Methods for protecting a web form without captcha

What are we talking about?

Recently on Habré quite a lot of ideas for captcha have been proposed. Complex, smart, funny, captcha remains one of the main ways to protect the form from bots.

However, at the same time, captcha is a usability problem, because it forces the user to perform an extra action.

In this overview post I would like to consider the methods of protection from bots that are invisible to the user.
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Protection methods

Minimum time to fill the form

The essence of the method is that the server notices the time to create the form. If the user has filled out a form in less than a certain time, then he is considered a bot. Time can vary depending on the complexity of the form.
It is also true that if the form was not filled out too long, then something is wrong.

Hidden field

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HTML

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