Whenever anybody feels the need to ask "Can I do X without using any modules?", the first thing to do is check the existence of modules in Pure Perl. You can always just lift the code out of the modules, and paste it into your script. There's nothing magic about modules, except maybe that they are "run" at compile time — taking care of proper initialisation. So maybe you'll have to tweak the code a little to make it work right: move it around a little, add BEGIN blocks, ...

The conventional nomenclatura for such modules, if one exists in XS, is to add "PurePerl" or "PP" to the name So, looking for crypt PP, I found these likely candidates:

• Crypt::Twofish_PP
• Crypt::Rijndael_PP
• Crypt::GOST_PP
• Crypt::Blowfish_PP
• Crypt::CAST5_PP
• Crypt::DES_PP

I don't know if they really all are encryption+decryption modules, but it looks like enough choice to get you started.