Filter::decrypt - template for a decrypt source filter

NAME

Filter::decrypt - template for a decrypt source filter

SUPPORTED PLATFORMS

This module is not included with the standard ActivePerl distribution. It is available as a separate download using PPM.

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.

The best you can hope to achieve by decrypting your Perl source using a source filter is to make it impractical to crack.

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. 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.

AUTHOR

Paul Marquess

DATE

19th December 1995

 Filter::decrypt - template for a decrypt source filter