Thoughts on Tiny Encryption Algorithm (TEA, anyone)?

Question

I'm planning to implement Tiny Encryption Algorithm and exchange data between two PHP sites.

Specifically, using the code presented at PHP-einfach.de

Does anyone have any experiences / thoughts / comments on using this algorithm? (other than why are you using it)

Background: Its some code that i'm planning to put in a drupal module. the drupal module allows authenticated users of that site to connect to my site. I'm planning to give secret apikeys to these drupal site owners. The code on the drupal site will encrypt data: "userid-commandid-time" with the apikey and show this as a hidden field on a form. Their users submitting that form will be redirected to my site with this encrypted data.. When i get it, i'll decrypt it using my copy of the apikey and check the other data values. Time is being sent to make the request valid for a limited time only.

Also, i did consider oauth, but we're later in our development cycle and work is pretty much done. the system does not have to be 100% secure, its going to be exposed to a limited set of people (the drupal module users). it just has to be reasonable enough to dissuade most wannabe pranksters. gaining access to the key and our encryption methodology would at most allow the hacker spoofed (but limited) access to our system.

Answer 1

TEA is cryptographically broken. Its successors XTEA and XXTEA fare much better in that respect. Also, the performance of the *TEA algorithms is not very good.

The good point of TEA is the tiny code size -- this is a great asset in situations where code size is constrained, e.g. when you want to learn the code by heart, or when the code is part of often-downloaded software (e.g. a Javascript implementation). This does not apply to a PHP implementation, which stays server-side. So there is little point in using TEA or XTEA, and my advice would be to stick to a faster, more secure and more standard encryption algorithm (i.e. AES, also known as Rijndael).

(Also, encryption is only part of a data exchange protocol; you also need some kind of integrity protection, and there is the whole issue of key management. Usual recommendation is to rely on an existing protocol which already covers all the tricky details, first and foremost being SSL/TLS.)

Answer 2

I agree whit Thomas Pornin previous post.

I also have some comments on how you do the cross site authentication method. I recommend that you do the authentication on the server side. instead of giving the user the authentication info give it a unique random value instead. Then also send that value to the site you want the client to login to and ask it to return the user that present that random value.