Here's another attempt.
For each "approved" plugin, you would compute the file length and MD5 Message Digest. You would have a file on your site that contains this information for each of the plugins. Each line of the file might look like this: PLUGIN_NAME,LENGTH,MD5. You might also want to include a version number or date for the file in the first line.
Users of your script would download that file from your site, and your core script would read it to validate the plugins.
