$_GET Variables - How to ensure privacy?

Question

I've written a licensing script for some modifications I release freely, and part of my terms and conditions states that upon installation the modification will send the following information for storing in my database:-

Domain Name where the mod is installed
Email address of the site owner
Modification name
Modification version number

All of my modification code is ioncube encrypted, and I'm sending the values through an image URL like so:

http://www.mydomain.com/licenses.php?modname=".$modname."&modversion=".$modversion."&domain=".$domain."&email=".$email."

Im using an image so that I can return an image to the users browser showing whether a code update is available based on what the user currently has installed.

I'd like to ensure user privacy when sending between domains and send these values encoded, but I'm unsure what encryption is the most appropriate for this situation? I need to be able to decode the values for comparison against the database, but anything that can be decoded by me could easily be decoded by anyone else? What am I missing here? Im sure others have tried to do this thousands of times before

Any pointers would be most appreciated :) Thanks

Answer 1

Don't reinvent the wheel. Use HTTPS/TLS/SSL. If you tell us what web server you're using for the destination, we can point you to setup guides. You may want to use POST in addition. However, it's not really necessary, because TLS will protect the entire query string. A common reason to use POST for secure data is to prevent people accidentally storing data in bookmarks, history, etc., which I don't think applies here.

Answer 2

"I need to be able to decode the values for comparison against the database, but anything that can be decoded by me could easily be decoded by anyone else?"

You can check mcrypt.* and mdecrypt.* which provide symmetric encryption and decryption using libmcrypt (available at the php manual).

Answer 3

If you have a dedicated server running Apache, you can always rewrite htaccess modules.

This way you can hide Get variables or show to users whatever you want.

@Matthew Flaschen is a viable option too.

Answer 4

you might try to alter your app a bit so that the customer registers on your system and then they get something like an API key which is passed back to you. That way you don't have to pass back their personal information on every request - just pass the anonymous API key.

otherwise you can crypt the data, but people will be able to see your crypt key. you could use public key encryption, of course but that may be tricky to implement