Filter::decrypt - phpMan

NAME
    Filter::decrypt - template for a decrypt source filter

SYNOPSIS
        use Filter::decrypt ;

DESCRIPTION
    This is a sample decrypting source filter.

    Although this is a fully functional source filter and it does implement
    a *very* simple decrypt algorithm, it is *not* intended to be used as it
    is supplied. Consider it to be a template which you can combine with a
    proper decryption algorithm to develop your own decryption filter.

WARNING
    It is important to note that a decryption filter can *never* provide
    complete security against attack. At some point the parser within Perl
    needs to be able to scan the original decrypted source. That means that
    at some stage fragments of the source will exist in a memory buffer.

    Also, with the introduction of the Perl Compiler backend modules, and
    the B::Deparse module in particular, using a Source Filter to hide
    source code is becoming an increasingly futile exercise.

    The best you can hope to achieve by decrypting your Perl source using a
    source filter is to make it unavailable to the casual user.

    Given that proviso, there are a number of things you can do to make life
    more difficult for the prospective cracker.

    1.   Strip the Perl binary to remove all symbols.

    2.   Build the decrypt extension using static linking. If the extension
         is provided as a dynamic module, there is nothing to stop someone
         from linking it at run time with a modified Perl binary.

    3.   Do not build Perl with "-DDEBUGGING". If you do then your source
         can be retrieved with the "-DP" command line option.

         The sample filter contains logic to detect the "DEBUGGING" option.

    4.   Do not build Perl with C debugging support enabled.

    5.   Do not implement the decryption filter as a sub-process (like the
         cpp source filter). It is possible to peek into the pipe that
         connects to the sub-process.

    6.   Check that the Perl Compiler isn't being used.

         There is code in the BOOT: section of decrypt.xs that shows how to
         detect the presence of the Compiler. Make sure you include it in
         your module.

         Assuming you haven't taken any steps to spot when the compiler is
         in use and you have an encrypted Perl script called "myscript.pl",
         you can get access the source code inside it using the perl
         Compiler backend, like this

             perl -MO=Deparse myscript.pl

         Note that even if you have included the BOOT: test, it is still
         possible to use the Deparse module to get the source code for
         individual subroutines.

    7.   Do not use the decrypt filter as-is. The algorithm used in this
         filter has been purposefully left simple.

    If you feel that the source filtering mechanism is not secure enough you
    could try using the unexec/undump method. See the Perl FAQ for further
    details.

EXAMPLES
    See /usr/share/doc/libfilter-perl/examples/decrypt/ for examples of
    encryption and decryption scripts using this filter.

AUTHOR
    Paul Marquess

DATE
    19th December 1995