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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
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
Also, with the introduction of the Perl Compiler backend modules, and the B::Deparse mod-
ule in particular, using a Source Filter to hide source code is becoming an increasingly
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 pres-
ence 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 pur-
posefully 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.
19th December 1995
perl v5.8.0 2003-01-27 decrypt(3)
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