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