I'm building a website and my payment methods will be Google Checkout and Paypal. There will be links/buttons which will redirect the user to the secure Google/Paypal sites for processing the payments. This means I do not need the $150/year added expense and complexity of installing SSL certificates for my site.

However I would like to encrypt user's passwords as they are logging in so that if they are on a network some malicious person running FireSheep etc can't read the user's actual password as it is being sent to the server. The rest of the site doesn't need encryption as it's not really sensitive data and would probably slow the user experience down significantly.

My thoughts are this could be implemented with public key cryptography. Lets say the process goes something like this:

Public key is in the JavaScript external file, private key in PHP on the server User enters their username and password into the form and clicks submit The JavaScript runs and encrypts the password, storing it back in the text field Form is submitted to server and the password is decrypted with PHP Plain text password in PHP is salted & hashed then compared to hash in database. Possible similar process for the registration/change password functions.

I'm thinking something like RSA would do the trick. But I've hunted around the net for a working JavaScript library to do it but none seem to be compatible with the PHP libraries available. At any rate it needs to generate a set of keys that are compatible with the JavaScript and PHP.

Anyone know of an actual working solution for this? If not how about we write one then open source it. Unfortunately writing encryption/decryption code is pretty complex so I don't really know exactly what the existing libraries are doing and how to modify them to make it work. I already have protection for session fixation/hijacking so I'm not interested in that. Just interested in encrypting the data before it gets to the web server.

NB: Please don't post a bunch of links to standalone Javascript or PHP encryption libraries, I've found those already on Google. That's not actually useful. What I need is code for JavaScript encryption AND PHP decryption that actually works together harmoniously to produce the intended result outlined above.

Also if you could refrain from posting comments like "just use SSL". I'd actually like a solution to this exact problem even if it's not best practice, it would be interesting none the less.

Many thanks!