non whitespace), split(/(...)/, (split per 3 chars).

It's a nice showcase 😃 even if a lot of things are actually useless and only there to obfuscate the Perl code.

The Japh creation could be simply reduced to :

my $str = join('', map {chr()} ( split(/(...)/, '074117115116032097110111116104101114032080101114108032104097099107101114044')));
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The join seems useless but forces the string context.

So here is the complete Japh unfolded :

use MIME::Base64;

# Base 64 encoding
# am9pbignJywobWFwe2NocigpfShncmVwL1xTLyxzcGxpdCgvKC4uLikvLCcwNzQxMTcxMTUxMTYwMzIwOTcxMTAxMTExMTYxMDQxMDExMTQwMzIwODAxMDExMTQxMDgwMzIxMDQwOTcwOTkxMDcxMDExMTQwNDQnKSkpKQ==
# join('',(map{chr()}(grep/\S/,split(/(...)/,'074117115116032097110111116104101114032080101114108032104097099107101114044'))))

my $str = eval { join('', ( map {chr()} ( grep/\S/, split(/(...)/, '074117115116032097110111116104101114032080101114108032104097099107101114044')))) };
# Take 3 chars (...), do not keep space (\S), tranform to corresponding string representation, concat without space, eval (useless)

$str =~ /.*/; # Match everything
print $&; # The string matched by the last successful pattern match
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Maybe you noticed that there is also some fun with regex match and captured match print. It is just for fun ! 😃

Open Format Write

open(P,"|perl");print P"format=\nJust another Perl hacker,\n.\nwrite"
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After some ordering, it looks like this :

open(P,"|perl"); 
print P "
format=
Just another Perl hacker,
.
write"
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There are 2 tricks here :

  • The open filehandle to another perl process
  • The "format", an old Perl function that is less and less used nowadays

It is actually the same than :

format=
Just another Perl hacker,
.
write
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Kisses

Here is a simple but interesting japh with some kisses inside ! 😘 😘 😘 😘

@_=("Just another Perl hacker," =~ /(.*) (.*) (.*) (.*)/);print"@_";
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Kisses

This japh demonstrates default variable @_ and matching.

I think it can be rewritten like this :

my $str = "Just another Perl hacker,";
my @a = $str =~ /(.*) (.*) (.*) (.*)/;
print join(" ", @a) . "\n";
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Perl developers should be comfortable with this form 😄

It's probably time to address regex greediness.

This kind of greedy .* matches is "dangerous" (depends what you want), because it will try to match as much as possible.

For instance "Just another Perl 7 hacker," with the greedy version will produce a string split like the following :

Just another Perl 7 hacker, (because the .* happily eats also the space)

x25

Let's have a look at a simple japh using printf, x and ASCII conversion.

printf "%c"x 25,74,117,115,116,32,97,110,111,116,104,101,114,32,80,101,114,108,32,104,97,99,107,101,114,44;
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The operator x is magic :

printf "perl " x 10
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Will produce :

perl perl perl perl perl perl perl perl perl perl 
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In the japh, the x 25 is applied to "%c", therefore it can be translated like this :

printf("%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c",74,117,115,116,32,97,110,111,116,104,101,114,32,80,101,114,108,32,104,97,99,107,101,114,44);
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Arrow here

Again a printf "%c" but this time the input being in hexadecimal and the whole thing being stored in a string assignment :

$_ = <<'-- '; s/../printf "%c",hex($&)/ge;
4a75737420616e6f74686572205065726c206861636b65722c
-- 
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It starts with a "here-doc" delimited by -- (dash + dash + space not represented by DevTo).

What is weird, is that the whole thing looks only like a string assignation...

Actually, the printf inside the regex will print to stdout. At the end $_ does not even contains the string, but sprintf would have done the job.

And what is exactly s/../printf "%c",hex($&)/ge ?

  • s/// is for substitution.
  • $& is what have been matched.
  • The /e modifier allows the right part of the substitution to be actually executed as normal Perl code.
  • The /g modifier is the "looping modifier" or "global modifier" and just repeats the match along the string !

Finally, I propose this rewrite of the japh :

my $str = "4a75737420616e6f74686572205065726c206861636b65722c";
while($str =~ /../g) { # Or $str =~ s/..//
       printf("%c", hex($&));
}
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Cristal clear like this isn't it ?

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