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Steve Hay


Filter::Crypto - Create runnable Perl files encrypted with OpenSSL libcrypto


    # Encrypt a Perl script using the crypt_file script. Run it as usual:
    $ crypt_file --in-place hello.pl
    $ hello.pl

    # Create a PAR archive containing an encrypted Perl script. Run it as usual:
    # (This example assumes that you also have PAR installed.)
    $ pp -f Crypto -M Filter::Crypto::Decrypt -o hello hello.pl
    $ hello

    # Display the Filter-Crypto distribution version number:
    use Filter::Crypto;
    print "This is Filter-Crypto $Filter::Crypto::VERSION\n";


The Filter-Crypto distribution provides the means to convert your Perl files into an encrypted, yet still runnable, format to hide the source code from casual prying eyes.

This is achieved using a Perl source code filter. The encrypted files, produced using the Filter::Crypto::CryptFile module, automatically have one (unencrypted) line added to the start of them that loads the Filter::Crypto::Decrypt module. The latter is a Perl source code filter that decrypts the remaining (encrypted) part of the Perl file on the fly when it is run. See perlfilter if you want to know more about how Perl source code filters work.

These two modules can be built and installed separately, so it is possible to set-up two separate Perl installations: one containing the Filter::Crypto::CryptFile module to be used for encrypting your Perl files, and another containing only the Filter::Crypto::Decrypt module for distributing with your encrypted Perl files so that they can be run but not easily decrypted. (Well, not very easily, anyway. Please see the WARNING below.)

Encrypted files can also be produced more conveniently using the crypt_file script, or (if you also have the PAR module available) using the PAR::Filter::Crypto module. The latter can be utilized by the standard PAR tools to produce PAR archives in which your Perl files are encrypted. The Filter::Crypto::Decrypt module (only) can also be automatically included in these PAR archives, so this is perhaps the easiest way to produce redistributable, encrypted Perl files.

The actual encryption and decryption is performed using one of the symmetric cipher algorithms provided by the OpenSSL libcrypto library. The EVP library high-level interface functions to the various cipher algorithms themselves are used so that your choice of algorithm (and also what password or key to use) is made simply by answering some questions when building this distribution. See the INSTALL file for more details.

This module itself only contains this documentation and the version number of the Filter-Crypto distribution as a whole.


Some people regard the whole area of Perl source code encryption as being morally offensive, given that Perl itself is open source. However, Perl's Artistic License does specifically allow the distribution of Perl "as part of a larger (possibly commercial) software distribution," and many people producing commercial Perl software are uneasy about distributing the source code in easily accessible form for anyone to see, and want to take more practical action than involving intellectual property rights lawyers.

That is where software like this comes in, but a word of warning is in order regarding the security provided by this (and, indeed, any other) source code decryption filter.

Some of the points below come from a discussion on the perl5-porters mailing list, in the thread starting here: http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/2003-10/msg01169.html; others are taken from the Filter::decrypt manpage.

In general, it is hopeless to try to prevent everyone from getting at the source code, especially when it is being run in an environment that you have no control over, and even more so when the software running it (Perl) is open source itself.

This technique can never completely hide the original unencrypted source code from people sufficiently determined to get it. The most it can hope for is to hide it from casual prying eyes, and to outdo everyone who is using a precompiled perl (at least from "regular" sources) and everyone who is not knowledgeable enough to suitably modify the Perl source code before compiling their own.

Perl source code decryption filters work by intercepting the source stream (read from the encrypted file) and modifying it (in this case, decrypting it) before it reaches the Perl parser. Clearly, by the time the source reaches the parser it must be decrypted, otherwise the script cannot be run. This means that at some stage every part of the script must be held in memory in an unencrypted state, so anyone with the appropriate debugging skills will be able to get it.

If perl was built with DEBUGGING then running the script with the perl's -Dp command-line option makes this much easier. Even without a DEBUGGING perl, the script can still be run under the Perl debugger (perl's -d command-line option), whose l command will list the (decrypted) source code that was fed to the parser.

In fact, with the introduction of the Perl compiler backend modules it is now easy to get at the decrypted source code without any debugging skills at all. To quote B::Deparse:

    B::Deparse is a backend module for the Perl compiler that generates perl
    source code, based on the internal compiled structure that perl itself
    creates after parsing a program.  The output of B::Deparse won't be exactly
    the same as the original source, since perl doesn't keep track of comments
    or whitespace, and there isn't a one-to-one correspondence between perl's
    syntactical constructions and their compiled form, but it will often be

To make debugging and deparsing more difficult, the source code decryption filter implemented in this distribution contains checks to try to disallow the following:

  • Running under a perl that was built with DEBUGGING (-DDEBUGGING);

  • Running under a perl with DEBUGGING flags enabled (-D or $^D);

  • Running under the Perl debugger (-d);

  • Running under the Perl compiler backend (-MO=Deparse).

You should also not use a perl that was built with C debugging support enabled (e.g. gcc's -g option, or cl.exe's /Zi option) and should strip the perl executable to remove all symbols (e.g. gcc's -s option).

None of the above checks are infallible, however, because unless the source code decryption filter module is statically linked against the perl executable then users can always replace the perl executable being used to run the script with their own version, perhaps hacked in such a way as to work around the above checks, and thus with debugging/deparsing capabilities enabled. Such a hacked version of the perl executable can certainly be produced since Perl is open source itself.

In fact, it is not difficult for suitably experienced hackers to produce a modified perl executable that makes it absolutely trivial for them to retrieve the original unencrypted source code with comments, whitespace and all (i.e. not just a deparsed reconstruction of it). One example that was mentioned in the perl5-porters thread cited above is to modify the perl executable to simply print each line of the decrypted source stream that is fed to the parser, rather than parsing and running it!

A typical hacker's opinion of all this is perhaps the following delightful message that I received off-list during that perl5-porters thread from someone who shall remain anonymous:

    "If you don't want anybody to see your source code, why don't you
        -- Klortho


Before version 2.00 of this distribution, encrypted source code was simply the raw output of the chosen encryption algorithm, which is typically "binary" data and therefore susceptible to breakage caused by perl reading source files in "text" mode, which has become the default on Windows since Perl 5.13.11 (specifically, Perl core commit #270ca148cf).

As of version 2.00 of this distribution, each byte of encrypted source code is now output as a pair of hexadecimal digits and therefore no longer susceptible to such breakage.



Patches, bug reports, suggestions or any other feedback is welcome.

Patches can be sent as GitHub pull requests at https://github.com/steve-m-hay/Filter-Crypto/pulls.

Bug reports and suggestions can be made on the CPAN Request Tracker at https://rt.cpan.org/Public/Bug/Report.html?Queue=Filter-Crypto.

Currently active requests on the CPAN Request Tracker can be viewed at https://rt.cpan.org/Public/Dist/Display.html?Status=Active;Queue=Filter-Crypto.

Please test this distribution. See CPAN Testers Reports at http://www.cpantesters.org/ for details of how to get involved.

Previous test results on CPAN Testers Reports can be viewed at http://www.cpantesters.org/distro/F/Filter-Crypto.html.

Please rate this distribution on CPAN Ratings at http://cpanratings.perl.org/rate/?distribution=Filter-Crypto.



Filter::Crypto::CryptFile, Filter::Crypto::Decrypt, PAR::Filter::Crypto;

perlfilter; Filter::decrypt;

PAR; PAR::Filter.

In particular, the Filter::decrypt module (part of the "Filter" distribution) contains a template for a Perl source code decryption filter on which the Filter::Crypto::Decrypt module itself was based.


The latest version of this module is available from CPAN (see "CPAN" in perlmodlib for details) at

https://metacpan.org/release/Filter-Crypto or

http://www.cpan.org/authors/id/S/SH/SHAY/ or


The latest source code is available from GitHub at https://github.com/steve-m-hay/Filter-Crypto.


See the INSTALL file.


Steve Hay <shay@cpan.org>.


Copyright (C) 2004-2010, 2012-2015, 2017 Steve Hay. All rights reserved.


This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself, i.e. under the terms of either the GNU General Public License or the Artistic License, as specified in the LICENCE file.


Version 2.08


31 Jul 2018


See the Changes file.