Question: I've a website where I'm providing email encryption to users and I'm trying to figure out if there's a way to detect **if a user is human or a bot**.
I've been digging into **$\_SESSION** in php but it's easy to bypass, I'm also not interested in **captcha**, **useragent** or **login** solutions, any idea of what I need ?

There are other questions very similar to this one in SO but I couldn't find any straight answer...

Any help will be very welcome, thank you all !

Answer: 
I've seen/used a simple arithmetic problem with written numbers ie:

Please answer the following question to prove you are human:
"What is two plus four?"

and similar simple questions which require reading:

"What is man's best friend?" 

you can supply an endless stream of questions, should the person attempting access be unfamiliar with the subject matter, and it is accessible to all readers, etc.
I suggest getting the Growmap Anti Spambot Wordpress plugin and seeing what code you can borrow from it or just using the same technique. I've found this plugin to be very effective for curtailing automated spam on my WordPress sites and I've started adapting the same technique for my ASP.NET sites.

The only thing it doesn't deal with are human cut-and-paste spammers.