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what is the translation?

by roadtest (Sexton)
on Jul 20, 2011 at 02:51 UTC ( #915568=perlquestion: print w/replies, xml ) Need Help??
roadtest has asked for the wisdom of the Perl Monks concerning the following question:

Inside an solaris utility(change x86 hostid), there is some perl code as following:

echo "0x0ddb00b5" | /usr/bin/perl -e 'print("\""); while(<STDIN>){chop +;tr/!-~/P-~!-O/;print $_;} print("\"\n"); exit 0;'

What is the "tr/!-~/P-~!-O/" doing? I have difficulty to find the explanation.

Thanks in advance!

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Re: what is the translation?
by BrowserUk (Pope) on Jul 20, 2011 at 03:23 UTC
    What is the tr/!-~/P-~!-O/ doing?

    tr is perl's transliteration function.

    It translates the characters in the string it is applied to, according to the two tables of characters given as its arguments.

    For $string =~ tr/123/ABC/, it would change any and all occurrences of the characters '1','2', or '3', for the characters 'A', 'B', and 'C' respectively. Ie. If $string contained "a1b2c3c3b2a1", after the above code it would then contain "aAbBcCcCbBaA".

    In your example, the first list is specified as: !-~ which is a short-hand notation meaning all the characters between '!' and '~', That is all the visible characters in the (7-bit) ASCII character set. Eg:

    !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdef +ghijklmnopqrstuvwxyz{|}~

    The second list, P-~!-O specifies the same characters, but in a different order, as two ranges. 'P' through '~' and '!' through 'O'. Effectively,

    PQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~!"#$%&'()*+,-./01234567 +89:;<=>?@ABCDEFGHIJKLMNO

    The effect of these tables is that '!' will be translated to 'P' (and 'P' will become '!'); '"' will become 'Q' (and 'Q' becomes '"') and so on.

    This is a reversible (obfuscation) process similar to ROT13. Effectively "ROT47".

    However, why this is being applied to the input data, your guess is as good as mine. Probably better :)

    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.
      Awesome! Thanks for your reply!
Re: what is the translation?
by Khen1950fx (Canon) on Jul 20, 2011 at 05:10 UTC
    I can confirm that it's ROT47. I translated the regexp into simple perl:
    #!/usr/bin/perl use strict; use warnings; use Data::Dumper::Concise; my $string = '0x0ddb00b5'; my $rot = rot47($string); print Dumper($rot); sub rot47 { my $str = shift; $str =~ tr/!-~/P-~!-O/; return $str; } exit 0;
    The output is the same as your shell script:
      I translated the regexp into simple perl

      What regexp?    There is no regular expression in either the question or your reply.

      Thanks for the effort! I didn't know rot47 is a encryption method of ASCII text file.

        I didn't know rot47 is a encryption method of ASCII text file.

        s/encryption/obsfucation/. This is a very weak method that is not secure from anyone but the most honest of users.


Re: what is the translation?
by jwkrahn (Monsignor) on Jul 20, 2011 at 06:52 UTC
    echo "0x0ddb00b5" | /usr/bin/perl -e 'print("\""); while(<STDIN>){chop +;tr/!-~/P-~!-O/;print $_;} print("\"\n"); exit 0;'

    That could be written more simply as:

    echo "0x0ddb00b5" | /usr/bin/perl -lne'tr/!-~/P-~!-O/; print qq/"$_"/'

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