decrypt(3)            User Contributed Perl Documentation           decrypt(3)

       Filter::decrypt - template for a decrypt source filter

           use Filter::decrypt ;

       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.

       It is important to note that a decryption filter can never provide com-
       plete 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 con-
            nects 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 "",
            you can get access the source code inside it using the perl Com-
            piler backend, like this

                perl -MO=Deparse

            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 fur-
       ther details.

       Paul Marquess

       19th December 1995

perl v5.8.0                       2003-01-27                        decrypt(3)